Need some electrical help

If it's 25A, I would suggest they install a 14-30 (or a 10-30, but that might not pass a new electrical inspection), which is a 30A outlet, good for up to 24A max. If it's actually just 25A, you will probably have to dial down the power draw on the Model S screen to 20A or so. The tesla online store has both 10-30 and 14-30 adapters for $45 for the Model S UMC. They are sold out of the 14-30, so the 10-30 will work as well, you can build an adapter from 14-30 to 10-30 or Visa Versa as well, as long as you get one of them, build an adapter to the other, then you can handle either.
Shop Tesla Gear NEMA 10-30

TNEVol said:

This confirms much of what I expected. I had already figured that I would need to dial down to 20 A. Could he put a 14-50 or 6-50 receptacle in for a one time charge since I have both adapters and could dial down to 20 A?

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I don't think they would do that. People will be expecting to draw 40A from a 50A outlet, resulting in calls to their maint. dept. to "reset" the tripped circuit breaker. It's already questionable putting a 30A outlet on a 25A circuit, if indeed it's 25A, as that's an odd size for a circuit breaker. You could build yourself a 14-50 receptacle to a 14-30 plug, as long as you dial down the amperage.
I would suggest they install a 30A outlet though, to minimize future "issues" with other EV owners using it and causing nuisance circuit breaker trips.

TNEVol said:

This confirms much of what I expected. I had already figured that I would need to dial down to 20 A. Could he put a 14-50 or 6-50 receptacle in for a one time charge since I have both adapters and could dial down to 20 A?

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Wow, that's really generous of them. Cool to find a place that is this friendly to EVs.

But the answer to your question is: Not legally. He's not even going to be able to install a 14-30 or 10-30 since the breaker and wiring has to be rated at the same or greater than the outlet. I don't think he can legally install a 30A plug on a 25A circuit and Tesla doesn't make a 6-20 adapter.

So ask if he can install a 10-30 or 14-30 and see what he says.

strider said:

Wow, that's really generous of them. Cool to find a place that is this friendly to EVs.

But the answer to your question is: Not legally. He's not even going to be able to install a 14-30 or 10-30 since the breaker and wiring has to be rated at the same or greater than the outlet. I don't think he can legally install a 30A plug on a 25A circuit and Tesla doesn't make a 6-20 adapter.

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This isn't correct, and bear with me, because this is complicated.

When multiple receptacles are placed on the same branch circuit (for these loads, that's atypical), you are limited in the receptacles that may be connected. The code doesn't even list a 25A branch circuit, but following the guidelines for 40A circuits with 50A receptacles, an inspector would likely clear the installation of a NEMA 14-30. NEMA 10 series receptacles may only be used as a direct replacement, and new circuits may not use them under any circumstances anymore.

However, if a single receptacle is placed on a branch circuit (typical installations), the only rules that apply are that the receptacle and wiring must be rated at or above the branch circuit's rating, *AND* the branch circuit must be rated for the load that will be attached. And *that* is where you run into the code problem with the Tesla, because the use of the 14-50 adapter creates a minimum circuit size requirement of 50A (despite the fact you can "dial down" the current).

So, in summary - it is code-compliant for him to install a NEMA 14-50 receptacle to AWG 10 conductors attached to a 25A circuit breaker, because conductors and receptacle are rated above branch circuit overcurrent protection. Attaching the Tesla Model S to said receptacle is what breaks the NEC, because you're attaching a 50A nameplate load to a smaller circuit.

Now, should you insist on going down this path, well, dial it down! And be aware of insurance and liability implications should something happen.

FlasherZ, he's going to run into the same set of legal issues (connecting the UMC of the car at a rating over the breaker), whether the hotel installs a 14-50 or a 14-30, right?

So might as well go with the 14-50 and save yourself the hassle of getting a 14-30 adapter from Tesla. And with a 14-50 adapter he's going to be able to allow other Teslas to charge in the future - just need to put up a sign to 'dial it down'. (Again, with a 14-30 he'll also have to put up a sign to 'dial it down'). So not saying that it's specifically good, just that 14-30 won't buy anything over 14-50.

deonb said:

FlasherZ, he's going to run into the same set of legal issues (connecting the UMC of the car at a rating over the breaker), whether the hotel installs a 14-50 or a 14-30, right?

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Correct. Both receptacle types are legal for him to install, so ask him to put something for which you have an adapter already.

FlasherZ said:

Correct. Both receptacle types are legal for him to install, so ask him to put something for which you have an adapter already.

