I'm buying the material needed for an electrician to install a 240V outlet in my garage for a Tesla charger. I need to buy wiring that has to go through an underground conduit, and I was wondering what type I need to buy?

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    Is there a reason the electrician not getting/providing the materials? – cano Jul 5 '17 at 0:04
  • How much current is this charger pulling? – ThreePhaseEel Jul 5 '17 at 0:13
  • @ThreePhaseEel 40 amp. – eclipsis Jul 5 '17 at 1:42
  • And the charger circuit is all that's in the conduit, right? – ThreePhaseEel Jul 5 '17 at 1:44
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    @mmathis and I would recommend to OP that he go either 6Cu or 4Al so he can use future chargers which are indeed 50A. Both of those give you headroom up to 60A. – Harper - Reinstate Monica Jul 5 '17 at 3:30

I won't write a detailed answer since ThreePhaseEel nailed it. But to confirm, yes, you want THWN wire, which is a single wire, not a multiconductor cable. It is designed to operate in wet locations (which is to say: immersed) and to endure the physical strain of being pulled through a conduit. It has a slick nylon outer jacket for that purpose.

You don't want to pull a multiconductor cable through conduit. Electrical cables are not flexible like cordage. They have no nylon jacket, they are stiff (even 10 AWG is like wrestling an alligator) and will want to bind and snag.

Since Tesla insists on using the NEMA 14-50 connector, you will need four conductors including ground.

  • A ground wire which is natively green, green/yellow stripe, or bare wire. It must be copper.
  • A neutral which must be natively white wire (or gray).
  • Two "hot" wires which are either colored wire (any color except the above), or white wire which has been marked with colored tape on both ends.

You can use metal conduit itself as the ground wire, however for an underground run, I would only do that if the conduit is Rigid type. Otherwise, the hardware store sells 8 AWG bare solid copper ground wire, which will suffice for anything we are talking about here.

You will be using a 50A receptacle even though the Tesla charger only wants 40A. That is a practical workaround; 40A receptacles don't exist. A 40A circuit has a 40A breaker (duh) and requires 8 AWG copper wire or 6 AWG aluminum. Ther are several reasons to upsize, though. First is to reduce transmission loss over long distances.

The second is future flexibility. If you run 6 AWG copper or 4 AWG aluminum, you can make it a 50A circuit just by changing the breaker, and support a larger charger or large RV. It could also support a 60A subpanel, allowing you to power other loads too.

Price it several ways, all these will require 3/4" conduit except 4Al will require 1" conduit. 1" will make the pulls easier in all cases.


THWN is the stuff you need

You'll need to get get 6 AWG copper or 4 AWG aluminum THWN in black and/or red for the live wires as well as white for the neutral and 10 AWG bare copper for the ground wire. While it sounds like "40A charger = 40A circuit", NEC 625.41 says that the charger is a continuous load, which requires upsizing the wire to 6AWG (or 4AWG for aluminum) and the breaker to 50A as a 40A breaker will eventually trip if subject to continuous 40A current through it.

If you know how long the conduit run is as-run -- you can order wire by-the-foot in each color (including bare) and add about 5' for terminations at each end. Or, you can get a bunch of white (say, a 250' spool should leave you and the sparky with plenty left over to negotiate over), and have the electrician treat it as the universal donor wire color it is (tape flags for hots, strip it bare for grounding duty).

Using wet-rated (UF) cable is a bad idea because it makes the pull harder for the electrician, and the wires inside aren't marked for individual use so you can't take the jacket off.

  • Thank you for the explanation Eel. Is there a jacketed cable that includes black, red, white, and bare copper within it? That way I'm not running separate cables. – eclipsis Jul 5 '17 at 2:17
  • @eclipsis -- pulling cable down conduit is more difficult/annoying, so it's best avoided (you could use 8/2 or 8/3 UF here, but why? You're in a conduit anyway, so there's no need for a jacket to group the wires.) – ThreePhaseEel Jul 5 '17 at 2:32
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    @eclipsis Jacketed cable may seem niftily packaged, but it's no convenience in conduit, not at all! It's a nightmare to pull, and it requires much larger conduit. For instance pulling four #8's down a 3/4" conduit is easy peasy. But 8/3 jacketed cable? It wouldn't even fit in 3/4" conduit! It might pull in 1" conduit, but the pull would be murder. Do that if you hate your electrician. The purpose of jacketed cable is to use without conduit, e.g. Stapled to joists or direct burial. – Harper - Reinstate Monica Jul 5 '17 at 3:27
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    Also you are not allowed to use tear the wires out of the jacket and use them individually, they are not marked properly and the inner wires' insulation is not rated for pulling through conduit or for wet locations. So don't buy cable because it's cheap expecting to shuck it for the wires. – Harper - Reinstate Monica Jul 5 '17 at 3:34
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    Downvoter you should explain. This answer looks great to me. Is there something we are both missing? – Harper - Reinstate Monica Jul 5 '17 at 15:01

Actually, if the charger is rated for 40 amps, you need a 50 amp circuit.

EV chargers have to be sized as a continuous load. So you take their current draw and multiply by 125%.

625.41 Rating. Electric vehicle supply equipment shall have sufficient rating to supply the load served. For the purposes of this article, electric vehicle charging loads shall be considered to be continuous loads.

So, if it draws 40 amps you need a 50 amp circuit with #6 THWN wire.

Good luck and stay safe!

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