Charging 2 Tesla’s on 100amp main - advice needed

[Stevs6382]

We have a model 3 performance and a new model S performance on the way very soon.

I am on a time of use rate plan with my cheapest rates only from 12am to 6am daily.

That leaves us with only 6 hours to charge 2 Tesla’s overnight.

We have installed 2 separate dedicated 50amp circuits (one for each car) each with their own NEMA 14-50 plug.

The main circuit to the house is only 100 amps.

When I look at the model 3 charging screen, it shows a max of 32amps when plugged into a 50 amp circuit.
Does the model S also max out at 32 amps when using the included mobile charging connector?

So if both Tesla’s were charging simultaneously, it would be 32a + 32a = 64a total coming off my main circuit, correct?
Am I missing anything here?

I’ve run some rough numbers, and based on our individual commutes, and a 32a charging speed. Our new model S will need approx 4 hours to recharge each night and my model 3 will only need 2 hours because my commute is only 1/2 as long.

Should I schedule charging so the model S charges from 12am to 4am and the model 3 charges from 4am to 6am?

Or should I just set the model 3 charge speed down to 16amps and let both cars start charging simultaneously at 12am?

Which would be optimal?

 

[davewill]

The current mobile connectors are limited to 32a, as is the Model 3 SR. The other cars can charge up to 48a with a 60a circuit and wall connector, or at 40a with a 50a circuit and wall connector. If your model S is older, it may have a 40a (or 72a or 80a) capable internal charger and a 40a capable mobile connector.

I take it you didn't have a professional install this? Juggling charging times is not a adequate solution. What I would do in your shoes is first, find out how much load your panel can actually handle by having a professional come out and figure it out. Once you know that you have a couple of choices:

1. If you can actually handle two 50a EV circuits, then just let them charge and stop worrying.

1a. if you can handle two 40a circuits, change the breakers to 40a breakers and do the same.

2. Assuming that one 50a EV circuit is the max you can handle, install two wall connectors on the two circuits, run a data wire between them, and configure them to share the 50a circuit. Then each car can charge at a max of 40a (or 32a for a 3 SR) if charging alone, and will split the 40a if charging at the same time.

3. if your panel can handle two 30a EV circuits, replace the breakers with 30a breakers, replace the outlets with 14-30s and charge each car at a max of 24a.

There are other combinations possible like a 40a circuit and a 20a circuit, or sharing a 40a circuit, it depends on your panel capacity and your daily charging needs.

 

[eladts]

The current mobile connectors are limited to 32a, as is the Model 3 SR. The other cars can charge up to 48a with a 60a circuit and wall connector, or at 40a with a 50a circuit and wall connector. If your model S is older, it may have a 40a capable internal charger and mobile connector.

I take it you didn't have a professional install this? Juggling charging times is not a adequate solution. What I would do in your shoes is first, find out how much load your panel can actually handle by having a professional come out and figure it out. Once you know that you have a couple of choices:

1. If you can actually handle two 50a EV circuits, then just let them charge and stop worrying.

1a. if you can handle two 40a circuits, change the breakers to 40a breakers and do the same.

2. Assuming that one 50a EV circuit is the max you can handle, install two wall connectors on the two circuits, run a data wire between them, and configure them to share the 50a circuit. Then each car can charge at a max of 40a (or 32a for a 3 SR) if charging alone, and will split the 40a if charging at the same time.

3. if your panel can handle two 30a EV circuits, replace the breakers with 30a breakers, replace the outlets with 14-30s and charge each car at a max of 24a.

There are other combinations possible like a 40a circuit and a 20a circuit, it depends on your panel capacity and your daily charging needs.

Instead of using two sockets and splitting the available power 50/50 even if only one car is charging, you can install two Wall Connectors sharing the same circuit splitting the power dynamically, thus optimizing the usage.

 

[Stevs6382]

We have a model 3 performance and a new model S performance on the way very soon.

I am on a time of use rate plan with my cheapest rates only from 12am to 6am daily.

That leaves us with only 6 hours to charge 2 Tesla’s overnight.

We have installed 2 separate dedicated 50amp circuits (one for each car) each with their own NEMA 14-50 plug.

The main circuit to the house is only 100 amps.

When I look at the model 3 charging screen, it shows a max of 32amps when plugged into a 50 amp circuit.
Does the model S also max out at 32 amps when using the included mobile charging connector?

So if both Tesla’s were charging simultaneously, it would be 32a + 32a = 64a total coming off my main circuit, correct?
Am I missing anything here?

