Garlan Garner
Banned
I am going to assume since you are posting in the Model 3 forum that you have a UMC Gen 2. The UMC Gen 2 as others have stated will only draw a max of 32 amps so it is completely fine on a 40a circuit. If for any reason you had a UMC Gen 1 which can draw 40 amps then you MUST put it on a 50a circuit.
Make sure the UMC plug is fully inserted in the wall receptacle. Make sure the UMC adapter is firmly shoved into the UMC box itself. Then if the receptacle is getting too hot the UMC can detect that and scale back charging - so check if it is getting hot while charging. Also, if voltage drops too much once charging starts then that too can cause it to reduce charging speed - this likely indicates an electrical issue. I am worried you have a loose wire or something causing the receptacle to heat up too much.
Turning down the charging amperage on the charging screen is not code compliant. I would not rely on remembering that (or making sure the Tesla does not forget it after a software upgrade which others have reported) to keep from burning my house down. But with a UMC Gen 2 you don't need to turn it down. A 40a circuit with proper ampacity wire (which can be as small as 8awg) is totally fine. (but if installing new I would generally always install a 50a circuit with the right wire)
So this same circuit and receptacle was fine at 32a before, but now the UMC has issues? It is possible that the UMC is detecting overheating of the receptacle or your car is detecting too much voltage drop. The old EVSE may just not have had those safety features and was not detecting a dangerous situation.
Yeah, the car has no idea if you are on a 40a or a 50a breaker. All the UMC knows is that it has a 14-50 adapter head plugged into it but since the UMC is only good to 32a, it only charges at 32a, so it does not matter if it is a 40a or 50a circuit.
My god. That was an unbelievably dangerous situation. As you point out, thank god it was in conduit. 10 gauge THHN wire in conduit is good to 35 amps (28 amps continuous). 10 gauge NM cable is only good to 30 amps (24a continuous). A 50a breaker was incredibly dangerous.
Ironically, 8 gauge in conduit is good to 50a, though 6 awg is even better. 6 awg you could do a 60a breaker if you had a Wall Connector instead of a 6-50.
Yes, please double check the wire gauge just to be sure, however, 8 AWG is completely fine for a 40a circuit. 8 awg is even fine for a 50a circuit if it is THHN in conduit and the distances are not too far to have to worry about voltage drop. A 6-50 or 14-50 receptacle is totally legal on a 40a circuit as long as the intended load is not over 40a (or 32a continuous like an EVSE). Though again, I would generally never install a new 6-50 or 14-50 on a 40a circuit if I had the choice. Best to have a full 50a circuit.
I agree you need to figure out what is going on here immediately. This could be a fire risk. An extremely common issue (other than wrong wire size) is loose wire connections at the breaker, or the receptacle, or any intermediate connections. We have seen a couple of cases recently where wire was not stripped sufficiently before being shoved into the terminals on a receptacle and so the insulation was pinched and so the terminal was not making good contact with the wire. This caused the receptacle to overheat and melt.
8 gauge THHN wire in conduit is allowed to be used at its 75c rating which allows for 50a. So it is perfectly legal to use on a 50a circuit.
8 gauge NM cable (romex) is only allowed to be used at the 60c rating which limits you to a 40a circuit.
A 40a circuit is utterly and completely legal on a 6-50 or 14-50 receptacle as long as the intended load is not greater than 40a (32a continuous). So a UMC Gen 2 is totally fine. A UMC Gen 1 is NOT fine.
Why change to 4 gauge? If you are going to use a Wall Connector and charge at some higher rate this might make sense, but at 6 awg you are already oversized for a 50a breaker. If your run distance was not too long you could have used 8 AWG. Here is the chart I am getting all my data from: (but you have to know whether you are allowed to use the 60c or 75c column)
Ampacity Charts - CerroWire
Ahh, ok. So you did 6 awg and are using a Wall Connector, so does that mean you put in a 60a breaker then? A 60a breaker is required to charge at 48a.
Ahh, just got down to this post. Ok- You need to check your connections at the receptacle (no insulation in the terminals and that they are tight), as well as at the breaker (or anywhere in between they are joined). The issue could also lie upstream in your panel our out to the utility transformer... If your car is getting low voltages then I would check them back in my panel with a multimeter while the car was drawing load.
Yeah, check the voltage on the Tesla display just as you plug it in and it starts charging (but has not ramped the amperage yet). Then watch the voltage drop as it fully ramps into charging (and perhaps say watch it for a while as the wires heat up - it could take a bit for the voltage to drop more due to some wiring issue). You will want to see voltage before the car cuts the charging speed down (as it does that to reduce voltage drop).
My bet is that you may have a loose connection to the 6-50 receptacle and it is getting hot causing the Tesla to ramp down charge rate. Either that or a loose connection elsewhere that is causing voltage drop which is causing the Tesla to ramp down charging rate.
I'm installing 4 gauge for 2 reasons.
One - Distance due to resistance. I can't just install 6 gauge and run it as far as I have and feel safe according to the charts. The charts say that being 65 feet away requires 4 gauge and that's what I'm going to do.
Secondly - One day we are going to have a Kia Nero EV which charges at 18 AMPs. So with my 48 AMP Tesla.....4 AWG would be required at 65 feet.