Transformer Installation

If a blueprint of a particular transformer installation is available to you, your job will be comparatively easy. All the construction and electrical specifications will be worked out for you beforehand and all you have to do is convert this information to the finished job. However, in some instances, a blueprint will not be available. Then, it will be up to you to determine the location and size of the transformer and install the transformer according to the latest specifications. You should be familiar with the rules and requirements of the most current electrical codes. Be sure to study any applicable code requirements carefully before installing a transformer.

DETERMINATION OF TRANSFORMER SIZE.\Let's suppose you are given the job of installing a single-phase transformer in a certain area of the advanced 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 5-15. The location of each pole as well as the barracks is noted. Lines representing the service leads are drawn between the poles 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 barracks (fig. 5-15).

Next, figure out the kVA load per pole. In this particular example, each pole serves 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.

Figure 5-15.\Transformer size calculations.

Now, calculate the total maximum connected load on the transformer. As you can see from figure 5-15, the total connected load is the sum of the kVA loads per pole. It amounts to 25.85 kVA. But don't jump to conclusions. You won't need a 37.5-kVA transformer to take care of the total load. The amount 25.85 kVA represents the amount of power that the transformer would have to supply if all the lights and motors were turned on 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 5-2. In this example, the maximum demand is 23.26 kVA (25.85 0.9). The transformer capacity required to meet this demand will be 25 kVA, since a 25-kVA transformer is the next larger standard size.

DETERMINATION OF TRANSFORMER LOCATION. \Your next problem is to find the most suitable location for the transformer. That doesn't mean finding the strongest pole but the one that is nearest to the ELECTRICAL CENTER of the area.

The electrical center is the point where a balance is obtained between the total kVA spans to the north and south of the location of the transformer. The kVA span is the product of the number of spans times the kVA load of the pole.

To begin with, assume that you are going to place the transformer on pole K (fig. 5-15). Then figure the total kVA spans to the north and south of this location. A chart will simplify your calculations.

You can see that if you placed the transformer on pole K, it would be at an imbalanced electrical center; that is, it would be too far away from the heaviest loads.. So pick another pole. This time choose pole L and make another chart.

Total kVA spans north ^{Total kVA spans south} of pole L = 12.55 ^{of pole L = 12.0}

Pole L is nearest to the electrical center of the area. That is the pole, then, on which you will mount the transformer.