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Can I run 3-#8(2 pole 40amp) breaker and 4-#10(2 pole 30amp breaker) in the same pipe

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    What size conduit (aka pipe)? – manassehkatz Apr 12 at 18:06
  • Yes, what size conduit are you dealing with here? – ThreePhaseEel Apr 12 at 22:14
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Yeah, that's fine but you have to follow the rules.

The fill rules

All wires count. (In switch loops, neutrals also count even if they're not even present).

There are various conduit fill calculators around the Web and on smartphone apps.

The derate rules

  • Grounds don't count.
  • If there are 2 or 3 hot wires, neutrals don't count because they only carry differential current
  • This means in 120/240V single or split phase, 1 circuit always counts as 2 wires.
  • You have 4 wires
  • NEC 310.15(B)(2) calls for an 80% derate. But wait.
  • The derate is off the 90C column in 310.15(B)(16), or to be more precise the highest column allowed by the wire type; so for instance THWN (not THWN-2) uses the 75C column. There is a special rule that NM uses the 90C column even though it is 60C wire, but NM better not be outside. I presume your wire is THWN***-2***, which is 90C, so here we go:
    • THWN-2 #8 gets 55A@90C, it derates 80% to 44 amps.
    • THWN-2 #10 gets 40A@90C, it derates 80% to 32 amps.

Well, that's not so bad. You are still capped at the 30A and 40A limits, so this doesn't really limit you.

3 circuits (6 wires) uses the same derate.

4 circuits uses a 70% derate, which will bite you on #10 and #8, (28A and 38.5A), however you get to round up to the next available breaker size, so you dodge a bullet.

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Yes, but there are rules for conduit fill based on the number and sizes of the conductors that will dictate the size of conduit you must use, plus you will have a de-rating factor that must be applied to the conductors because of having more than 3 current carrying conductors in a raceway, so depending on the length and the load, you may need to go up one size. The NEC (or CEC if you are in Canada) has all the information you will need.

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