220: Two outlets, two cars, one line

[Eric from NE]

There's a high probability our garage will be the home of a Roadster and an S by the end of 2013. I'm charging 220 exclusively at the office right now, and would like to wire the home garage with a pair of outlets on one 220 line.

Does anyone have any experience charging two cars on the same line simultaneously, particularly overnight? Obviously local codes may dictate otherwise, but is there any actual technical issue here, either with the electrical current or with the Teslas?

My reasoning is I'm in a common garage area in a condo building, and wiring costs aren't going to be cheap, like adding an outlet for a freezer in your private garage adjacent to your kitchen. Literally might have to run 75 feet of conduit across and down from a 25 ft ceiling. If I can cut the install cost in half it would be great.

The obvious workaround is to time one charge, for example, at 6p to 12p and the second from 12p to 6a, but that's not always going to be a possibility.

 

[jerry33]

That's really hard to say, but done right it shouldn't cost much for for two rather than one, assuming that you run one large line down to a sub-breaker box and then split out two receptacles from that. I did a similar thing to route power from my UPS to various devices.

Doing it that way will make that third Tesla really cheap to wire.

 

[dhrivnak]

I do not think you would save much and I will explain why. One could put in a 100 amp breaker, then use 4 guage wire and wire two NEMA 14-50 plugs. You would be limited to 80 amps max or 40 amps/car which would be more than enough for overnight charging.

Or one could run slightly larger conduit and run two circuits with less expensive #6 wire and you could also use less expensive 70-80 amp breakers. With a pair of 80 amp breakers you could charge each car at 64 amps and put in an HPC for each car. I would not run two HPC's on one circuit. While it can be done if you limit the current it is unlikely an inspector or electrician would install it. If you go the two NEMA 14-50 (basically a stove outlet) you should be OK.

 

[Eric from NE]

That makes sense. I'm obviously not an electrician, so forgive my ignorance on the details of this.

Could this work in a drop cord setup? As opposed to conduit and receptacles being mounted on the wall?

 

[dhrivnak]

Not really is my understanding. Assuming you have electrician install then you will likley need to have an inspection as well. They do not like having cords exposed as the wires should be inside a wall or inside conduit. They do sell easy to work with plastic conduit at Lowes/Home Depot and it is not a lot of money I would guess less than $30. Now if you do it yourself you have more flexibility as I installed my own HPC. But fortunately I have a basic understanding of electrcity and codes.

 

[hcsharp]

That's really hard to say, but done right it shouldn't cost much for for two rather than one, assuming that you run one large line down to a sub-breaker box and then split out two receptacles from that. I did a similar thing to route power from my UPS to various devices.

Doing it that way will make that third Tesla really cheap to wire.

+1
100 amp cable (or 125A) to a sub-panel is the way to go. Probably won't cost much more and would make it possible to put that freezer in front of your car.

 

[Eric from NE]

Here's the response I got from Tesla:

I contacted one of our Customer Experience Associates, and he recommends the following:

Although we recommend a dedicated circuit, you theoretically could run two electric vehicles off of one line and two outlets as only one line is live at a time (the vehicle doesn't care if it's getting the power.

Regardless if you have a High Power Wall Connector or two outlets, in either situation Model S will not charge if another is connected to a non-dedicated line.

All outlets will draw 80% of the breaker rating. In order to make all of your future charging options easier, I'll break down the charging equations and plug in a random outlet variable.

Voltage * Amps = Watts
Watts/1000 = Kilowatts (kW)
Then divide your battery pack (kWh) by kW

Voltage = the diameter of flow (as an example) Amps = flow or pressure of the flow if you visualize this process as a water hose.

With your NEMA 6/20 you can calculate the following.
240 volts * 16.8 amps = 4,032 watts = 4kW

60kWh/4kW = 15 hours to fully charge your battery pack.

If you opt for a 110 out outlet even with the same (or near) amperage you will still end up with twice the charge time.

I would think about what your daily commute looks like and most likely with that 240V outlet type you'll be fine for daily driving and just be topping off most of the time.

 

[Doug_G]

I'd just put in two 50-amp circuits each with a NEMA 14-50 plug.

You can simultaneously draw 40A on each circuit if you want to, and the full charge time for a 60 kWh pack is about 6.25 hours (using that crude calculation). Not that you'll need a full charge very often, but it means you can always recharge overnight.

I should also point out that this method of calculating charge time is ballpark but optimistic. You should probably add 15%.

 

[TEG]

...With a pair of 80 amp breakers you could charge each car at 64 amps and put in an HPC for each car.

You better check total power (electrical service) to the house. Many (most?) houses don't have 160+ amps of extra capacity available. Were your planning to pay you power company to upgrade your electrical service?

