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How hot is too hot for a 40A breaker?

15K views 36 replies 21 participants last post by  GregBrew 
#1 ·
Hi, all:

Newbie here - new to the Forum, new to EVs, new to the Bolt, and new to my new AV TurboDX 32A home charging station just installed today and using for the first time tonight to charge my Bolt from 63% battery to 100%.

I've noticed the following: The wall charger unit is warm (blue light blinking slowly, as expected). The cable from the unit to the charge port is warm. The Bolt itself in the vicinity of the chargeport is warm. From the charging stats, it seems the car pulled around 25 to 30 amps at the beginning, but has slowed down as it reaches full charge. Some fan inside the car cycles on and off slowly - otherwise, no odd noise. No unusual notices from the car display. Charge time is around 3-3/4 hours which seems about right. All of this seems normal as far as I can tell.

My question for the more experienced here is that the 40A breaker (on an unused 50A circuit for an electric range in a 200A panel) is almost too hot to touch. I don't know what the temperature is, but I can't lay my palm across the exposed face of the breaker plastic body for more than 10 secs without it being too uncomfortable to continue. There is no hot or burning smell - no vibration or buzz. It is just hot.

Is this normal? The breaker temperature has not increased while I've been watching it tonight. If anything, the temps seems to be moderating a bit as the car pulls less and less current toward full.

Thanks for any advice you can all provide. Just looking to know if I need to call the electrician back out.

PS: If this question has already been answered, feel free to direct me to the right discussion.

PPS: What a great car! It is like the diff between a rotary phone and a smartphone. Not likely to buy a gasoline car ever again.
 
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#2 ·
googled that number. 90F per UL.

Also googled that 140F is temp that someone can touch up to 5 seconds.

I'd get someone to look at the torque on your breaker connectors. Other issue that can happen is the breaker is not making good contact on the bus bar. Wrong matching breaker. Make sure you have the same manufacturer of the breaker as the power center.
 
#3 ·
The acceptable operating temperature of a circuit breaker is defined by UL in the UL489 standard (June 2011), which is listed below.
  • Terminations for standard rated breakers: UL 489 Paragraph 7.1.4.2.2 says the temperature rise on a wiring terminal at a point to which the insulation of a wire is brought up as in actual service shall not exceed 50°C (90°F).
  • Terminations for 100% rated breakers: UL489 Paragraph 7.1.4.3.3 says the temperature rise on the termination shall not exceed 60 deg. C (108 deg. F).
  • Handles, knobs and other user surfaces: UL489 Paragraph 7.1.4.1.6 says the maximum temperature on handles, knobs, and other surfaces subject to user contact during normal operation shall not exceed 60°C (140°F) on metallic and 85°C (185°F) on nonmetallic surfaces.
The circuit breakers in my house have nonmetallic faces/trip levers, so they fall under the 85C (185F) catagory. This info was taken from a Schneider Electric FAQ webpage.
 
#4 ·
I'm no expert...so that's out of the way...is this a separate subpanel with a 40A breaker attached to a 50A breaker in your main panel? Could be cable size from the main to that 40A breaker. Could be cable length for that size and desired power draw between the main and 40A breaker.
 
#5 · (Edited)
I'd suggest
a) tighten the screws in the buss inside the box, after removing the panel cover. Use a screwdriver with some tape around the shaft, to avoid big fat sparks,
b) then replace the breaker. It probably won't cost over $15, and should be a plug-n-play replacement. It should never be hot to the touch.
 
#7 ·
I'm all for self repair. That said, turn the main breaker off when servicing the 50a breaker. Don't rely on tape and a steady hand to keep you from a shock...

I recently figured I'd try to replace a fan switch with a timed switch with the breaker on. That was an illuminating experience.

Yes, you'll have to reset the time on the microwave and the coffee pot.
 
#8 ·
You've gotten lots of good advice in this thread already and yes that's way too hot. I also concur with red point if you're not an electrician you should never be working on a panel live.

