Does Tesla mobile connector work with 40Amp breaker

[Garlan Garner]

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.

 

[ChrisH]

@PatsFan : This is kind of an aside from your question (I don't know what might be the problem)...but...if you don't know already, personally I would double check what gauge wire is used for this circuit, as there is ambiguity - you have a 40A breaker with a 6-50 outlet on it. Having the lower amperage breaker is fine (and actually what I would prefer, if I knew I was never going to pull more than 32A on that circuit - it's safer), but I worry specifically that you have 8-gauge wire or something like that, with a 50A outlet. You need to have at least 6-gauge wire (for the main conductors) for this circuit. Otherwise the circuit shouldn't have a 50A outlet installed in the first place.

The UMC has the ability to check for excessive voltage drop (which would happen with insufficient wire thickness), and I believe it also can detect heating in the plug (due to a bad connection or some other problem), but others here who are experts can clarify those capabilities; I don't actually know. It's covered elsewhere in other threads.

Edit: TL;DR: My guess is your wire is undersized or something is wrong with your circuit. I would discontinue use until you figure out what it is. Do NOT put a 50A breaker on the circuit. It will just increase your risk of fire, which may be quite high already.

Hmmm I have a 6-50 with a 40amp breaker and I’m pretty sure 8 gauge wire. This was installed in a new construction home in 2016 so I’m guessing it is code acceptable - does that mean it’s still a fire risk? I actually only have the car draw 30A while charging.

 

[RedModel3]

Please just call an electrician already. What is it with you guys?

 

[Garlan Garner]

Please just call an electrician already. What is it with you guys?

View attachment 358905

Is this your house?

 

[ewoodrick]

I ran 10 gauge wire ( 240V ) from my 50A circuit breaker to my Tesla Wall charger. The Tesla Wall charger limited my charge rate to 32A.




After a tremendous amount of work and experimenting over 3 days ( charging at work only ) ....I changed my 65 feet of wire from 10 gauge to 6. NOW EVERYTHING works with no yellow lights...nor heat nor anything.


The Car didn't know anything...it just tried to pull everything it could and at some points during my 10 gauge wiring charging....and couldn't.
My voltage went down from 240V to 198V when the 10guage wires heated up my 3/4 conduit.

That's my experience.

The car DOES know.

The smaller the wire, the higher the internal resistance per foot. The longer the wire, the higher the total resistance.

Power = current * current * resistance (current squared times resistance)

By going to a bigger wire and shorter length the power used by the wire is decreased.

Power used by wire is directly converted into heat.

So therefore, the smaller the wire, the longer the wire, the more heat is generated in the wire.

Heat can become fire. And that's why so many people recommend that if you don't know what you are doing, let an electrician do it. The last thing that you want to do is burn down the house and the insurance company decline payment because of your negligence.


As to how the car knows, the voltage at the car is = voltage - (current * resistance)
So, as the resistance goes up, the voltage goes down.

You can easily figure the number out, the tables for the type wire that you are using is posted all over the Internet. Find resistance/ft and then you know the current and wire length.

 

[jdcollins5]

The 14-50 adapter would tell the car to limit it to 40 amps. If you have a Gen 2 UMC that is what tells the car to limit it to 32 amps.

All of the different adapters have internal circuits (resistors?) that pass the adapter limits to the car onboard charger. The 14-50 just happens to be the same as the UMC limit.

 

[Garlan Garner]

The car DOES know.

The smaller the wire, the higher the internal resistance per foot. The longer the wire, the higher the total resistance.

Power = current * current * resistance (current squared times resistance)

By going to a bigger wire and shorter length the power used by the wire is decreased.

Power used by wire is directly converted into heat.

So therefore, the smaller the wire, the longer the wire, the more heat is generated in the wire.

Heat can become fire. And that's why so many people recommend that if you don't know what you are doing, let an electrician do it. The last thing that you want to do is burn down the house and the insurance company decline payment because of your negligence.


As to how the car knows, the voltage at the car is = voltage - (current * resistance)
So, as the resistance goes up, the voltage goes down.

You can easily figure the number out, the tables for the type wire that you are using is posted all over the Internet. Find resistance/ft and then you know the current and wire length.

I just posted all of that.

I'm not sure what you are saying.

 

[jdcollins5]

As @LCR1 says, Gen2 UMC is limited to 32A.

For your circuit:
I guess this is allowed by code from what I just read elsewhere (40A wiring & breaker with 50A outlet) but it seems sketchy. And with a Gen1 UMC, you’d be in trouble, since presumably it would ramp to 40A with an 14-50 outlet, and that would exceed your wiring capability. And probably flip your breaker, which at least would be the saving grace here.

Agree with the recommendation to check the wiring though.

My setup is perfectly legal. The breaker matches the wire size and device load. If I were to change the UMC to a Gen 1 then I would increase the breaker and wire size. But I have no need to do that.

