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The Direct Current Distributing Systems of American Cities

( Originally Published Early 1900's )

THE INTENT of this paper is to describe the electric distribution system which is commonly in use in the central areas of the larger American cities and to state the reasons for the adoption and retention of that system. I approach the subject from the commercial standpoint, taking conditions as they have existed and as they now exist—not as they might exist. In a sense my paper is intended to be a justification of the methods of the larger electric light companies of the United States.

In its essentials the system of distribution which I describe is that which has been employed by the Edison companies of the United States for twenty years past. The independent steam power district stations of the original Edison plan have in many cases been replaced by substations receiving and transforming power transmitted from a single generating station. The distributing system is thereby affected only in that such a substation may be conveniently or profitably placed in a location where a steam plant would be unprofitable or intolerable and that consequently in any given area the number of substations is likely to be greater than the number of steam stations which would have been provided to serve the same area. The general method of distribution, however, stands unchanged.

The Edison distribution system as it exists today is a network of three-wire mains, underground or overhead, supplying continuous current (as we express it, direct current) at constant pressures of approximately 115 and 230 volts from the one system of mains for all purposes for which current can be used. The mains of the system are continuous and interconnected throughout the area to be served. There may be one or more generating stations or substations supplying the same network. Each station or sub-station feeds the network through a number of feeders; the feeders being so proportioned and so regulated as to maintain practically constant voltage between the several wires of the three-wire. system under all conditions of load, while the generated pressure is varied to compensate for the fall of potential in the feeders.

The three-wire method; the interconnected network supplying current at constant pressure; the furnishing of all demands from the same system of mains; the feeder and main method of equalizing the pressure throughout the network—these are now all accepted as standard throughout the world. It must not be forgotten, nevertheless, that in the days of fifteen to twenty years ago when electric lighting had its early commercial development in the United States, these methods were Edison methods and Edison methods alone. Today the Edison distribution system has general acceptance and approval, save only in respect of its adhesion to direct current and to a voltage which many engineers think should be doubled.

In preparing this paper I have made no attempt to collect statistics. I have drawn my illustrations primarily from personal knowledge of the distribution systems of several large American cities and where personal knowledge has not been adequate I have obtained specific statements from friends of whose knowledge I am surely advised. To a great extent the statements which I make are familiar to and will be accepted as correct by American electrical engineers. Many of them will indeed to that portion of the audience be truisms, and it is solely because of the international character of this meeting that I have not assumed them to be matters of general knowledge.

In speaking of large cities I have in mind cities of a population of 250,000 or upwards. It is to be noted, however, that what I may call the Edison system of distribution is to be found in many cities of a much smaller population. Generally speaking, that system will be found in any American city which has a well defined business district. It was in the business and shopping districts as distinguished from the residence and manufacturing districts of our cities that the Edison system had its early development. In these districts only could—at least in the early days of electric lighting —such loads be obtained as would justify the installation of under-ground mains. In later years, with the increasing use of electric light and power, the Edison distribution system has been extended to residence districts and in some cities throughout the entire settled area, these extensions being with underground or overhead mains as the demand for service, or as municipal regulations, might dictate.

The companies using the Edison system do not abjure the alternating current. They use it not only in transmission, but for general distribution in areas where the density of business is not sufficient to warrant the construction of an Edison network. They supply alternating current to motors, both polyphase and single-phase, in these outlying areas. In fact, the so-called Edison companies are among the largest distributors of alternating current for general use.

In the immediately following paragraphs I discuss my subject in detail; noting particularly such points as may be of interest to engineers not familiar with our methods; and stating reasons for our practice where reasons are not obvious or of general knowledge.

Stations and Substations

In the older steam-driven generating stations each engine carries two dynamos connected one on each side of the neutral wire of the three-wire system. These dynamos are shunt wound and can be regulated by hand for any voltage between the standard pressure of the network and the maximum pressure required to supply the longest feeder. In stations constructed within the last six or seven years the pairs of dynamos are usually only sufficient to provide for the balancing of the system and single dynamos of the voltage proper for connection across the outer wires furnish the greater part of the output. It is not customary to depend upon motor sets nor upon a storage battery for balancing but storage batteries are freely used as auxiliary or reserve sources of current and serve incidentally to maintain balanced voltage.

Battery Substations

Substations are in some instances purely storage battery stations in which case the battery is charged from the network by motor-driven boosters during the hours of light load or else through a direct current tie line from the nearest generating station. A well known instance of the storage battery substation is the Adams Street station of the Chicago Edison Company where are installed batteries capable of supplying current during the 1 1/2 hours of daily maxi-mum demand at the rate of 13,500 amperes on each side of the three-wire system. Substations receiving alternating current and transforming it to direct are frequently equipped with storage batteries. This is the standard practice of the New York Edison Company.

Rotary Converters and Motor-Generators

The apparatus for transforming alternating current to direct has thus far in the majority of cases been the rotary converter, but there is a large minority preferring and using motor-generators. As a rule, those companies whose alternating current transmission is at 25 cycles use rotary converters for transformation and those other companies whose alternating current transmission is at 60 cycles prefer motor-generators. There are, however, some notable exceptions; as (for instance) the Cleveland Electric Illuminating Company which uses 60-cycle rotary converters, as also do several Pacific coast companies; and the Buffalo General Electric Company which uses motor-generators to convert the 25-cycle Niagara supply. The most common size of converter or of motor-generator set is probably 500 kilowatts, but larger units are in use and are growing in favor. The standard rotary converter of the New York Edison Company has a capacity of 1000 kilowatts and one machine recently put in service by that Company and others now being manufactured for it have a capacity of 2500 kilowatts without exceeding a safe temperature. The Buffalo General Electric Company has had a motor-generator set of 1000-kilowatt capacity in service for three years. The Edison Company of Detroit has three motor-generator sets of 1000-kilowatt capacity in service and three others being built. When motor-generators are used, the transmission voltage is supplied to the motors without reduction, and the choice between the synchronous motor and the induction motor is usually in favor of the synchronous machine; but there are also a large number of induction sets of the average size. All the small sets—say below 250 kilowatts—have induction motors, and at least one set having an induction motor of 1000 kilowatts is in regular and satisfactory service. I have thought it well to call attention to the use of motor-generator sets because there appears to be among our European correspondents the impression that American engineers are definitely committed to the rotary converter for all uses, whereas it is only in railway work that the rotary converter can be said to be the accepted thing. The lighting companies of Boston, Buffalo and Detroit use motor-generators exclusively and the Philadelphia company uses them along with rotary converters.

