ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Top Posters(30 Days)
Potseal 11
Recent Posts
Electrode boilers question
by annemarie1. 04/27/17 01:40 PM
Why cables look like they do
by LongRunner. 04/26/17 09:36 AM
Dimmable LED 2x4 lay in fixtures
by HotLine1. 04/24/17 05:43 PM
Old decora style outlets
by Lostazhell. 04/22/17 07:59 PM
Permit Snafus...AHJs and Contractors Jump in
by gfretwell. 04/22/17 01:11 PM
New in the Gallery:
SE cable question
Popular Topics(Views)
234,887 Are you busy
169,688 Re: Forum
162,922 Need opinion
Who's Online Now
0 registered members (), 52 guests, and 8 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate This Thread
#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

2017 / 2014 NEC & Related Books and Study Guides

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


Member
Joined: Apr 2002
Posts: 6,809
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: 9,069
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: 4,259
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)



Member Spotlight
HotLine1
HotLine1
Brick, NJ USA
Posts: 6,809
Joined: April 2002
Show All Member Profiles 
Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box
Powered by UBB.threads™ PHP Forum Software 7.6.0
Page Time: 0.017s Queries: 15 (0.004s) Memory: 0.7684 MB (Peak: 0.9090 MB) Zlib enabled. Server Time: 2017-04-28 21:46:02 UTC