# Is it okay/safe to load a circuit breaker to 90% of its amperage rating?

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?

• I think it is the cable rating that limits the amount of power. Romex is usually at the lowest rating(60), so it can only use 80% continuously by code. A cable with a better rating(75) might be allowed to pass more power, but need the experts to confirm. Jun 11 at 15:36
• @crip659 running a CB at 100% continuous can cause nuisance trips due to thermal build up. That'll be totally unhelpful as far as getting the car charged. Jun 11 at 16:09
• A better thermal rating on the wire has a higher allowable ampacity. But you still only get 80% of the rating on a CB for continuous loads. So if it was 90c THHN #8 cu (55amps when NMt3CCC in C,R,or E), it could have been a 50a CB; running 40a continuous (assuming all other temperature considerations having been taken, including derates and voltage drop). OP has 'mystery meat' and doesn't even have the highest ratting on their CB that the NEMA 14-50 outlet was made for. - You'd be causing undue heat in the panel (screwing with all the trip curves) and possible wear on the insulation. Jun 11 at 19:32
• ^ @ Mazura - The ampacity rating of my 8/3 NMD90 cable allows for 40 amps continuous and then some. I chose the NEMA 14-50 receptacle to mate with the NEMA 14-50 plug that comes factory-installed on the supply cable of the Emporia EV Charger. I chose to buy/utilize a 40 amp breaker for price considerations as well as to align with the amperage specs of the 8/3 NMD90 cable I installed. Any sort of voltage drop utilizing 26 feet of 8/3 wire would be negligible. Any increased heat in the panel as the result of a 36 amp load versus a 32 amp load on the 40 amp breaker would also be negligible. Jun 11 at 23:24

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.

is it okay/safe

No, it's a code violation to run a continuous (more than 3 hours) load at more than 80%, (that includes 80.01% as being not permitted, much less 90%) and since code is written in blood, (meaning most things that are part of code are part of code because people died from the thing they are requiring NOT being in code) you can take that as being both unsafe and not OK.

If you are under the impression that you are the rare person that really needs maximum charge rate, you have to provision that correctly all the way back to the supply, including a load calculation on the entire house service to verify that you can support that capacity, and breakers and wiring to support that capacity.

• @ Ecnerwal - Running at a continuous (more than 3 hours) load at 90% of the breaker's rating, wouldn't the worst case result be that the breaker simply trips? Jun 11 at 16:33
• No, the worst case tends to be that the wires overheat from being run too close to the edge for too long and something catches on fire. You're trying to self-justify your intended (or active) code violation, rather transparently. Don't. Jun 11 at 16:37
• Most rules are not about things guaranteed to trip a breaker every single time you try. If only breakers were that smart! The newer the rule (hence anything to do with EVs) the more likely it is about something non-obvious that has already caused harm. The "worst case breaker pops" theory is like driving double the speed limit for 8 hours a day, every single day, with your seat belt on because "worst case air bag will save me". No .... that's not the worst case. It's not the best case either, it's somewhere in the middle. [1/2] ... Jun 11 at 17:06
• ... [2/2] Your 8/3 wire can handle 40 amps. No problem, all the time, continuously. In a lab. Electrical code is about how you can use it in a home. 8/3 wires installed in by human electricians in homes inhabited by humans, using all the techniques, parts, tools, and errors that humans make, is apparently more likely to cause deaths at 40A than at 32A. It's based on observation, not theory. Jun 11 at 17:10
• @jay613: Continuous loads fed by dedicated breakers are required to be below the level of current that might result in a breaker eventually tripping, which is below the level of continuous current at which it would be required to trip within two minutes, which would in turn be below the amount of current that a wire could handle indefinitely without overheating. Jun 12 at 21:36

TLDR: not legal or safe, but such a high rate is probably gross overkill anyway.

Many EVs and EVSEs have a convenient method for the user to adjust charging amps at will, for various reasons such as using excess solar or limiting battery heating (a Nissan Leaf problem). This is generally not permitted for setting circuit ampacity; that must be shielded from user tampering.

You asked about "Romex cable" which is a brand of cable, not a type, but it is fair to assume you mean NM-B. That is limited to 40A in all uses.

## NMD90 is weird

You mentioned NMD90 in a comment. NMD90 is special stuff rated at 90C thermal. That gives it superpowers. However, your panel and breaker are certainly limited to 75C. Further, your socket is limited to 60C unless it says otherwise. The lowest here sets the limit of the wire. If anything is limited to 60C, NMD90 is limited to 40 amps. If everything is 75C ready, then NMD90 is good to 50A rating.

That's a bonus you weren't expecting :)

The 125% rule still binds, however. The EVSE may only tell the car to draw 80% of (the lower of) circuit breaker and wire ampacity.

## Unfortunately, Ontario is also weird.

The Emporia EVSE is sold as being programmable to any ampacity from 15A to 60A. In Ontario, how this is done is very important. They do not permit "soft" methods like mentioned above, it must be a "firm" method resistant to user tampering. Ontario ESA Bulletin 86-1 dictates the allowable methods, and they're pretty strict. To start with, they require both "hardwired" and "bolted to the wall". There are also restrictions on the method of access -DIP switches are fine, "apps" are hit and miss - see the bulletin.

If the "firm" amp configuring method does not satisfy ESA standards as documented there, you must wire the circuit for 125% of the maximum amps the unit is physically capable of, which for the Emporia is 48A with a 60A wire and breaker. At that point you can use the "not firm enough for ESA" setting method to run any ampacity you please, since the hardware is set for any setting you could pick.

## EV charging starts at the Load Calculation

But to bleed off potential anxiety, it helps to watch a good "how to get started" video like this one, particularly the really pointy bit at 28:15. That can calm the feeling of urgency about having high power charging.

This is the calculation an electrician must do to determine the size of the electrical service your house requires, or alternately, the amount of electrical equipment that can fit on a service. This Load Calculation must be revisited whenever a large load is added, and an EVSE definitely qualifies.

Generally on an existing service, one does the Load Calculation without the EVSE, finds the difference between Load Calc and service size, rounds down to a muliple of 5 amps, selects that breaker and wire, and then chooses or commissions the EVSE to that breaker or 80% of that actual charge ampacity.

In Ontario, this can get ugly. Due to their peculiar "have to provision the maximum" rule, if you don't have 60A of service headroom and the EVSE can't comply with Bulletin 86-1, then you can't use that EVSE and that's that.

Other than that Ontario problem, if there is not headroom for EV charging in the Load Calculation there are still several possible wins. Another appliance that fits in the Load Calculation can be pre-empted (or pre-empt). Or, an energy monitor with current sensors can dynamically tell the EV how much headroom is available.

Note the Emporia EVSE is not designed or certified to do that last one, as their engineers posted to Reddit.