Feeder Size

[mstrlucky74]

Two questions.

1.Have a 120/208v 400A fused switch feeding sprinkler booster pump controller( the pump is 30HP) with 3#2 & 1#3 MU cable. According to Pyrotenax website(for the MI) and 310.17 #2 is only good for between 100A-150A 75 degree. Doesn’t the feeder(MI) need to be sized to the 400A switch.

2. See attached pic. Showing a bus tap from the 4000a board feeding another 400A bus. Isn’t a OCPD required? Also, this feeder should be sized to the 4000A bus, correct? Thanks.

 

[JoeStillman]

The feeder is sized for 1.25 x FLA (495.6(B)). The fuse has to carry LRA. The fuse is for ground-fault and short-circuit protection only, not overload. Overloads are in the pump controller. The ampacity mis-match between #2 MI & a 400A fuse is a red herring.

This is a common misconception with all types of motor feeders. 240.4 says feeders must have overcurrent protection according to their ampacity, but there is a variance at 240.4(G), which points you to article 430 for motors.

 

[infinity]

30 HP @ 208 volts is 88 amps. 88 amps * 125% = 110 amps which is the minimum conductor size. Assuming that this is designed as a fire pump circuit for NYC the OCPD is sized at 300%-600% (330 amps-660 amps) so the 400 amp OCPD is within the allowable range.

NYC Electrical Code
695.4(B) Disconnecting Means and Overcurrent Protection.
(1) Utility Service. Fire pumps and limited service fire pumps shall have overcurrent protection
selected as to allow the operation of the fire pump for as long as the fire pump remains capable of
running, except where direct connection is made in accordance with 695.4(A)(3) Exception 1:
(1) Fire Pump. Fire pump overcurrent protection shall be selected at between 300 percent and
600 percent of motor full load current.