Balancing between the two sides of the three-wire system is obtained in substations as in steam stations by the use of pairs of generators. Rotary converters connected across the outer wires have the neutral of their transformer system connected to the neutral wire of the three-wire mains. When there are numerous substations on the same network it is obviously not necessary to install a balancing set in each substation; the less so when batteries are installed.

Multiplication of Substations

The system tends toward the multiplication of substations. As noted in a previous paragraph it is a much more convenient and less expensive matter to build a substation and equip it with rotary converters or motor-generators than it would be to build and equip a complete steam generating plant. Many substations are now operated as one watch stations; that is to say, they are operated only during a short period daily, which period is covered by one staff of operators. The staff in a number of instances within my knowledge is reduced to one man. This condition prevails in districts such as residence districts where there is an evening load but no day load; or (to be more precise) where the day load is so small that it can be supplied through the network from distant generating stations. A similar condition obtains in the office building and wholesale districts of the larger cities where there is a heavy daylight and early evening load and little or no load during the night. Such substations may be operated by two 8-hour shifts of men. The duties of employes operating motor-generators or battery stations are very simple. These employes are under the control by telephone of the central or district operating chief to whom any question arising in practice is referred and by whom instructions are given in case of accident or emergency requiring unusual action. It follows that the wages paid substation operators are not large and that, therefore, there is little or nothing to be gained by the concentration of plant or by the installation of automatic regulators.

Obviously the multiplication of substations means the reduction of the investment in feeders. The network of mains will be the same whether it is served from one station or from several, but the cost of feeders will be much reduced if the service is from several substations instead of from one. In general it pays to increase the number of substations so long as the reduction in cost of feeders will pay for the land and buildings. The depreciation of land and buildings is less than the depreciation of feeders—sufficiently less in many cases to pay the additional wages required by the extra substation. This general rule has its obvious limitations. We cannot afford to put up a toy substation to save a few thousand dollars investment in cables; but when we are installing converting equipment in 500-kilowatt or 1000-kilowatt units, the cost of land and buildings is a small proportion of the total expense. It is to be remembered also that it is not necessary to carry complete reserve equipment in each substation. Any substation can by raising the generating pressure force current through the network into the area normally served by an adjacent station.

Regulation of Pressure

Pressure wires led back from feeding points are used to indicate in the station or substations the pressure maintained on the system. The regulation is performed by a switch board attendant. Neither automatic regulating devices nor the compound winding of generators have been approved by the Edison companies. The switch gear is so arranged that separate groups of feeders may be connected to separate generators or banks of generators. The longer or heavier loaded feeders are thus connected to and served by a generator which is operated at a higher pressure than that required by the shorter or less lightly loaded feeders. In some stations provision is made for as many as five groups of feeders. Obviously all the generators are in parallel with one another through the network although not directly connected in parallel at the station, and the division of load between the generators is accomplished in the usual way by increase or decrease of field strength. The variation of field strength sufficient to cause the generator operating any group of feeders to take its proper share of the load is not sufficient to cause any material variation of the pressure in the network. This method of grouping feeders, together with the customary proper adjustment of the cross section of each feeder to its length and load, provides sufficiently for ordinary equalization of pressure throughout the network. Rotary converters are, of course, regulated by induction regulators or by variable ratio transformers—not by variation of field.

Distribution Network

The network itself may be either overhead or underground, but in all cities of the first-class the mains in the central districts have been underground from the beginning. In small cities and in the less densely populated districts of the large cities overhead mains are customary. There is nothing unusual about the construction of these overhead mains, the methods common for many years in telegraph work being followed. The wires or cables are covered with two or three braids of cotton saturated with ozokerite or some asphaltic compound, the covering being intended for the convenience of the erecting and maintenance men rather than as an insulation. In dry weather these braided wires carrying currents of comparatively low potential can be handled or laid on the earth or even swung together without making trouble, whereas bare wires cannot be so dealt with and would require much greater care in handling. The neutral wire of the network, whether overhead or underground, is always connected to earth. This has been customary for ten years past.

Underground Conductors

The Edison tube is still in use as a distributing main. It is likely to continue in use for many years to come. As manufactured during the last ten years it has proved to be a reliable and convenient form of conductor. The only radical difference between the older and newer form of Edison three-wire tube is that in the new tube more space is left for insulating compound around the coppers. Tubes of 2 inches internal diameter containing three 200,000 circular mils (0.157 sq. in.) coppers, and tubes of 2 1/2 inches internal diameter containing three coppers either of 350,000 circular mils (0.275 sq. in.) or of 500,000 circular mils (0.392 sq. in.) are the sizes in most common use. When larger carrying capacities are required in the one main it is now customary to employ cables, such cable mains and all feeders laid in recent years being usually of paper insulated and lead covered cable drawn into vitrified tile ducts. There is in service much cable laid "solid," but the solid method is not now approved. The "draw-in" method has been found not only more convenient but cheaper.

