Size wire for 125 amp breaker?

unioncreekSeptember 28, 2009

I own a commercial building that we're turning into a restaurant that has two apartments above it. We've updated the electrical to international fire code, but I still need to run the wiring from the meters to the breaker box. The run will be about 100 feet and the breaker boxes are 125 amp. I'm looking for suggestions on wire size, this is what was given to me by a friend of mine. All wiring will be copper two #2 THHN for hot, one #3 THHN for neutral and one #4THHN for the ground. Do these sound right, I've looked at some of the online calculators and they all say to run #3 for everything.



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What does your electrician say?

    Bookmark   September 28, 2009 at 4:27PM
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I hope you know that just about every (if not all) municipality prohibits unlicensed people from doing electrical work in commercial establishments and/or multifamily dwellings. The logic is (I think) is you can burn down your own home or kill your own family but not put others at risk.

Good luck.

    Bookmark   September 28, 2009 at 9:06PM
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Tell your friend to keep his day job. I will assume since this is commercial, it is a 120/208V., 3-phase system, but will keep in mind since there is a restaurant it might be 277/480V., 3-phase system.

#2s are only good for 95A without ambient exposure(interior run)and 77A with ambient exposure(exterior run). The 125A MCB will NOT flip if there is a load higher than 95A and you will have a grave situation on hand.

The MINIMUM this needs to be is (4)#1/0's Cu.(three hots, 1 neutr), (1)#6 cu. e.g.(ground) in 1-1/2" conduit for interior run, and (4)#2/0's cu., (1)#6 cu. e.g. in 1-1/2" conduit for an exterior run.

This of course is for a 125A MCB board. If you were to use a 100A MCB board, the copper wires would be smaller.

    Bookmark   September 29, 2009 at 12:45AM
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Ron Natalie

From his post it sounds like the 125A breaker is the main on the subpanel NOT the OCD for the feeder. Hence, it's a gigantic leap of assumptions to make any recommendation on feeder sizes with this limited information. WIthout knowing what the loads are, the distances involve, etc... it's impossible for even an electrician to answer this question.

    Bookmark   September 29, 2009 at 5:05AM
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azlighting, You are also assuming a lot.
#2 conductors rated at 60 deg C are rated for 95a. Almost nothing anymore is rated 60 deg C, unless you are using old wire.
And, so what if the breaker will not trip at 95A??? We round this UP to 100A since there is no such thing as a 95a breaker.

Also, maybe in AZ you have to regularly adjust for ambient temps, but MOST Of the rest of the country this is not that much of an issue.

Also also, Ron is right, just because the panels are 125a does NOT mean they need to have a feeder rated at 125A. As long as the conductors are rated for the size of the service disconnect OCP.

    Bookmark   September 29, 2009 at 6:51AM
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...but I still need to run the wiring from the meters to the breaker box.

I'd be inclined to think that Bob is installing panels to serve those two apartments.

    Bookmark   September 29, 2009 at 7:30AM
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I'm not assuming anything. Title of the post is "Size wire for 125A breaker", to which I gave an answer.

Where did you get a 95A breaker from, or even a 100A breaker? OP is not asking about those, did you read the title of the post?

    Bookmark   September 30, 2009 at 12:52AM
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The OP does not know what he needs. He is asking for the size wire for a "125a box".

By assuming I meant you are assuming 60 deg C conductors (HIGHLY unlikely), and extremely high ambient temps (quite unlikely).

You make it sound like if the current exceeds 95A the place will burn down.
Read the last sentence of my post. If I am wrong fine, I'll admit that.

    Bookmark   September 30, 2009 at 6:21AM
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