[atc98092]

When I built my house in 2019, I ordered a 50A/220V circuit with the plug-in inside my garage, getting ready for an EV. The wires come from the main box outside the house and the circuit breaker is labeled 40A, not 50A.

Then you didn't get what you ordered. They only provided you with a 40 amp circuit, and its max continuous load is 32 amps.

 

[Pushpaw]

The electricians who came for my Qmerit install were nice enough to install a 60 amp circuit for my Chargepoint Flex. As stated earlier in the thread the Flex can deliver up to 50 amps, but that would require even higher than my 60 amp breaker. As it is, though, I can (and do) charge at 48 amps. 40 would have been fine honestly, and I told the electricians that but they had the part and were happy enough to install it, so all good.

@mtbguy07 - As others have said, if your breaker is 50 amps then your max charging rate should be 40 amps. Unless you are going from nearly empty to full all the time then the charging time difference is pretty inconsequential.

 

[Vinson]

There are so many aspects to know about wiring. The wires have temperature ratings for the coverings and other aspects of ambient temperature etc.
I don't have a problem running max power continuously except when I'm sleeping. Most people have plenty of time to charge overnight at lower rates. Save the continuous max for when you really need it. Preferably when you're not asleep.

 

[jmcbrew]

I've not heard of 50 amp level 2 EVSEs (one that outputs 50 amps). Have the make and model?

Autel makes 50 amp EVSEs. I have at least two of them in the basement right now. I also have a Schumacher 50 amp EVSE, but that sucker was DOA. Getting ready to part it out if anyone needs a J1772 connector with 50 amp rated cord.

 

[avery_larry]

Hi, I just had a local electrician install my charger through QMERIT. I have a 50a charger so it had to be hard wired. But techs installed a 50a breaker and the charger is only delivering 44a max. I heard of 80% rule for continuous load however they said this is fine when I reached out. Is this fine? It’s more like 88% of cont load. Thanks in advance.

Ironically, I'm going to offer possible reasons for the electrician to tell you this, despite the fact that personally I generally champion strict adherence to the 80% rule.

IF everything the electrician installed (wiring, connections, receptacle) is rated for at least 55 amps other than the breaker, then possibly a hardwired circuit (not a receptacle circuit) could safely sustain 44 amps continuously. I'm not an electrician. I don't know if a 50 amp breaker can safely pass 44 amps continuously (perhaps the 80% rule is for wiring/connections/receptacles but not for breaker).

This seems to imply that everything is, of course, complicated. The 80% rule does seem to apply to circuit breakers, though possibly with exceptions.


In the end, though, I would have to recommend to you that you limit your EVSE to 40 amps. OR get an independent electrical inspector (probably from your city) to agree, in writing, with the electrician.

 

[enslay]

If I understand what you're saying, you're wrong. The breaker is designed to trip when the amperage exceeds the breaker's max amperage rating. A 50 amp breaker is designed and expected to allow 49 amps to pass without tripping, even continuously. Breakers do not, and are not designed to, protect continuous loads. That's not their job. Though some breakers might also have temperature protections, I see no way that those would be able to protect all the various parts of a circuit (wiring, junctions, receptacles) in continuous load scenarios.

OK, while I don't really see this explicitly covered in General Definitions of NEC for Circuit Breakers (either way), I had a look at UL. It seems to me that if you have a circuit breaker that can overheat and cause a fire without tripping, sounds like you have a defective breaker and should replace it.

Now it's all buried in UL 489, and I had to give my personal information to look at the document... for this... Jeez...

So firstly... in Calibration Tests (7.1.2), a 50A breaker has to trip within 4 minutes running 200% of its rated current (7.1.2.2.1). When it's running 135% of its rated current, it has to trip within 1 hour (7.1.2.3). In other words, the UL says 50A circuit breakers need to be able to run for a maximum of 1 hour at 135% of its rated current before tripping.

What about temperature? Well, that's covered in Temperature Tests (7.1.4). I assume that our 50A circuit breaker isn't a 100% circuit. So it's tested without its enclosure at 100% of its rated current (and then optionally at 80% of its current in its enclosure). What does it say?

7.1.4.1.4 When tested without an enclosure under the conditions described in 7.1.4.1.5 - 7.1.4.1.23, or in an enclosure as described in 7.1.4.2.4, a circuit breaker shall operate without tripping until constant temperatures are attained, and materials used in the construction of a circuit breaker shall not be affected adversely by the temperatures to which they are exposed during the test.

It basically says that the circuit breaker can't melt or catch fire during operation while running at 100% of rated current without an enclosure.