Fusible Links

The connections between sections of the underground network are fusible links of sheet copper of such dimensions that they will fuse at twice the "overload rating" of the mains they are intended to protect. The overload rating is the current which the main will carry for a short period—say 2 hours—before it attains an unsafe temperature. This is a much greater current than could be continuously carried. The copper fuse link has a considerable' time constant—4 or 5 seconds—and obviously it will not be melted by any momentary overload. Neither will it be melted by the extra current flowing when the failure of a feeder causes the supply of a district to reach it through adjacent mains instead of by the normal course. But the copper link melts promptly in case of a short circuit between the conductors of the underground system, and that is all that is expected of it. In practice a fault in the mains is either burned off, or "blown loose" by the melting of the fuses connecting that particular length of mains to the network. A fault in a feeder is usually dealt with by the operator pulling the switch at the station. The return current from the system then melts the fuse at the distant end of the feeder. These links are a convenient means of disconnecting sections of mains for testing or during repairs. Similar links are used to some extent on overhead mains, but it is much more common to connect up the overhead network without fuses. It goes without saying that every service into the premises of a customer has fuses at the entrance point.

Direct Current Distribution

Distribution at constant pressure is universally accepted. Distribution of direct current is, however, an Edison practice much criticised or challenged in the past and present by engineers who believe in alternating current. In later years, when transmission by alternating current is so frequently combined with distribution of direct current, the challenge usually takes the form of a query as to what justification there can be for the expense of converting machinery requiring substations and attendance for its housing and care. The challengers assume or state that alternating current will meet all practical demands and that, therefore, its conversion to direct current requires needless investment and needless operating expense. That is an assumption which I will discuss later.

The immediate justification of our adoption of direct current is historical and commercial rather than technical. Nevertheless the technical justification is in itself sufficient and seems likely to continue sufficient for several years to come.


I give first the historical justification. It is that our industry obtained its first great development in the years 1885 to 1890. Before that time and during that time the Edison system was the only complete practical system of distribution known to the art of electric lighting. By 1885 the system had been developed in all its essentials. Generators which would operate in multiple, the feeder and main system, three-wire mains, distribution at constant pressure, practical underground conductors, practical incandescent lamps of high resistance, practical motors—all of these were found in the Edison system and nowhere else. By 1890 this condition was not materially changed. During the years 1885 to 1890 the alternating current dynamo and the alternating current transformer were assuming their practical form. Not until 1890 could it be said that they were satisfactory operative devices. It is well to have this fact firmly impressed upon your minds—that many thousand kilowatts of direct current central station machinery and of direct current motors still in regular and efficient service were in service before the alternating current transformer could be deemed a practical and reliable device.

The advocate today of alternating currents has in the alternating current transformer of today a most efficient and reliable piece of apparatus; but the engineer who as late as 1890 dispassionately considered the possibilities of alternating current transmission was compelled to recognize in the transformer of that day the weak point of his system—weak both in respect of reliability and of efficiency. And when he turned to consider arc lamps and motors he found himself compelled to admit that alternating arc lamps were impracticable and alternating motors were nonexistent.

Commercial Considerations

From the commercial point of view the adoption of direct current distribution was warranted in the beginning by the fact that the capitalist who desired to invest his funds in an electric lighting enterprise could obtain an operative direct current system complete in all its details from the generating plant to the last translating device without engaging in any experiment, and with a positive certainty as to the operative efficiency and a reasonable certainty as to the extent of the repair bill. The capitalist who invested in direct current central station apparatus in the early days of the industry usually made money. The capitalist who invested in alternating current apparatus almost invariably lost money. That this was due to any inherent difference between direct and alternating current is not to be understood by us as engineers, but that such was the understanding or belief of many of the men who supplied the money for the early electric light stations is a fact still vividly impressed upon the memories of some of us. The truth of the matter was that in municipal areas where the conditions predicated the installation of an early Edison system there was likely to be sufficient business to make the installation profitable. An exactly similar district might have supported an alternating station equally well, but the direct current men were first in the field and preempted all the best locations. Alternating current men, imbued with belief in the long distance capabilities of their system, too often undertook commercial impossibilities.

It is necessary in recalling the causes which led to the commercial success of the Edison companies to recognize the effect of the establishment among them almost from the beginning of a uniform system of accounts and reports and of arrangements for the confidential distribution and exchange of practical information. The system of accounts although far from complete served for the making of intelligent comparisons. The correspondence between different companies and the meetings held for educational purposes built up a community of interest which has continued in an active and useful form to the present day. The commercial value of this community of interest—of personal and technical, not financial interest—has been and continues to be very great. That it existed in the beginning only among the so-called Edison companies tended materially toward the permanent establishment of direct current distribution throughout the United States.

The Persistence of Direct Current Distribution

The reasons for the persistence of the use of direct current are not merely commercial, but are technical. The immediate and obvious commercial reason is that the existing investment in direct current apparatus is so great that only some tremendous advantage to be gained by a change—an advantage very much greater than the most extreme advocates of alternating current have yet suggested—could possibly warrant the wiping out of our present equipment. That equipment includes not only our central station machinery but the immense investment of our customers in motors and the investment made both by our customers and ourselves in arc lamps. How great these investments are must be obvious even to our most recent visitor. It is well, perhaps, to give some idea of the investment in direct current motors which, scattered around in customers' premises, are not so apparent to a chance visitor. On the Island of Manhattan, for instance, there are connected to direct current distributing circuits approximately 15,000 motors, having a capacity of 85,072 horse power. In the direct current area of Chicago the connection is over 9000 motors, having a capacity of 43,230 horse power. In the comparatively small city of Detroit the connection is 872 motors of 4541 horse power. You will note that the average horse power is small. You will please also understand that this connection does not include fan motors, dental motors and similar little machines. It includes only motors of 1/2 horse power and upwards. The expense of substituting alternating current machines for these direct current machines would have to be borne by the electric light company. The customers would not stand it.

