# Sizing a subpanel

• posted on January 2, 2005, 8:17 pm
Let's say I want a subpanel in the garage. It will drive 220V and 110V outlets. How should I determine the amp on the breaker on the main panel that would serve the subpanel? That would also determine the gauge of the wire leading to subpanel, obviously. The simplest way is the obvious one, add all amp ratings of various outlets, being mindful that the 110V outlets would be on different legs. Is that the best way though?
Specifically, what I want is a 30A 220V outlet, a 20A 220V outlet, and two 20A 110V outlets on two legs. The sum is 70A. Note that it is unlikely that I will use both 30A and 20A 220V outlets at the same time. So, this means that I should be able to get away with 50A service.
Makes sense?
i
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• posted on January 2, 2005, 9:23 pm
Ignoramus14233 wrote:

You'll use a 100A or 125A "main lug" load center. (70A panels do not have enough spaces) The panel will cost you about \$20 to \$30, depending on the brand.)
The wire size you use is determined by the maximum load you intend to use all-at-the-same-time and the allowable voltage drop (which is dependant on the length of the wires, and whether you use copper or aluminum.) There are tables where you can look up the voltage drop.
Then the breaker in the main panel is determined by the maximum ampacity of the wire.
My code book is in the garage somewhere, so I will just make some guesstimates. Your maximum load is probably something like 40A, so you could use #8 copper cable. But depending on the distance to the main panel you might wanna use the next larger wire size to reduce the voltage drop. The ampacity of #6 copper wire is 55A, and you get to round that up to a 60A breaker. If you use #6 copper SER (service entrance cable) instead of 6-3 NM cable, I *think* the ampacity is 65A and you could use a 70A breaker, but don't quote me on that.
If you think you might ever want to run a full-sized arc welder out there, just go ahead and run #6 copper or #4 aluminum even if something smaller is adequate for now.
Best regards, Bob
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• posted on January 2, 2005, 9:49 pm

Thanks, that's completely sensible. For a few extra bucks, I see no reason not to go with #6 cable.
i
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• posted on January 2, 2005, 11:06 pm

You need a 4 wire cable. Sub panels must have the neutral and ground separated. 6-4 SER would be what you looking for. use this page to calculate the voltage drop for the right size wire. http://www.electrician.com/vd_calculator.html
A true load calculation is done at the service. I will bet after you add this load to the service you could use #8 (40 amps) and be fine. Just in-case you want to save a buck or two
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• posted on January 2, 2005, 11:26 pm

That's my intention, yes.

I would rather spend a few bucks extra, but get something super powerful. The real expense, anyway, is the time spent.
Do I need to enclose the cable into a metallic conduit?
i
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• posted on January 3, 2005, 1:29 am
Ignoramus14233 wrote:

The answer to that last question depends on the code that is enforced in your area. In Chicago and environs conduit is required for almost everything. In most other areas you can run SER or any other cable that is suitable for use indoors. -- Tom H