So it has to be able to trip within 4 minutes at 200% of its rating, within 1 hour operating at 135% of rating and it can't overheat and melt/catch fire while operating at 100% of rating without its enclosure. You should be fine at 99% of the rated current.

That isn't to say that I advocate violating code! It should be sized correctly!

To suggest that a circuit breaker could violate its own protective purpose, overheat and cause a fire while operating within its ampacity sounds too silly to be true. It would suggest a corner condition where a malfunctioning properly-sized device could overheat the breaker and cause a fire. That just can't be true (I hope)!

While UL doesn't seem to mandate a temperature sensor to trip the breaker, the simple bimetalic piece of metal in the video that bends with heat seems like one good solution to catch those "mild" continuous overload conditions (like the 135% load condition specified in UL... which has to trip within 1 hour for 50A circuit breakers).

 

[jmcbrew]

I’ve never come across a 44 amp EVSE. What exactly do you have?

As others have mentioned, it is likely that you can turn down the amperage either through an app or with an internal dial or dip switches. I have one that can be adjusted from a touch screen, too. In my opinion, all EVSEs should be adjustable so that you can connect them to lesser circuits.

 

[avery_larry]

. . .
It basically says that the circuit breaker can't melt or catch fire during operation while running at 100% of rated current without an enclosure.
. . .
To suggest that a circuit breaker could violate its own protective purpose, overheat and cause a fire while operating within its ampacity sounds too silly to be true. It would suggest a corner condition where a malfunctioning properly-sized device could overheat the breaker and cause a fire. That just can't be true (I hope)!
. . .

But the wiring or the receptacle or junctions or connectors can catch on fire while the breaker functions fully and properly. Otherwise there wouldn't need to be different rules for continuous loads.

 

[enslay]

But the wiring or the receptacle or junctions or connectors can catch on fire while the breaker functions fully and properly. Otherwise there wouldn't need to be different rules for continuous loads.

Well, if that's actually true, one of your appliances could malfunction and pull up to (but not over) the breakers' rating and cause the wires/junctions to catch fire and burn the house down. But that doesn't seem right. Why would the UL be so lax with the 135% trip test if the wires/junctions were so delicate?

So if I spend another bunch of hours looking in the NEC and studying the physics (because I'm not an expert!), you're promising that the wire gauges and junction boxes dictated by NEC for specific-sized breakers will almost certainly be liable to catch fire for such mild continuous load conditions?

 

[cwerdna]

I’ve never come across a 44 amp EVSE. What exactly do you have?

As others have mentioned, it is likely that you can turn down the amperage either through an app or with an internal dial or dip switches. I have one that can be adjusted from a touch screen, too. In my opinion, all EVSEs should be adjustable so that you can connect them to lesser circuits.

It was answered in post 6. I had a reply afterward. The one they say they have has no option for 44 amp output. It has 40 and 50 amp choices, hence my followup question in post 20.

 

[jmcbrew]

It was answered in post 6. I had a reply afterward. The one they say they have has no option for 44 amp output. It has 40 and 50 amp choices, hence my followup question in post 20.

Gotcha. I missed that. I have the same one, lol! FYI, those Autel units have a selectable high limit internally, then the app lets you select anything from your high limit down to 6 amps in 1 amp increments.

 

[mmascari]

So if I spend another bunch of hours looking in the NEC and studying the physics (because I'm not an expert!),

What you would you'll probably find is that the requirements for all of the different parts are over engineered with safety margins.

• The breaker is likely able to deal with conditions well beyond it's rating.
• The wiring in the wall likely has a safety margin beyond it's rating.
• The wiring in the wall likely has a bunch of nuance around it beyond just the size but also related to the insulation itself.
• The receptacle may be the weakest point for it's overengineering. Since it may not have contemplated many insert and removals in it's design at all, which could weaken the rest.
• The 80% continuous rule likely has an extra safety margin as part of the rule.

Taking all of that into consideration, someone could run a system well into those extra safety margins and be just fine without crossing an actual threshold and burning something down. They could also get unlucky and run it a little too far into that extra margin to spectacular conclusion.

Beyond the breaker, and that it's hard wired not using a receptacle, nobody likely knows any of the wire nuance in this specific install. Maybe it's fine, maybe it matches the undersized breaker.

If it was my house and I was sleeping right next to it, I would prefer to have the extra safety margins built into each of the components provided by following the appropriate guidelines. For others, that's up to them. For the insurance company that determines something was hooked up against the guidelines encroaching on that safety margin and was the cause of destruction, they may choose not to cover the loss.

Meaning "can you do it" and "should you do it" are two very different questions.

