Electrical – Nema 14-50 40A outlet for electric car: one gang box (plastic or metal) and cable size …code

electric vehicleelectrical

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?

Thanks

Best Answer

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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Electrical – Provisioning for a hypothetical electric car

See SAE J1772

In the 2009 standard, two charging levels are defined.

            Voltage  Phase          Peak current
AC Level 1  120 V    Single phase   16 A
AC Level 2  240 V    Split phase    32 A (2001)
                                    80 A (2009)

Nissan say

Most Nissan LEAF owners recharge their cars at home. The 6.6kW charger option lets you charge your Nissan LEAF at home in just 4 hours using a 32A Home Charging Unit

For that option you'd need a dedicated 240 volt 32 amp circuit.

Until you actually have an electric car it may be best to leave that circuit terminated in a junction box of some sort and leave the fitting of a charging-unit until later. You probably shouldn't fit a regular 240 V 32A outlet because the J1772 system requires some intelligence be built in to the supply side (the charging unit).

In some countries the government will pay you up to $1600 to have a charging point fitted in your garage. If you live in such a country, it might be worthwhile taking advantage of such offers whilst they are still available.

Electrical – Hooking up a 50A hot tub breaker to a 60A subpanel

Regardless of what all the individual breakers add up to, the maximum you can draw through that subpanel is 60amps. Realistically you want to be a bit lower then that to allow for some spikes.

To determine if you can support the tub, you need to take a measurement of the current (amps) on the circuit with some or all of the existing devices in use (i.e. normal use). Ideally you would use a clamp meter around the hot wires in order to measure this, but alternatively you could add up the loads on the circuit. If you subtract this value from 60, and you have enough capacity for the tub, then it should be OK to run from that sub-panel.

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Electrical – Provisioning for a hypothetical electric car

Electrical – Hooking up a 50A hot tub breaker to a 60A subpanel

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