ECN Forum Forums General Topics of the Trade General Discussion Area Wye/delta fire pump motor question

So in case you didn't get that last posting, YES, you can size the conductors for 58% of the motor nameplate current. All 6 of the conductors going to the motor will be carrying current all of the time. The issue of the contactor shorting 3 of them during the Start sequence is irrelevant; the conductors are sized for the running condition and when in Delta (Run), each of the 6 conductors will carry 58% of the motor current. But don't let the 58% confuse you as to the sum being 116%, it is not. The issue at hand is that the current through each conductor path is the motor current divided by the square-root of 3 (1.732). So if the motor FLA were 100A, the Delta current in each of the 6 conductors would be 100 / 1.732 = 57.736, which is rounded up to say "58%".

Fusing for Fire Pumps is sky high.

Life Safety equipment is NOT fused to protect the motor.

IIRC my last fire pump was 40 HP and it was fused at 1200 A 480 delta across the line.

Generally fusing is set high enough so that the smoke will come out of the motor.

The conductors don't have to be huge, just the breaker/fuse.

A fire pump is designed to suck up a carp from the pond, cook it down to bouillabaisse in a locked rotor condition and send it on down the line when the impeller frees up.

poorboy, I love those rock quarries. But all the one's I have worked with have many large motors. And yes, soft starts are the consensus choice. Also, some drives (VFD's) on the screens. I worked with Vulcan and Hanson aggregates.
Fortunately the wye/delta set up is mainly on european eqiupment. But we will never see them gone completely. I'm glad to see you learned something with your project. Learning is the key to success. But Jon (Winnie) is correct in the statements she made. Put an amp clamp on the motor when it is fully loaded.

Jon.....Very good description. Thanks

LOL, Greg. And Testa's more serious explanation makes a lot of sense. Put the fire out!!

Thanks to all for the answers, and it's good to be able to run stuff by such a collective mass of experience and knowledge---both from the engineering and tradesman's side of things. Ken