For instance, can you hook up a 1600W space heater and run it for hours on a 15 amp circuit. However, how often do we hear about house fires started by space heaters? Probably wouldn't stop any of use from running one for a little while anyway. I know that I have personally run a space heater, on low not the full wattage, for hours and hours in the winter inside of an outdoor hot tube enclosure because the hot tub heater was broken. A probably questionable use of electricity.

 

[enslay]

What you would you'll probably find is that the requirements for all of the different parts are over engineered with safety margins.

• The breaker is likely able to deal with conditions well beyond it's rating.
• The wiring in the wall likely has a safety margin beyond it's rating.
• The wiring in the wall likely has a bunch of nuance around it beyond just the size but also related to the insulation itself.
• The receptacle may be the weakest point for it's overengineering. Since it may not have contemplated many insert and removals in it's design at all, which could weaken the rest.
• The 80% continuous rule likely has an extra safety margin as part of the rule.

Taking all of that into consideration, someone could run a system well into those extra safety margins and be just fine without crossing an actual threshold and burning something down. They could also get unlucky and run it a little too far into that extra margin to spectacular conclusion.

Beyond the breaker, and that it's hard wired not using a receptacle, nobody likely knows any of the wire nuance in this specific install. Maybe it's fine, maybe it matches the undersized breaker.

If it was my house and I was sleeping right next to it, I would prefer to have the extra safety margins built into each of the components provided by following the appropriate guidelines. For others, that's up to them. For the insurance company that determines something was hooked up against the guidelines encroaching on that safety margin and was the cause of destruction, they may choose not to cover the loss.

Meaning "can you do it" and "should you do it" are two very different questions.

For instance, can you hook up a 1600W space heater and run it for hours on a 15 amp circuit. However, how often do we hear about house fires started by space heaters? Probably wouldn't stop any of use from running one for a little while anyway. I know that I have personally run a space heater, on low not the full wattage, for hours and hours in the winter inside of an outdoor hot tube enclosure because the hot tub heater was broken. A probably questionable use of electricity.

Please go back and read the rest of the thread. I am not suggesting you do anything against code... I am defending this statement:

Won't it just trip the breaker a lot? It would be annoying...

EDIT: I will update as I go. I invest hours of my time reading these documents and I am not amused by walls of text that deviate from anything but my first statement... which is, using an oversized appliance on an undersized circuit will trip the breaker and probably nothing will happen (especially not burn down your house).

Things that are wrong

If you don't exceed the breaker's current rating, it might not trip, but it can heat up quite a lot under sustained loads nearing the rated capacity.

UL 489 debunks this concern.

NO. The breaker is for peak overages. The 80% rule means you can run 80% of the circuit's rated amperage continuously. If you run 95% of the circuit's rated amperage, you are violating the 80% rule and risking fire because it gets hot. But it won't trip the breaker because it does not exceed the breaker's max amperage.

NEC does not seem to define circuit breakers this way and UL 489 debunks this completely.

Things that are probably wrong (pending)

Tell whoever "they" are, that you want it in writing, for when your house burns down.

Will probably just trip the breaker and be annoying. Probably won't burn down the house.

 

[mmascari]

Please go back and read the rest of the thread. I am not suggesting you do anything against code... I am defending this statement:

I don't think I implied that you were suggesting doing something against code. I certainly didn't mean to. I was commenting on what you're likely to find if you dive deep into the code and UL processes. That you'll likely find all the products are over engineered vs the guidelines.

My bad for no separating this from the rest of the comment more clearly. Which wasn't really a reply to what you were finding, but to the people suggesting it is no big deal to just run the EVSE at over 40amps on a 50 amp circuit all the time and it'll just work. It might just work, and it might not. Even if it just works for a long time, it may not forever.

Will probably just trip the breaker and be annoying. Probably won't burn down the house.

You're probably right.

It'll cause a little wear on the breaker and the wiring that heated up causing the trip too. Something that with enough cycles could cause other issues.

 

[enslay]

I don't think I implied that you were suggesting doing something against code. I certainly didn't mean to. I was commenting on what you're likely to find if you dive deep into the code and UL processes. That you'll likely find all the products are over engineered vs the guidelines.

My bad for no separating this from the rest of the comment more clearly. Which wasn't really a reply to what you were finding, but to the people suggesting it is no big deal to just run the EVSE at over 40amps on a 50 amp circuit all the time and it'll just work. It might just work, and it might not. Even if it just works for a long time, it may not forever.

I'm sorry for the snappy response.