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That just seems completely bizarre to me. Why on earth would you be allowed to connect a 14-50 outlet on a circuit w/ 30A-rated wiring and 30A breaker? If I see a 14-50 plug I assume I can draw 40A continuous. I would think it would be the other way and you couldn't only install a lower rated outlet than the wiring and breaker would support.

strider said:

That just seems completely bizarre to me. Why on earth would you be allowed to connect a 14-50 outlet on a circuit w/ 30A-rated wiring and 30A breaker? If I see a 14-50 plug I assume I can draw 40A continuous. I would think it would be the other way and you couldn't only install a lower rated outlet than the wiring and breaker would support.

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If you see a 5-15 do you assume you can draw 12A continuously? You can't. It depends on whatever else other people, potentially in different rooms, have plugged into the same circuit. It may trip at 0.5A if you're unlucky.

It's not common, but it's similarly legal to share a 14-50 circuit across multiple outlets*. So imagine you have 2x 14-50 outlets on an ordinary 50A circuit, and in one side someone plugged in a 20A load. What you're left with at the second outlet is exactly the same experience as with a single 14-50 backed by a 25A breaker.

Outlets should NEVER be rater lower than the breaker. Otherwise the outlet itself can start melting before giving the breaker a chance to interrupt the circuit.

Rule of thumb: In any given circuit the breaker should be the weakest link. (Unfortunately, extension cords frequently are).


* Actually, 14-50's may be tricky to share because of the neutral wire, and the fact that they're often associated with inductive loads, which shouldn't be shared. So not 100% sure if NEC makes an exception for this - can't find anything. You can definitely share a 6-50 though (I've passed an inspection recently with a shared 6-50 circuit). The issues that would make a 14-50 unsharable doesn't apply to my argument above .

The code is different depending on whether the circuit has a single receptacle vs. more than one, but in the case where there is a single receptacle on the circuit, the NEC section 210.21(B)(1) states that the receptacle must be rated at least what the wiring/breaker is rated for, but it's okay for it to be rated higher than the wiring. It's probably not a smart idea to put a receptacle rated for more power than the breaker for obvious reasons - you could end up tripping the breaker if you plug something in that needs more power than the circuit is capable of delivering. That said, there is no safety issue which is what the NEC primarily tries to prevent. If you try to draw 40 A on a 30 A breaker, the breaker will trip and that's the end of the story.

You can think of it just like you are allowed to put multiple (say 8 - 10) separate 15A outlets on a single 15A circuit. There is nothing stopping you from trying to plug in 2 devices that each use 15A, but the breaker will trip if you do. Same idea here.

strider said:

That just seems completely bizarre to me. Why on earth would you be allowed to connect a 14-50 outlet on a circuit w/ 30A-rated wiring and 30A breaker? If I see a 14-50 plug I assume I can draw 40A continuous. I would think it would be the other way and you couldn't only install a lower rated outlet than the wiring and breaker would support.

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It's all about the nameplate rating on the load you want to attach, and ensuring the components used are rated above that.

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pgiralt said:

The code is different depending on whether the circuit has a single receptacle vs. more than one, but in the case where there is a single receptacle on the circuit, the NEC section 210.21(B)(1) states that the receptacle must be rated at least what the wiring/breaker is rated for, but it's okay for it to be rated higher than the wiring. It's probably not a smart idea to put a receptacle rated for more power than the breaker for obvious reasons - you could end up tripping the breaker if you plug something in that needs more power than the circuit is capable of delivering. That said, there is no safety issue which is what the NEC primarily tries to prevent. If you try to draw 40 A on a 30 A breaker, the breaker will trip and that's the end of the story.

You can think of it just like you are allowed to put multiple (say 8 - 10) separate 15A outlets on a single 15A circuit. There is nothing stopping you from trying to plug in 2 devices that each use 15A, but the breaker will trip if you do. Same idea here.

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That's what I tried to explain here -- thanks for making it clear! I heard at TESLIVE that I can be a bit overly complicated sometimes.

FlasherZ said:

And *that* is where you run into the code problem with the Tesla, because the use of the 14-50 adapter creates a minimum circuit size requirement of 50A (despite the fact you can "dial down" the current).

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Won't you have the same problem charging a Tesla from a 5-15 on a shared circuit? i.e. You can plug the UMC into some random 5-15, dialed down to 5A because you just want to maintain & balance, and unbeknownst to you, the circuit may be shared with another 7A load. Let's say a fire starts - does that mean insurance won't pay out for it?

deonb said:

Won't you have the same problem charging a Tesla from a 5-15 on a shared circuit? i.e. You can plug the UMC into some random 5-15, dialed down to 5A because you just want to maintain & balance, and unbeknownst to you, the circuit may be shared with another 7A load. Let's say a fire starts - does that mean insurance won't pay out for it?

