Home charging question

[ewoodrick]

I wonder why 40 amp circuits can't do 40 amps.
Why call them 40 amp then say you must subtract 20%? Why not call it a 32 amp circuit?
Must be some historical reason.

Can you go up three flights of stairs?
Can you do it constantly for 10 hours?
You might get warm after three flights, you get downright hot after an hour.

 

[miimura]

Oh, and it is stupid that Tesla does not offer a 6-30 adapter for the UMC.

They have the 6-15, 6-20, and 6-50. They should also include a 6-30...

A 14-30 (which they sell) is dumb because it requires a wasted neutral wire. The 10-30 is not usable because it has no ground. As others stated, you are not allowed to install new ones anymore.

It is an oversight they need to fix. Folks with 10 gauge wire available, but only two current carrying conductors (plus ground) really deserve a better option for using the UMC Gen 2.

Actually, I find that Tesla's choice of adapters is completely logical. The logic is based on what is commonly deployed in the real world. NEMA 6-30 is very rare because there are very few appliances that use that plug. Most loads in that power range also require 120V, so they must use a different plug like a 14-30 that has the neutral line. NEMA 14-30 is extremely common because of electric clothes dryers. A large fraction of the housing stock in California has the laundry hook-ups in the garage, so it is crucial for Tesla to offer that adapter.

 

[eprosenx]

Actually, I find that Tesla's choice of adapters is completely logical. The logic is based on what is commonly deployed in the real world. NEMA 6-30 is very rare because there are very few appliances that use that plug. Most loads in that power range also require 120V, so they must use a different plug like a 14-30 that has the neutral line. NEMA 14-30 is extremely common because of electric clothes dryers. A large fraction of the housing stock in California has the laundry hook-ups in the garage, so it is crucial for Tesla to offer that adapter.

Totally agree that 14-30 is more important (due to current installed base), but 6-30 allows for re-use of existing wiring in some (limited) cases and saves from having to install a neutral wire in other greenfield cases. So it is also useful and it would be nice to have as an option.

TT-30 (camp trailers at 120v) is also one that has a huge install base and would be useful.

Edit: I went back and re-read my previous post and realized it came across as me saying Tesla offering a 14-30 adapter was a bad idea. That is not at all what I was meaning to say. The 14-30 was the right choice if you could only do limited adapters since it is the most widely deployed. I was just saying that from a pure technical standpoint the 14-30 is dumb due to the wasted conductor wire, but that ignores the context of the situation.

 

[Shock-On-T]

Can you go up three flights of stairs?
Can you do it constantly for 10 hours?
You might get warm after three flights, you get downright hot after an hour.

I don't disagree, but I would have rated outlets based on sustained load.
So you could comfortably plug in a 40A appliance into a 40A outlet without having to do any further calculations.

 

[Dan123]

I don't disagree, but I would have rated outlets based on sustained load.
So you could comfortably plug in a 40A appliance into a 40A outlet without having to do any further calculations.

You don't have to do any calculations. The designer of the appliance and the electrician do the calculation. Your 40amp appliance will come with a plug that can only be plugged into a circuit that is designed for it. If it is a continuous 40amp appliance, then the plug will be a 50amp plug (or it will be hardwired into a 50 amp circuit by the electrician). If it is not a continuous 40amp draw, then it will have a plug for a 40amp circuit.

If you plug a 40amp continuous load into a "40amp" circuit, what that means is that both the wire and the breaker need to be oversized beyound 40 amps to handle the overheating that is caused by a continuous load. That means that if your 40amp appliance malfunctions and draws 50 amps or 60 amps, the circuit breaker is not going to trip right away. THat's not very safe.

 

[Rocky_H]

I don't disagree, but I would have rated outlets based on sustained load.

That is how the European electric code is set up. They have the ratings and wire sizes set for continuous 24/7 load at maximum circuit rating. The U.S. doesn't do that though.

So you could comfortably plug in a 40A appliance into a 40A outlet without having to do any further calculations.