Another commercial consideration is that our new alternating construction would cost but little less than direct current construction. Our generating equipment would not be altered in cost. Our equipment of motor-generators or rotary converters would, of course, be replaced by transformers and there we would have a material saving; but the motor equipment hereafter sold to our customers would cost more, as also would arc lamps. In respect of meters we would, because of our ordinary use of the Thomson integrating wattmeter for direct current service, save some money. The induction type of meter for alternating currents is cheaper both in manufacturing cost and selling price and better maintains its initial accuracy of registration. The present difference in cost of arc lamps is considerable. A cheap but practical direct current arc lamps costs less than $10. A similar alternating lamp costs approximately $13. The same ratio obtains between the cost of direct and alternating arc lamps of the better qualities. As to the difference in cost of motors, I think that is well shown by Table II, in which I show actual prices quoted on a recent purchase of motors. The motors priced are of similar workmanship and efficiencies.

At this point I expect to be challenged by some engineer, perhaps one of our European visitors, who will insist that the lower price of the direct current motor is unnatural and artificial. I admit that the difference in price between direct current and alternating current motors is at present greater than is warranted by their respective shop costs. On the other hand, I doubt seriously whether even the polyphase motor—not to say the single-phase motor—will ever be built in American shops at a cost materially less than the direct current motor. The single-phase motor, afflicted either with a commutator or a condenser, is obviously an expensive machine to make. The polyphase motor, however, with a simple rotor and no commutator may in time be built as cheaply as- the direct current ma-chine; but it is to be remembered that the two or four or six field coils of the direct current machine are much simpler propositions in the machine shop and in the winding room than is the wound stator of the induction motor. The rotor of the induction motor should not cost more than the armature of the direct current ma-chine. Either motor requires a starting device. The direct current motor is by long custom equipped with a starting rheostat combined with an automatic disconnecting device. The polyphase motor, if it is to be started without disturbing the regulation of the lighting system, must have either a device for interpolating resistance in the rotor circuit or a variable ratio transformer commonly called a compensator. The cost of either of these is greater than the cost of the exceedingly simple direct current rheostat. I am inclined to believe that the cost of the commutator and brushes on the direct current machine will continue to be offset by the greater cost of the stator winding and the starting device of the polyphase motor. Commutators are built cheaply in American motor shops and make very little trouble in operation. This latter statement is true since the adoption of the carbon brush. Our motor inspectors find very little commutator trouble nowadays, and the maintenance of the motor commutator is an exceedingly small item in the expense of operation.

Alternating Current Distribution

It is claimed by the advocates of alternating distribution that substations are unnecessary if the alternating transformer system is used. In practice we do not find this to be so. We find that while the number of substations may be reduced, yet the requirements of regulation make some substations not only desirable but necessary. It is out of the question to control the distribution throughout a large urban district from one regulating point-the more so if that point be located at a distant generating station. We hear from time to time rumors of this being successfully done but when we follow up those rumors we find either that the load is essentially constant or that the exactness of regulation required is far less than our standard of 2 per cent plus or minus. We do not see our way to change our standard of regulation; neither is it possible for us to modify the load requirements of our urban areas. The variation of load in these areas is so effectively dealt with by our substation system that we do not look kindly upon any less effective method.

Moreover, the advocate of alternating current does not nowadays approve of scattered transformers. He used to tell us that transformers might be placed any place, that the space required was negligible, and that the less the length of secondary mains between transformer and translating device the better would be the service. Now we are told that a secondary network is as desirable with alternating current as it is with direct current and that the transformer capacity should be concentrated in as few units as possible. When these units reach the 500 or 1000-kilowatt size (as they certainly must do under our urban conditions) some kind of a substation is necessary to hold them and the step from a shelter substation to a regulating substation is a small one.

Motors and Arcs

There will, therefore, be little or no change in our network nor in our system of feeders. The principal change would be the substitution of large transformers for the motor-generators or rotary converters. But when we reach the translating devices there must be radical changes. The difference in cost of motors and in arc lamps has already been noted. The difference in efficiency deserves consideration. That there will be any material difference of motor efficiency does not appear. For equal ultimate cost, over the range of sizes required in a commercial distribution, the efficiency of either type of motor will be practically the same. The amount of light given for equal energy by the alternating arc lamp is probably equal to that given by the direct current lamp. The steadying resistance of the direct current lamp can be dispensed with and thereby a considerable saving of energy effected but the character of the light given by the alternating arc is thoroughly unsatisfactory. When the two services are operated side by side, the user invariably demands the direct current service even when he has to pay for the loss in the steadying resistance. This is not a theory. This is actually a commercial condition and its technical reasons are so well known that I need not state them here. The change of interior illumination from direct current arcs to alternating current arcs can practically be accomplished only by making concessions in price to the customer. To change from direct to alternating arcs for exterior illumination is easier but still it is an undertaking requiring much tact.

Nernst and Incandescent Lamps

With Nernst lamps the alternating current has the advantage. In the United States the Nernst lamp is used solely on alternating circuits, and its experimental use on direct current circuits has not been a commercial success. The lamp gives good light on direct current but the life of the glower is shortened to an extent which makes a serious difference in the operating cost. Nernst lamps are not yet a large factor in our business although their use is steadily increasing. Incandescent lamps are, of course, equally effective with either direct or alternating current.

