To charge my EV I have a Level 2 Emporia EV Charging station (aka "EVSE") that is capable of outputting a maximum of 40 amps when using its included NEMA 14-50 plug on the supply side. Accordingly, in my home, the circuit that feeds that NEMA 14-50 plug includes a NEMA 14-50 receptacle, 26 feet of 8/3 romex wire and a 40 amp circuit breaker. For the time being I have set the Emporia EVSE to output a maximum current of 32 amps (i.e. 80% of 40 amps).
Using the Emporia EVSE's associated app, I am able to change/increase the EVSE's output current to max out at 36 amps. My question is… is it okay/safe to put a continuous (i.e. 5-8 hours) load of 36 amps through my 40 amp circuit breaker? At a continuous (5-8 hour period) load of 36 amps would I be "stressing" my 40 amp breaker?
Is it okay/safe to load a circuit breaker to 90% of its amperage rating
circuit breakerelectricalelectrical-panel
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Hard "no". The only receptacle you can install on a 30A circuit is a 30A receptacle. NEC 210.21B3.
You mentioned the EV industry's tendency to put 14-50 on every charger. That creates a real mess. You cannot install a 50A receptacle unless you have 50A wire and breaker. As you note in the above table, there's an exception where it's also allowed on 40A circuits, but the reason for the exception is there's no such thing as a 40A receptacle, and that exception is bullseyed at the special case of range/ovens. For a new -50 in a garage, where anything could be plugged into it from a better EV supply to a welder, best to use 50A cable so people get exactly what is on the tin.
Contemplating a situation where replacing the 10AWG cable is not practicable... I would get a "generator transfer switch" that takes the form of a small subpanel with an interlock so two specific 240V breakers can't be on at once, and takes readily available breakers (because I'd change them). Normally, those panels backfeed; I'd not. The 30A supply would go to its buses. I'd change one breaker to a 30A and feed the 14-30. The other to 20A and feed the 20A receptacle.
If forced to plug into 30A service, I would use a 30A receptacle and a cheater cable. This is not quite legal, but since it's not part of the building, it's out-of-scope for the permitting and inspection process. I'd specifically aim to make it as obvious as possible that it is a cheater cable -- to warn "the next guy" that this is not 50A as advertised. A receptacle installed in a building should always be what it looks like. I shudder to think of people solving this problem with a neat 14-50 installation onto 10AWG wire; later they sell the house to a guy motivated by that sweet 50A service in the garage.
This gets a little tricky. First NEC Table 310.15 (B) (3) (a) says 4-6 conductors in a conduit need to be adjusted to 80% of their normal current carrying capacity. So 30*.8 = 24A which is the continuous load of your charger. In NEC speak the definition of continuous is a load lasting over 2 hours. If that is true then you have to add 25% load to your charge for conductor sizing. If all of what I just stated is true, then your #10 conductors are under sized and this might be why your conductors are buzzing.
Buzzing is caused by the conductors emf (electromotive force) opposing each conductor and causing it to vibrate at 60 hertz per second. This causes two things to happen first they generate heat, which we know heat + electricity = bad, and second your conductors are moving causing splices and connection points to loosen up (also not good). Start up buzzing is common in industrial sites and sometimes allowed, but these sites have skilled maintenance crews who are supposed to have scheduled routines to maintain these circuits. In a dwelling this is not so common.
It might be a good idea for you to invest in an infrared thermometer which are fairly inexpensive nowadays, so you can scan your entire circuit while you are charging to see if it is heating up above average room temperature. It might turn out you will need to either up grade your conductor size or downsize your charger.
Remember our main concern is to prevent a burn or shock hazard so stay safe.
Best Answer
A 40 amp breaker shouldn't have more than 80 percent continuous load. My Tesla is capable of charging at 48 amps which would require a 60 amp breaker and the appropriate cable to support it. I only have #8 wire with a 40 amp breaker. So I am limited to 32 amps which is almost 7.7kw. That gives me a solid 10 percent battery increase per hour which is plenty. I've made it through four Minnesota winters and drive 20000 miles or more a year with no issues. Set it to 32 amps and forget about it. No reason to stress your electrical system for a few more amps.