I’ve run some rough numbers, and based on our individual commutes, and a 32a charging speed. Our new model S will need approx 4 hours to recharge each night and my model 3 will only need 2 hours because my commute is only 1/2 as long.

Should I schedule charging so the model S charges from 12am to 4am and the model 3 charges from 4am to 6am?

Or should I just set the model 3 charge speed down to 16amps and let both cars start charging simultaneously at 12am?

Which would be optimal?

The current mobile connectors are limited to 32a, as is the Model 3 SR. The other cars can charge up to 48a with a 60a circuit and wall connector, or at 40a with a 50a circuit and wall connector. If your model S is older, it may have a 40a (or 72a or 80a) capable internal charger and a 40a capable mobile connector.

I take it you didn't have a professional install this? Juggling charging times is not a adequate solution. What I would do in your shoes is first, find out how much load your panel can actually handle by having a professional come out and figure it out. Once you know that you have a couple of choices:

1. If you can actually handle two 50a EV circuits, then just let them charge and stop worrying.

1a. if you can handle two 40a circuits, change the breakers to 40a breakers and do the same.

2. Assuming that one 50a EV circuit is the max you can handle, install two wall connectors on the two circuits, run a data wire between them, and configure them to share the 50a circuit. Then each car can charge at a max of 40a (or 32a for a 3 SR) if charging alone, and will split the 40a if charging at the same time.

3. if your panel can handle two 30a EV circuits, replace the breakers with 30a breakers, replace the outlets with 14-30s and charge each car at a max of 24a.

There are other combinations possible like a 40a circuit and a 20a circuit, or sharing a 40a circuit, it depends on your panel capacity and your daily charging needs.

The current mobile connectors are limited to 32a, as is the Model 3 SR. The other cars can charge up to 48a with a 60a circuit and wall connector, or at 40a with a 50a circuit and wall connector. If your model S is older, it may have a 40a (or 72a or 80a) capable internal charger and a 40a capable mobile connector.

I take it you didn't have a professional install this? Juggling charging times is not a adequate solution. What I would do in your shoes is first, find out how much load your panel can actually handle by having a professional come out and figure it out. Once you know that you have a couple of choices:

1. If you can actually handle two 50a EV circuits, then just let them charge and stop worrying.

1a. if you can handle two 40a circuits, change the breakers to 40a breakers and do the same.

2. Assuming that one 50a EV circuit is the max you can handle, install two wall connectors on the two circuits, run a data wire between them, and configure them to share the 50a circuit. Then each car can charge at a max of 40a (or 32a for a 3 SR) if charging alone, and will split the 40a if charging at the same time.

3. if your panel can handle two 30a EV circuits, replace the breakers with 30a breakers, replace the outlets with 14-30s and charge each car at a max of 24a.

There are other combinations possible like a 40a circuit and a 20a circuit, or sharing a 40a circuit, it depends on your panel capacity and your daily charging needs.

Thanks Dave!
We had an electrician install the 1st one 6 months ago when I got the model 3, and at the time he said if I got a 2nd Tesla he could come back and install a 2nd 50amp later in the last available open slot in the panel. He told me the panel would be able to handle it.

After I saw how easy it was to install the 1st one, we just copied his work and made a second identical circuit ourselves.

Maybe for safety, we should switch both the breakers from 50a down to 40a?
The mobile connector will only pull 32a anyway, so it won’t reduce our charging speed, correct?

My model 3’s mobile connector is from
July 2019 when I got the car brand new.

And the model S is a brand new car - delivery expected mid December 2019

both should be the newest version mobile connector, with the NEMA 14-50 adapter (sold separately)

I don’t want to have to go purchase the Tesla wall connector(s) at $500+ each. I would much rather just use the included connectors that come with each car.

 

[davewill]

I would only change the breakers if you had to, not as a guess.

Truly it's hard for someone to be sure from photos. My answer is still the same. Be sure what the panel can handle instead of guessing. That means having someone do a proper load calculation. It helps that I don't see a lot of big energy users in there like a spa or electric heat. Mainly just the AC and I'm guessing that it is only counting for 20-30a, not the 40a of the breaker. It's not impossible that the electrician was correct and the second circuit is fine.

 

[Stevs6382]

Also Dave,

To clarify, my goal is to put the lightest load (wear & tear) possible on my electrical panel, while still getting our charging done within the 6 hour “super off peak” window for both cars.

32amps charge speed is more than enough to cover the daily use of both our cars. I don’t need to charge them any faster.

 

[Stevs6382]

I would only change the breakers if you had to, not as a guess.

