EV Charging Approach - Difficult Choice

[wwhitney]

So that #2 Aluminum is good for 90 amps? What do you mean by "Unless there is further derating by a factor smaller than 0.9?"

Ampacity correction due to high temperature (above 30C) around the cable, and/or ampacity adjustment due to more than 3 CCCs (which would only occur if you bundled that cable with another cable for more than 24").

Cheers, Wayne

 

[tortuga]

I suppose given that limitation I could also propose this option: Wire a pigtail 10-30 plug to a Tesla Wall Charger and run through wall ...

Probably a violation of 400.12(2) and possibly 400.12(1)

 

[jaggedben]

So that #2 Aluminum is good for 90 amps? What do you mean by "Unless there is further derating by a factor smaller than 0.9?"

If the cable were bundled with other cables or subject to a higher ambient temp you might have that. I would assume you can ignore that stuff and use 90A.

 

[ESolar]

If the cable were bundled with other cables or subject to a higher ambient temp you might have that. I would assume you can ignore that stuff and use 90A.

I am also in N. Calif. The #2 feed for the subpanel runs between ceiling rafters of a one-story section. I would imagine that it gets hot in there. But how hot? Nobody really knows.

 

[tortuga]

So that #2 Aluminum is good for 90 amps?

I think so

but the SER was run through the walls. With insulation contact that wire has a 60 C, 75 amp rating.

This part is incorrect since the 2017 code, I think for a code cycle they did de-rate all SER but not anymore.
See 338.10(B)(4)

 

[wwhitney]

TLDR: If the ambient is up to 106F, and the cable isn't bundled, then #2 Al ampacity is still 90A. If the temperature is up to 123F, it's still fine to protect the unbundled cable with a 90A breaker, but the ampacity will be between 81A and 90A, and the calculated load still needs to be below the ampacity.

I am also in N. Calif. The #2 feed for the subpanel runs between ceiling rafters of a one-story section. I would imagine that it gets hot in there. But how hot? Nobody really knows.

So, the 90C ampacity of #2 Al is 100A, but you are limited to a maximum 90A breaker based on the 75C termination ampacity. The 100A ampacity is based on an ambient of 30C, or a 60C temperature rise. Temperature rise is proportional to the square of the current, since heat loss is proportional to temperature rise and heat gain is proportional to I²R, and at equilibrium heat gain will equal heat loss.

That means the temperature corrected ampacity goes down to 90A when the allowable temperature rise is down to 60 C * (0.9)² = 48.6C, or the ambient temperature is up to 90C - 48.6C = 41C = 106F. Up to 106F you can ignore ambient temperature if there's no bundling.

But if the temperature corrected ampacity is at least 81A you can still protect the cable at 90A, as long as your calculated load is no more than the temperature corrected ampacity. And 60C * (0.81)sup[2][/sup] = 39C, so the ambient is up to 90C - 39C = 51C = 123F.

For residential, I expect this issue is usually ignored. And as long as the cable is on the conditioned side of any attic/ceiling insulation, I wouldn't expect it to get above 106F. If it's on the unconditioned side of the insulation, and in an attic, I expect it could well exceed 106F.

Cheers, Wayne

 

[DanS26]

Guys...here's real world experience. I run #4AL on a 60 amp breaker to a garage sub panel. From there I run #4CU to the two Tesla Wall Chargers that communicate with each other.

I set the Wall Chargers to 48 amp max. Early on...three years ago I would charge at 48 amps to my Model 3. That was too much....things got too hot. I set the max in the Model 3 to 42 amps and things settled down. So IMO 48 amps to the Wall Charger and 42 amps max at the car is the best setup.

I have used and abused my Model 3 for three years now.

 

[ESolar]

Guys...here's real world experience. I run #4AL on a 60 amp breaker to a garage sub panel. From there I run #4CU to the two Tesla Wall Chargers that communicate with each other.

I set the Wall Chargers to 48 amp max. Early on...three years ago I would charge at 48 amps to my Model 3. That was too much....things got too hot. I set the max in the Model 3 to 42 amps and things settled down. So IMO 48 amps to the Wall Charger and 42 amps max at the car is the best setup.

