3

I understand (at least what I assume to be) the reasoning for de-rating wiring in circuits (heat build up and also headroom for variable loads), but what is the reasoning for also de-rating circuit breakers (particularly for constant loads). For example, I just installed an EVSE, and the breaker is 50A but the wiring (edit: 6 AWG copper THHN) is rated at 65A (edit: for the 75ºC column; 55A/75A for 60º/90º respectively). The breaker de-rates to 40A, but even after de-rating the wiring there’s plenty of room for, say, 45A, and because of the controlled nature of the load there’s no way you’re going to exceed the breaker and trip it.

What’s the reasoning being needing to de-rate the breaker in this situation?

6
  • What country are you in? If in Canada, the electrical code requires you to use the wire ampacity chart’s 60°C column in certain cases. A good example is Tesla’s gen3 wall connector. It may technically be required in the US as well but the NEC and CEC are not 100% equivalent. Oct 2, 2022 at 3:24
  • Breaker is likely 75 C rated. Is the EVSE? Oct 2, 2022 at 17:52
  • @manassehkatz-Moving2Codidact indeed in the CEC if a fixed appliance does not have explicit temperature markings on its connectors, and the device calls for less than 100A, then you must use the 60°C column in the copper wire ampacity chart. That means that a Tesla gen3 charger (no temperature marking) requires #4 copper if you want to utilize all 48A of actual maximum draw. #4 copper is rated for 85A in the 75°C column. Most electricians who don’t usually recall the rule around unmarked terminals requiring the 60°C column think it’s overkill. But otherwise it’s not code-compliant. Oct 2, 2022 at 22:55
  • 1
    @RibaldEddie Not an issue for cord-and-plug. For hardwired, I would solve that with a junction box 12 inches away from the EVSE, allowing me to use cheap aluminum for the long haul. Oh, wait. There's already a box 12" away from the EVSE, it's the disconnect switch mandatory in hardwired installations! And it takes #2, so I'll just use 2-2-2-4. That stuff, well, was a buck a foot. Neutral for future subpanel use. Oct 2, 2022 at 23:04
  • @Harper-ReinstateMonica Except in Montgomery County, MD where as I understand it you could only do that to a subpanel. Which is really minimal cost, the catch often being the working space requirement. Oct 2, 2022 at 23:28

1 Answer 1

5

Edit: Your 50A limit is due to your choice to use a 50A socket instead of hardwiring, which you say flows from belief that this will allow you to plug your EVSE into solar panels during outages. You'll want to carefully fact-check that belief. I cannot emphasize this enough.

All continuous loads are derated to 125% (40A load needs 50A breaker) because consumer tier gear is just not rated for continuous use. Certain loads are always treated as continuous loads, including EVSEs per NEC 625.42.

NEC 210.19(A)(1) General. (a) Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load in accordance with 310.14.

There is an exception for industrial gear which is rated for continuous 100% rating, however that is very costly and a 25% larger breaker is a lot cheaper.

There are no other derates, unless you are dealing with thermal derates due to environment.

The normal approach is to use 50A wire to a 50A socket to an EVSE commissioned for a 50A circuit. The EVSE tells the EV to take 40A, which x 125% = the 50A given in the circuit.

Of course there is nothing wrong with using larger wire than is required, but such optional behavior is not a derate.

I have no idea why your installer used a 50A breaker with 65A wire. But I can make a few guesses.

First, the most popular cable for hooking up 50A EVSE's is 6 AWG copper NM-B or UF-B. That is not 65A wire. That is 55A wire per NEC 334.80 and Table 310.15(B)(16) which does not have any 60A wires in it. For reasons which seem utterly bizarre to me, EVSEs have a 60A setting but don't have a 55A setting. Clearly, nobody looked at "El NEC" when designing EVSEs.

Second, it may be an arbitrary limit of your EVSE. EVSE's are only rated for so much current internally. Handling more requires a larger contactor and heavier wires both internally and in the costly J1772 cord.

Third, perhaps someone has weirdly specced this thing to be cord and plug connected (why?), and the NEMA 6-50 or 14-50 socket maxes out at 50A per NEC 210.21. So that is a code requirement.

Fourth, perhaps your EV, wiring and EVSE may all be ready for 65A, but your house's service is not. Really, that should be the first thing to check. You can't just keep dogpiling on more and more load onto a house's electric service - A/C, hot tub, tankless water heat, large EVSE, compressor, yadayada. There are rules for determining what can fit on a service called a Load Calculation. So your installer may have properly done a Load Calc and determined 50A is all that can be safely added.

Fifth, it might have been a long circuit and larger wire was warranted for voltage drop reasons.

Or sixth, the installer might have been a novice who simply conformed with random Web (or worse: Youtube) advice, saw "everybody" using 6/3 NM to a 14-50 socket, and as a result selected #6 THHN when #8 THHN is good enough. To that I cite Harper's Rule: Buy the wire last. Implying one should seek advice from the community and let all information be gathered before making purchase commitments, especially for non-returnable wire!

Seventh could be dealing with a thermal derate either due to hot spaces (Arizona attic) or multiple circuits sharing a conduit longer than 2 feet. For 2-3 circuits in a conduit, #6Cu derates to 60A. For 4 circuits it derates to 52.5A. For 5-10 circuits it derates to 37.5A.


Editorial time. People often think they need way bigger charge points then they actually need. A 240V/20A is enough for almost everyone almost all the time - 100-120 miles in a 10 hour charge session. And for those "but Sometimes..." moments, there is DC fast charging becoming exponentially more available.

As such, the 65A capacity wire puts you in a good position if you get a second EV. Now you can use Share2 technology EVSEs to share the 65A among two different cars, splitting dynamically - even if both cars charge all night (unlikely) they'll each get 32A (provisioned; actual 25.6A). When one is charging it gets system limit, probably 60A (provisioned; actual 48A).

14
  • 1
    Hey, Harp, great and complete answer as usual. But did you really mean 125% de-rating? Mathematically that would result in a negative number. I think the better way to state it, if I understand the code correctly is to say that a continuous load can only be 80% of circuit capacity or must be de-rated by 20% (choose your language). I'm picky about this bc so darn often ads will say "my ABC widget is better than my competitors XYZ widget and its 5 times less expensive. " Mathematically that would mean the ABC guy would be paying 4 times the price back to the customer to buy his product. Oct 2, 2022 at 1:41
  • 1
    @George NEC doesn't mention 80% anywhere. It says you must provision for 100% of non-continuous loads + 125% of continuous loads (and applies 125% to long list of specific loads including heaters and EVSE). Oct 2, 2022 at 3:50
  • Harp: I don't want to belabor this point but I maintain that "de-rating" by 125% would result in a negative number. Maybe yet another better way of saying it is " continuous loads must be supplied by at least 125% of anticipated load." Mostly a matter of semantics. but I do bristle at language that, when you do the math, results in an irrational result. Oct 2, 2022 at 4:09
  • @George it's a derate to 125%. Edited. Oct 2, 2022 at 4:39
  • 1
    @Harper-ReinstateMonica It is indeed a NEMA 14-50 outlet because I also have a critical loads sub panel, and in rare situations with prolonged outages (2x a year max) I'd like to be able to divert PV generation to the charger. Service load calculation is well within bounds (we have gas appliances everywhere possible, so much fewer high load electrical devices). I also agree re: necessary charge speed, but nonetheless it's interesting. The EVSE is configurable up to 48A. I've done the entire install, and I followed code, but I had wanted to know the "why" behind that particular part.
    – jcoleman
    Oct 2, 2022 at 17:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.