Now #8Cu/#6AL is pinched worse down to 38.5A, and #12 copper is limited to 21A which is still no problem. If you have 4 circuits (7-9 wires which count) you must derate to 70% of ampacity. That pinches on the formerly 50A #8Cu/#6Al, but doesn't bother the #12 wire at all since it's already limited to 20A. Now #8Cu/#6Al is limited to 44A, and #12Cu is limited to 24A. If you have 2-3 circuits (3-6 wires that count) you must derate that number to 80% of ampacity. In 'normal' 120/240V split-phase power, all circuits count as 2 conductors under the thermal rules. #12 copper NM-B is allowed 90C (for this calculation only) which is 30A. However in practice you can only get 50A out of it because of 75C thermal limitations on terminals. You look this up on Table 310.15(B)(16).įor instance #8 copper and #6 aluminum THWN-2 is allowed 90C which is 55A.
TLDR: with 3 or fewer circuits, you don't have much to worry about.įor any given wire or cable, you look up the highest thermal rating the insulation permits, except with NM cable you use 90C (even though it is 60C wire) per 334.80. So wires must be thermally derated with many wires in same pipe. With conduit, you now have a bunch of wires making heat in a confined space.
PIPE FILL CALCULATOR SKIN
That is why 1000 kcmil aluminum is barely more than double the ampacity of 250 kcmil aluminum - while it has 4x the cross-section it has less than 2x the skin for radiating heat. It's also about skin surface area - the wire must be able to shed the heat made. Wire ampacity limit isn't just about copper/aluminum cross-section. The wire cross section is summed up and compared to the conduit. Then the diameter of the wires (or wide dimension of cables) to figure its cross-section. Gory details? OK, you consider the inside diameter of the conduit to determine cross-section in area. You don't need to crunch numbers - there are online "conduit fill calculators" which will do that for you, such as Southwire's. With the above settled, we can talk about physical conduit fill, or how many wires you can have in the pipe. However, you can only "fork" the ground with a splice, and a splice requires you treat the junction box as a junction box and follow the "box fill rules" (which I won't cover here). #10 ground will suffice for 25-60A circuits. The ground wire must be large enough for the largest circuit.
Multiple circuits can share a single ground wire. If you have a multi-conductor cable with a sheath, you must take the widest width of the cable and treat it as a single wire of the wide dimension.
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