Storage Batteries

The storage battery is not a practical auxiliary to an alternating current system. Its use on such a system requires the interpolation of motor-generators or converters having a capacity equal to the maximum output of the battery. The Edison companies of the United States have for years been free users of the storage battery. The number of electric automobiles is constantly increasing. The custom of the owners of electric automobiles is to charge their vehicle batteries from their house supply. Obviously a house supply of alternating current would mean the installation and maintenance of converting apparatus.

Minor Uses of Direct Current

There are a number of minor uses of direct current as in electrochemical processes, electromedical apparatus, mercury arcs used by photographers, etc. These uses are not now important but deserve mention. Some of them may become important.

Regulation Required by Incandescent Lamps

In a preceding paragraph I have noted that our rule is to maintain a regulation within 2 per cent plus and minus of the declared voltage. Our practice is somewhat better than our rule. We have learned by experience that the efficiency and durability of incandescent lamps depend mainly upon exact regulation of voltage. I desire to remind our foreign visitors that most of the large American companies own the incandescent lamps used by their customers, and that all of them renew those lamps either free or at a price considerably less than the cost of the lamps. I desire to remind our visitors also that the great majority of our lamps are 16 candle power lamps, using 50 watts, the candle power being in terms of the English standard candle measured horizontally when the lamp is rotated in an upright position. Carbon filament lamps run at the temperature corresponding to the figures just given are sensitive to variations of voltage—blackening rapidly if they are overrun and failing to give satisfactory light if underrun. The failure to give satisfactory light brings immediate complaints from customers. Overrunning increases rapidly the cost of lamp renewals. Please remember that our custom is to renew a lamp as soon as it is blackened to such an extent as to displease the customer's eye. We do not require that the filament be burned out; in fact some of us so effectively encourage customers to bring in their lamps for renewal that comparatively few lamps are burned out and that the average service of a 16 candle power lamp before it is exchanged is little in excess of 400 hours. Our policy in respect to incandescent lamps is dictated by our belief that what our customers expect to purchase from us is light, and that we cannot supply their wants by selling them electric energy and permitting them to select and maintain their own incandescent lamps.

I expect that my statement as to our customary use of lamps having an efficiency of 3.1 watts per candle will be challenged by some of our visitors. I find that my British correspondents are generally incredulous on this point; their incredulity being based upon their own experience rather than upon a knowledge of ours. We have, however, been purchasing lamps sold as of an efficiency of 3.1 watts for fifteen years past, and for more than seven years past we have known positively that we got what we purchased. Our present specifications for incandescent lamps—that is to say, the specifications adopted by sixty or seventy of the large lighting companies forming the Edison group and also by several companies outside of that group—require that lamps of a candle power of 16 and upwards shall have horizontal candle power at marked voltage of 1 candle for each 3.1 watts supplied. Under this condition the lamp is required to last in continuous service over 470 hours before its candle power is reduced by blackening below 80 per cent of the initial candle power. The specification is filled in practice. That it shall be filled is secured by factory inspection and tests made by a special testing bureau which has been in operation for over seven years. The customary tests include (interalia) the measurement of watts and candle power at marked voltage of 5 per cent of all the lamps manufactured for us, the lamps measured being selected at random before they are packed—not from the top of the barrel after packing. Out of this 5 per cent, one-tenth, that is to say, one-half of one per cent of the total product, are given a continuous life test, being run at marked voltage until the candle power has fallen to 80 per cent of its initial value. The testing organization which I refer to inspected, tested and approved for shipment during the last twelve months for which the records are complete, five and three-quarters millions of lamps of 16 candle power at 58 watts, in addition to a very considerable number of lamps of higher candle power and similar efficiency. Nor are the Edison companies the only users of such lamps. There are other users who purchase lamps subject to similar tests; and over and above these inspected and tested lamps there are sales by reputable factories of lamps not similarly tested but made to conform to similar specifications. I have thought it necessary to give these details because so many of my British friends believe a 3.1 lamp to be commercially impossible. It certainly is commercially impossible so long as you leave your customers to buy their own lamps.

The reason for our use of an incandescent lamp of this comparatively high efficiency is our desire to furnish the most light at the lowest cost. We have many times and under many conditions calculated the effect on our investment and on our operating costs of the substitution of a lamp of lower efficiency but the general result is invariably the same; the precise values obtained by the calculation varying only with the minor conditions. Please under-stand that we recommend and use lamps requiring more current for the same light—that is to say, lamps of lower efficiency—when good regulation is hopeless; as for instance, on circuits where a large proportion of the load is of necessity in the form of motors having unsteady or irregular loads; or where the total work to be done is not sufficient to warrant the cost of good regulation. In our central station practice, however, we have always set reliability of service first; good regulation—that is to say, good quality of service—second; and low operating cost third. And although, as already said, our central station calculations invariably justify the use of the 3.1-watt lamp as giving the most light for the least money spent in fixed charges and operating charges, I am inclined to believe that even if our calculations gave results warranting the use of a lamp of lower efficiency we would still insist for business reasons on the same effective regulation of the light given. A company which gives good steady clear light deserves and can expect a constant growth of its business. A company which offers an inferior service even at a reduced price fails to meet the commercial requirements of American cities.

