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#167751 - 08/19/07 12:26 PM Circuit Design
windmiller Offline
Junior Member
Registered: 12/05/06
Posts: 43
Loc: Alaska
Whats the rule on feeding a 300A Fused Disconect from a 400A breaker?

thanks

Windmiller
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#167759 - 08/19/07 05:05 PM Re: Circuit Design [Re: windmiller]
HotLine1 Online   content


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Registered: 04/03/02
Posts: 6778
Loc: Brick, NJ USA
More info required for answer.
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#167768 - 08/19/07 07:39 PM Re: Circuit Design [Re: HotLine1]
windmiller Offline
Junior Member
Registered: 12/05/06
Posts: 43
Loc: Alaska
Hi,
This is from a 480 V MCC to a maint. shop.

This is a feeder to supply power to the building.

No 300A plug in bucket is available and no other place to get power except from 480V MCC with a new section and a frame mounted 400 CB.

The feeder is 3/C 500 MC.

Thanks,

Windmiller
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#167770 - 08/19/07 07:56 PM Re: Circuit Design [Re: windmiller]
gfretwell Offline


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Registered: 07/20/04
Posts: 9039
Loc: Estero,Fl,usa
500 MCM is 380a @ 75c
I don't see a real problem. If you can get in under the feeder tap rules it is a slam dunk and you can probably use 240.4(B) if it comes down to it.

From a practical sense, you are really only providing short circuit protection in the 400a breaker since overload protection will come with the 300a. That is how they rationalize the "tap rules".


Edited by gfretwell (08/19/07 07:57 PM)
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#167907 - 08/22/07 11:36 AM Re: Circuit Design [Re: windmiller]
electure Offline


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Registered: 12/24/00
Posts: 4259
Loc: Fullerton, CA USA
Quote:
This is a feeder to supply power to the building.


If so, it may qualify for an outside the building tap of unlimited length per 240.21(B)(5)
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