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  • I had not considered this, but you're right - 12/3 is not rated for 40 amps, it's rated for 20 amps. The line from the junction box to the breaker box should be 10-3. Of course if you're only powering two electrical boxes - one each per line - there's very little real risk of loading up 40 amps on the line, but of course it is possible. Commented Aug 27, 2012 at 19:27
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    It sounds like the two hots are on different legs (which is how it should be done), so the neutral will only ever carry the unbalanced load between the two circuits. If you have 20A draw on black, and 20A draw on red, the neutral will have 0A. Commented Aug 27, 2012 at 19:32
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    Aha! Reversing the polarity for the win! 20+20 isn't 40, it's 0! :D Commented Aug 27, 2012 at 19:44
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    @bib no, alternate busses in the panel are on different legs. This is how single breakers (on one side of the panel) send 220v. Therefore in this case the two hot conductors in the same run should be connected to two adjacent bus prongs. Commented Aug 27, 2012 at 19:57
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    @bib Most panels are set up where every other pair is on a different leg. You'll notice one side will be odd numbers and the other even, so you'll have 1-2=A, 3-4=B,5-6=A,7-8=B... So if you connect a double pole breaker at 3 and 5, you'll have Leg B on 3 and Leg A on 5. Next time you're at HomeDepot, open up a service panel package and look how the terminals on the legs are configured. Commented Aug 27, 2012 at 20:09