Taps for a motor starter

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ktm400sx

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I have a MCC that I want to use a 9' tap from (from the MCC buss bars) to supply a pump panel.
The breaker in the pump panel is 400 amp.
From 240.21 (b) I see that I have to size the tap wire for the overcurrent protective device. In this case the 400 amp beaker.

My question is: can I use the next standard size rule to size the tap wire? I want to use 500 mcm but of course its only rated at 380 amps at 75 degrees.

Thanks for any help
 
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It is common for the OCPD in a motor control center to be a motor protector, not a circuit breaker. If so, I don't think you could protect a pump panel with it. Check the type of breaker in your MCC.

Jim T
 
I was more concerned with the device in the MCC. Are you tapping the bus, or coming off of an existing cubicle?

As far as rounding up with the ampacity, I don't see a problem.

Jim T
 
My question is: can I use the next standard size rule to size the tap wire? I want to use 500 mcm but of course its only rated at 380 amps at 75 degrees.
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No, you can not use the next larger standard OCPD for this type of application.
(B) Feeder Taps Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). The provisions of 240.4(B) shall not be permitted for tap conductors.
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This wording is new for the 2005 code, but was always a code requirement. The previous wording said that the conductor had to be protected at or below its ampacity. When you are using the "round-up" rule you are protecting the conductor above its ampacity. There were many violations found with the old wording, so the new words were added to make the rule very clear.
Don
 
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