With regard to the 50amp EVSE adjusted to draw 40amps on a 50amp circuit... I do know there is a complicated rule in NEC just after 625.41 that covers these adjustable amperage EVSEs. I don't know enough to understand it though (I'm not an expert!)... I'll have to login and look it up.

It'll cause a little wear on the breaker and the wiring that heated up causing the trip too. Something that with enough cycles could cause other issues.

That's the part I don't know... @avery_larry may have it right on the wires and junction boxes.

 

[mmascari]

I'm sorry for the snappy response.

No worries at all. Sorry that my post was not clear.

With regard to the 50amp EVSE adjusted to draw 40amps on a 50amp circuit... I do know there is a complicated rule in NEC just after 625.41 that covers these adjustable amperage EVSEs. I don't know enough to understand it though (I'm not an expert!)... I'll have to login and look it up.

If it's adjusted down to only use 40, it's probably totally fine. All the adjustable EVSE installation manuals include directions on how to set the maximum current based on the breaker and circuit they're wired too. Including UL listed models. I would guess that UL wouldn't list a product if that wasn't a valid use case to install.

It would be running it higher than that that creates problems. Reading some of the manuals, they all universally say to set the max to 40 if they are not hard wired. However, if you look closely, it isn't like the box knows it's hard wired. It's trusting the user to set it correctly. Looking at the Chargepoint Flex manual, it requires a hardwire for 48 or 50 amps. It also requires a 60 amp breaker for 48 amps and a 70 or 80 amp breaker for 50 amp charging. But, it's not like it "knows" what it hooked up to. Someone throwing all care to the wind and going for it, YOLO, could configure it to 48 amp while plugged in and using a 50 amp breaker. It would probably work just fine, for a little while at least. (Anyone reading this, please do not do this.)

In this sense, the connection from the EVSE to car is actually smarter. Since, while the car could draw an assortment of different amperage it's limiting itself to what the EVSE says is the available maximum. The car is counting on the EVSE being correctly configured and then communicating to the car what that maximum is.

 

[avery_larry]

Well, if that's actually true, one of your appliances could malfunction and pull up to (but not over) the breakers' rating and cause the wires/junctions to catch fire and burn the house down. But that doesn't seem right. Why would the UL be so lax with the 135% trip test if the wires/junctions were so delicate?

So if I spend another bunch of hours looking in the NEC and studying the physics (because I'm not an expert!), you're promising that the wire gauges and junction boxes dictated by NEC for specific-sized breakers will almost certainly be liable to catch fire for such mild continuous load conditions?

Sizing a Circuit Breaker

Circuit breakers are designed to carry 100% of their rated current while the NEC dictates an 80% application. Why the difference?

www.ecmweb.com

A CB is designed and evaluated to carry 100% of its rated current for an indefinite period of time under standard test conditions. These conditions, per UL 489, Underwriters Laboratories Standard for Safety for Molded-Case Circuit Breakers and Circuit Breaker Enclosures, include mounting the CB in free air (i.e.: with no enclosure) where the ambient temperature is held at 40°C (approximately 104°F). Under these conditions, molded-case CBs are required not to trip at rated current.

The 1996 NEC recognizes that overcurrent protective devices will be affected by heat in the system. As such, it defines the concept of continuous loads and the 80% rule to try and offset the effects of heat in the system when sizing a CB.

Emphasis mine.

My non-expert basic understanding. A circuit breaker is designed to handle 100% of it's rated amperage forever in "ideal" circumstances -- most notably if it can sufficiently dissipate heat. Therefor, I am led to extrapolate that the breaker will not trip if it's running 95% of it's rated capacity as a continuous load provided it has sufficient heat dissipation. I just see no way for the breaker to know if anything else (wiring, wiring jacket, conduit, connections, etc.) is getting too hot because it can't sufficiently dissipate heat under continuous load.

At the end of the day I'm not an expert. But I'm not going to take a chance that my circuit breaker is able to sufficiently dissipate heat but some other aspect of the electrical system is not able to. After all, we are literally talking about burning down your house. These quotes are just a little too cavalier for my taste -- perhaps I'm overly sensitive.

Will probably just trip the breaker and be annoying. Probably won't burn down the house.

Won't it just trip the breaker a lot? It would be annoying...

In other words, I don't think anyone should presume that their circuit breaker will protect them from a continuous load running between 80% and 100% rated amperage (especially when wiring/connectors/receptacles are in hot garages or outdoors). UNLESS a bunch of additional requirements have been met.

[Circuit breaker]s that are 100%-rated are permitted to be loaded continuously at their full rating as long as the assembly is listed and conductors are properly connected.

