Home > Construction Training Manuals > Construction Electrician Intermediate > Figure 4-15.Transformer Size Calculations.

strength of the pole or the crossarms and bolts supporting them.

2. When more than one transformer is installed on crossarms, the weight should be distributed equally on the two sides of the pole.

3. Single-phase distribution transformers of 100 kVA or smaller are usually placed above the secondary mains if conditions permit. Those larger than 100 kVA are usually platform or pad mounted.

4. Lightning arresters and fused cutouts have to be installed on the primary side of all distribution transformers except the self-protected type.

5. Ground wires are required to be covered with plastic or wood molding to a point 8 feet above the base of the pole.

What is involved in the determination of the transformer size? Let’s suppose you are given the job of installing a single-phase transformer in a certain area of the base. This area contains 10 barracks that receive power from a 2,400-volt overhead primary main. The electrical equipment in the barracks consists of single-phase lights or motors operating at either 110 or 220 volts. A three-wire overhead secondary main distributes the secondary voltage alongside the barracks. Service leads complete the connection between the secondary main and each building.

The first thing you should do is make a rough drawing of the area. When you are finished, it should look like figure 4-15. The location of each pole, as well as the barracks, is noted. Lines representing the service leads are drawn between the pole and the building.

Your next step is to determine the total connected load of each service. It sounds complicated, but what it actually amounts to is summing up the power required by the lights and motors in each barracks. This power demand is noted in each square representing a barrack (fig. 4-15).

Next, figure out the kVA load per pole. In this particular example, each pole services two barracks; therefore, the kVA load of a pole will be the sum of the total connected loads of the two barracks served by that pole.

Now, calculate the total maximum connected load on the transformer. As you can see from figure 4-15, the total connected load is the sum of the kVA loads per pole. It amounts to 35.05 kVA. This amount of 35.05 kVA represents the amount of power that the transformer would have to supply if all the lights and motors were consuming power at the same time. Although that possibility exists, the time interval would be small compared to the length of time that only a portion of the total load would be on. Therefore, it is necessary to calculate only the maximum demand load and then use this figure as a basis for determining transformer size.

An approximation of the maximum demand load can be computed by multiplying the total maximum connected load by the demand factor listed in table 4-1. In this example, the maximum demand is 35.05 times

Figure 4-15. - Transformer size calculations.

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