What wire do I need for a 48A EV charger?

Question

I have my electrical panel in my drywalled garage, and want to install a 48A EV charger on the wall next to the panel. My plan was to run wire through the bottom of the panel, sideways through one stud, and then up the drywall space into the back of the EV charger.

My question is, since 48A is just above the continuous current rating for 6/2 NM-B, what kind of wire would be best to use? I get could get three conductors of 6awg THHN, however since it's supposed to be run through conduit or raceway, what would be the best way to cross that stud?

I would prefer to keep everything behind the drywall for aesthetics.

Answer 1

EV wire sizing is tricky. As you may have noticed, "there's a myth out there", well all sorts of myths actually, about wiring up EV charging, and they're getting very pervasive.

One of those myths is that #6 wire will suffice, and so most EVSE makers permit a maximum size of #6. Another myth is that #6 "Romex" (NM) is the right stuff, and its insulation is simply not good enough for running 48A continuous. Here we have direct reports of #6 wire running positively above 60C. And we know the car is not drawing more than 48A because that is the hard limit of both the EVSE and the charger onboard the car in that case. So clearly, #6 NM and UF are not suitable for 48A. I generally recommend #4 for anyone obsessed with the Fastest Charge Possible.

Let's segue to that for the benefit of our other readers. The fastest speed the car is capable of is gross overkill for everyday non-travel driving. It's for travel - when you arrive at a hotel at 10% and want 100% by morning for onward travel. Pushing your home to that kind of speed creates many "headaches you hadn't bargained for" including the melty crispy kind!

Unfortunately, most EVSE manufacturers have bought into the myth of #6 Romex for 48A charge/60A breaker, and do not support #4 wire. (it further muddies the water that 55A breakers are not a standard size, so even though #6 Romex is only 55A, it's allowed to use a 60A breaker under the "Round Up rule". Many EVSE instructions tell you to set the charge speed based on breaker size, which is improper and I'm surprised UL let it slip.)

All this to say, NM and UF are poor choices for EV wiring if you're running 48A/60A breaker. #4 NM would be fine, but most EVSE's won't accept it. So, that forces us into either copper SER or SEU cable, or individual wires - which in turn require a complete and proper conduit run be completed empty of wire, and then wires pulled in after it is complete. Hold that thought.

And for reasons which boggle my mind, literally no EVSE on the market has terminals cross-listed for copper and aluminum. Aluminum lugs are a cheap no-brainer - they play nicely with both aluminum and copper wire. Heck, if they just had you land the wires on the internal contactor, most contactors have AL rated terminals.

For you, your panel is right next to your EVSE so I don't care. But for everyone else, there's a huge wrinkle coming down the pike - California's obsession with "Vehicle To Home" is going to force to happen what's already easy to do - set up cars to back-feed your house e.g. during outages. That's going to require additional wires, for signaling and probably to carry DC battery current (THHN isn't going to cut the mustard for that). As such, I strongly advise (others) leave a nice big conduit between garage and panel.

Answer 2

First: I'm going to ignore whether or not 48A fits in your load calculation or makes sense. If you're planning an all-electric home for carbon reasons, you might want to go smaller even if the higher ampacity charging fits, just to save yourself room in the load calc for other appliances.

The easy way to do this is by running individual conductors through conduit. If you can easily run metal EMT conduit between the two locations, you can use the EMT as your ground conductor and two #6 THHN wires for your current conductors. This means you get the ease of running 1/2" conduit. Alternately, you can run two #6 THHN current carrying conductors and a #10 THHN ground through a 3/4 conduit of basically any type. I would suggest taking a peek at ENT for that purpose, since it's relatively easy to run even with very limited access, i.e. without having to punch a big hole in the wall, due to being flexible. It doesn't require any tools to bend either. If your conduit run is less than 2 feet, you can even use the higher conduit nipple fill allowance, so 1/2 conduit will work for your three conductors.

Answer 3

Wire ampacity ratings are for the breaker, not the de-rated load size. 6AWG copper NM may not be used for a 60A circuit / 48A continuous load. For that load, you need 4AWG copper NM, 6AWG copper THWN, or 4AWG aluminum THWN.

That’s all assuming you really need 48A charging. You probably don’t, it’s gross overkill for most people. 48A at 240V will restore around 300 miles of range in 8 hours for a 3mi/kWh EV. Do you really need to charge that fast at home?