so I realized that my electrician ran Romex 8/3 wire (about 20 feet) to a new Nema 14-50 outlet. I know Tesla recommends 6 gauge. But I will be only charging at 32 amps. Anyway- I noticed that it’s on a 50 amp circuit breaker. Shouldn’t it be on a 40? If I only plug in the mobile charger would it matter?
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Electrical question..
- Thread starter SouthFLGuy
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I have that same set up at at different location where I charge only with my mobile connector. I ran 8/3 wire and have a Nema 14-50 plug. It draws 32 amps and by the accepted electrical rule of 80% your breaker should be 40 amps so that it will trip and close the circuit safely should there be an unexpected current surge. In your setup the 50 amps breaker would not trip until much later (50 amps instead of 40 amps) and there could be a risk of overheating and possibly fire by the wire overheating or melting. It would be safer and in compliance to accepted building code and electric standards to change that breaker to a safer 40 amps breaker.
ralph142
Member
Not an electrician, but my understanding is that a 14-50 requires a 50 amp breaker, and at least 8 gauge wire, otherwise you could plug something in trying to pull a 50 amp load and cause all kinds of problems.
NEMA 14-50 allows for either a 40 amp breaker with 8 gauge or a 50 amp breaker with 6 gauge.
Most people reference this chart for that question. Romex is also called NM-B so use the 60 degree column.
Ampacity Charts - Cerrowire
There has been some long discussions on if this violates code or not. Long story short I always use 6 gauge just to be safe.
You are also correct that the Tesla mobile connector will only draw 32 amps so that should be fine on a 40 amp breaker.
Ampacity Charts - Cerrowire
There has been some long discussions on if this violates code or not. Long story short I always use 6 gauge just to be safe.
You are also correct that the Tesla mobile connector will only draw 32 amps so that should be fine on a 40 amp breaker.
Thanks for your help. I'm going to get it changed out to a 40a breaker so I don't need to worry. In the meantime would it still be safe to charge with the 50a breaker in? I will get it changed out in a few days. Sorry I don't know much about electric.
It is currently not up to code. Change it to a 40A breaker and it will be correct. Suggest putting a label on the outlet that it is a 40A circuit.
mrau
Authorized Driver
You may want to put a label/sticker on the 14-50 plug that says something like "40 amp Circuit". That way you and any future users of that plug will know what it is rated.
You should be OK to use on a 50 amp breaker for a few days. If really concerned about 50 amp breaker, don't use it while you are sleeping.
You should be OK to use on a 50 amp breaker for a few days. If really concerned about 50 amp breaker, don't use it while you are sleeping.
M3BlueGeorgia
Active Member
Since the car won't pull more than 32A continuous, having the 50A breaker poses no risk in the short term. However, because you have 8 gauge wire, you should quickly switch to a 40A breaker.
eprosenx
Active Member
Everyone here has covered this pretty well! Just a few extra notes:
It is perfectly code legal to install a 14-50 receptacle on only a 40a circuit (actually technically their are no restrictions on how low an amperage circuit you can install one on as long as it is sufficient for “the load to be served”).
Yeah, this is generally true for romex (NM-B) cable.
You actually can use 8awg wire in conduit on a 50a circuit as long as you don’t have to derate it for any other reason (ambient temp, number of conductors in a raceway, etc...)
It is very clear that 8 awg romex is allowed for a 40a circuit and that is allowed on a 14-50 receptacle if your load to be served is a UMC Gen 2 (32a continuous limit). If a UMC Gen 1 Brian would not be allowed.
I think more of the debate has been about 8 gauge on a 50a circuit, but this is also clearly allowed by code if it is in conduit and meets all the other specifications.
Yeah, I would be fine operating this for a short period of time with the 50a breaker. The magnetic trip functions (for short circuits) in a 40a vs a 50a breaker are probably identical or nearly identical. It is the thermal overload functions that are tuned differently. We have never seen a UMC Gen 2 give any hint of allowing a car to charge over 32a so I think you are very safe for a short term.
The more I learn about electrical installations the more errors I catch where too large a breaker was used. It is rampant in residential electrical and yet you only very rarely have fires. This is far from the most egregious error you could have.
Also, the limitation of romex to the 60c level is likely kind of silly. The history on that probably does not apply to your installation. There is an argument to be made that your 8awg wire is fully sufficient for a 50a circuit anyway.
It is perfectly code legal to install a 14-50 receptacle on only a 40a circuit (actually technically their are no restrictions on how low an amperage circuit you can install one on as long as it is sufficient for “the load to be served”).
Yeah, this is generally true for romex (NM-B) cable.
You actually can use 8awg wire in conduit on a 50a circuit as long as you don’t have to derate it for any other reason (ambient temp, number of conductors in a raceway, etc...)
It is very clear that 8 awg romex is allowed for a 40a circuit and that is allowed on a 14-50 receptacle if your load to be served is a UMC Gen 2 (32a continuous limit). If a UMC Gen 1 Brian would not be allowed.
I think more of the debate has been about 8 gauge on a 50a circuit, but this is also clearly allowed by code if it is in conduit and meets all the other specifications.
