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Old 12-15-2010, 11:18 AM   #1
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Breaker size for subpanel


Okay, I've bought my sub-panel and my feeder cable, I bought 2-2-2-4 aluminum SER from Home Depot, the manufacturer has this table on allowable amps (except the cable I have has a 4th conductor for ground):
http://www.cerrowire.com/default.aspx?id=31

The table is from 310-16 of NEC.

Where I'm a little confused is as to which column applies. Under aluminum there is a 60 Celsius column which says 90 amps is allowable, and there's a 75 Celsius column which says 100 amps is allowable.

The footnotes indicate the 75 Celsius is for termination to equipment rated 100 amperes or marked for conductors larger than No. 1. This is going into a main breaker panel rated for 200A, does that mean I can use the 75 Celsius column?

If I can't use a 100A breaker, I would either use a 60A or need to figure out where I can get something larger, but 60A would be okay because my ampacity calculations come out at 57 amps (after the 125% calculation) so I can live with 60A, but I'd prefer to have the 100A because later I plan to feed the garage with a 100A subpanel and I'll need the 100A for a welder and 60A wouldn't be sufficient. That and I know it's possible that my plans on upgrading to 200A service could change to plans to sell the house before upgrading, and having the extra capacity available would be better in that case.

And yes I'm also working on asking the AHJ,

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Old 12-15-2010, 12:35 PM   #2
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Breaker size for subpanel


Are you installing this sub-panel now and then planning to add another sub-panel for you garage? What kind of welder are you planning that takes a 100 amp feed?

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Old 12-15-2010, 12:42 PM   #3
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Breaker size for subpanel


Table 310.16 lists #2 Al at 90A in the 75C column, so you may use a 90A or smaller breaker. You can get a 90A breaker, but you will have to visit an electrical supply house or order online.
Check with your AHJ to see if it is allowed to use Table 310.15(b)(6) to size the breaker for a subpanel in your area. If so, then you may use a 100A breaker.
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Old 12-15-2010, 12:51 PM   #4
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Breaker size for subpanel


The current service is a newer 100 amp main panel located in the first floor bedroom, I'm installing the 200 amp panel as a sub-panel in the utility room, but it will be fed from this breaker I'm trying to figure out if I should use 100 amp or whatever.

For the time being, the sub-panel is serving updated new circuits for the 2 second floor bedrooms and the kitchen, all of which I'm bringing up to current code... The old wiring for these particular areas have issues, the rest of the house is lower risk.

Later, I plan to move the service entrance and have the service upgraded to 200 amps, then the sub-panel in the utility room will become the main panel. I'll probably move all the circuits into the 200 amp main panel at this time and do any other necessary code updates.

The garage will have a 100 amp subpanel, most likely I'll do this after or at the same time as the service entrance is moved and upgraded. Eventually I plan to have a TIG welder, probably something like a Lincoln square wave tig 175 but possibly up to 225, so that will need a 60A breaker, and I'll probably have up to a 7.5 HP compressor, that's a 40A breaker, and that's before lights, garage door and outlets.

I usually just say I'm planning to run a welder in the garage because that is enough to avoid the question getting derailed into asking why I need so many amps for a garage.
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Old 12-15-2010, 01:03 PM   #5
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Originally Posted by HouseHelper View Post
Table 310.16 lists #2 Al at 90A in the 75C column, so you may use a 90A or smaller breaker. You can get a 90A breaker, but you will have to visit an electrical supply house or order online.
Check with your AHJ to see if it is allowed to use Table 310.15(b)(6) to size the breaker for a subpanel in your area. If so, then you may use a 100A breaker.
that table is for a dwelling only.a detached garage is not a dwelling.and ser cable cannot be burried or in conduit as both are wet locations by code.to bury or conduit you need use cable.USE is rated for bury or conduit.is this panel in your attached gar?table 310.15(b)(6) is for dweling service entrance only.

Last edited by oleguy74; 12-15-2010 at 01:07 PM.
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Old 12-15-2010, 01:07 PM   #6
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Breaker size for subpanel


The question at hand though is for a subpanel in the same house that is serving circuits for the upstairs bedrooms and kitchen.

When I wire the subpanel for the detatched garage at a later time, I expect I'll be buying a new cable and burying in PVC conduit.
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Old 12-15-2010, 01:13 PM   #7
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Breaker size for subpanel


SER to a sub panel is rated the same as NM cable. USE THE 60 DEGREE COLUMN! #2 AL SER 60 degree is rated at 75 amps.

Last edited by brric; 12-15-2010 at 01:15 PM.
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Old 12-15-2010, 03:21 PM   #8
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Breaker size for subpanel


I don't think I'm any less confused than I was before I posted this question.

The data I have says the conductors on the cable I'm using are XHHW-2, which shows under the 90 degree C column... Is the 60 degree C column really needed anyway?
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Old 12-15-2010, 04:16 PM   #9
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Breaker size for subpanel


You can only use the 90 degree column if the terminals on the equipment are rated at 90 degrees, of which you will find few. Most equipment has terminals rated at 75 degrees. SER used for other than service entrance conductors must use the 60 degree column. See NEC 338 and 334. SER is governed under the same ratings as NM cable. NM cable is governed by the 60 degree column. They are cable assemblies, not individual conductors.

Last edited by brric; 12-15-2010 at 04:19 PM.
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Old 12-15-2010, 07:04 PM   #10
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Breaker size for subpanel


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Originally Posted by WillK View Post
I don't think I'm any less confused than I was before I posted this question.

The data I have says the conductors on the cable I'm using are XHHW-2, which shows under the 90 degree C column... Is the 60 degree C column really needed anyway?
Which is why those charts are pretty much useless. It is never as simple as you think.

Newer codes require SE cable run as a branch circuit or feeder to be rated the same as NM cable as far as ampacity goes, which means using the 60 deg C column.
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Old 12-15-2010, 07:45 PM   #11
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Breaker size for subpanel


Well, what I see is that 338 refers to the installation requirements of 334 part II, but the only place I read that it has to be 60 celsius is where it refers to NM cable.

What I do see is that the panel which I presume is what would be referred to as the equipment to which the cable terminates is rated for 75 celsius... If I can use the 75 celsius column it says the most I could go with is 90 amps, 60 amps covers me regardless so unless I find an electrical supply place to get my breaker, I'm getting 60 amps since 100 amps is out whether I can use the 60 celsius column or the 75 celsius column.

It raises another question... When I do upgrade to 200 amp service and I'm using this same panel, am I still going to have to use the 75 celsius column since that's the temperature rating of the panel? If so, 4/0 aluminum isn't large enough and I'd have to go with 3/0 or 4/0 copper... Am I getting that right?

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