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Aluminum sub feed


fyrmnk
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Hi all,

Home has 100A breaker in the main on #2 aluminum feeding a sub panel in a detached garage. Originally I was thinking this was OK since the garage is independent with its own ground system (no metal contact such as sidewalk, phone line, grounbd wire, etc.) back to the home, therefore making it essentially having its own main service.

Then I decided it should be considered a branch circuit same as a standard sub panel and not be any larger then a 70A breaker for the wire, since #2 aluminum on a branch is 70A max.

Right or left brain right on this one? Thanks

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Off the top of my head I would say that this is still a subpanel that would qualify for the "separate building" exception. If there's a panel set out there I couldn't think of it as a branch circuit in the usual sense.

The capacity would be limited by the smallest component, between the feed, main, and panel rating. If the wire has the least rating, that's the limit for the panel until someone changes it. I would write it up as such too (70 amp, 120 / 240 volt) and warn the client about it.

Someone smarter than I may know better, so wait for more opinions.

Brian G.

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Originally posted by fyrmnk

Hi all,

Home has 100A breaker in the main on #2 aluminum feeding a sub panel in a detached garage. Originally I was thinking this was OK since the garage is independent with its own ground system (no metal contact such as sidewalk, phone line, grounbd wire, etc.) back to the home, therefore making it essentially having its own main service.

Sounds right. I presume that this was a three-wire feeder? (two hots and a ground)

Then I decided it should be considered a branch circuit same as a standard sub panel and not be any larger then a 70A breaker for the wire, since #2 aluminum on a branch is 70A max.

The ampacity of the wire depends on the type of insulation and the cable assembly. Was this SE cable or romex? If it's SE, I think you can use the 75-degree column. That'd give you 90 amps.

Right or left brain right on this one? Thanks.

I don't know about right or left, but I think your first reaction was the right one.

- Jim Katen, Oregon

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