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Old 08-17-2008, 04:09 PM   #1
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Sub Panel and Wire Guage


Hello,

I will likely be installing a sub panel that will provide 100-120 amp service to a section of my house. What is the appropriate wire guage to supply the sub panel, and what would I need to run the wire in (conduit, flex?)?

Thanks.

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Old 08-17-2008, 06:42 PM   #2
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Unless the conductors are part of a listed cable assembly, yes you would use an approved conduit wiring method, which is the way I would do it. Tbl 310.15(B)(6) allows residential services and feeders (certain conductor types:double check, but you'll most likely be using one of those types) to use sizes:

You cant use table 310.15(B)(6) for a subpanel.

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Old 08-17-2008, 06:58 PM   #3
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Originally Posted by chris75 View Post
You cant use table 310.15(B)(6) for a subpanel.
We do.
At least for 100A. We use #2AL or #4CU.
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Old 08-17-2008, 06:59 PM   #4
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Sub Panel and Wire Guage


I thank you chris, and I apologize. I re-read 310.15(6) and agree with you, and see my mistake. I retract my post.
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Old 08-17-2008, 07:02 PM   #5
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Sub Panel and Wire Guage


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We do.
At least for 100A. We use #2AL or #4CU.

A lot of people do.... i'm just posting the correct information. I just install a 90 amp breaker.
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Old 08-17-2008, 07:13 PM   #6
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Sub Panel and Wire Guage


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A lot of people do.... i'm just posting the correct information. I just install a 90 amp breaker.
I know, and I agree. I was more playing than anything.
Again, it comes down to giving the broad correct advice as opposed to what they let you get away with at home.
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Old 08-17-2008, 07:20 PM   #7
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We do.
At least for 100A. We use #2AL or #4CU.

Well I've had my couple more and now I'm fine. We are also allowed to use T310.15(B)(6) for residential garages, Damn I am lucky to have my inspectors
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Old 08-17-2008, 07:26 PM   #8
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Well I've had my couple more and now I'm fine. We are also allowed to use T310.15(B)(6) for residential garages, Damn I am lucky to have my inspectors
If the garage is detached then its not even a dwelling unit.... how messed up is that, so you actually have a state amendment for that?
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Old 08-17-2008, 07:43 PM   #9
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No state amendment, just been accepted practice for umpteen years. One of the new inspectors around 3 years ago tried to enforce it, and all the contractors went over his head to the regional inspector and he got spanked and eventually quit. I guess our inspectors are more reasonable because we don't rely on building inspectors. We use electrical inspectors who are all licensed electricians so they understand the concepts behind the NEC and are not just enforcing the rules to the letter.

If you understand that T310.5(B)(6) is derated because the total load will more than likely never be used, then the same concept holds true for a residential detached garage were most people will only be using a couple of 8 foot flourescent lights and maybe be recharging their weedwacker on a 100 amp panel.

If the homeowner is known to be a welder or mechanic or some other type of tradesman then we all know that the inspector would prefer that we don't use the table because he will probably be moonlighting out of his garage/shop.

So you see, if everybody in the loop is competent, then a little leeway is not over the top. As professional tradesmen we should be allowed to use some of our own best judgements, otherwise any old monkey could just "follow the rules".

I could tell you a few more doozies, but it's Sunday an I wouldn't want anybody to get jealous because I think it's against "the rules" to "covet thy neighbors inspector"

Last edited by Silk; 08-17-2008 at 08:59 PM.
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Old 08-18-2008, 10:10 AM   #10
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Originally Posted by Speedy Petey View Post
We do.
At least for 100A. We use #2AL or #4CU.
Same here in MN.
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Old 08-18-2008, 04:02 PM   #11
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Same here in MN.

If you guys are using SER type cable, the 2008 nec will require it to be rated at 60 degree table, so it will only be good for 75 amps, instead of the current 90 amps that everyone still uses 100 amp breakers for, even though its a code violation.
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Old 08-18-2008, 04:24 PM   #12
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If you guys are using SER type cable, the 2008 nec will require it to be rated at 60 degree table, so it will only be good for 75 amps, instead of the current 90 amps that everyone still uses 100 amp breakers for, even though its a code violation.
I do agree with you there Chris. 2008 does clarify this.

Without getting into the same old debate, I do strongly disagree with you, that using a 100A right now under our circumstances, is a code violation.
It is merely a difference in interpretation by our AHJs.
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Old 08-18-2008, 04:34 PM   #13
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It is merely a difference in interpretation by our AHJs.
No big deal, my local inspectors dont know squat, I could use a 125 amp breaker if I wanted too...
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Old 08-19-2008, 02:53 PM   #14
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No big deal, my local inspectors dont know squat, I could use a 125 amp breaker if I wanted too...
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Old 08-19-2008, 04:18 PM   #15
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I know my post was supposed to be funny, but in actuality, its the sad truth, we have building inspectors (usually some wanna be fire department volunteer, that inspects every trade.

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