I have used and abused my Model 3 for three years now.

What got too hot: The #4 AL, #6 CU (you wrote #4 but I assume that you meant #6), Tesla cable, or the Wall Charger box? And how hot is "too hot"? Given that the wires are near their ampacity limit at 75 C (48/.8 = 60; ampacity of wire = 65 amps at 75 C), they should run hot: am I correct? And is that really a problem given that the wire is designed to? I guess if you dont like it then you go with #2 AL and #4 CU. But the Wall Charger will probably still get hot unless you reduce the amperage like you did. #6 CU is called for by Tesla. One of the experts here probably has something to say about this.

 

[HuntNJ]

Why do you need a sub panel

 

[Electrical Geek]

OP said this earlier.


It is apparently outside the garage. Don't know if that makes a difference in MA or not.

Still needs to be GFCI protected.

 

[ESolar]

Why do you need a sub panel

I am assuming that you are responding to DanS26. He is running two chargers with "power sharing" off of the same circuit sized for one charger. But each charger needs its own breaker. In that case, a subpanel is required and the subpanel circuit only needs to be sized for one charger. Then the circuit is split at the panel to the two chargers. I believe that the power sharing makes it code and prevents overloading the circuit to the panel. I have not verified this - but that is my understanding.

 

[ESolar]

Here is what I did: The dryer circuit was NEVER code compliant (not even grandfathered). It was 10/2 solid with a solid bare ground to a 10-30R. The existing dryer is gas so that circuit was not being used. Because they want the option to use an electric dryer (they have solar), I installed a new 8/3 NM-B 14-30 for a dryer from the subpanel (very short run) and repurposed the 10/2 with bare ground, conected to 10 THHN and running through (5 ft) EMT to a 6-30R inside the garage (and on the adjacent garage wall). They had a Tesla Wall Charger new in box. I wired a 6-30P pigtail with #8 SOOW to the Tesla Wall Charger and mounted that on the wall adjactent to the 6-30R where it is now plugged in.

So some of you are probably thinking that the wall charger can do 60 and supplying 30 is selling them short. Maybe. However, the 30 amp does 21 miles/hr while a 50 amp, for example, will do 29 miles/hr (Tesla Y). The client is well aware of the differences and does not see the need for 50 or 60 amps because the 30 amp circuit allows for more than 315 miles per day off-peak (9pm - 12pm) and 230 miles per day on work days (off-peak 9pm - 8am).

Now you may be thinking - but what if one needs to juice up very fast. In that case, home charging doesn't really cut it at 60 or less, period. In that case one uses a nearby supercharger. However, how often are you driving more than 200 - 300 miles per day? This would likely be a rare occassion for most people.

Thoughts, comments and critique appreciated as always.

 

[ramsy]

The client is well aware of the differences and does not see the need for 50 or 60 amps because the 30 amp circuit allows for more than 315 miles per day off-peak (9pm - 12pm) and 230 miles per day on work days (off-peak 9pm - 8am).

My clients feel the same way.

Anyone who must commute > 200mi day in a POV needs to reevaluate their job, much less commute in traffic.

 

[retirede]

Here is what I did: The dryer circuit was NEVER code compliant (not even grandfathered). It was 10/2 solid with a solid bare ground to a 10-30R. The existing dryer is gas so that circuit was not being used. Because they want the option to use an electric dryer (they have solar), I installed a new 8/3 NM-B 14-30 for a dryer from the subpanel (very short run) and repurposed the 10/2 with bare ground, conected to 10 THHN and running through (5 ft) EMT to a 6-30R inside the garage (and on the adjacent garage wall). They had a Tesla Wall Charger new in box. I wired a 6-30P pigtail with #8 SOOW to the Tesla Wall Charger and mounted that on the wall adjactent to the 6-30R where it is now plugged in.