It is very common for the screw connections on the circuit breakers to the wire to actually back themselves out overtime with temperature cycling. It is excellent preventive maintenance to turn a panel off and run down every screw terminal with the screwdriver retorquing all the connections.

As already stated it could also be the interface between the breaker in the bus bar.

A common diagnostic tool in an electrician's tool box is a thermal camera, you point it at sockets, electrical connections, and breaker boxes, and look for problem spots that are high temperature.
 
#13 ·
My 32A L2 charger specifies that it needs a 50A breaker. If it was my house, I'd replace the 40A breaker with a 50 and make sure #8 wire is run all the way. That's the way mine is wired and I've checked everything from the charge cord to the breaker and nothing is beyond just barely warm.

Mike
 
#15 ·
Thanks to all for your replies - really appreciate the info. After I posted about the hot breaker, I got a notification that the charge cycle had ended so I went out to the garage. Surprise! The car had started a battery conditioning cycle. I'm sure I don't have to tell you: that is a surprise when it first happens. This sophisticated, quiet, clean, modern device suddenly turns into a shop vac! Anyway, it only lasted for 10 mins or so with the fan noise decreasing throughout. When finished, the charger stopped blinking and things were back to normal. I will say that the garage was noticeably warmer after charging & conditioning finished.

On the breaker topic: Seems that it should not be that hot, but also from the Schneider info, a temp rise of 60 degF (from 80 to 140) is within the specs for the breaker if I understand the data. My neighbor has a Tesla so I've asked him if his breaker gets warm/hot while charging (didn't know). I've also asked one of the execs in my company who has a Bolt if that happens on his charger unit. This should confirm what MikeyBolt says - that the circuit should not be hot while charging. Anyone else done the touch test while charging?

Btw, Duke installed a digital meter earlier this year. I looked online and could see that the unit was pulling 6-7 kW during those hours it was charging. I also realized that my AC unit was running fully during that period. The ganged 40A breaker for the AC is right across from the charger breakers at the top of the box. Next time, I'll see if they are warm although it seems from online that the compressor will not pull as much current as the charger.

I will ask the electrician to come back out and inspect the circuit given your advice and will report back.

Thanks again for the info.
 
#17 ·
I had an unused 40A breaker in my pannel that I used for my OpenEVSE. It got hot. I also discovered that my 40A stove breaker was hot. Two breakers latter, all is fine. I took the bad breakers apart and the internal contacts we burnt in both units. They were both a disaster waiting to happen.
 
#18 ·
... I took the bad breakers apart and the internal contacts were burnt in both units. They were both a disaster waiting to happen.
Similar here. Running two 15 amp portable heaters tripped a 100 amp subpanel's main breaker. The breaker was ugly inside, badly corroded and the springs that open the contacts were badly rusted.

Disaster averted. That barn subpanel will eventually host an EVSE.
 
#21 ·
Hi, all:

Newbie here - new to the Forum, new to EVs, new to the Bolt, and new to my new AV TurboDX 32A home charging station just installed today and using for the first time tonight to charge my Bolt from 63% battery to 100%.

I've noticed the following: The wall charger unit is warm (blue light blinking slowly, as expected). The cable from the unit to the charge port is warm. The Bolt itself in the vicinity of the chargeport is warm. From the charging stats, it seems the car pulled around 25 to 30 amps at the beginning, but has slowed down as it reaches full charge. Some fan inside the car cycles on and off slowly - otherwise, no odd noise. No unusual notices from the car display. Charge time is around 3-3/4 hours which seems about right. All of this seems normal as far as I can tell.

My question for the more experienced here is that the 40A breaker (on an unused 50A circuit for an electric range in a 200A panel) is almost too hot to touch. I don't know what the temperature is, but I can't lay my palm across the exposed face of the breaker plastic body for more than 10 secs without it being too uncomfortable to continue. There is no hot or burning smell - no vibration or buzz. It is just hot.

