Size sub panel for each apartment in a 3 family house

fotostatNovember 18, 2009

In a 3 family house there are three main panels in the basement, one for each apartment. Some circuits are taken out of those panels, but there is also a sub panel inside of each apartment that the rest of the circuits are taken out of.

Is there any calculation that needs to be done to determine the size of that sub panel?

Right now the panels are fed with a 12-2 (that's right, single leg) and there has never been a problem.

The panels need to be upgraded, would it be alright to refeed them with 10-3, which should be more than enough ampacity for them? Or should it be bumped up to 8-3 just to be safe?

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Just so you know all (or at least the vast majority of) municipalities prohibit unlicensed persons from doing electrical work in multi-family dwellings. You really need to hire a licensed electrician who could answer your questions and ensure the work is done safely and to code.

    Bookmark   November 18, 2009 at 11:52AM
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Thanks for the info, but my area of PA doesn't require contractor licensing.

FWIW, I am an electrician, but I do commercial work and unfortunately, as an IBEW member, our apprenticeship and continued work doesn't deal with the code all that much. Most of the code issues are dealt with by the engineers, very little is left for us to decide, some people call us "electrical installers" which is true to a point.

On an issue like this thread topic, I'd be happy to learn if people could help by citing the code or giving opinions.

    Bookmark   November 18, 2009 at 12:29PM
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There's a table in Article 220.49 that might help with the load calculations. Properly sized feeders is covered in Article 215.2 and you can find wire ampacity tables Article 310.16.

Hope this helps!

    Bookmark   November 18, 2009 at 2:31PM
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220.49? I don't see that in 05 or 08.

In 310.16 where is NM listed?

    Bookmark   November 18, 2009 at 10:09PM
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    Bookmark   November 22, 2009 at 8:53PM
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220.49? I don't see that in 05 or 08

Maybe Mike meant Table 220.42 ??

In 310.16 where is NM listed?

Cable types aren't listed, only wire sizes. See 334.80 which says to use the 60C columns in 310.16 for NM to select wire ampacity. (In other words size NM the same as TW, UF.)

    Bookmark   November 23, 2009 at 5:33AM
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