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Circuit Design #167751
08/19/07 03:26 PM
08/19/07 03:26 PM
windmiller  Offline OP
Junior Member
Joined: Dec 2006
Posts: 43
Whats the rule on feeding a 300A Fused Disconect from a 400A breaker?



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Re: Circuit Design [Re: windmiller] #167759
08/19/07 08:05 PM
08/19/07 08:05 PM
HotLine1  Offline

Joined: Apr 2002
Posts: 6,936
Brick, NJ USA
More info required for answer.

Re: Circuit Design [Re: HotLine1] #167768
08/19/07 10:39 PM
08/19/07 10:39 PM
windmiller  Offline OP
Junior Member
Joined: Dec 2006
Posts: 43
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.



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Re: Circuit Design [Re: windmiller] #167770
08/19/07 10:56 PM
08/19/07 10:56 PM
gfretwell  Offline

Joined: Jul 2004
Posts: 9,186
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 10:57 PM.

Greg Fretwell
Re: Circuit Design [Re: windmiller] #167907
08/22/07 02:36 PM
08/22/07 02:36 PM
electure  Offline

Joined: Dec 2000
Posts: 4,265
Fullerton, CA USA
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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