Adding a new subpanel, have some questions

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Forum topic by Vrtigo1 posted 08-25-2011 11:33 PM 952 views 0 times favorited 2 replies Add to Favorites Watch
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434 posts in 3046 days

08-25-2011 11:33 PM

My main panel is full, so I’m planning on adding a subpanel. It’s in my garage, where my makeshift shop is, so it should be pretty straightforward. I’m just going to put the subpanel right next to the main panel.

A friend of mine does commercial electric work and is going to help me install everything, but for purposes of knowing what I need to buy, I have some questions.

In terms of finding the proper wire size for my application, can anyone provide links to a good reference chart or calculator?

Also – when you’re talking about 240V circuits and calculating wire size, when someone refers to total circuit amperage, is that the amperage marked on the breaker, or double that amount? In other words, if you have a 240V breaker marked 20A, that’s actually two 20A breakers giving you two 20A 120V legs, yielding 40A total circuit amperage, right?

I found this calculator online:

According to that, if I have a 240V circuit which has 40A total circuit amperage (as in, a 240V breaker marked 20A) and is 30’ long, I can get away with #14 wire? That doesn’t seem right to me. I’ve always seen people recommending #12 or even #10 if you want to overkill it.

Now, next question in terms of sizing circuits…one of the tools I will be hooking up is a 240V compressor which is labeled 15A on the motor. I assume that means that I buy a double pole 15A breaker and the device is actually able to draw 30A (two 120V legs at 15A each), but in terms of 240V that would be called a 15A circuit, right?

So, last question, pertaining to the question above – do I buy a 15A breaker for a tool marked 15A or do I need to size it up to 20A so the breaker won’t trip if the device pulls its full 15A? Or does the label already take that into account, for instance the device may only pull 10A but they label it 15A? In short – is the label on the machine indicative of the circuit breaker capacity needed?

2 replies so far

View Mark Shymanski's profile

Mark Shymanski

5621 posts in 3767 days

#1 posted 08-26-2011 05:58 AM

I would think local codes also would affect your wire and breakers selection also. I second the idea of getting your buddy to help you draw up that shopping list.

-- "Checking for square? What madness is this! The cabinet is square because I will it to be so!" Jeremy Greiner LJ Topic#20953 2011 Feb 2

View NewfieDan's profile


50 posts in 2703 days

#2 posted 08-27-2011 04:42 PM

You aren’t correct in assuming that a 240V breaker labeled 20A gives you a total of 40A. Each leg will only give you 20A. what it will give you is more wattage, not more amperage. This where your buddy will be able to help. Your tools will also have a HP rating. This is wattage. One HP=745Watts. So, a TS that is 2 HP will need 1500 Watts. At 240V this will translate to a 2pole breaker rated at 15A. But at 120V it will need a breaker rated at 20A.

Even though a 15A breaker will work there is rule in the NEC thar breakers can only be filled to 80% capacity. So the 12.5 amps at 120V exceeds the 80% so a 20 is needed. Also at 20A a larger wire size is needed to caary the additional load. When wires carry too much current they get hot (read house fire). A larger wire will be able to carry more electricity without getting hot. This is also the principle behind electric heating.

You are correct in calling a 15A breaker a 15A circuit.

But like the others here have said. get your buddies help. When he comes to give you a hand ask…most guys I know will be glad to explain the basics. Probabay easier and better than I can on here…

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