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If you knowingly do it, and the insurance company determines you are knowingly doing it (for example, you admit that the circuit breaker would trip now and then but you did nothing about it), then yes, you could have that problem because you violated 210.19(A)(1) - "branch circuit conductors shall have an ampacity not less than the maximum load to be served". Technically, you're never supposed to connect the Model S to a shared 15A circuit at all, since the code requires you to take the 125% adjustment factor into account in 210.19(A)(1) and the 12A continuous load rating means you have to assume the full 15A rating dedicated for your load.

I've said a number of times that there's a difference between what the code reads and what will work. We all know that if you dial down the current, you can even make 4A work if that's all you can get; and that the circuit breakers are likely to protect you if you overload a circuit accidentally. At the same time, I'm sure we've all heard the odd stories of technicalities - e.g., the burglar that successfully sues a homeowner because he slipped on an untended spill in the kitchen and injured himself while burglarizing the house. Take the risks you feel comfortable with.

In the wild, it may require a little bit of a hack to charge, and if it means being stranded vs. getting to your destination, I don't blame people for using these hacks to get charged. My goal would be to let you know the dangers involved, the technicalities of the NEC, and the liability and insurance risks involved.

Likely, in your case (as in most cases here), you won't run into any issues unless you give the investigators a sign that you knew you were doing something wrong, or you negligently plugged it in.

deonb said:

If you see a 5-15 do you assume you can draw 12A continuously? You can't. It depends on whatever else other people, potentially in different rooms, have plugged into the same circuit. It may trip at 0.5A if you're unlucky.

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But I would assume that if nothing else is plugged into that circuit that I could draw 12A. But the example above would be like putting a 10A breaker and wiring on a 5-15 circuit. If I plugged in 12A of Christmas lights and the breaker kept tripping and I made sure nothing else was plugged in I would be frustrated. Yes, I should look at the breaker value when I went to reset it but still seems weird.

deonb said:

It's not common, but it's similarly legal to share a 14-50 circuit across multiple outlets*. So imagine you have 2x 14-50 outlets on an ordinary 50A circuit, and in one side someone plugged in a 20A load. What you're left with at the second outlet is exactly the same experience as with a single 14-50 backed by a 25A breaker.

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Yes but I can remove the 20A load and then pull a full 40A continuous. But if the breaker and wiring are only 25A there is NO WAY I can pull up to the rated output of the 14-50.

deonb said:

Outlets should NEVER be rater lower than the breaker. Otherwise the outlet itself can start melting before giving the breaker a chance to interrupt the circuit.

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Ah. Ok, hadn't thought about this angle but I still don't buy it. This would require someone pulling more power than the outlet is rated - ie doing some wrong. If you had an 80A breaker/wiring and a 14-50 outlet you could safely pull 40A continuous. If someone did something wrong such that they were pulling more than 40A through a 14-50 then yeah the outlet could melt/short.

Guess it makes sense that they would choose fire prevention over the nuisance of constant breaker tripping but it would still drive me nuts if I saw a 14-50 and the breaker tripped when I tried to pull 40A.

strider said:

But I would assume that if nothing else is plugged into that circuit that I could draw 12A. But the example above would be like putting a 10A breaker and wiring on a 5-15 circuit. If I plugged in 12A of Christmas lights and the breaker kept tripping and I made sure nothing else was plugged in I would be frustrated. Yes, I should look at the breaker value when I went to reset it but still seems weird.

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You'd be surprised at how many people report on some electrical forums that they reset a tripping breaker 1, 2, 4, even 8 times a day and ask if they should replace the breaker (when it's obvious they have some type of a circuit break or ground fault or something).

Yes but I can remove the 20A load and then pull a full 40A continuous. But if the breaker and wiring are only 25A there is NO WAY I can pull up to the rated output of the 14-50.

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Take, for example, some medium-sized cooktops that require 20A. You'll find that they may have a 14-30 plug, but the nameplate says minimum circuit ampacity is 25A. This is permitted, along with a 25A breaker.

Also note that when multiple receptacles are involved on a single branch circuit, there are limitations (210.21(B)(3)) on the circuit ratings and receptacles that can be used. Multiple 14-50's on a single circuit requires the circuit be rated at 40 or 50A only. This limitation doesn't exist for a one-receptacle branch circuit (treated as an "individual" load).

