Derating a 240v circuit for EV charger
I have a garage with a 60amp sub panel. I would like to install a 240v plug for
my 32amp EV charger. My EV charger has multiple charge settings 32/16/10/8/4amps. The receptacle will be located about two feet from the sub panel. I want to install a 14-50 receptacle which matches the plug on my EV charger.
My question is: can I use a smaller breaker size (ie 30 or 40) with a 14-50 receptacle or does code require a 50amp breaker. Again, the breaker for this circuit will be easily accessible so I am not concerned with tripping that, more concerned with tripping the breaker at my main panel. I do plan on running 6ga wire so it could in theory accommodate a 50amp breaker if needed.
in my mind it should be safe to use a smaller breaker, you just risk tripping it more often.
Thank you in advance for your help.
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Replies
I don't think this will work with a plug in charger. You can de-rate with a hard wired unit.
Since you have the 60A panel, I would put in the required breaker and de-rate the charger. The chances of this tripping is pretty much never.
First, you have to follow manufacturer's instructions, check the instruction manual for what it says.
You should be properly sizing the breaker to the load. Breakers are not designed to be tripping more often than they need to be.
You're only permitted to run these breakers at 80% of rated capacity in continuous operation. "Continuous" is defined by the electric code as "more than 3 hours". An EV charger counts as a "continuous" load here, because it can be reasonably expected to run for 3 hours or more while charging an EV.
Going by the "80% rule", as it's commonly known, would permit your 32A max car charger to be supplied by a 40A breaker, and you'd still be within both ratings and code-allowable continuous loading. This depends more on the CHARGER than it does on the RECEPTACLE though. If the CHARGER's namplate says it needs a 40A circuit, then I'd go with a 40A breaker. If the charger wants a 50A breaker, then use that. If you use a 50A receptacle fed from a 40A breaker, then you should label the receptacle like that, such as "40A 240V from CKT 3 PANEL B" or something similar that fits your specific installation.
You should not use anything less than a 40A breaker here regardless.
Bill
The instructions are here:
https://www.tesla.com/sites/default/files/pdfs/NEMA_14-50_Overview.pdf
It says "50 Amp circuit breaker / 40 Amp continuous current." You have to follow the instructions.
Notwithstanding that, there is this in the NEC:
"26-700 General (see Appendix B)
2) Except as provided for by other Rules of this Code, receptacles having configurations in accordance with Diagrams 1 and 2 shall be connected only to circuits having a nominal system voltage and ampere rating corresponding to the rating of the configurations."
I don't have access to diagrams 1 and 2, nor the exceptions, but there is a general rule that a receptacle has to be connected to a circuit capable of powering it. Which in this case would be a 50A circuit.
If the instructions say it neesd 50A, then it does. I didn't bother to look up the instructions myself :-) I am surprised they are marking it as a 40A load when it only supplies 32A, so they appear to be derating twice (32->40 and again 40->50). There are various reason this may be being done that wouldn't be readily apparent though, which is the reason the code wants you to install what the device says it requires.
Regarding #2, it is generally accepted to wire receptacles for different CURRENTS (NOT different VOLTAGES), IF they are labeled. The most common example of this is using 15A 120V (NEMA 5-15R) receptacles on a 20A circuit. Larger ones are typically labeled with what feeds them if it's different than the rating of the receptacle.
Bill
I think that two of the listed exceptions are 15A receptacles on a 20A general purpose circuit, and a 50A receptacle on a 40A stove circuit.
You're also permitted to run 208V to a receptacle labeled "250V". The listed ratings on the receptacles are maximums. The code mentions some exceptions, the "label it as special" is typically also exceptable, although discouraged. Somewhat similar to the volt drop part of the code.
Bill
What would be the purpose/advantage of installing the smaller breaker in the first place?
OP said so it would trip before the main breaker, because the subpanel is easier to reach.
But that's 60 amps. I guess if other significant things are running off the sub it could matter. But if the charger isn't drawing the higher amp rating it won't matter anyways and a smaller breaker for the charger doesn't limit the amperage of the other loads on that panel.
The charger likely only actually ever uses a maximum of 32A, or just very slightly over that. These EV chargers work by communicating how much power than can deliver to the car, and the car's electronics are the actual "charger", which adapts to only draw what the wall "charger" can deliver. The device on the wall that we like to call an "EV charger" is really just a fancy plug, a GFCI, and a little brain to communicate current capabilities to the car. The car does EVERYTHING else.
I think there is very little chance of the 60A feed to that subpanel tripping, unless the OP has some other large loads that weren't mentioned. Normal lighting and plug loads would be unlikely to add another 28A or so on one or both legs of that subpanel, unless there are a lot of things plugged in (i.e. at least two space heaters on one of the legs).
Bill