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I have read that I should use 8 gauge wire for a 40 amp circuit. If I had extra 12 gauge wire from a 20 amp circuit, could I simply use two lines of 12 gauge wire instead?

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    Why are you even trying to do this? Commented Jan 4, 2018 at 3:03
  • @ThreePhaseEel, to save a little money and time, of course. The reason for many safety issues, even when the dollar amounts are very small.
    – JPhi1618
    Commented Jan 4, 2018 at 18:57
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    1/0 is the smallest wire size that can be paralled
    – Ed Beal
    Commented Jan 4, 2018 at 19:42
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    Besides safety and Code issues, 2 - #12 wires comes up short of the total cross sectional area of 1 - #8
    – ArchonOSX
    Commented Jan 5, 2018 at 0:08
  • To be honest, I wasn't planning on doing it. I figured it would be a bad idea, but couldn't think through all of the reasons. I searched on google for a bit, but couldn't find anything, but I wasn't sure what to google to find my answer. So I figured I would just ask on a forum and people who already knew the answer could help me figure out where to look. And, future people with this question can probably find this answer easier next time. Thanks for your help! Commented Jan 5, 2018 at 15:30

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NO! This would be a serious safety issue and Code violation.

Paralleling of conductors under 1/0 is not allowed according to the National Electrical Code.

310.10(H) Conductors in Parallel. (1) General. Aluminum, copper-clad aluminum, or copper conductors, for each phase, polarity, neutral, or grounded circuit shall be permitted to be connected in parallel (electrically joined at both ends) only in sizes 1/0 AWG and larger where installed in accordance with 310.10(H)(2) through (H)(6). [emphasis added]

Get the right sized wire, it is not worth you and your family's safety.

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You should not even think of attempting to wire a 40A circuit with two 12 AWG (20A) parallel conductors. If you did this and then by chance one of the 12 AWG conductors failed to an open circuit for some reason you would have a serious overload condition possible on the other lone conductor. That conductor could very well overheat and cause a fire when the circuit is still backed up with a 40A breaker.

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    It's also a very blatant code violation.
    – Ecnerwal
    Commented Jan 4, 2018 at 4:53
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    It doesn't even need to be a open circuit failure, just a bad connection on one wire could force most of the current to go through the other wire and overheat it. Commented Jan 4, 2018 at 14:18
  • @ratchetfreak - Very true
    – Michael Karas
    Commented Jan 4, 2018 at 17:30
  • Awesome. I figured it would be a bad idea, but these reasons make a lot of sense. Commented Jan 5, 2018 at 15:31
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It's forbidden with wire that small.

If you had a 400A circuit and wanted to use paralleled 4/0 instead of having to go buy 1000kcmil, then there are ways to do that. Particularly, you need to use a listed enclosure that is specifically designed for paralleling. This will include both a common shutoff switch and separate fusing for each wire. That way if one conductor breaks and you are pulling 400A down a single 4/0, that fuse will blow, protecting the wire.

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