strength of the pole or the crossarms and boltsas the barracks, is noted. Lines representing the servicesupporting them.leads are drawn between the pole and the building.2. When more than one transformer is installed oncrossarms, the weight should be distributed equally onthe two sides of the pole.3. Single-phase distribution transformers of 100kVA or smaller are usually placed above the secondarymains if conditions permit. Those larger than 100 kVAare usually platform or pad mounted.Your next step is to determine the total connectedload of each service. It sounds complicated, but what itactually amounts to is summing up the power requiredby the lights and motors in each barracks. This powerdemand is noted in each square representing a barrack(fig. 4-15).4. Lightning arresters and fused cutouts have to beinstalled on the primary side of all distributiontransformers except the self-protected type.Next, figure out the kVA load per pole. In thisparticular example, each pole services two barracks;therefore, the kVA load of a pole will be the sum of thetotal connected loads of the two barracks served by thatpole.5. Ground wires are required to be covered withplastic or wood molding to a point 8 feet above the baseof the pole.What is involved in the determination of thetransformer size? Let’s suppose you are given the jobof installing a single-phase transformer in a certainarea of the base. This area contains 10 barracks thatreceive power from a 2,400-volt overhead primarymain. The electrical equipment in the barracks consistsof single-phase lights or motors operating at either110 or 220 volts. A three-wire overhead secondarymain distributes the secondary voltage alongside thebarracks. Service leads complete the connectionbetween the secondary main and each building.Now, calculate the total maximum connected loadon the transformer. As you can see from figure 4-15,the total connected load is the sum of the kVA loads perpole. It amounts to 35.05 kVA. This amount of 35.05kVA represents the amount of power that thetransformer would have to supply if all the lights andmotors were consuming power at the same time.Although that possibility exists, the time intervalwould be small compared to the length of time thatonly a portion of the total load would be on. Therefore,it is necessary to calculate only the maximum demandload and then use this figure as a basis for determiningtransformer size.The first thing you should do is make a roughdrawing of the area. When you are finished, it shouldlook like figure 4-15. The location of each pole, as wellAn approximation of the maximum demand loadcan be computed by multiplying the total maximumconnected load by the demand factor listed in table 4-1.In this example, the maximum demand is 35.05 timesFigure 4-15.—Transformer size calculations.4-11

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