Comparative Regulation of Alternating and Direct Systems

This discussion of the regulation required by incandescent lamps leads to consideration of the comparative regulation of alternating and direct current systems. The broad difference between the two is well known to you. The regulation of a direct current system between dynamo and translating device depends solely upon the resistance of the circuit. The regulation of an alternating current system depends upon resistance, inductance and power factor of the load. The power factor for incandescent lamps and for Nernst lamps is identical with either current. The power factor for arc lamps and for motors under average central station conditions is very seriously different. A mixed load, supplied through transformers, of incandescent lamps, arc lamps and motors in the proportions common in central station practice will have a power factor seldom better than 0.9 and frequently lower than 0.8. These figures are not theories—they are observations. Please do not forget that these American direct current central station companies are among the largest distributors of alternating current not only in the United States but in the world. We, therefore, have experience to speak from. Please do not forget, either, that the motor load in American cities is very large and that the arc lamp load is always respectable. During the evening peak the incandescent lights predominate. At other hours the motor load is likely to be one-half or more of the total service. Our direct current methods allow us to follow closely by hand regulation the variations of the evening load. The copper which is sufficient to carry without overheating the maximum evening load is amply sufficient to carry without disturbance of pressure the rushes of current due to direct current elevator motors and similar intermittently operated devices. But that same copper would not be sufficient to take care either of the evening load or of the intermittent service motors if the distribution should be by alternating currents. An increase of at least 10 per cent in copper would be necessary and if the proportion of intermittent service motors were high a still greater increase would be required either in the general network or in the form of a special circuit for intermittent services.

Elevator Service

I suspect that the point of view from which the large American companies regard the elevator motor is not recognized by some of our friends. We do not look upon the elevator motor as an undesirable load, nor as necessarily a disturber of the system. On the contrary we seek to connect so many elevator motors that their individually intermittent operation will provide (because of their number) a continuous load. Momentary loads due to the simultaneous starting of several machines are readily taken care of by the engine flywheels. That seems rather a broad statement but it is literally true. I admit that we are liberal in our provision of flywheel capacity. When we use batteries, of course, the batteries take care of these rushes. Our distributing mains are habitually made so large that elevator motors of from 7 to 30 horse power can be connected without special provisions. The larger motors used for very heavy duties, and the exceedingly rapid acceleration required for the service of the fifteen or twenty-story office buildings, are almost invariably taken care of by a storage battery located in the same building. We do not offer to supply current for such express elevators unless a local battery is a part of the equipment. Our welcome to the ordinary elevator of the six to ten-story office building or apartment house is because we recognize in our acceptance of that electric elevator the certainty that the owner of the building will not install his own steam and electric plant but will depend upon the central station service for his requirements of light and power. At this writing the makers of alternating current elevator motors have not quite succeeded in meeting the requirements of the service. There are many elevators operated from our alternating circuits, but these, unless they are of the smallest size, require a complexity of starting gear which compares badly with the standard direct current elevator equipment, or the acceleration of the elevator is less than would be acceptable under average conditions.

Distribution Losses

It may be claimed that the loss of energy in the distributing network will be less with an alternating supply than with direct current. I question this; in fact I deny it as regards the great majority of existing distributions. In our present practice we may have a loss between station or substation and translating device of 15 per cent during the 1 or 2 hours of the evening peak load in the winter months. Fifteen per cent is an exceptionally high figure-virtually a figure which represents the limit of our practice. Ten per cent represents better the ordinary case. With the multiplication of substations this loss is reduced. The condition of maximum loss in transmission obtains only during a total of 200 to 300 hours per annum. During all other hours of the year the transmission loss is but a small fraction of the maximum. All the copper of the network stays in service and at half load the loss is one-quarter of the maximum, and less in the same order for less loads. The annual loss of energy in an Edison network is some-thing like 4 per cent.

The losses in motor-generators or rotaries are obviously proportional to the load. As the load decreases the machines are switched out of service; and during the longer periods of light load not only individual machines but entire stations or substations are shut down. Assuming that the transformers required for an alternating distribution are located similarly to the motor-generators or rotaries of a direct current distribution, similar switching would then provide similar economy and the greater efficiency of trans-formers as compared with converting apparatus would be realized. But if the transformers were scattered over the network and were not switched on and off as required, the core losses would in a year amount to at least as large a proportion of the output as the total losses in the direct current network. Assume 25 per cent load factor and 1 per cent core loss (the latter being 1 per cent of the total transformer capacity) and the core losses are obviously 4 per cent of the output.

Cost of Generating Apparatus

In a previous paragraph I said that our electric generating equipment would not be altered in cost by a change from direct to alternating current. It may be claimed that the generating equipment would be reduced in cost to the extent of the difference in efficiency which exists between transformers and machinery for converting alternating to direct current. This difference might in an extreme case be 10 per cent, and I have been told that to the extent of this 10 per cent of eliminated conversion loss we could reduce the capacity of our generating plant. I don't see it. We might reduce the capacity of our boilers and engines but we could not reduce the capacity of our electric generators, because the capacity of generators—that is to say,, their size—is a function of volts times amperes, not of kilowatts. The power factor of the alternating current system, 0.8 to 0.9, would (and does) require greater capacity in generators to an extent more than sufficient to offset the saving in substation losses. Please note that I have said nothing about losses in the regulating apparatus required in sub-stations. When rotary converters are used, the regulating devices employed are substantially identical with those which would be required by substation transformers of similar capacity. When motor-generators are used, regulation by variation of field strenght is all that is requisite.

The ratio of units sold to units generated is a useful figure and deserving of study. But that the ratio is low does not necessarily mean that the commercial efficiency of the system is low. Most of those extra units which look so bad in a comparison between units generated and units sold cost but little to generate—so little that the most precise measurement and analysis are requisite to identify the saving made when we cease to supply some part of them. Thus, the energy to be saved by the substitution in sub-stations of transformers for converters requires for its production but little extra fuel and no extra labor. It is a proportional increase in the output of every hour of the twenty-four. But the possible reduction in fixed charges which would follow the reduction of steam plant, by say 10 per cent, added to the reduction of fixed charges following the substitution of transformers for converters —this total reduction of fixed charges would be well worth having. A calculation made for Detroit conditions shows that the reduction in fixed charges might be 7 per cent—not 7 per cent of the total fixed charges, but 7 per cent of the fixed charges chargeable against direct current business. This would permit a reduction of 3 1/2 per cent in the selling price of the same amount of energy or might add proportionately to the profits.

