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Circuit Design #167751 08/19/07 02:26 PM
Joined: Dec 2006
Posts: 43
W
windmiller Offline OP
Junior Member
Whats the rule on feeding a 300A Fused Disconect from a 400A breaker?

thanks

Windmiller


We all live under one King
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Re: Circuit Design [Re: windmiller] #167759 08/19/07 07:05 PM
Joined: Apr 2002
Posts: 7,216
HotLine1 Offline
Member
More info required for answer.


John
Re: Circuit Design [Re: HotLine1] #167768 08/19/07 09:39 PM
Joined: Dec 2006
Posts: 43
W
windmiller Offline OP
Junior Member
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


We all live under one King
Re: Circuit Design [Re: windmiller] #167770 08/19/07 09:56 PM
Joined: Jul 2004
Posts: 9,541
G
gfretwell Offline
Member
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".

Last edited by gfretwell; 08/19/07 09:57 PM.

Greg Fretwell
Re: Circuit Design [Re: windmiller] #167907 08/22/07 01:36 PM
Joined: Dec 2000
Posts: 4,283
electure Offline
Member
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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