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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.

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    Have you done a load calculation to see if you have an extra 48 amps of power available?
    – crip659
    Commented Sep 14, 2023 at 16:50
  • It's your garage, cut open the drywall, mount a junction box to a stud, run the conduit from the panel to the stud, pull wire, repair drywall, mount EVSE to the JB. But, are you sure you need 48V of charging?
    – FreeMan
    Commented Sep 14, 2023 at 16:57
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    Where's that canonical ev post... Commented Sep 14, 2023 at 17:27

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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.

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  • What about #6 NM-B? My understanding is that NM is not rated to 90 C, but NM-B is. Also, how does the length of the run affect this?
    – Lunyx
    Commented Jan 1 at 1:47
  • @Lynux NM-B changes nothing about the thermal characeristics of NM. You may be thinking of Canadian NMD90, which we call SER here. In the US, wire length changes nothing about how much heat a given foot of wire will make. The sum total heat will be more on a longer wire, and that means less gets to the car. But why charge that fast? It's expensive, and comes with headaches you hadn't bargained for. See Technology Connections. youtube.com/watch?v=Iyp_X3mwE1w&t=1695s Commented Jan 1 at 23:44
  • I see, that makes sense. The only reason is really to have the capability to charge faster should it be needed. For example, forgetting to charge and realizing you're low, and wanting to top up as much as you can in half an hour or so. The EVSE has controls for limiting power through software, so for me, it's a matter of making sure that I can support the higher speed if needed, and as a backup in case software control fails or something.
    – Lunyx
    Commented Jan 3 at 15:02
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    @Lunyx yeah, we hear that a lot from EV first-timers, that abundance of caution. Once you get into the groove of EV ownership, you'll get into the ABC habit (Always Be Charging) i.e. plug in every night… and your car being under 50% will be a rare occurrence. Commented Jan 8 at 1:39
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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.

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  • Thank you for the answer. I generally only charge at 30a at home, but since I have room in the panel and few other high-current draws, I'd like to wire for 48a in case I ever need it in the future. I have never installed EMT through studs. Is it worth doing that, or is flex conduit an option?
    – John K
    Commented Sep 14, 2023 at 17:49
  • ENT flex is exactly what I would recommend, either 3/4 if you need more than two feet, or 1/2 if you don't need that much. Both can go through 2x6 construction with enough clearance. And you really aren't likely to need 48A - but hey, if you have room in the load calculation and it doesn't lead to buying a more expensive EVSE, this is one of those few cases where it might make sense to do.
    – KMJ
    Commented Sep 14, 2023 at 19:41
  • ENT is plastic flex conduit. Often nicknamed smurf tube due to its typically blue color.
    – Milwrdfan
    Commented Sep 14, 2023 at 19:43
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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?

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