[enslay]

I'm not going to lie... perusing formidable and boring documentation like NEC and UL 489 was greatly aided by Internet searching (which hinted on chapters/articles). And one particular forum pops up w.r.t. NEC stuff a lot (see below). Those guys seem to be very knowledgable... and they could probably tear me a new one since I'm just some caveman interpreting some complex code I don't fully understand.

So here's some interesting discussion on the 80% rule. One poster seems to suggest that the 80% rule stems from limitations in thermal magentic circuit breakers not being able to run at 100% continous load (i.e. they trip). The other posters suggests NEC wrote the requirements for breakers and manufacturers designed circuit breakers around the 80% rule. I don't know which is true.

80% rule

would anyone mind explaining the 80% rule for sizing breakers/wire. someone asked me recently and i couldnt exactly explain it well

forums.mikeholt.com

And here's a fascinating article from the same forum with lots of information about temperature and circuit breakers:

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www.mikeholt.com

Maybe you can just join this mikeholt.com forum and ask these guys about EVSEs and electrician shenanigans.

 

[CharleyTX]

Ahh those pesky circuit breakers + 80% rules + temp variations.
Aren't most, if not all residential breakers trip because over current cause the internal bi-metal strip to reach its thermal limit? As it is shown in the video in a few posts back?

 

[enslay]

Ahh those pesky circuit breakers + 80% rules + temp variations.
Aren't most, if not all residential breakers trip because over current cause the internal bi-metal strip to reach its thermal limit? As it is shown in the video in a few posts back?

Or as @avery_larry mentioned, probably due to heat dissipation concerns. This is certainly echoed by another post on that mikeholt forum. The UL test does perform 100% load tests on circuit breakers without their enclosures... it's possible they get hotter with their enclosure. Given all the numerous tests done for UL 489, I still don't think your circuit breaker is going to catch fire.

EDIT: Someone does mention the bimetalic strip as another reason for the 80% rule. It might be a combination of both... motivated by thermal magnetic breakers and then something like a self-fulfilling prophecy because manufacturers design the breakers with the 80% rule in mind.

 

[Electric Personality]

As you indicated, NEC 625.14 "requires electric vehicle supply equipment be considered a continuous load. This means that the EV charging load must be added at 125% of the full load charging capability of the electric vehicle supply equipment."

If your charger is rated at 50A delivery, you would need a 65A breaker (they don't make a 62.5A breaker).

You should only operate your EVSE at 40A max delivery if its on a 50A breaker.

The Emporia 48 amp evse specifies a 60 amp breaker, and iirc, 6 gauge cable.

 

[CharleyTX]

Or as @avery_larry mentioned, probably due to heat dissipation concerns. This is certainly echoed by another post on that mikeholt forum. The UL test does perform 100% load tests on circuit breakers without their enclosures... it's possible they get hotter with their enclosure. Given all the numerous tests done for UL 489, I still don't think your circuit breaker is going to catch fire.

EDIT: Someone does mention the bimetalic strip as another reason for the 80% rule. It might be a combination of both... motivated by thermal magnetic breakers and then something like a self-fulfilling prophecy because manufacturers design the breakers with the 80% rule in mind.

Indeed. my 50 amp breaker + 6 ga wire direct wired to my juicebox EVSE, it will trip on 32 amp (BOLT charging) during summer heat. The garage is well insulated the main electrical panel is inside, but direct sun beat on the outside wall. 105F ambient temp is not helping. Possible solutions: I will need to slow the charge rate, or need to shield the sun's radiant heat or go up to a 60 amp breaker with 4 ga wires.

 

[Electric Personality]

Indeed. my 50 amp breaker + 6 ga wire direct wired to my juicebox EVSE, it will trip on 32 amp (BOLT charging) during summer heat. The garage is well insulated the main electrical panel is inside, but direct sun beat on the outside wall. 105F ambient temp is not helping. Possible solutions: I will need to slow the charge rate, or need to shield the sun's radiant heat or go up to a 60 amp breaker with 4 ga wires.

You very well have a weak breaker, have you tried a different one of same rating?

 

[Lite2]

The Emporia 48 amp evse specifies a 60 amp breaker, and iirc, 6 gauge cable.

Running my Emporia on 50a breaker on 4ga wire @ 40amp charge rate. Barely warm at all check points.

 

[Electric Personality]

Running my Emporia on 50a breaker on 4ga wire @ 40amp charge rate. Barely warm at all check points.

That’s great, just don’t forget and set it to 48 amp on the app.