 

[jefro]

An evse backfeeds non-linear harmonics back. Circuit design used to be rms @60hz.
Just be safe, lower load. Very few people NEED hi amp charging I'd think.

 

[enslay]

I just see no way for the breaker to know if anything else (wiring, wiring jacket, conduit, connections, etc.) is getting too hot because it can't sufficiently dissipate heat under continuous load.

They essentially stick the thing in an oven when they test it. It's not going burn down your house at 99% of its rated load. Heck, it won't even burn down your house if it's running at 135% of its rated load for at most 1 hour (it is required to trip within 1 hour at this load).

Yes, it's very silly to put an appliance on an undersized circuit... you shouldn't do it. But it probably won't burn down your house and it may nuisance trip the breaker.

In other words, I don't think anyone should presume that their circuit breaker will protect them from a continuous load running between 80% and 100% rated amperage (especially when wiring/connectors/receptacles are in hot garages or outdoors). UNLESS a bunch of additional requirements have been met.

The breaker will not catch fire for any of those loads... it's required by UL 489. They test the thing at 100% of its rated load continously. The only thing I have to double check are the wires and junction boxes. And if those are fine within those ranges, then there's no fire risk... none. That an electrician put an undersized circuit in is very unfortunately... probably won't burn down your house. Probably no danger whatsoever.

Now using an oversized breaker... the one UL says can run at 135% of its load for up to 1 hour (50 amps and less) or 2 hours (greater than 50 amps) before tripping... will make your malfunctioning undersized EVSE extra toasty like a marshmallow. That's a very real fire risk.

 

[enslay]

DISCLAIMER: I am not an expert. I am just digging around NEC.

OK, so with respect to the wires. Here's what I could scrounge up from the NEC

So first, the NEC does make this peculiar statement:

210.22 Permissible Loads, Individual Branch Circuits. An individual branch circuit shall be permitted to supply any load for which it is rated, but in no case shall the load exceed the branch-circuit ampere rating.

It's confusing (as noted by @avery_larry 's article) because it then stipulates that breakers be sized to 125% of the rating of the appliance/EVSE plugged into it.

There is also "210.23 Permissible Loads, Multiple-Outlet Branch Circuits" but we're generally just dedicating a circuit to the EVSE

So what of conductors? Conductors are seemingly mandated to have a maximum operating temperature. There is a table listing various kinds of conductors and their maximum operating temperature for various AWGs. Most have a maximum operating temperature of 90C. And one of them has a 150C operating temperature. Most have flame-retardant insulation. Aside of AWG, it's not clear to me which of these complicated-sounding chemical-named conductors are being used.

EDIT: Actually, the 150C entry, like any others that are above 90C all seem to be categorized "special applications." So it looks like 90C for more general uses.

So as summarized

300.2 (B) Temperature. Temperature limitation of conductors shall be in accordance with 310.14(A)(3)

310.14 (A)(3) Temperature Limitation of Conductors. No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved. In no case shall conductors be associated together in such a way, with respect to type of circuit, the wiring method employed, or the number of conductors, that the limiting temperature of any conductor is exceeded.

Informational Note No. 1: See Table 310.4(1) and Table 315.10(A) for the temperature rating of a conductor that is the maximum temperature, at any location along its length, that the conductor can withstand over a prolonged period of time without serious degradation. The ampacity tables of Article 310 and the ampacity tables of Informative Annex B, the ambient temperature correction factors in 310.15(B, and the notes to the tables provide guidance for coordinating conductor sizes, types, ampacities, ambient temperatures, and number of associated conductors. The principal determinants of operating temperature are as follows:
(1) Ambient temperature - ambient temperature may vary along the conductor length as well as from time to time.
(2) Heat generated internally in the conductor as the result of load current flow, including fundamental and harmonic currents.
(3) The rate at which generated heat dissipates into the ambient medium. Thermal insulation that covers or surrounds conductors affects the rate of heat dissipation.
(4) Adjacent load-carrying conductors - adjacent conductors have the dual effect of raising the ambient temperature and impeding the heat dissipation.

Information Note No. 2: Refer to 110.14(C) for the temperature limitation of terminations.

Seems to me that it's saying: don't use the wrong type of wire. Use the right type and this is the maximum temperature it can get... which generally looks like 90C for almost all conductors listed. I don't think 90C can cause fires and I would be surprised if wires could get anywhere near their maximum temperature from running at near the rated load (e.g. 99% load). I was kind of hoping to study some physics to predict the heat generated in a wire.

And I guess I'll have to study the NEC for the terminations. Jeez...

I'll dig around for junction boxes.