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

thanks

Windmiller


We all live under one King
2017 / 2014 NEC & Related Books and Study Guides
Re: Circuit Design [Re: windmiller] #167759
08/19/07 07:05 PM
08/19/07 07:05 PM
HotLine1  Offline

Member
Joined: Apr 2002
Posts: 7,069
Brick, NJ USA
More info required for answer.


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


We all live under one King
Re: Circuit Design [Re: windmiller] #167770
08/19/07 09:56 PM
08/19/07 09:56 PM
G
gfretwell  Offline

Member
Joined: Jul 2004
Posts: 9,338
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".

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

Greg Fretwell
Re: Circuit Design [Re: windmiller] #167907
08/22/07 01:36 PM
08/22/07 01:36 PM
electure  Offline

Member
Joined: Dec 2000
Posts: 4,279
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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