I'm getting ready to install a nema 14-50 outlet for an electric car/vehicle (EV) and I'm looking at the details. I found out that NEC 2017 section 314.17 talks about enclosures but I'm a bit confused. The outlet will be in my (attached) garage so it would be indoor. I'm planning to run NM-B cable (AWG 6 or 8, about 15 ft inside drywall) from the breaker box (upstairs) where I'll have it connected to a 40A breaker. I can't do 50A breaker because my SE cable is rated at 75A and the main breaker is 60A. The city permit people had no problem with this when I consulted them. I can use it at night when nothing else is being used. Do I need a metal box for the outlet? Will attaching the cable to the stud within 8in of the box be required? Am I missing any other details?


  • I think you will need a 4" box for your outlet, off the top of my head a 30 amp outlet is the largest that will fit in a single gang box. If your box has clamps you can secure the wire at 12" from the box or if fishing wires down finished walls no anchoring is required to the studs. Metal or plastic box, both are legal but consider if you will be plugging and unplugging a heavy duty plug in metal will last longer. Just an FYI these chargers are considered a continuous load so your wire size needs to be sized at 125% of the nameplate amperage.
    – Ed Beal
    Commented Oct 28, 2018 at 16:35

1 Answer 1


I'm betting this is a Tesla charger, which draws 40A. You cannot load a 40A breaker at 40A continuously (defined as 3 hours or more), you can only load it to 32A (80%) because technically, a 40A breaker is designed to protect 40A rated conductors, which by code must be sized at 125% of the continuous load. So you need conductors rated for 50A and a 50A breaker, it wasn't a suggestion on their part. If you ran #6 conductors and wanted to protect them with a 40A breaker, technically that would not violate Code, but you are likely to have nuisance tripping of that breaker. That's a nasty surprise to find out about in the morning when you want to drive to work...

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