Comparison Between Systems

To sum up the case in favor of the distribution of alternating current in our urban districts: The advantages to be obtained are a reduction in the first cost of engines and boilers; a reduction in the cost of substation apparatus because of the substitution of transformers for converting machinery; and a longer life for the glowers of Nernst lamps. The disadvantages are inferior regulation of voltage or (as an alternative) greater cost of mains and feeders; inferior arc lighting and greater cost of arc lamps; special contrivances required for charging small storage batteries; and the requirement that to each large storage battery be added converting machinery capable of carrying the maximum discharge of the battery. These advantages and disadvantages seem to be permanent consequences of the difference between the two currents. Temporary disadvantages of the alternating current are the greater cost of constant speed motors and the lack of satisfactory motors for elevator work. I am perhaps optimistic in terming these motor disabilities "temporary." My conclusion is, after in many specific cases comparing these advantages and disadvantages, that the direct current is at least equal and likely to remain equal to the alternating current in its suitability for general distribution, and over and above its inherent equality it has the immense advantage of prior occupation of the territory. What this means to us as owners or operators of central station properties I have already indicated. What it means to the general consumer in that every device which he is likely to use has been for years standardized for direct current and is manufactured for direct current by a score or a hundred manufacturers, each and all anxious to obtain the consumer's trade—what this means cannot be expressed within the limitations of a paper for this audience. It may be indicated to you by the fact that in every large city of the United States where direct current and alternating current have been offered competitively to the public, the direct current system is today in possession of the business of the central territory.

The Question of Voltage

There remains the question of voltage. In large American cities we have generally adhered to an incandescent lamp voltage under 125 volts and our three-wire systems are arranged to maintain the selected lamp voltages between outers and neutral. Many of our friends in Great Britain and a very few in the United States have undertaken the supply of urban areas with lamps of double the voltage which we use, and have doubled the difference of pressure between neutral and outer wires of the three-wire system. The reasons for our practice in this respect are (as in respect of our adherence to direct current) both commercial and technical. Technically, our main reason is that there is not made a double voltage incandescent lamp of an acceptable efficiency and life. To use an incandescent lamp of lower efficiency would mean the reduction of the earning capacity of our present networks and generating apparatus; or, conversely, would mean that we would require a greater capacity in generators and mains to do the same amount of business. Another technical reason is the inferiority of the double voltage arc lamp. We formerly used two arc lamps in series. We welcomed the advent of the enclosed arc with its higher voltage because (among other reasons) it allowed us to get rid of these series lamps. Such series arc lamps, moreover, cost more than single lamps and are likely to continue to cost more. The high voltage motor—that is to say, the 440 to 500-volt motor—is a more expensive machine to build and costs more in the market than the motor of 220 to 250 volts—that is to say, it is so in the average sizes used by our customers. Moreover, it tends to have commutator troubles, from which the 220-volt motor is notably free. The insulation which must be maintained for double voltage is markedly greater than that which is permissible at the present voltage. Moreover, our practice of handling our mains alive—making connections and disconnections and minor repairs—would have to be changed if we carried twice the present pressure. Our customers' meters are all watt-hour meters having a potential circuit connected across the mains. In three-wire meters the potential circuit is connected from one outer to the neutral wire. The loss of energy in this potential circuit is as great as we are willing to accept and we do not take kindly to the idea of doubling it. Of course, we could use ampere-hour meters as do so many of- our British friends and thereby do away with all losses in potential circuits, but to do so would be to get out of the frying pan into the fire. We would in that case in doubling the voltage double the "slip"—that is to say, we would double the amount of energy passing through the consumer's meter unregistered.

Incandescent Lamps of Double Voltage

The proposed use of double voltage incandescent lamps deserves special discussion. In preceding paragraphs I have recited our practice in respect to ownership of incandescent lamps, and renewal of lamps blackened or burned out in service. Under our present conditions, for each kilowatt-hour sold to customers for use in incandescent lamps, our expense for lamp renewals is between three-tenths and four-tenths of a cent. The difference between these two figures is due to the varying practice of different companies—not to any appreciable difference in voltage regulation in different cities. Three-tenths of a cent represents the lowest expense at which customers can be satisfied. Four-tenths of a cent represents service at a high standard when the customer is encouraged to renew his lamps frequently. Taking either figure, it is obvious that lamp renewals form a large proportion of our operating costs—after fuel, the lamp renewals are the heaviest single item. Taking the figure previously given of five and three-quarter millions 50-watt, 16 candle power lamps—with the further statement that the Testing Bureau referred to in that paragraph inspected during a recent twelve months for use on our systems of distribution over ten million incandescent lamps of all kinds—it will appear to you that a reduction in the effective life of incandescent lamps would be to us a very serious matter.

