What guage wire is needed (both aluminum and copper) to feed a 125 amp subpanel from a 200 amp main circuit panel.
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It depends on what kind of wire you use. If you use wire with a temperature rating of 75 degrees C, the Code says #1 copper wire or #2/0 aluminum wire. For 60 degree wire, the sizes go up to #1/0 for copper and #3/0 for aluminum. However, that doesn't take into account the distance between the main panel and the subpanel...if they are more than 100 feet or so apart, you may have to upsize the wire to avoid excessive voltage drop. There are a number of voltage drop calculators online, the one I use is this one: http://www.electrician.com/vd_calculator.html
The subpanel is one stud bay away from the main.
In that case, no problem.
just a question...what is the temp rating based on?
Sorry if it's a naive question.
Nap
The basic is the temp rating of the insualtion on the wire.And secondardy the terminals that the wire that it connects to has to be rated at or above that temp.Most breakers will be rated 75, and common wire (THHN) is rated at 90.So you are limited to using the 75.However, NM-B cable although rated for 90 is limited to using the 60 degree ratings. I don't have any ideas.But if you need to derate it because of going through an area of high temps or multiple wires bundled together you can use the 90 degree column to start your derating from.
Thanks. Although, I might be more confused now than I was before. I guess that's why I'm not an electrician.
The temp rating is the temperature at which the insulation will melt if you run too much current through it. The whole deal with sizing a wire for a certain ampacity depends on heat generation - if you have a bare wire in free air (i.e., stretched between two terminals without touching anything) it can carry an amazing amount of current, compared to what the code books says about the same size wire covered in insulation and buried in your wall. It'll get really hot, but it will still work.Edited 6/8/2005 3:51 pm ET by Stuart
Edited 6/8/2005 3:52 pm ET by Stuart
I disagree, unless the code has changed. Table 310-15(b)(6) (at least in the 1999 NEC) specifies wire sizes for services and feeders. For THHN, THWN, SE, USE, etc, # 2 AWG copper or # 1/0 AWG aluminum will feed the 125 amp panel.
My choice would be Cu if he's only going a couple feet to the subpanel.
That table is only good for the main feeder cable.IE, where the cable supplies all or most of the power to the building.
The text of 310-15(b) reads "For application of this section, the main power feeder shall be the feeder(s) betwen the main disconnect and the the lighting and appliance branch-circuit panelboard(s)..."
That sentence was not in the 1996 NEC - it was added to the 1999 NEC. To me, it seems like they added it and worded it to specifically include feeders to what we refer to as 'subpanels'.
Burt you stopped mid sentence."and the feeder conductors to a dwelling unit shall not be required to be larger than their service-enterance conductors.""Someplace" I "think" that I read that the change was to allow "minor" load to be at the main disconnect, such as a tap for an AC, and then normal full household load, with all of it's normal diversity, to be assumed when figuring the feeder from the disconnect to the main "distribution" panel.And it is limited to certain types of wire/cable.We have not details of the application other than it is a sub-panel.But it is area that is not very clearly defined.
Edited 6/9/2005 9:49 am ET by Bill Hartmann
Use Table 310-16. Don't forget the ambient temp. correction factors. This will probably result in #1 Cu, in most cases.
Edited 6/9/2005 4:01 am ET by BarryO