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Old 02-14-2009, 11:12 AM   #1
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Subpanel wire size


Hi,

I want to add a subpanel with four 20 amp/120 volt breakers to feed various equipment (mini-fridge, computers/laser printer, attic fan, ect). I have an unused 50 amp/240 volt breaker in the main panel which I would like to use to feed the subpanel to be located only a few feet away, so about 5-10 feet of wire. Could I use 8-3+G or should I use 6-3+G?

Thanks

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Old 02-14-2009, 11:48 AM   #2
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Subpanel wire size


You'll need to use the 6/3. I would just use 6/3 Al SER personally.

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Old 02-14-2009, 11:55 AM   #3
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Quote:
Originally Posted by wirenut1110 View Post
You'll need to use the 6/3. I would just use 6/3 Al SER personally.
I thought 6/3 AL was only suitable for 40 amps, not 50. The op would need 6/3 copper for 50 amps or 4/3 AL for 50 amps.
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Old 02-14-2009, 12:14 PM   #4
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Subpanel wire size


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I thought 6/3 AL was only suitable for 40 amps, not 50. The op would need 6/3 copper for 50 amps or 4/3 AL for 50 amps.
It may be a 2008 thing. This cat knows very little about 08. Hoping to find something else to do when that starts around here.

I wish we could use the POCO's way of determining conductor size. Job I'm doing now I'm using 500 Cu and they're using 4/0 Al.

Last edited by wirenut1110; 02-14-2009 at 12:18 PM. Reason: I wish
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Old 02-14-2009, 12:23 PM   #5
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Subpanel wire size


Yeah, I just looked. It is an 08 thing. 05 didn't include 334.80 it looks like 08 does.
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Old 02-14-2009, 12:50 PM   #6
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Subpanel wire size


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Yeah, I just looked. It is an 08 thing. 05 didn't include 334.80 it looks like 08 does.
Exactly.
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Answers based on the 2008 & 2011 NEC. If you're on the '14 already I feel sorry for you.
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Old 02-14-2009, 01:13 PM   #7
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To add to that the CMP was forced thru proposal to realize that SE cables with essentially the same construction as other non-metallic sheathed cables were being used in other than 310.15 (B)(6) applications such as downstream panelboard feeders that were not mains calculated. The lack of the this controlled calculation required by inspection removed diversification considerations for after the fact sub panel feeders and similar. So it accepted the proposal to put Se in the 60 degree column as for ampacity for interior wiring. Underground USE is also 60 C ampacity depending on application. The point is it is no longer 75C but must be either 60C or table 310.15 B6 one or the other.
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Old 02-14-2009, 01:30 PM   #8
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Subpanel wire size


I guess 310.15 (B)(6) will be next. We'll end up using 350's for a 200 amp service.
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Old 02-14-2009, 02:10 PM   #9
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Lets hope they don't mess with individual conductors in conduit...
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Old 02-14-2009, 02:45 PM   #10
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Subpanel wire size


Depends on how much money the copper industry has to pony up to buy the rule change!

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Old 02-14-2009, 04:43 PM   #11
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Thanks for the help. I bought the 6/3 NM copper. One more question, the sticker on the panel door indicates a wire size range of 2/0 - 4 for the main terminals, so is there a way to use the 6 gauge wires and the 10 gauge ground, or should I buy a different panel?

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Old 02-14-2009, 07:34 PM   #12
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You have Siemens panel but it must be a 125 amp panel as most 100's will take a #6. The only option I see is to use a backfed double pole breaker in the mains position. You will need the hold down kit ECMBR2 as shown below....or get a smaller rated panel to reduce the main lug size do you need one this big??

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Old 02-14-2009, 08:01 PM   #13
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No, I don't need a 125A panel, as I only plan on putting 4 or 5 20A 120V breakers in it. I think it will be easier and cheaper to return this panel and get a smaller one. Thanks for your help
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Old 02-14-2009, 08:09 PM   #14
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Stubbie:

Why do you need a hold down kit?
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Old 02-14-2009, 08:46 PM   #15
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It's a requirement by the NEC for a breaker that is backfed serving as the main disconnect for the panel. The specific code is 408.36(D). They simply don't want it to be easily removed by just pulling it out by hand.

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