# Adding a new subpanel, have some questions

 Forum topic by Vrtigo1 posted 08-25-2011 11:33 PM 952 views 0 times favorited 2 replies
 Vrtigo1434 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: http://www.csgnetwork.com/wiresizecalc.html 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?