Calculations of pillars and transformers

A residential area for population of 4,000 is divided into four parts. According to the population percentage occupying each type; our task is to:

     1.      Arrange their houses and service centers.

     2.      Arrange their supplying boxes so as to increase the reliability of the supply and also its continuity.

     3.      Connect the boxes to their distribution transformers.

Calculations in this part depend on trial and error concept, and there are many solutions. One of them is acceptable and the others are refused.

The calculation is done by the following steps:

1-    Select the number of buildings to be fed by one pillar. Also select the number of feeders to feed these buildings

2-    Find the diversified load of one building (from the chart or using diversity factor of 1.05à1.1) then multiply it by the number of buildings fed from the same feeder to get the actual load of the feeder.

3-    Find the diversified load of one feeder by dividing the actual load of the feeder by 1.05à1.1, then multiply it by the number of feeders fed from the same pillar to get the actual load of the pillar. To check that the assumption is correct; the actual load of the pillar shouldn't exceed 80% of its ratings.

4-    To select the feeder c.s.a, divide the load of the feeder by ×380 to get the feeder actual current which shouldn't exceed 70% of the feeder current carrying capacity. Use the tables of the cables to get the appropriate c.s.a and the corresponding voltage drop of this cable. (If the feeders of the same pillar have different loads, then calculations are based on the more loaded feeder). Notice that if more than one cable is to be laid in the same tunnel then an appropriate derating factor should be used.

5-    Now that the load of the pillar is selected; it is desired to select the distribution transformer rating and the feeders to the pillars. Select the number of pillars to be fed from the same transformer. Also select the number of feeders to feed these pillars. Use the appropriate diversity factor to get the diversified load of the pillar then multiply it by the number of pillars on the same feeder to get the actual load of the feeder. Find the feeder current. It shouldn't exceed 70% of its current carrying capacity. Use the tables of the cables to get the appropriate c.s.a and the corresponding voltage drop of this cable. Notice that if more than one cable is to be laid in the same tunnel then an appropriate derating factor should be used.

6-    To select the transformer, find the diversified load of the feeder then multiply it by the number of feeders from the transformer to get its actual load. It shouldn't exceed 80% of the transformer rating.

7-    Selecting the route of the feeders is done by trial and error to fulfill the constraints mentioned before.

8-    Finally the total voltage drop in the LVN is calculated. The design is good if the voltage drop is  5% of 220 V, where

9-    The voltage drop = the voltage drop on the riser of the building box + the voltage drop on the feeder between the furthest building box and the distribution box + the voltage drop between the furthest distribution box and the transformer.