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#97857 - 04/05/06 08:47 PM min ampacity of feeder for sub panel 4 ckts  
gserve  Offline
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Joined: Mar 2003
Posts: 92
On a sub panel installation in a residential garage of 4 two wire branch circuits I say that the min feeder ampacity should be 60A. Based on the fact that the disconnecting means must be rated at 60 A by code. My co-worker says that it is based on the computed load and could be 40A or 50A. Please advise with code reference. Thanks


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#97858 - 04/05/06 10:07 PM Re: min ampacity of feeder for sub panel 4 ckts  
gfretwell  Offline


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I would be more interested in a reference for the idea that a feeder to a sub panel has to be 60a.
It would not have to be any more than the computed load as far as I know and that may easily be less than the combined breaker handle amps in the sub panel.

... but I amy owe someone an appology.


Greg Fretwell

#97859 - 04/06/06 01:17 AM Re: min ampacity of feeder for sub panel 4 ckts  
gfretwell  Offline


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Estero,Fl,usa
I was just over at Joe's house and I saw Mike's answer to you. Are we talking about the ampacity of the feeder or the rating of the garage disconnect?


Greg Fretwell

#97860 - 04/06/06 08:30 AM Re: min ampacity of feeder for sub panel 4 ckts  
luckyshadow  Offline
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In the '05 NEC - 225.39 (D)
I think this is what everyone is referring to.


#97861 - 04/06/06 09:08 AM Re: min ampacity of feeder for sub panel 4 ckts  
gfretwell  Offline


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That only refers to the interrupting rating of the disconnect, not the ampacity of the feeder.


Greg Fretwell

#97862 - 04/07/06 07:38 AM Re: min ampacity of feeder for sub panel 4 ckts  
luckyshadow  Offline
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Maryland USA
225.39 The feeder or branch-circuit disconnecting means shall have a rating not less than the load to be supplied, determined in accordance with parts 1 and 2 of article 220 for branch circuits, parts 3 or 4 of article 220 for feeders, or part 5 of article 220 for farm loads. In no case shall the rating be lower then specified in 225.39 (A)(B)(C)(D).
(A) - One circuit Installation - does not apply to this thread
(B) - two circuit Installation - Does not apply to this thread
(C) - One Family Dwelling _ Does not apply to this thread
(D) - ALL OTHERS. For all other installations( this is us) the FEEDER or branch-circuit Disconnecting means shall have a rating of NOT less than 60 amperes.

I read this as saying that the FEEDER must be at least 60 amperes because it states the FEEDER or branch-circuit DISCONNECTING means ....
Isn't the the circuit breaker in the main panel the DISCONNECTING MEANS for this FEEDER ?


#97863 - 04/07/06 12:01 PM Re: min ampacity of feeder for sub panel 4 ckts  
gfretwell  Offline


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Posts: 9,140
Estero,Fl,usa
You have to read the NEC in context. You don't simply cherry pick words out of context.
This section is titled "More than one building or other structure" and 225.39 is titled "rating of disconnect" (in that other building) so it does not refer to the breaker in the main building (see "location" in 225.32) or the ampacity of the feeder, only the disconnect in the "other" building. They just want to insure that you can safely disconnect the "other" building, under load, at that building if you need to.

[This message has been edited by gfretwell (edited 04-07-2006).]


Greg Fretwell

#97864 - 04/07/06 01:06 PM Re: min ampacity of feeder for sub panel 4 ckts  
George Little  Offline
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Joined: Jan 2004
Posts: 1,489
Michigan USA
One could also be using a 60a. fuse type disconnect and install smaller wires and fuses in said disconnect at the house.


George Little

#97865 - 04/07/06 10:34 PM Re: min ampacity of feeder for sub panel 4 ckts  
gfretwell  Offline


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Joined: Jul 2004
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Estero,Fl,usa
George, I suppose I would have to hold my nose and approve one of those 60 a "pull out only" disconnects you see in HVAC units but I don't think I would actually want to pull one under load without PPE.


Greg Fretwell

#97866 - 04/08/06 11:44 AM Re: min ampacity of feeder for sub panel 4 ckts  
George Little  Offline
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Joined: Jan 2004
Posts: 1,489
Michigan USA
Yeah, Greg, I'd agree with you on the "hold my nose". I've been an inspector for 20 years and a contractor prior to that for 12 years. One of the toughest things I had to adjust to was that everyone has a different level of professionalism and some of them are not at the same level as I am but they still meet minimum code. Of course there are those who are really perfectionist and they exceed code by a wide margin making the inspectors job easier.


George Little

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