So some of you are probably thinking that the wall charger can do 60 and supplying 30 is selling them short. Maybe. However, the 30 amp does 21 miles/hr while a 50 amp, for example, will do 29 miles/hr (Tesla Y). The client is well aware of the differences and does not see the need for 50 or 60 amps because the 30 amp circuit allows for more than 315 miles per day off-peak (9pm - 12pm) and 230 miles per day on work days (off-peak 9pm - 8am).

Now you may be thinking - but what if one needs to juice up very fast. In that case, home charging doesn't really cut it at 60 or less, period. In that case one uses a nearby supercharger. However, how often are you driving more than 200 - 300 miles per day? This would likely be a rare occassion for most people.

Thoughts, comments and critique appreciated as always.

Comment: On a 30A circuit, the EVSE must be set to supply a max of 24A. It cannot “supply 30”.

 

[ESolar]

Comment: On a 30A circuit, the EVSE must be set to supply a max of 24A. It cannot “supply 30”.

YES! The Tesla Wall Charger, for example, defaults to 60A (48A max) upon install. In the interface one has to select 30A and it will allow a max of 24A. I did that upon install. BUT!!! If I had been quick to test it without configuration, I would have plugged it in to the Tesla with the default 48 Amp draw and maybe caused some damage (but hopefully just have blown a fuse). But in this case the Tesla itself also has control over the draw. So I also set that toa max of 24A. In theory that was a fail safe. However, I am not sure how those two play. Also, because the Wall Charger and Tesla settings are via software, its unclear to me if that will ever revert to the default for some reason (maybe the charger has a battery and CROM and the battery dies). In any case, this issue should be front and center in the manuals and it is not.

 

[retirede]

YES! The Tesla Wall Charger, for example, defaults to 60A (48A max) upon install. In the interface one has to select 30A and it will allow a max of 24A. I did that upon install. BUT!!! If I had been quick to test it without configuration, I would have plugged it in to the Tesla with the default 48 Amp draw and maybe caused some damage (but hopefully just have blown a fuse). But in this case the Tesla itself also has control over the draw. So I also set that toa max of 24A. In theory that was a fail safe. However, I am not sure how those two play. Also, because the Wall Charger and Tesla settings are via software, its unclear to me if that will ever revert to the default for some reason (maybe the charger has a battery and CROM and the battery dies). In any case, this issue should be front and center in the manuals and it is not.

There is a hardware setting in the EVSE to limit the max charge rate. That must be set to not exceed the branch circuit rating in order to be code compliant.
The charge rate may be further reduced via software, but software alone cannot be relied upon to keep the charge rate within the capabilities of the branch circuit unless specific exceptions are met.

 

[ramsy]

There is a hardware setting in the EVSE to limit the max charge rate. That must be set to not exceed the branch circuit rating in order to be code compliant.

Is that hardware setting user adjustable per EVSE instruction manual, or a factory setting only ?

 

[retirede]

Is that hardware setting user adjustable per EVSE instruction manual, or a factory setting only ?

User adjustable, but if it requires tools to access the settings, the NEC allows the branch circuit to be sized based on the setting.
If it were a switch on the exterior (I’ve never seen one), the branch circuit would have to be sized for the max setting.

 

[ESolar]

User adjustable, but if it requires tools to access the settings, the NEC allows the branch circuit to be sized based on the setting.
If it were a switch on the exterior (I’ve never seen one), the branch circuit would have to be sized for the max setting.

There is nothing in the Tesla manual for their Wall Charger about a hardware setting. It's all software.

https://www.tesla.com/sites/default/files/support/charging/Gen3_WallConnector_Installation_Manual.pdf

 

[ESolar]

Based on this conversation, I gave Tesla a call. As you are probably aware, the car itself is the actual charger. The Tesla Wall Charger (WC) is just a plug with some communication. When commissioned, the WC has you set the amperage. That info is not stored in the WC. It goes to Tesla. It is used in communication and monitoring. The car (the actual charger) must be set to the correct ampacity so as to not overload the circuit. However, that communication with Tesla adds another layer of protection. The car knows which plug is being used and the maximum ampacity of that plug. It limits you to that max. But none of the settings are through hardware.