National Electrical Installation Standards

Standards as High as Your Own

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Monday, December 16, 2019

Question:

Sizing a 480V 3-phase 4-wire service for a waste water pump station. There are (4) 60 HP motors however the mechanical system will only allow (2) pumps to ever run simultaneously. What is the correct way to size the main utility service using the load for the (2) pumps? or do you have to size the service using the load for all (4) pumps.

Bob Poteste
A

Answer:

Hey Bob thanks for your question. If the pumps are interlocked so that only 2 pumps can run simultaneously then you only need to size the service conductors for the motors that will run at the same time as stated in 220.60. Based on 430.6(A)(1) use the full load current (FLC) rating of a 60 HP motor from Table 430.250. Each motor branch circuit conductor is sized at 125% of the FLC as stated in 430.22 and the service conductors, in your example, would be sized at 125% of one of the motors plus 100% of the second motor as stated in 430.24. Any additional loads would need to be added.

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