And the U.S. way makes more sense generally, because well over 90% of devices are not continuous loads. So intermittent use at maximum rating is fine. A stove heating element cycles on and off. A refrigerator runs for a while and shuts off. The heating element in the clothes dryer cycles on and off for periods of time. That's just how most high draw equipment works.

You don't have to do any calculations. The designer of the appliance and the electrician do the calculation. Your 40amp appliance will come with a plug that can only be plugged into a circuit that is designed for it.

Yes, this is all handled by the ratings published on the equipment telling you what size circuit and outlet it requires.

 

[eprosenx]

So an interesting detail in this conversation about continuous loads vs non continuous loads.

My understanding is that a NEMA 14-50 receptacle is actually tested to 50 amps continuously. The wire ratings are also tested to continuous loads.

Where the 80% / 125% derating / upsizing comes in to play is actually on the breaker sizing. Standard thermal breakers are not very precise. They may break somewhere before their rating when exposed to continuous loads (between 80% and 100% of their rating) and so in order to avoid nuisance trips you are required to upsize them by 125%. As a fallout from this, you need to upsize the wire and receptacle in order to not cause a dangerous situation due to the larger breaker.

You actually can get 100% rated breakers which don’t have that derating requirement! (But I don’t think any exist for residential breakers - so it is not gemane to home EV charging) I believe they are generally microprocessor controlled so they don’t suffer from the same thermal issues.

So it does not hurt that this breaker limitation ends up causing the wire and receptacle to have additional headroom on capacity (an extra margin of safety), but it is the breaker limitation that drives the requirement.

 

[eprosenx]

I don't disagree, but I would have rated outlets based on sustained load.
So you could comfortably plug in a 40A appliance into a 40A outlet without having to do any further calculations.

One additional interesting thing about the US NEC is that you can put a NEMA 14-50 receptacle on a 40a circuit (actually, for dedicated use cases I think you can do even a 30a or less breaker which is just silly - you should use a lower rated receptacle but code does not require it) as long as the intended use case only requires 40a (or 32a continuous).

I am not a fan of this, but it is what it is. The only time I would do it is if there is existing installed wire that is insufficient for 50a but is fine for 40a.

 

[Tz00]

As a few others have said, the 80% derating is to prevent nuisance trips.

 

[Tz00]

And further to that, think of fuses. A nuisance fuse blowing means homeowners would just put in larger fuses until they stop blowing. 80% prevents nuisance that causes unsafe work-arounds by homeowners.

 

[miimura]

One additional interesting thing about the US NEC is that you can put a NEMA 14-50 receptacle on a 40a circuit (actually, for dedicated use cases I think you can do even a 30a or less breaker which is just silly - you should use a lower rated receptacle but code does not require it) as long as the intended use case only requires 40a (or 32a continuous).

I am not a fan of this, but it is what it is. The only time I would do it is if there is existing installed wire that is insufficient for 50a but is fine for 40a.

40A on 50A receptacles is a special case because there are no defined 40A receptacles. I think you will be hard pressed to find an inspector that will allow a 50A receptacle on a 30A circuit since there are plenty of proper 30A receptacles.

This is very common for 30 and 32A EVSE that requires a 40A breaker. Ones that come with a plug will have a 6-50 or 14-50 plug fitted.

 

[Dana1]

That makes sense, although if I were designing the code I would have assumed constant current.
Ie a 40A circuit should be able to handle a sustained 40A.

No, motors have a high starting current to overcome inertia and then taper off. A short period at higher amps doesn’t heat up the way a long draw at the same amps will. I interpret that a given wire size can do a burst of 40a or 32a practically forever without melting.

 

[eprosenx]

40A on 50A receptacles is a special case because there are no defined 40A receptacles. I think you will be hard pressed to find an inspector that will allow a 50A receptacle on a 30A circuit since there are plenty of proper 30A receptacles.

This is very common for 30 and 32A EVSE that requires a 40A breaker. Ones that come with a plug will have a 6-50 or 14-50 plug fitted.

I am on my phone so I don’t have the code reference in front of me, but I have posted it several times on the forum.

It is really weird, but the code as written for a dedicated single receptacle circuit simply says the receptacle must be equal to or greater in rating to the circuit it is attached to. I will try to remember to post the code snippet later.