Ah. Ok, hadn't thought about this angle but I still don't buy it. This would require someone pulling more power than the outlet is rated - ie doing some wrong. If you had an 80A breaker/wiring and a 14-50 outlet you could safely pull 40A continuous. If someone did something wrong such that they were pulling more than 40A through a 14-50 then yeah the outlet could melt/short.

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Or, you have a short generated by heat. Let's say one of the line conductors isn't torqued properly. Running even 20-30A through this can cause severe heat and melting.

You'll frequently find range outlets (10-50's and 14-50's) connected to 40A breakers in houses.

FlasherZ said:

This isn't correct, and bear with me, because this is complicated.

When multiple receptacles are placed on the same branch circuit (for these loads, that's atypical), you are limited in the receptacles that may be connected. The code doesn't even list a 25A branch circuit, but following the guidelines for 40A circuits with 50A receptacles, an inspector would likely clear the installation of a NEMA 14-30. NEMA 10 series receptacles may only be used as a direct replacement, and new circuits may not use them under any circumstances anymore.

However, if a single receptacle is placed on a branch circuit (typical installations), the only rules that apply are that the receptacle and wiring must be rated at or above the branch circuit's rating, *AND* the branch circuit must be rated for the load that will be attached. And *that* is where you run into the code problem with the Tesla, because the use of the 14-50 adapter creates a minimum circuit size requirement of 50A (despite the fact you can "dial down" the current).

So, in summary - it is code-compliant for him to install a NEMA 14-50 receptacle to AWG 10 conductors attached to a 25A circuit breaker, because conductors and receptacle are rated above branch circuit overcurrent protection. Attaching the Tesla Model S to said receptacle is what breaks the NEC, because you're attaching a 50A nameplate load to a smaller circuit.

Now, should you insist on going down this path, well, dial it down! And be aware of insurance and liability implications should something happen.

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Sorry to dig up an old thread - I found this doing a search, and I think it answers my question, but I'm asking a couple of follow up questions to be sure.

I have a detached garage with its own subpanel. The sub is fed with 10 ga and a 30A breaker.

1 - If I install a 14-30 (or L6-30) with a 25A breaker and 10ga wire, that is NEC compliant, correct?
2 - If I plug in a Tesla using a 14-30 adapter, that is not NEC compliant, even if I dial back the charge current to 20A or less, correct?
3 - If I plug in an EVSE that limits the charge current to 20A (via the pilot signal - such as an LCS-25), is that now NEC compliant (I think it is)?

I know I can direct wire the EVSE with 10/2 (instead of using an L6-30), but I think I want to use the plug for an additional disconnect. I suppose a 30A double pole switch is also an option.

Finally, an additional code question:

4 - NEC requires downstream breakers to be smaller, correct ("less than", not "less than or equal")? Or can I install a 30A breaker in the sub fed by a 30A in the main panel? I thought all the breakers in the subpanel had to be smaller than the one feeding it in the main (hence my plan to use a 25A).

If it were me, and there is conduit to the subpanel in the detached garage, I would pull a larger wire (#6? or better depending on the size of your conduit) and upgrade your subpanel. It will be a minor cost and give you many more options going forward.

Lloyd said:

If it were me, and there is conduit to the subpanel in the detached garage, I would pull a larger wire (#6? or better depending on the size of your conduit) and upgrade your subpanel. It will be a minor cost and give you many more options going forward.

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Thanks, that would be the ideal approach, but, unfortunately, it won't work.

I should have added that there is no conduit; the feeder is direct-buried #10 UF. After the wiring was installed, the driveway paving was extended and wraps around the garage, between the house and garage, right up to the edge of the woods.

There is no way to lay new cable/conduit without going under the driveway or through the woods. If I didn't have pavement in the way, I'd have a new conduit installed in an afternoon (I have a small backhoe for my tractor; trenching is easy), with a 50A feed to the sub panel and a separate 100A feed for a HPWC.

Edit:

Also, I forgot to add that this is a vacation/weekend home. I will always arrive at ~50% SOC (assuming 85kWh) and spend at least one night. The ~15mph charge rate at 20A is more than adequate to get me back to "return trip" charge levels with an overnight charge, so I'm not too worried about charging speeds. If I ever need to make a return trip without stopping to charge, there are planned superchargers along the route.

tga said:

There is no way to lay new cable/conduit without going under the driveway or through the woods. If I didn't have pavement in the way, I'd have a new conduit installed in an afternoon (I have a small backhoe for my tractor; trenching is easy), with a 50A feed to the sub panel and a separate 100A feed for a HPWC.

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My understanding is that there are now methods of running electrical lines under the driveway without having to tear up the driveway.