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#167751 - 08/19/07 03:26 PM Circuit Design  
windmiller  Offline
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

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


Member
Joined: Apr 2002
Posts: 6,858
Brick, NJ USA
More info required for answer.


John

#167768 - 08/19/07 10:39 PM Re: Circuit Design [Re: HotLine1]  
windmiller  Offline
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

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


Member
Joined: Jul 2004
Posts: 9,105
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 10:57 PM.

Greg Fretwell

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


Member
Joined: Dec 2000
Posts: 4,260
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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