strength of the pole or the crossarms and bolts
as the barracks, is noted. Lines representing the service
leads are drawn between the pole and the building.
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.
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
4. Lightning arresters and fused cutouts have to be
installed on the primary side of all distribution
transformers except the self-protected type.
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
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? Lets 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.
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
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
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.