Truly it's hard for someone to be sure from photos. My answer is still the same. Be sure what the panel can handle instead of guessing. That means having someone do a proper load calculation. It helps that I don't see a lot of big energy users in there like a spa or electric heat. Mainly just the AC and I'm guessing that it is only counting for 20-30a, not the 40a of the breaker. It's not impossible that the electrician was correct and the second circuit is fine.

Thanks Dave!

 

[eprosenx]

To kind of add to what Dave was saying:

There is no way that install is actually code compliant if indeed you only have a 100a electrical service. Each of those cars is calculated as 32a continuous which means it comes off the load calculations directly as 40a each. So that leaves you 20a for everything else in your house.

Now with that being said, it may be workable mostly. You don't have a lot of large loads, but that AC compressor is a serious load. If it is running at night along with the cars that right there could put you very close to the tripping range.

First off, do you have an external main disconnect or something? This looks like a subpanel. I would want to 100% verify what the breaker size on that is (and make sure it is relatively modern and you think it is going to actually work right if needed) and also what size and type of wire was used from the outside into this panel (and what size feed comes from the utility, though I worry about that less since it is really the utilities issue mostly). Pictures of all that stuff (and of the panel uncovered if you are comfortable doing that) is always useful. Also pictures of the breaker panel specs from the door/inside.

Here is one jurisdictions form they want folks to fill out for load calculations based off the NEC formula:
https://www.ladbs.org/docs/default-...oad-calculation-form-in-form-00.pdf?sfvrsn=16

Two EV's on a 100a service is pretty tight (especially since you have AC) and even MORESO since you have time of use windows you are trying to fit into.

Really this is a great use case for load sharing Wall Connectors. You could figure out what the load calcs will safely allow as a max and then set the two to not exceed that in aggregate. No rewiring necessary (but in aggregate you can't use over the ampacity of the wire to a single unit). You would need a data cable between the two.

If you were really desperate, there are things that can monitor your main feed and "shed" the EV charging load if you are maxing things out. Though unless you are in a high rise condo or something, I generally find it is cheaper and easier right now to just upgrade your main service (this also could be something to consider - if your meter base is on the opposite side of the wall and already happened to be conductored from the utility for 200a, upgrading may not be that bad - also, if your external box with the main breaker has more slots, you may be able to feed an EV directly from it - post pictures!)

I personally would put in a Sense Energy Monitor to let me monitor my power usage in great detail. In these marginal situations, it would give you a good idea of how close to capacity you actually are. There are provisions in the NEC to let you violate the load calculations if you have historical data showing that load is lower than the calculations would otherwise indicate.

P.S. Nice job on the conduit! Looks good. Looks like 3/4in. You might be a really good candidate for 6 AWG to a pair of Wall Connectors so they can both share 48a max (if the load calcs allowed for it). Would require upgrading both breakers to 60a. Odds are though you would leave both at 50a and have a combined max of 40a allowed between the two.

 

[M3BlueGeorgia]

Also Dave,

To clarify, my goal is to put the lightest load (wear & tear) possible on my electrical panel, while still getting our charging done within the 6 hour “super off peak” window for both cars.

32amps charge speed is more than enough to cover the daily use of both our cars. I don’t need to charge them any faster.

Unless you do a long commute each day, you don't need to charge them that fast.

Try each at 16A. Should be good for 14mph.

 

[DiyEvRepair]

I'd say don't worry about charge times, and have both cars start charing at 12am. You can lower the max charge rates if you will be tripping the main breaker.

But yea, two wall connectors would be most ideal.

 

[Stevs6382]

To kind of add to what Dave was saying:

There is no way that install is actually code compliant if indeed you only have a 100a electrical service. Each of those cars is calculated as 32a continuous which means it comes off the load calculations directly as 40a each. So that leaves you 20a for everything else in your house.

Now with that being said, it may be workable mostly. You don't have a lot of large loads, but that AC compressor is a serious load. If it is running at night along with the cars that right there could put you very close to the tripping range.

First off, do you have an external main disconnect or something? This looks like a subpanel. I would want to 100% verify what the breaker size on that is (and make sure it is relatively modern and you think it is going to actually work right if needed) and also what size and type of wire was used from the outside into this panel (and what size feed comes from the utility, though I worry about that less since it is really the utilities issue mostly). Pictures of all that stuff (and of the panel uncovered if you are comfortable doing that) is always useful. Also pictures of the breaker panel specs from the door/inside.

Here is one jurisdictions form they want folks to fill out for load calculations based off the NEC formula:
https://www.ladbs.org/docs/default-...oad-calculation-form-in-form-00.pdf?sfvrsn=16

Two EV's on a 100a service is pretty tight (especially since you have AC) and even MORESO since you have time of use windows you are trying to fit into.

