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I got my hands on a 40A EV "charger" that I'd like to attach to a sub-panel in the garage. (I put "charger" in quotes because the cord with the bump is just a fancy relay; the charger is in the car). Can #8 NM in-wall wire handle 40A continuous?

The standard reply may be to size up the circuit to 50A. However there are other considerations that make me hesitate.

  • The sub-panel has a 60A breaker. Putting a 50A circuit on there would eat up too much of the budget.
  • I plan to convert the gas water heater to a hybrid electric water heater in the future. While that normally sips power, the backup resistive element can pull ~20A.
  • The sub-panel services the kitchen, living room, and a bedroom. Normally the total load is under 10A, but worst case it could double (microwave, stereo, TV, space heater...).
  • If total load becomes a problem, I could schedule charging during off-peak hours.

It seems better to limit the charger circuit's max draw to 40A instead of 50A. Is there a breaker that allows 40A max continuous but not more, not even peak? The sub-panel is an Eaton CH load center.

Attached is a photo of the main panel with 225A service breaker. The two 60A sub-panel circuits are at the top left. Main panel 225A

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    Any possibility of upgrading the subpanel feed so that you can put in a bigger breaker? Feb 28, 2022 at 2:13
  • What gauge is the feeder wire to the subpanel? Also, is using a different wiring method for the EV circuit an option? Feb 28, 2022 at 2:15
  • "Is there a breaker that allows 40A max continuous but not more, not even peak?" A breaker is a safety feature, not a throttle. The specs for the charger should indicate the correct breaker size to use.
    – LShaver
    Feb 28, 2022 at 3:14
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    2 AWG aluminum is a very popular feeder size because it's at a price point "sweet spot" due to very wide use as 100A service wire. As feeder it's only good to 90A though. If the guy before had asked us, we'd have advised #2Al to the subpanel and a 60A breaker (it takes #2 and it's cheaper than a 90A breaker) if the full 90A is not needed. Feb 28, 2022 at 3:46
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    @pmont yes! Just make sure it is not NM or UF (that's not likely). Feb 28, 2022 at 20:34

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The term you're looking for is EVSE, or Electric Vehicle Service equipment. You are correct; it is a relay, computer-controlled GFCI and a computer that talks to the charger on the car, including telling the car the current it is allowed to draw. This is a "soft setting" and is configured in the EVSE at commissioning time, either through DIP switches or in a special commissioning screen.

The proper charger inside the car listens to the EVSE data and charges at the rate authorized. If the car ignores this, the EVSE senses it and cuts power.

Therefore you are not under any obligation to have an EVSE be a particular size. You can simply determine the surplus ampacity available, and set the EVSE per Code requirements given that ampacity.

Start with a Load Calculation

using the NEC approved procedure for doing those. That is a science-based formula that determines probable loads for a given dwelling.

You do two Load Calculations, actually. One for the house's entire service, and the other for the loads inside the subpanel. Once you have finished that, you know how many amps of "headroom" you have to give to an EVSE.

For instance if you calculate to 144A on a 200A service, you have 56A of spare service amps. If you calculate to 24A on a 65A subpanel, you have 41A spare subpanel amps. This would call for a 40A EV charging circuit off the subpanel.

Provision the breaker and wires on this basis.

So continuing the 40A example, you wire the circuit with a 40A breaker and 40A wires (that being #8 copper or #6 aluminum).

At these large sizes there is nothing wrong with aluminum. There was an issue with 1970s small branch circuit wiring, but that isn't applicable here.

Then, derate the EVSE 125% / 80%

Any EVSE circuit requires a 125% derate, NEC 625.14. So you take the circuit size and multiply by 80% (the inverse of 125%). For instance, if you have a 40A EV charging circuit, you take 80% of it or 32 amps. This will be the actual charging rate.

Note that 32A x 125% = 40A.

They're not "singling out" EVs. This 125% / 80% thing is a requirement on any continuous load. NEC also imposes this on many other kinds of appliances that are arguably not continuous, to silence such arguments.

Now, this is configured into the EVSE. The EVSE manual will have a procedure for setting the maximum charge rate allowed; follow the procedure (a NEC 110.3 requirement). For instance, Tesla EVSEs have you set the breaker size (40A) not the actual-charge-rate (32A) and the EVSE figures out the 80% thing on its own.

The gory details: so much more than a relay.

If you want to be picayune, the EVSE doesn't actually limit current. The EVSE sends a signal (a square wave on the Control Pilot pin) which tells the EV how much it is allowed to draw. (32A in our running example). The EV onboard charger detects the signal and chooses to limit the current draw to less than that.

So the EVSE just passes on the message to the car, but the EVSE is the only place the charge rate can be set.

That is on purpose, as UL will not approve any setup where the consumer could change the max charge rate in software. It has to be "crack open the EVSE and change DIP switches" or some equally elaborate procedure.

It also means any car can be plugged into any EVSE and the right thing just happens.

If you have a limited amp allocation you want to dynamically share between two EVSE's, they have tech for that called Share2 that also works slick.

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  • It's a UL rated EVSE. Limiting the circuit to 40A is for sub-panel budgeting.
    – pmont
    Feb 28, 2022 at 3:24
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    @pmont if you have 40A to provision, then the circuit is 40A and the actual EV charge rate is 32A. If that EVSE won't let you configure it for 30-32A, I'd be surprised, but then it's the wrong EVSE for the job. Feb 28, 2022 at 3:41
  • The EVSE is fully configurable. I appreciate your concern, it's the wrong focus. This exercise is about limiting the circuit's current in order to not exceed the sub-panel's capacity. Eventually I'll sell the house and I don't want the next owner puzzling over why the panel's breaker trips. Or rather, I want to avoid the inspector flagging an issue with breaker sizing, giving the buyer a reason to hesitate.
    – pmont
    Feb 28, 2022 at 4:54
  • @pmont Sorry, I did sort of mislead you by saying your view of EVSE's was correct. Only the "relay" part was correct, there's actually a whole lot more to it that you have overlooked, and that's where the magic happens. I have restructured my answer to explain how you get from "subpanel" to "allowable circuit" to "EVSE" configuration". To alleviate your concern about inspections, buyers and breaker trips, you won't have a problem if you follow Code. Feb 28, 2022 at 6:34
  • Additionally you can add current sensing probes on the feed into the panel and the EVSE will then decide how much headroom it is allowed to use based on the other consumption of electricity. That way when for example the electric heat is in the off part of its duty cycle the car gets to draw the surplus amps that are usually assigned to that electric heat. Feb 28, 2022 at 14:05

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