Optional School Service Calculation
The calculation to determine minimum service size for a school with
the optional method is difficult to understand from the NEC directly. So this
summary and calculator technique will clear up this daunting calculation for you
once and for all. Test constructors love to include an optional school
calculation because most working electricians will throw up their hands when
confronted with this sort of question. Here goes.
If you are not familiar with the store and recall buttons on your
calculator, take a minute to read the booklet about the STO and RCL keys. You
need to know how to use them for this calculation. Nothing to it. Just clear the
calculator. Enter a number. Press the STO key. You may have more than one memory
in your calculator so you may need to press STO then 1. Next, press the RCL or
RCL and 1 key to get the number to show back up in the display window. There,
you can use it. If you are disturbed by having your calculator STO a number even
if you turn it off, then press 0 and STO it. That way you are storing 0.
First, find the total connected load that will
be supplied by the service. This includes inside and outside loads. Do not apply
any sort of reduction to any of the loads. The total should be strictly
Second, Determine the square feet of the school
from outside measurements.
Third, divide the total connected load in
volt-amperes by the square feet to obtain va/sq. ft.
Fourth, if this number is 20 va/sq. ft or less,
enter the va/sq. ft. number minus 3 = STO 1. (This is the
store feature on your calculator.)
Clear 3 times sq. ft. + (RCL 1 times square feet times .75) =
divided by volts the service is to be and if 3 phase divided
The result is the service size minimum.
If the va/sq. ft. number is 20.1 va/sq. ft. or more:
Clear 3 times sq. ft. + (17 times square feet times .75) +
(RCL 1 times square feet times .25) =
divided by volts and if three phase divided by 1.732.
The result is the minimum service size.
Find practice problems HERE.
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