Really this is a great use case for load sharing Wall Connectors. You could figure out what the load calcs will safely allow as a max and then set the two to not exceed that in aggregate. No rewiring necessary (but in aggregate you can't use over the ampacity of the wire to a single unit). You would need a data cable between the two.

If you were really desperate, there are things that can monitor your main feed and "shed" the EV charging load if you are maxing things out. Though unless you are in a high rise condo or something, I generally find it is cheaper and easier right now to just upgrade your main service (this also could be something to consider - if your meter base is on the opposite side of the wall and already happened to be conductored from the utility for 200a, upgrading may not be that bad - also, if your external box with the main breaker has more slots, you may be able to feed an EV directly from it - post pictures!)

I personally would put in a Sense Energy Monitor to let me monitor my power usage in great detail. In these marginal situations, it would give you a good idea of how close to capacity you actually are. There are provisions in the NEC to let you violate the load calculations if you have historical data showing that load is lower than the calculations would otherwise indicate.

P.S. Nice job on the conduit! Looks good. Looks like 3/4in. You might be a really good candidate for 6 AWG to a pair of Wall Connectors so they can both share 48a max (if the load calcs allowed for it). Would require upgrading both breakers to 60a. Odds are though you would leave both at 50a and have a combined max of 40a allowed between the two.

Hi eprosenx
Thank you for the info. Greatly appreciated!

This is a townhouse and the panel in the garage is a sub-panel. The 100amp main breaker is outside with the main breakers 4 other attached units - see pic

Yes we do run the AC overnight in the summer mainly because at super off peak rates it’s so cheap to run, I figure why not? If need be, I can program the AC to shut off when the 2 cars will be overlapping their charge times (maybe 3am to 5am)

I thought Maybe having the model S start charge at 12am, and set the model 3 for a 6am departure charge. The potential overlap would happen between 3-5am when the AC is programmed to shut off.

 

[boriszima]

Unless you do a long commute each day, you don't need to charge them that fast.

Try each at 16A. Should be good for 14mph.

I'd say don't worry about charge times, and have both cars start charing at 12am. You can lower the max charge rates if you will be tripping the main breaker.

But yea, two wall connectors would be most ideal.

I agree. I come from non-electrical background unlike what i assume Dave knows more about it. You got 2 circuits, now that its "winter" in our lovely SoCal likely you are not using AC. So start both cars at 32amps charging and lower each day or week to minimum Amps for each car so you get your needed charge. Since Model S will be less efficient it will likely need more amps unless its driven shorter distance. This way you will be ready when summer comes around. Also congrats on 2 Performance Teslas.

 

[Stevs6382]

Found more pics- inside of panel before electrician added the 50 amp at the bottom
And the label in the inside of the panel

 

[dk10438]

although 2 HPWC would be ideal, the cost is not insignificant. That being said, I think the the most cost efficient would be to get a Gen 1 mobile connector that would charge at 40A and just charge each car every other night. You didn't say how long the commute was or the total round trip mileage so not even sure if that's an option...

If not, then just charge each car simultaneously with 2 chargers. Even if you can't top off the battery to the desired level given the time constraints, you should be able to get through the week and top off over the weekend.

 

[eprosenx]

Hi eprosenx
Thank you for the info. Greatly appreciated!

This is a townhouse and the panel in the garage is a sub-panel. The 100amp main breaker is outside with the main breakers 4 other attached units - see pic

Yes we do run the AC overnight in the summer mainly because at super off peak rates it’s so cheap to run, I figure why not? If need be, I can program the AC to shut off when the 2 cars will be overlapping their charge times (maybe 3am to 5am)

I thought Maybe having the model S start charge at 12am, and set the model 3 for a 6am departure charge. The potential overlap would happen between 3-5am when the AC is programmed to shut off. View attachment 485495View attachment 485496View attachment 485497

Great pictures and data! Thanks! I have some additional ideas...

The primary line of defense in the NEC against electrical fires (or at least a massive factor) is to size things properly so they are not ever overloaded. This is where the load calculations come in. Breakers are another line of defense, but the NEC does not want you relying on them (as they don't always work!). Generally the NEC is super conservative and they assume no smarts on the part of the owner. So while you are smart enough to manage the load and set schedules for the car and maybe manually set the charging amps down, the next owner or a visitor, etc... may not be.