 

[Shock-On-T]

nd the U.S. way makes more sense generally, because well over 90% of devices are not continuous loads.

I would have thought a system that works for 100% of cases makes more sense than one that works for 90%.

 

[Shock-On-T]

You don't have to do any calculations. The designer of the appliance and the electrician do the calculation.

I should have said “one doesn’t have to make calculations” but didn’t want to sound pretentious
My argument is that if a circuit can handle a 32A load then it should be called a 32A circuit, not a 40A circuit. The buffer should be built intrinsically into the code, rather than having to be recalculated again and again and again for every new installation.

 

[SpiceWare]

TT-30 (camp trailers at 120v) is also one that has a huge install base and would be useful.

There is an aftermarket option. They buy one of Tesla's 30 amp adapters and replace the plug, so it's a bit more than if Tesla offered it themselves.

TT-30 Adapter for Tesla Model S/X/3 Gen 2

 

[eprosenx]

No, motors have a high starting current to overcome inertia and then taper off. A short period at higher amps doesn’t heat up the way a long draw at the same amps will. I interpret that a given wire size can do a burst of 40a or 32a practically forever without melting.

Motors are a completely different story. They are a bad example to use here. A better comparison is a stove top (not used for more than 3 hrs typically) vs an electric car (often runs for more than three hours).

Motors are special in that they have to have breakers that are quite a bit bigger in order to provide for that surge startup current, but then when running they don’t need that much power.

So there are special rules that let you size the wire to the motors continuous running current needs and the breaker to its surge current needs. Then to protect the wire there is an additional component in the circuit that protects the wire. Specifically like ”thermal protection” built into the motor or fuses external to it that won’t blow instantly.

They are a very special case in the NEC...

 

[eprosenx]

40A on 50A receptacles is a special case because there are no defined 40A receptacles. I think you will be hard pressed to find an inspector that will allow a 50A receptacle on a 30A circuit since there are plenty of proper 30A receptacles.

This is very common for 30 and 32A EVSE that requires a 40A breaker. Ones that come with a plug will have a 6-50 or 14-50 plug fitted.

2017 NEC 210.21 "Outlet Devices" is the relevant section.

210.21 (A) Does not apply.
210.21 (B) (1) applies. Exception 1 is about motors so ignore that.
210.21 (B) (2) Does not apply since you will only have one receptacle on a single circuit (which is a requirement for EV charging in Article 625 I think)
210.21 (B) (3) Does not apply since you will only have one receptacle on a single circuit (which is a requirement for EV charging in Article 625 I think)
210.21 (b) (4) Does not apply.

So while it would be stupid and I would not do it, I see nothing that restricts you in the NEC from putting a 14-50 receptacle on a 30a circuit for instance (as long as whatever you plugged into it did not draw more than 30a (24a continuous). Though the UMC is a good example here. It draws 32a continuous (when connected to a 14-50 receptacle with the right Tesla adapter which is included) so if that is your intended load you would have to have a 40a or 50a circuit.

I guess having a 14-50 on a 30a circuit never really happens in practice since any device that only needs 30a is not going to have a 14-50 plug on them, they would have a 14-30.

 

[Rocky_H]

I would have thought a system that works for 100% of cases makes more sense than one that works for 90%.

My argument is that if a circuit can handle a 32A load then it should be called a 32A circuit, not a 40A circuit. The buffer should be built intrinsically into the code, rather than having to be recalculated again and again and again for every new installation.

There is a very obvious reason for this, though. If you were to do this, requiring everything to be built to 100% continuous use, then you are requiring extra cost of thicker wire size in that 90% of cases where it is just useless and unused. And that is 90% of every circuit in every building in the country. That is a gigantic waste of money.

 

[Dan123]

My understanding is that a NEMA 14-50 receptacle is actually tested to 50 amps continuously. The wire ratings are also tested to continuous loads.

.

The receptacle and wire rating are also not precise. They depend on things like the wire length, the outdoor temperature, on the bend of the wire, on subtle manufacturing variances, and on how well the electrician makes the connections. There is a big safety margin built into everything.