Also, the different adapters have internal circuits (resistors?) that pass current limit information to the car. The 14-50 happens to be the same as the UMC limit, the others are not. Whether the adapter passes to the UMC and then to the car, it is the adapter that has the limiting circuitry.

 

[LCR1]

All of the different adapters have internal circuits (resistors?) that pass the adapter limits to the car onboard charger. The 14-50 just happens to be the same as the UMC limit.

We're talking about the model 3 so the gen 2 limits it to 32 amps, not the adapter.

If you plug a gen 1 into a 14-50 it will pull 40 amps.

 

[eSpiritIV]

The Model 3 connector is only 32 amps so you are fine with a 40 amp breaker.

By code though a 50A breaker is required for a 50A plug. I highly recommend you get rid of that plug for the future owners of your house who made plug in a car that doesn't have the feature to turn down there charging rate like Tesla can.

Otherwise do spend the money and upgrade your panel breaker and wire to 50 amp.

 

[ChrisH]

By code though a 50A breaker is required for a 50A plug. I highly recommend you get rid of that plug for the future owners of your house who made plug in a car that doesn't have the feature to turn down there charging rate like Tesla can.

Otherwise do spend the money and upgrade your panel breaker and wire to 50 amp.

Then how did my house pass code inspection when it has a 40amp breaker for a 6-50? Poor code enforcement/inspection?

 

[AlanSubie4Life]

Hmmm I have a 6-50 with a 40amp breaker and I’m pretty sure 8 gauge wire. This was installed in a new construction home in 2016 so I’m guessing it is code acceptable - does that mean it’s still a fire risk? I actually only have the car draw 30A while charging.

See @eprosenx post above. He is an expert; I am not, on the exact gauge requirements and the code - he linked to the tables too. 8-gauge is ok sometimes, depends on the details. And as long as the intended load is supposed to be less than 32A you are apparently good, there may be other caveats. Personally I would just use 6-gauge to allow 40A and less voltage drop, but it’s apparently not required by code. And as long as all is done correctly it should not be a safety issue.

The OP seems to have a different problem than wire gauge; one that must be diagnosed ASAP.

 

[wws]

Then how did my house pass code inspection when it has a 40amp breaker for a 6-50? Poor code enforcement/inspection?

There is an exception in the NEC (Nat'l Electrical Code) allowing a 50 amp receptacle on a 40 amp circuit. I'd guess it is there because there are no common NEMA 40 amp receptacles. So you are 'code compliant'.

 

[AlanSubie4Life]

My setup is perfectly legal.

Yes. As I said, I gathered that it was legal from elsewhere. I can see why it makes sense that it would be since there is a gap in available NEMA plug capacities.

Unlike the OP, your setup seems fine. Just wouldn’t be my choice for something new.

The 14-50 happens to be the same as the UMC limit, the others are not. Whether the adapter passes to the UMC and then to the car, it is the adapter that has the limiting circuitry.

As mentioned elsewhere, 14-50 and 6-50 adapters should request a limit of the current to 40A. The Gen 2 UMC is “hard-configured” to request a current limit of 32A . So 32A, the minimum number, is communicated to the car as the max available. At least that is how I understand it.

In any case, the OP should figure out his potentially dangerous problem. It sounds like everything about his setup SHOULD be code compliant...but something is wrong. The Tesla and the UMC really try to avoid fires through detection, so they may be detecting something that the original EVSE did not.

 

[jdcollins5]

We're talking about the model 3 so the gen 2 limits it to 32 amps, not the adapter.

If you plug a gen 1 into a 14-50 it will pull 40 amps.

I agree with what you are saying about the limits of the Gen 1 and Gen2 and the 14-50 limit would be 40A, so the UMC would set the 32A limit. For the limits for the 15, 20, and 30 amp adapters though, the circuits in the adapters set the current limits, not the UMC.

 

[jdcollins5]

As mentioned elsewhere, 14-50 and 6-50 adapters should request a limit of the current to 40A. The Gen 2 UMC is “hard-configured” to request a current limit of 32A . So 32A, the minimum number, is communicated to the car as the max available. At least that is how I understand it.

OK, I understand what you are saying that the 14-50 adapter circuit limit of 40A but the UMC ford limit is 32A.

 

[PatsFan]

I just tested for any voltage drop in circuit. Reading shows 243 at breaker and 246 in outlet. So definitely there’s no drop. Im guessing it’s faulty 6-50 adapter that Tesla sent me.

 

[AlanSubie4Life]

View attachment 358975 View attachment 358977

I just tested for any voltage drop in circuit. Reading shows 243 at breaker and 246 in outlet. So definitely there’s no drop. Im guessing it’s faulty 6-50 adapter that Tesla sent me.