We have studied and experimented with the double voltage lamp for many years. It is at least seven years since we made our first earnest effort to utilize it, and during the seven years more than one experiment on a large scale has been made by us. In addition to our experimental central station work there have been many commercially successful installations of double voltage lamps in the United States, and such installations are increasing. These commercial installations have been under conditions where lower lamp economy was permissible and where a reasonable life could, therefore, be obtained by the use of a lamp of 3.6 to 4 watts per candle. It will appear from the foregoing that we do not speak without knowledge. I may say to our British friends that our knowledge includes a respectable acquaintance with double voltage lamps manufactured for the British market both by British and continental makers. Our conclusion always has been that we could not under present conditions afford to double our standard voltage. Our further conclusion has been that the differences between standard voltage and double voltage lamps were inherent in the nature of a carbon filament, and not to be done away with by improved manufacturing methods. So far as we can foresee we will always have the choice between a greater investment in distributing mains with the use of lamps of a superior efficiency, and a reduced investment in distributing mains with lamps of inferior efficiency. The alternative of shorter life of the lamp is not acceptable; for the reason (over and above its increased cost to us) that we cannot require our customers to make exchanges more frequently than they do now. We find that shorter lamp life brings complaints. Even when the exchange of a lamp is not inconvenient—that is to say, when it is not located in some practically inaccessible position, say a dressed show-window, customers will complain if they have to change lamps too frequently. It is not the expense they object to. The expense falls on the company. It is merely the trouble of making the exchange.

So far I have spoken particularly of lamps of 16 and greater candle power. But a large and increasing part of our business is decorative and sign lighting which requires lamps of 2 candle power and 4 candle power. This is profitable business because of the long hours of service. The lamps are of lower efficiency than lamps used in regular lighting, our custom being in such work to reduce the efficiency of the lamps until the cost per kilowatt-hour for renewals is approximated to the average. Double voltage lamps of these candle powers are simply impossible. They can be made. They have been made for us, but the cost of manufacture is prohibitive. A lamp of short life will not serve. The extinction of a few lamps spoils the decorative effect and the replacement of lamps in signs or displays is always relatively inconvenient. And a lamp of very low efficiency would put the cost of energy up to a figure beyond what the business will stand.

Double Voltage Comparisons

What are we to gain, then? Reduced cost of mains and feeders or alternatively a much longer radius of distribution from each station or substation and thereby a reduced number of stations. That is about all. Our regulation is now excellent—we do not need double voltage to keep our lamps up to candle power.. Our losses in transmission are now so small that their reduction will not warrant any great investment. And the reduction in cost of mains is merely a reduction in copper, obtained at some expense for additional insulation if the former factor of safety is to be retained. As between copper and insulation we would rather lock up money in copper. It is one of the very few things we buy which does not deteriorate. The reduction of the number of substations means some saving in buildings, but nothing in machinery and only a trifle in labor. Per contra to these gains we shall have a serious reduction in the earning capacity of our machinery, the reduction being measured by the additional capacity necessary to illuminate the double voltage incandescent lamp. And the capital expended in changes of lamps, both arc and incandescent, in changes or reconstruction of generators and motors, and in minor changes of customers' house wiring, would be a clear loss. We so far have never been able to figure a profit in the change. On the contrary, one of the most notable double voltage installations in the United States, that of the former Imperial Electric Company of St. Louis, is about to be changed over to standard voltage.

Concerning underground Mains and Double Voltage

It goes without saying that each central station has its own local conditions and that no one rule, nor even the frequent demonstration of that rule, can settle such a question as this for any case not specifically calculated. Far less does the rule or practice of one country settle the question for another. On the contrary, local conditions which obtain in one country and not in another may justify many radical differences in practice. I think I have recognized in British practice a well established cause for the acceptance of double voltage—to wit, the relatively great ratio of cost of mains to total investment. This ratio is occasionally such as to control the engineering of the entire plant. It is not so with us. Not only is the relative cost of mains small because of our liberal use of over-head wires, but also because our methods of underground construction are comparatively cheap, and because with us the proportion of copper cost to total mains cost is large. Our ordinary paper insulated lead-covered feeder cable has in it 50 to 60 per cent of copper value, only 40 to 50 per cent standing for insulation and lead covering. The tile duct into which we pull a pair or three cables costs us complete (including manholes) 16 to 30 cents per duct foot. Sixteen cents represents recent trunk line work with twenty to thirty ducts in the run. Thirty cents per duct foot represents the cost of branch runs or runs laid in streets where obstacles are numerous and labor cost goes high. Even the self-contained Edison tube with its complex provisions for flexibility and for frequent service connections is completed in place at a reasonable cost. Wherefore a reduced expense for mains (or rather a reduced investment in copper) does not tempt us as it might a British engineer whose underground constructions (possibly of necessity) required the major part of his capital expenditure. Moreover we have (the most of us) been foreseeing in the matter of conduits. When we opened a ditch wherein to lay three immediately required ducts we have laid nine more to provide for the future, and we have profited by the foresight. To lay an additional feeder in such circumstances is not only simple but comparatively inexpensive. If it meant the tearing up anew of streets paved on concrete foundations or if it meant when it was completed that our new investment would stand 20 cents for copper and 80 cents for insulation and laying down it may be that we would look more favorably upon the double voltage expedient.

In Conclusion

It is necessary to omit from a paper of this class the discussion of many minor but interesting technical matters. I have limited myself to the salient features of that central station practice which I proposed to describe and discuss. It would be profitable to consider also some of the conditions beyond the boundaries of electrical engineering which have affected the evolution of our Edison lighting systems—for instance the comparative price of gas, the common use of elevators, the demand for a bright light which has made the 16 candle power lamp our standard, and the toleration of overhead wires in all but the most densely settled areas. It would likewise be profitable to consider the business and manufacturing methods of our people which caused them to welcome the electric motor when it was first offered and, by reason of which, supply to electric motors in many cases is now half the output of a station and one-quarter to one-third of the station's earnings. These considerations, external to electrical engineering, have been potent in the evolution and differentiation of our distribution methods, and if it were our present affair to determine to the ultimate limit why American central station engineers have done those things which are recognized as distinctly American, we should find it necessary to give full consideration and weight to these and to many kindred causes. This present paper has now, however, covered the ground it was proposed to cover and reached its intended conclusion.

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