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?
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.
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!