So that is not to scare you away from stuff, but just to give you the background on how NEC approaches it. (I have been known to do some not-code-compliant things under highly supervised conditions)

I am seeing some potential good news here:

First off, I hope that the breaker bus bars were totally cleaned before those breakers were installed. It looks like the sheetrock/painter folks may have sprayed over the bus bar which is maddening....

Second, can you get a wire size off the main feed wire in? It is hard to tell, but that wire might even be 2/0 aluminum which is potentially good for 150 amps in this use case (residential setting, feeding entire house load). There is actually a pretty good chance you could have the main breaker outside swapped to 125 amps and just magically get another 25 amps of capacity. The panel is 125a rated. The question is what the outdoor meter base/main is rated to and what the other tenants load is. If that outside panel is rated for it I would upgrade to 125a for sure. If the utility feed is not big enough, that is a problem that the utility needs to solve (probably is just fine though).

My personal thoughts are that I never want to limit my AC use just to charge the car (even at night!). I would want to make sure I had enough capacity. ;-)

One important thing: We have not talked about how much you drive each day each car. If it is not a lot, even each car being on a 20a circuit may be just more than enough (16a @ 240v for six hours). Tons of options here...
Pictures of all the engineering details for the outside panel would be useful if you can find them. (like a sticker inside the covers or somewhere? - might be only accessible after removing the main cover)

 

[eprosenx]

although 2 HPWC would be ideal, the cost is not insignificant. That being said, I think the the most cost efficient would be to get a Gen 1 mobile connector that would charge at 40A and just charge each car every other night. You didn't say how long the commute was or the total round trip mileage so not even sure if that's an option...

If not, then just charge each car simultaneously with 2 chargers. Even if you can't top off the battery to the desired level given the time constraints, you should be able to get through the week and top off over the weekend.

Given that he already has the wiring and will have two mobile connectors, I would just plug both vehicles in and set them to 20a and tell them to start charging right when the power becomes cheap at night. Hopefully they would both be full by morning each day... (this still is not code compliant since you are trusting the car to not accidentally reset to 32a - which has happened with software updates - or someone to bump it) No real benefit to buying a Gen 1 mobile connector that would then need to be alternated (you don't want to plug/unplug a NEMA 14-50 daily - it is not designed for that duty cycle - though obviously they could potentially switch where each vehicle was parked each night)

 

[Rocky_H]

Because I find these fascinating, I'll chip in with some thoughts:

I can't imagine having two full 50A circuits for those two outlets is code legal at all. Do a load calculation, but I'm about 80% sure that is not OK.
I agree that spending $1,000 for two wall connectors is pretty unnecessary.

I would expect that a load calculation will show that you could probably have something like 40 or 50 amps total for those two circuits. So I would replace the breakers and outlets to split that appropriately. So maybe you end up with a 20A outlet and a 30A outlet. Put the higher one on the side for the car that usually drives more miles per day. That's basically what I would do.

 

[animorph]

Very doubtful that is a code compliant setup. Two cars charging at 40A, allowed by the 14-50 outlets, hog all the power and leave nothing for the rest of the house. You may not to do that, but a future owner may.

Anyway, staggering the charge times will minimize the current draw, which would be a good thing. It will be slightly more energy efficient, since power loss in the wires depends on the square of the current. When you get the chance, check out the voltage in one charging car as you start charging the other car. You'll see the voltage drop a little bit (a few volts?) when the second car starts charging at full current. And there is the added advantage that you're reducing the load on your 100A mains.

 

[Tessaract]

I would only change the breakers if you had to, not as a guess.

Truly it's hard for someone to be sure from photos. My answer is still the same. Be sure what the panel can handle instead of guessing. That means having someone do a proper load calculation. It helps that I don't see a lot of big energy users in there like a spa or electric heat. Mainly just the AC and I'm guessing that it is only counting for 20-30a, not the 40a of the breaker. It's not impossible that the electrician was correct and the second circuit is fine.

Interesting that you're considering two 50A EV circuits on a 100A service. Even more interesting that an electrician thought this was OK. There is no possible load calculation in my jurisdiction that would allow this. I had to upgrade my 100A service to 200A service to add only 1 EV charging circuit.

 

[davewill]

Interesting that you're considering two 50A EV circuits on a 100A service. Even more interesting that an electrician thought this was OK. There is no possible load calculation in my jurisdiction that would allow this. I had to upgrade my 100A service to 200A service to add only 1 EV charging circuit.

I was thinking that maybe he was mistaken on the 100a since the electrician said it would work. Having seen more, I concur that there's no way he can support the two 50a circuits and needs to scale it back. Personally, I'd go ahead and load share, but converting to two smaller circuits is certainly workable as well.