Can I downsize wire when using multiple distribution blo

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dilkep

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In a control panel I am using 2 distribution blocks. The power coming in is fused at 250A in a separate disconnect enclosure and wired into the first distribution block using #250kcmil wire(this dist. block has 2 sets of input terminals).

I want to wire the second distribution block input power from the first block's second set of terminals.

If I figure the total amp draw used in the outputs of the second distribution block & multiply by 125% it comes out that I need #2/0 wire for the input into the second distribution block. However, the only previous circuit protection would be the 250A fused disconnect.

I have a colleague who tells me this is acceptable, but I can't find it in the NEC book.

Any help is definitely appreciated.

Thanks.
 
Re: Can I downsize wire when using multiple distribution blo

You don't say what the overcurrent device is for the 2/0 but it would seem to be covered by the tap rules of 240.21B
 
Re: Can I downsize wire when using multiple distribution blo

This sounds like it comes under the tap rules.

usually, control panels for machines come under nfpa79. nfpa79 does not specify anything at all about taps other than to define it the same way the nec does.

i think it is safe to use the tap rules in the enc for these applications.
 
Re: Can I downsize wire when using multiple distribution blo

I have used the tap rule to provide multiple distribution blocks in UL Listed panels.

Just remember you cannot tap a tap. If you plan to run smaller conductors from your second distribution block then the conductors feeding it need to be fully protected by the 250A main device.
 
Re: Can I downsize wire when using multiple distribution blo

Originally posted by jim dungar:
I have used the tap rule to provide multiple distribution blocks in UL Listed panels.

Just remember you cannot tap a tap. If you plan to run smaller conductors from your second distribution block then the conductors feeding it need to be fully protected by the 250A main device.
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So, I could tap the first distribution block into a breaker that would then protect the second distribution block, correct?

It's either that or find a distribution block with enough outputs to cover what I need.

Thanks for all replies, BTW.
 
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