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Given a 240V 40A circuit is there an official wire type that should be used? I assume there is a building code standard for this somewhere, but I don't really know where to look. I'd love if I could get some sort of official source.

For clarity, it would be helpful if the size could be given for copper and for aluminum, just so I'm not guessing at the material type of the answer.

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    Size is #8 copper, you asked “type” tho, and that’s more involved—what is the application? Is this single family residential or something else? Are you planning conduit? – Tyson Aug 27 '18 at 14:41
  • @Tyson So I've got a 40A breaker in. But I need to wire an electric vehicle charger. The charger is 32A, and it'll be the only thing on the circuit. Single family home, yes. – Jonathan Mee Aug 27 '18 at 15:00
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    How long is the run and do you need to run it outside or is it all inside? – JimmyJames Aug 27 '18 at 16:58
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    @JimmyJames The charger is actually located like 3' from the circuit panel. So there should be almost no load contribution from the wire. The ev charger is wired directly. With the wire being encased in conduit running directly from the panel to the charger. The charger is located "outside" but really in a porch. – Jonathan Mee Aug 27 '18 at 17:16
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    @Harper Noted, comment removed – UnhandledExcepSean Aug 27 '18 at 21:55
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Since the entire route is conduit, you need 8 AWG THWN-2 copper. This will be individual wires, not a cable in a sheath.

You can also use 6 AWG Aluminum, but you won't find it. You might find it if you shopped at a proper electrical supply house, but you'd get funny looks and an "are you sure". In fact, Just don't use aluminum - installation is more sensitive to proper procedure, and at these small sizes, is really for wizards only.

If both panel and charger have steel chassis and the conduit is non-flexible and metallic, then you do not need a ground wire. If you do need a ground wire, follow instructions but 10AWG Cu should suffice, in bare wire, green, or green/yellow. These are the only permissible colors for grounds, and vice versa. Bare aluminum grounds are illegal.

If neutral is needed (check instructions), use the same size wire as the main conductors, unless the instructions permit a smaller size. Neutrals must be white or gray. And vice versa.

Hot wires may be any other color(s) and your two hots may be the same color, feel free to be fanciful. There really isn't any value in distinguishing them here.

  • I only hear bad things about aluminum. Is there any reason why anyone (including wizards) would choose to use it? I know there have been periods when price was a strong incentive but that aside... – JimmyJames Aug 27 '18 at 21:57
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    @JimmyJames because if you go a lot deeper into the history of the aluminum wire problem, you realize the problem was non-wizards and the fact that copper is the bad actor. Landing Al on Cu-only lugs caused the problem, but what do you use when you want a universal-donor lug? Aluminum. Seriously, look in your panel. Al also has twice the conductivity of copper by mass and 12x the conductivity by dollar. Copper is only better by volume due to its high density, but that only matters in armatures and cyclotrons. And those hardly matter to the masses. – Harper Aug 27 '18 at 22:05
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    The problem with Al in small sizes is, you would need to be living inside an ecosystem where people who build small-wire terminations are expecting you to use aluminum, and have optimized for Cu-Al. That is not our world. Even if they claim Al compatibility, you have to get the goop and the torque exactly right, and that's wizard stuff. – Harper Aug 27 '18 at 22:09

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