Is this normal? The breaker temperature has not increased while I've been watching it tonight. If anything, the temps seems to be moderating a bit as the car pulls less and less current toward full.

Thanks for any advice you can all provide. Just looking to know if I need to call the electrician back out.

PS: If this question has already been answered, feel free to direct me to the right discussion.

PPS: What a great car! It is like the diff between a rotary phone and a smartphone. Not likely to buy a gasoline car ever again.
That is definitely not normal. What size wire is on the breaker? If it's #6 upgrade the breaker to 50 amps and should fix the problem. If wire is #8 then I'd check the screws on the breakers to be sure they're tight, if they are then replace it with another 40amp breaker because for whatever reason it's getting hot and not tripping is a fire hazard. Now some warmth is normal, but where you can fry an egg on it is not normal.
 
#22 ·
I have a Juicebox Pro 40 on a 30 Amp breaker. Normally I limit output to 24 Amps to comply with the 80% rule. Sometimes, though, I'm in a pinch for all the electrons I can get owing to a late night return and need a full charge early, or a short home stop before moving on. (I drive 150-250 miles / day)

On those occasions I'll bump the JB to full tilt boogie (33 amps; the car takes 31.7 max). Of course that represents a moderate overload of the 30 Amp circuit. Breaker and cable are new; run is short. Breaker and cable get slightly warm to the touch.

Of course don't try this at home...always comply with the 80% rule for long duration continuous loads. That said, I present this info to point out that a properly installed 40A circuit shouldn't be more than ever so slightly warm when operating at 32 amps.

NOTHING in an electrical system should EVER be anywhere near too hot to touch!

Advice to replace breaker and occasionally tighten everything in a panel box is spot on. In the course of replacing breaker examine panel bus bar for signs of heating / arcing damage.

I second the notion that working in a hot panel is not for the faint of heart - been there, done that with similarly illuminating experiences.

If any of this is unfamiliar or scary, back off and call a pro...downside risk is simply too high - lethal voltage and current lurk within awaiting the unwary and ill-informed.

If possible, future proof the circuit to 60 amps, allowing 48 amps / 11+ kW for next gen EVs
 
#23 ·
Update: I drained the car to less than half-full (Isn't Sport mode fun??), then had the electrician back to inspect the charger installation work. The guy who came back out was the original electrician who quoted the work - it was actually installed by his colleague and an apprentice. These guys work for the same company that installed the chargers at the dealer where I bought the Bolt and who recommended them. They seem to do quite a bit of industrial work with some residential. They come in a clean lettered van and have all the equipment. They seem confident, working on the hot panel with gloves and goggles without cutting any of the electricity to the house. Point being: The guy does this for a living.

He inspected the installation, re-torqued the connection points to the breaker, and also moved the AC breaker across from the charger circuit down one slot (swapping with the 20A 110V breakers that were below). He let everything heat up and verified that the circuit was feeding 32A to the charger at 240V. With IR, he showed me the contact temp at the back of the breaker to the wires was 150 degF. (ambient maybe 80-85). The temp on the plastic face of the breaker was noticeably warm, but not as hot as before. I could leave my palm against it without discomfort - quite warm but not hot. (A few days later, I ran another charge cycle - same result.)

He did not think there was anything out of the ordinary and was pretty non-chalant about the temps. He did not see anything wrong with the breaker externally (no distortion or evidence of overheating) and did not think it needed to be replaced.

So there you have it. For what its worth, the cord in the charge unit (on the other side of the garage from the panel) is also warm(ish) when charging.

This whole EV experience is pretty interesting. We took our first longer trip last weekend (about 150 miles RT) returning with slightly less than half the battery left. The car is so quiet - we actually had a conversation! We took the backroads (state highways) instead of the interstate so that helped. Although the drive took about 20 mins longer one-way, both of us felt much less tired when we arrived there and when we came back. Driving the car is really a great user experience.

