Insulation - Select the thermal rating of the insulation on the wire.Ĭonductor - Choose the material used as a conductor in the wire. To calculate ground wire size, use the Ground Wire Size Calculator. For three-phase circuits, four wires are required. Single-phase circuits, three wires are required. This is typically single-phase or three-phase. Phases - Select the number of phases in the circuit. To multiply the nameplate FLA by 1.25 for wire sizing. While three-phase voltages are typically 208V, 230V or 480V.Īmperes - Enter the maximum current in amps that will flow through the circuit. Single-phase voltages are usually 115V or 120V, However, we also price #1Al, and it turns out to be not much more expensive, so we decide to bump to #1Al and 100A breaker.Voltage - Enter the voltage at the source of the circuit. After pricing #4Cu and #2Al, we decide to go #2Al and 75A. But we decide to breaker at 70 or 75 amps for some future expansion room. Remember the voltage drop calc we did above, where we figured wire size based on voltage drop for our practical load, and we got a wire size out of that too? If that conflicts, use the size that is larger.įor instance in part 1, we arrived at #6Cu/#4Al. 70A: #4 Cu (rather pricey wire, aluminum is 1/3 the cost at this point)Īgain this is dictated by the breaker size.īut wait.Anyway, once you know the feed breaker you want, that dictates the wire size to use: You said "100A" off the cuff, and that's a good, cautious, safe choice (although a bit more expensive in terms of wire). Fire up a heater and those loads get big fast.
125% of that is 59A, so 60A would be bare minimum there, but even that overlooks lighting and other routine potential loads. In the example above, the higher leg was 47A. In fact, it should be at least 125% of the maximum routine load you ordinarily expect. You want to size that large enough that it won't cause nuisance tripping from peak loads. What matters is the size of the feeder breaker inside the main panel. A "plenty of spaces" subpanel will probably have a main breaker bigger than 100A.
It depends on you being in a commercial application where AFCI and GFCI breakers are not required. Some panels specify x spaces and y circuits (a larger number). Whereas avoiding this problem (with a big panel) will set you back a few latté's. That creates an expensive, high-labor problem when you add things in the future, like a 240V table saw, a better welder, a water heater, electric car charger etc. The size of your panel in terms of number of spaces is very important to not running out of spaces. Its size doesn't matter - it's not even required! What is required is a shutoff switch, and the cheapest way to get a shutoff switch in a subpanel is to use a panel that features a main breaker. Now, at the subpanel in your outbuilding, there is also a main breaker in the subpanel. One is at the main service panel, and that has a very important job - protecting the wire run to the subpanel, and protecting the subpanel itself if the subpanel doesn't have its own main breaker. I get 1.95% at 4 AWG Aluminum wire, as the minimum needed to carry practical current in this example. Take the average of that - 41A - and put that into the voltage drop calculator. The two 240V loads put 23+12A=35A on both legs, and the 12A dust collector adds 12A to one leg so your two legs are 35A and 47A.
If the number is pushing 5%, then change it to 4% and see what it says.įor instance let's say you have a "30A" 240V compressor that draws 23A, a "20A" 240V table saw that actually pulls 12A, and a 12A/120V dust collector. How much do you actually intend to load up this panel? (remembering that 120V loads only accrue to one leg or the other).Ĭalculate voltage drop based on that, and use 5%+ for your voltage drop number, because otherwise the calculator might find the standard wire is 3.06% and make you bump a size over 0.06% - that's ridiculous. You have two separate questions to think about.
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