Yeah, I would be fine operating this for a short period of time with the 50a breaker. The magnetic trip functions (for short circuits) in a 40a vs a 50a breaker are probably identical or nearly identical. It is the thermal overload functions that are tuned differently. We have never seen a UMC Gen 2 give any hint of allowing a car to charge over 32a so I think you are very safe for a short term.
The more I learn about electrical installations the more errors I catch where too large a breaker was used. It is rampant in residential electrical and yet you only very rarely have fires. This is far from the most egregious error you could have.
Also, the limitation of romex to the 60c level is likely kind of silly. The history on that probably does not apply to your installation. There is an argument to be made that your 8awg wire is fully sufficient for a 50a circuit anyway.
I’m learning a lot too. So essentially the danger is if someone were to plug in an appliance that draws 50 amps. The 8 gauge Romex is only ok up to 40 amps. So the wires would be the dangerous part. They could overheat, etc. With a 40a breaker if someone were to plug in a 50 amp appliance the breaker would go off preventing the wires from heating up.
Well, no. The electrician was probably correct as long as derating was not required for other reasons. BUT meeting the absolute minimum code and best practices are not necessarily the same thing. Chargers can put out a lot of current for a long time. This is why Tesla recommends specific industrial 14-50 receptacles rather than what is found inexpensively at home depot.
6ga and the recommended receptacles is the way to go. But it is probably going too far to say that the lesser parts some people have installed are dangerous.
I guess my understanding from here and doing some research is that the Romex is the issue. It’s only rated to 40a. So the corresponding breaker should be 40. Aside from the fact it should have been done with 6gauge, I think Tesla is limiting the chargers to 32a in cases like mine?
If you had a short for any reason, your 8 AWG wire with 50A breaker could be a problem. That's the primary thing to worry about, I think, until you switch to the 40A breaker.
Yes, I believe Tesla backed off to 32A to safely accommodate 14-50 outlets on 40A breakers. From reading here on TMC, 40A circuits were especially common in Canada.
Yes, I believe Tesla backed off to 32A to safely accommodate 14-50 outlets on 40A breakers. From reading here on TMC, 40A circuits were especially common in Canada.
eprosenx
Active Member
I should have been more clear that this would not be code compliant and even if they did remove the 60c limitation on romex (and let you use the 75c rating) it would be right on the limits (86 degrees Fahrenheit ambient).
No. Under basically no circumstances could 8awg be good for a 60a circuit. 6awg in conduit can be.
Correct. Though a breaker is just a backup safety mechanism. The primary safety mechanism is wiring things with the proper gauge for the intended load. (But a 40a circuit on a 14-50 is acceptable if the intended load is only 40a / 32a continuous) Though if running new wire I will always tell you to run a 50a circuit for a NEMA 14-50 regardless of what code allows.
8 gauge romex on a 50a circuit is not ok per code, so regardless of what Tesla recommends, the install was not compliant.
Yeah, so modern NM-B cable (Romex) actually carries a 90c rating on the insulation, but the breaker terminals and receptacle terminals are likely only rated 75c. Code then further has this odd requirement that you can’t use romex at a rating above the 60c insulation rating, so that limits you a lot. But what is interesting is that you can use the 90c insulation rating still for the purposes of ambient temperature derating.
But yes, I think there were multiple reasons for Tesla’s change to 32a limited UMC units, but one reason was likely safety related due to circuits with only 40a capable wire (which is good for 32a continuous).
I was reading about derating (sorry I still don't fully understand) - ambient temperature derating? Just curious.
Getting into the weeds a bit but ambient temperature derating basically says if you put the conductor somewhere extra hot then you get less out of it.
A conductor on the roof, in a metal pipe (EMT), and exposed to the sun will be a super hot environment, so in these cases the NEC requires you derate the conductor proportionally to the heat of the environment. A wire in phoenix AZ vs Ottowa will have different ampacity as well.
The reason for all of this is called thermal runaway. Power creates heat. If the wires heat up so much that the resistance increases. This increase in resistance then adds to the problem in a feedback loop where the wires just keep getting hotter as the resistance continues to increase due to heat.
Wires have different kinds of insulation, which the plastic can tolerate different heat limits before it gets gooey and burns or fails. So different kinds of wire are limited to different temperatures. Different installations more easily shed heat too.
So someone decided that NM-B uses the 60C degree ampacity chart, and that wires in other situations can use the 75C or 90C charts.
A conductor on the roof, in a metal pipe (EMT), and exposed to the sun will be a super hot environment, so in these cases the NEC requires you derate the conductor proportionally to the heat of the environment. A wire in phoenix AZ vs Ottowa will have different ampacity as well.
The reason for all of this is called thermal runaway. Power creates heat. If the wires heat up so much that the resistance increases. This increase in resistance then adds to the problem in a feedback loop where the wires just keep getting hotter as the resistance continues to increase due to heat.
Wires have different kinds of insulation, which the plastic can tolerate different heat limits before it gets gooey and burns or fails. So different kinds of wire are limited to different temperatures. Different installations more easily shed heat too.
So someone decided that NM-B uses the 60C degree ampacity chart, and that wires in other situations can use the 75C or 90C charts.
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