Fire pump phase conductor size increase voltage drop how to calculate new EGC 250.122(B)

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Senior Member
I have 20HP, 208V three phase fire pump. The full load amps according to table 430.250 is 59.4A. 59.4Ax1.25=74.25A. The conductor size of #3 awg cu would be sufficient. However whats provided is #2 awg cu due to voltage drop.

The LRA of the fire pump is 360A so fused disconnect with fuse size of 400A is provided.

The EGC provided is #3 gnd copper. The question I have is since phase cable is increased due to voltage drop would EGC of #3 gnd be sufficient per 250.122(B) or it would need to be increased?

Under normal circumstances this would be simple calculation but size of EGC is already increased due to fire pump LRA size fuse. For motors, fire pump its hard to know 250.122(B). Anyone know?
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Staff member
State Electrical Inspector (Retired)
IMO, since 250.122(B) states "phase conductors increased from the minimum size that has sufficient capacity" you would base you would use that as a base number and the #3 would be sufficient.


Senior Member
Cleveland, Ohio
Was it stated if the VD correction was for “run” or “start”. There is a specific max VD allowed per NEC for starting.
I believe this start VD should be indicated on plans to show compliance.