 

[CharleyTX]

You very well have a weak breaker, have you tried a different one of same rating?

yes bought a new one at home depot.
The breaker gets warm/hot to the touch. The entire 200 amp metal breaker panel soaks in the sun's heat over an exterior wall, -> with the panel being inside the garage. 50 amp breaker only trips in the peak of summer months, now it's fine. I verified it with a clamp on meter, only 32 amp max is drawn by my 2020 Bolt. I can duplicate it many time, just watch it for <5 min and it will trip.

Staying o the OP's topic. As it was mentioned in a post, can 60 amp breaker be used with 6 ga wire on a dedicated circuit that only uses a 40 amp max direct wired EVSE?

 

[no-gas]

Plant a tree near the panel so it keeps it in shade.

 

[Bill14051]

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.

It is per code. Continuous load is to be 80% of maximum.

 

[Pktrusty]

I would say the breaker is set to run the wiring. That is all it is there for. That being said, anything running more than 3 hrs is considered a continuous load and handled different than the NEC. If it is a 50amp circuit breaker, the 80% is 40amps. That would be the max I would run on the setup. Your insurance could find a reason to reject a claim if you run a 50a charger on a 50a circuit.
What a lot of people don’t realize is that loads don’t alway draw the same current. A electric motor can draw 2-3 times amperage at LRA. I absolutely have seen breakers trip under the rated breakage rating. Heating elements, microwave might say and calculate out to less than 80% of a rated load when in like but if you run 18+ amps on a 20amp circuit, you will likely be safe but be prepared to reset it all the time.

 

[Vinson]

What turned out to be interesting to me going through a solar/home battery storage and EV journey plus Geothermal heating is learning a lot about electricity, AC and DC. Having a solar app that tells me everything coming and going through the main service panel. I've learned what devices are running and what that looks like to some degree. I've really enjoyed it.

 

[GregBrew]

Plant a tree near the panel so it keeps it in shade.

Or hang a pretty picture (or some other decorative item) on the wall opposite the panel. Get the sunlight off of the wall, with an air gap between the cover and the wall.

Or drill a hole into the wall and fill it with expanded foam insulation.

 

[atc98092]

Or drill a hole into the wall and fill it with expanded foam insulation.

Most breaker panels are full wall depth, assuming a 4” nominal wall. A 6” wall might have some space behind the panel, but I’d rather have the air gap than foam. Plus, the foam might try to push the panel out from the expansion.

 

[SDirickson]

Staying o the OP's topic. As it was mentioned in a post, can 60 amp breaker be used with 6 ga wire on a dedicated circuit that only uses a 40 amp max direct wired EVSE?

Only if you're using wiring with a higher thermal rating; AWG 6 NM-B/TW/UF is only allowed to be connected to a source of 55A or less. How much load you plan to connect is irrelevant; the code cares about how much the branch circuit can deliver, which is determined by the breaker. If your ambient temp is over 95F, even AWG 6 won't cut it. And only copper works; none of the aluminum types are rated above 55A for AWG 6.

 

[Deant]

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.


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.

The breaker protects the wire from overheating.
To size the wiring including it’s over current protection fuse or breaker for a continuous load, multiply the load rating (amps) on the equipment label by 125% and round up to the next available fuse/breaker size and wire per NEC. Hire or work under the supervision of a licensed electrician.
Get a permit. Get it inspected and passed by the local authority having jurisdiction. Check all your smoke alarms and install according to your current fire safety bulletin.
Advise your insurance provider of the improvement.
When you follow these guidelines you will be able to rebuild your house with the insurance settlement when it catches fire.

 

[robcomp]

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.

Hello, I am looking at my mt first EV, Bolt EUV and have a lot to learn about home charging. My house is 50 yrs old but the good news is that the box was upgraded so there is plenty of extra space in it. Did you resolve your issue? I would like to take advantage of Qmerit but I am concerned about the permit aspects. Did the local town inspect your whole house for the permit? I don't' want to get stuck making thousands in modifications to save a few hundred on a charger. Also I would need to find out if my electrician participates in Qmerit. Thank you for any info you can give me. How do you like your Bolt? Rob

 

[atc98092]

Unless the inspector spots something wrong inside the breaker box, they will only be looking at what the permit covers. Assuming the box is in the garage, they won’t even go into the house itself.

 

[robcomp]

Unless the inspector spots something wrong inside the breaker box, they will only be looking at what the permit covers. Assuming the box is in the garage, they won’t even go into the house itself.

Thank you for your reply. Unfortunately, the box is in the house, the basement. We finished the basement about 40 years ago ourselves. It's not an apartment or anything, just a living room so I am not worried about that. However, newer requirements require an escape window which we do not have and I also can't afford my property taxes to go up from the already ridiculous levels here in NY.