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I have a 40 amp circuit. Can I use 2x10AWG wire for each conductor instead of 1x8AWG?

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No

The reason (aside from "code says you can't") is that if somehow one of the two conductors breaks or (more typically) disconnects at one end, instead of a dead circuit (annoying but safe), you have an overloaded wire that can set your house on fire under normal load without ever tripping a breaker.

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  • It is under 1/0 AWG and less than 3 paralleled conductors. Don't even consider it unless you can meet those two requirements. The NEC has more conditions. But these are the reason I say no.
    – Dan D.
    Jun 21, 2019 at 2:05
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    If one of the two conductors breaks or becomes disconnected it will not be safe. It will still be live as the other end is still connected.
    – Peter Jennings
    Jun 21, 2019 at 11:38
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Hell no. Not least it violates the "All related conductors must be in the same cable or conduit" rule, causing imbalance/eddy current problems under normal conditions, plus the safety factor manassehkatz discusses.

However, if you really want to, and are willing to run some 3" conduit...

  • You must increase the conductor size to 1/0 AWG as a bare minimum, and realistically nobody does this until they're above 4/0, so you won't find a 1/0-sized...

  • specialized distribution panel specifically designed for paralleling on the supply side, and that type of equipment generally starts in the $1000 price range and 300A+.

  • You must follow all the other rules for paralleling.

  • You must get the AHJ to sign off on this, even though he's going to reject it out of general crazy-factor.

  • However, you can still feed the $1000 distro panel from a 40A breaker.

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  • thanks. the conductors are in the same cable btw.
    – Walrus the Cat
    Jun 21, 2019 at 3:51
  • @WalrustheCat where did you get a 4-conductor #10 cable? I have never seen one of those.
    – Harper - Reinstate Monica
    Jun 21, 2019 at 4:58
  • Estonia ... :-(
    – Walrus the Cat
    Jun 25, 2019 at 1:01

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