Are you drawing current when you make this measurement? It appears you are not.
Technically, what you measure isn't physically possible, and in any case the answer at each end of the wire should be the same if you're not drawing any current from the circuit, even if the circuit were horribly broken & ready to catch on fire. So you have some measurement error or there's some sort of grounding difference...or something...which results in incorrect readings.

Regardless, you should make this measurement UNDER LOAD (with current draw)

For practical reasons, I would suggest using the CAR to measure the voltage, as suggested above. There are a couple ways to do it outlined above, but in any case it can all be controlled from the car. If your UMC doesn't work, even when the car current limit is set to 5A or something, then we'll have to think about why that might be. I'm not saying it's NOT the adapter, but you should definitely do the quick experiment with the ramp up of current using the car before you start sending things back to Tesla.

Good luck.

 

[AlanSubie4Life]

If the picture of the 242(3)V number at the breaker is under load (how much current does the car indicate?), and 246V is an open-circuit measurement (no load), that's also bad, and points to the breaker being bad, or something upstream of the breaker. 4V of drop across a breaker is a problem, especially if you're drawing 32A (you would feel it getting very hot). You can measure (carefully!) on the main bus bars on the panel at the same time you are drawing current to debug, if it gets to that point. But not yet.

In any case, just look at the car's voltage measurement at various current draws. That's step 1.

 

[eprosenx]

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.

I just ran the math on 48 amps of draw at 240v across a 6 awg wire at 65 feet of length. I get 1.15% of drop which is well below the recommended maximum drop of 3% (technically it is 3% or a total of 5% between the feeder and the branch circuit and no more than 2% for each in either case, but you are well below both - a feeder only applies if you have a subpanel).

So you are vastly within limits at 6 AWG. Also, the voltage drop aspect of NEC is actually only a recommendation and not a code requirement. In the case of a Tesla, it will ONLY draw "X" number of amps regardless of voltage, so it is not like if the voltage drops due to conductor length that it will draw more amps. It still will draw the same number of amps and just charge slower.

The energy that is lost due to that voltage drop is lost over the entire length of the wire and so it can easily dissipate it over such a long distance. Properly sized wires do get slightly warm sometimes, but not HOT.

There is absolutely nothing wrong with upsizing to #4 AWG if you want to, but I would have zero concerns sleeping at night with #6 AWG when used for 48 amps of EV charging (this is literally exactly what I have at my house - also in 3/4in conduit to a Wall Connector).

As to the Kia Nero EV- How would you hook that up? Would you put in a subpanel with a second charger? Or would you do a second Wall Connector and have them load balance?

Hmmm I have a 6-50 with a 40amp breaker and I’m pretty sure 8 gauge wire. This was installed in a new construction home in 2016 so I’m guessing it is code acceptable - does that mean it’s still a fire risk? I actually only have the car draw 30A while charging.

Absolutely code compliant and not a fire risk as long as your EVSE does not tell the car it can draw anything above 32 amps. Setting it in software in the car I do not believe counts.

I personally recommend against installing new 6-50 receptacles or 14-50 receptacles on anything less than a 50a circuit, but I also don't tell people to go spend money changing it if they already have it installed since it is fully code compliant and as long as they don't use an EVSE that is capable of drawing 40 amps.

All of the different adapters have internal circuits (resistors?) that pass the adapter limits to the car onboard charger. The 14-50 just happens to be the same as the UMC limit.

Yes, technically the resistor is in the adapter on the end of the UMC. It tells the UMC how many amps the receptacle it is plugged into is in theory rated for (this of course has nothing to do with what the wire or breaker ACTUALLY are). Then the UMC passes a signal to the car over the J1772 protocol telling it the max charge rate limit it is allowed to draw. It is the CAR that is actually choosing what to draw. It should NEVER go over what the EVSE tells it that it is allowed. The car of course can choose to draw less if it wants (like based on your software setting).

By code though a 50A breaker is required for a 50A plug. I highly recommend you get rid of that plug for the future owners of your house who made plug in a car that doesn't have the feature to turn down there charging rate like Tesla can.

Otherwise do spend the money and upgrade your panel breaker and wire to 50 amp.

This is incorrect. You absolutely are allowed to have a 40a breaker and wire on a 50a receptacle as long as the thing you intend to plug into it only needs 40a or less (32a continuous). (so no UMC Gen 1's) As stated before, I don't recommend folks install these kinds of circuits new, but sometimes if there is existing wiring in the walls it can be an extremely cost effective (and safe) option. I personally would probably label it clearly if this was the case.

Here is the relevant code sections from the 2017 NEC:

Here is (B) referenced from above (A did not apply)

And here is just a blurb about what you are allowed to connect in. This becomes vastly more complicated with a lot more rules if you have more than one receptacle on a circuit (which is not allowed by article 625 of the NEC which pertains to EV charging).