And the technology is eye-opening in comparison to gas: On household power, you can put 7kW into the car taking 10 hours to fill from empty. Gasoline is a miracle compound, really - in 5 minutes you can put 20 gallons in the car with 600 kWh equivalent going into the tank. That's a whale of a lot of energy, either way - by comparison,1 kg of TNT holds about 1 kWh. (Of course, that 1 kWh in TNT all comes out in a millesecond...) Gasoline holds so much energy, we have grown accustomed to being wasteful - I used 33kWh to go 150 miles as opposed to burning 180kWh of gas in our other car. Good thing gas is pretty stable as a liquid.

I suppose that as EVs become more commonplace, there are going to be houses burnt down from overloaded circuits just like there were probably barns burnt down in the first generation of horseless carriages a century ago. Interesting times! Porsche's 900V chargers can't come fast enough.
 
#24 ·
He let everything heat up and verified that the circuit was feeding 32A to the charger at 240V. With IR, he showed me the contact temp at the back of the breaker to the wires was 150 degF. (ambient maybe 80-85). The temp on the plastic face of the breaker was noticeably warm, but not as hot as before.
I'm no electrician and I believe the temps you are reporting are within specification but in all honesty it seems warmer than I expected them to be. I once posted IR temps for cable warmth, way lower than yours but primarily because my wire gauge is different. I just remember reading after I installed my 22kW Generac generator by myself that wires heating up are normal but it is loss of $ and efficiency and whenever possible the rule of thumb was to use a wire gauge one better than the instructions/NFPA/code calls for.

I would like to check mine at my next charge which should be Wednesday. How many minutes was the system delivering power to the car when he IR tested the wires and saw 150 degrees? I'd like to test at the same point.
 
#25 ·
There’s a difference in design and testing specs and what is normal. Breakers should remain very close to room temperature in operation. They are designed to work at elevated temperatures as a safety measure so they don’t cause a fire, but a warm or hot breaker used in a circuit it’s designed for needs replacing.
 
#26 ·
Breakers do wear out over time. I've got a 240V 40A breaker that feeds an EV charging circuit that's been in steady use since 1997. I've replaced it twice, and both times the internal contacts were corroded. Electrons don't flow through corrosion very well...
 
#30 ·
Good to hear, my current plan is adding 50A breaker and use #6 wire to make a NEMA 14-50 receptacle. Then, use outlet adapter to let me hook up my OEM EVSE, which would draw at max 12A. I would expect the breaker to be only slightly warm, if any.
 
#31 ·
The JuiceBox 40 Pro wire in the cable and specifically in the pigtail is 8AWG. I ran 6AWG to that. Here is a picture of the temp of the 6AWG wire 1 hour into a charge of my Bolt. So, figure that wire is taking 31 amps realistically. Or 32, not worth debating.

Granted, the IR thermometer is not exactly scientific but it was showing the surrounding air/environment/surfaces at 72 degrees which was accurate. Moving over the load carrying conductors showed a 17 degree increase. Again, 31-32 amps on 6AWG. There was no other load active in that load center so no heat was carrying from other wires and the cover was OFF the entire time so no heat build up in the cabinet. The surrounding wires were room temp., this load center really exists just for the car and serves as a disconnect if I go above 60 amps. Prior to this, I personally would not have guessed that 6AWG THHN wire, new wire, the THHN2 to be specific, would ever be 'warm' at 31-32 amps. My project used 75 degree C ratings per code, because circuit breakers are rated at 75 degree's C, not 90 which is what most amperage charts will lend you to rate wire for.

Electrical wiring Wire Electronics Electrical supply Electronic engineering
 
#36 ·
I may get a faster EVSE right before taking a long trip or buying a 2nd EV. No need for speed at this time. Daily recharging on 240V 12A is 2x of my needs right now.
 
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