 

[hcsharp]

Here's the response I got from Tesla:

If that isn't confusing, then what is?

Voltage = the diameter of flow (as an example) Amps = flow or pressure of the flow if you visualize this process as a water hose.

Sorry, but No. "Voltage = diameter of flow" makes no sense. And "Amps = flow or pressure of the flow..." is half wrong. Amps = flow rate is OK but NOT amps = pressure. Volts are equal to pressure, not amps.

It sounds like you asked Tesla about comparing an HPC to having two plugs. One HPC will only charge one car at a time, so you'll have to come out and swap which is not convenient. Further, if your garage gets cold, you need to keep both of them plugged in at the same time. A sub-panel would require about the same size cable as an HPC, but the sub-panel gives you more options, including an HPC if you decide to go that way.

 

[smorgasbord]

How many miles will each car be driven a day?
What size panel are you pulling from? What's its rating? What breaker spaces are available? What other draws happen at night?

Here are two options:

1) Cheapest
a) Find out the max current you can pull with 6gauge wire at the length you need (75'?) and put that size breaker in. Remember to use the 75Cdegree rating since that's probably what your connections will be rated at. Put two NEMA 14-50 outlets in series on this circuit. You may find a 60-amp breaker is legal/safe (do NOT take my word for it, I do not know).
b) Restrict the current on Roadster and Model S each to be 40% of the breaker you put in. That's because you're only supposed to run at 80% of the rating. Set each car to charge at plug-in time. You only need to do these settings once - the car will remember.
Now, both cars will charge as long as they need to.
The danger here is that if one car is driven a lot more than the other, it may not fully charge in time. Let's say you were able to install a 60-amp breaker. That means each car can charge at 24amps, or around 20 miles per hour. If you get home at 7pm and need to leave the next morning at 7am, you'll put up to 240 miles into each vehicle. That's a total full charge for Roadster. If you were only able to put a 50-amp breaker in, then you'd get about 16 miles per hour, or up to 192 miles. You can extend the math from there.
If you know one car was driven more than the other, you can vary the draws accordingly, as long as the total of the two cars doesn't exceed 80% of the breaker rating.

2) Better
a) Put a subpanel in. That'll be a single run with bigger breaker and thicker wire. That'll cost more and be harder to pull.
b) Put in two NEMA 14-50 outlets, each on their own 50-amp breaker in the subpanel.
c) See #1 above for math. If you were able to put a 100amp line in, then you can let the cars run at their maximum UMC pull (best).

Before you go ahead, make sure you either get an electrician to do the work, or at least talk with an experienced electrician familiar with codes in your area. Get permits and get it inspected afterwards.

The really interesting future possibility is if both cars support something like OVMS. Then you can have the cars "talk" to each other, doing things like having the less essential car start its charge only after the other car has completed its charge.

 

[Lloyd]

Problem with above is that you can't put a 60 amp breaker behind a receptacle that is only rated for 50. This would enable you to pull more than the receptacle rating, possibly overheating it.

 

[smorgasbord]

Problem with above is that you can't put a 60 amp breaker behind a receptacle that is only rated for 50. This would enable you to pull more than the receptacle rating, possibly overheating it.

Yeah, that should have been obvious. Sorry.

That means the best bet is to put a subpanel in. Even if it's only a 60 or 70 amp thing, that's still better than a single 50 amp line.

 

[Eric from NE]

How many miles will each car be driven a day?
What size panel are you pulling from? What's its rating? What breaker spaces are available? What other draws happen at night?

So I think I may have asked this in a manner that made it more complicated than it actually is.

The Roadster is driven about 80-90 miles (on a typical commute day), the S would be substantially less, probably less than 30-40 most days. This number could skew dramatically (although not routinely) on weekends, though. Both commutes would be Monday-Friday. Also, I currently charge the Roadster at the office, so it will come home most days only needing 40-50 miles of charge.

I'm thinking most days the S could be charged fully by 10p, even midnight at a worst case scenario, at which point the Roadster could be set to start charging. Again, the only time we might need more draw than this at home is on an odd weekend, which we could easily adjust the charge times for.

How would a fully charged car on the same line with a charging car work out?

I guess I would just like to have the benefits of keeping both cars plugged in as much as possible when not in use. Once both are fully charged, there should be no issue with having them plugged in to the same line on separate outlets, correct?

 

[Lloyd]

There is not an issue as long as you have a 50 amp breaker for 50 amp outlets. If both cars are set to charge at 40 amps, and they both happen to be charging at the same time, then the breaker will trip, and you will not get a charge on either car. Best would be to have separate runs, separate breakers for separate outlets for each car.

 

[smorgasbord]

Again, your best bet is going to be to run the biggest line you can to the garage and put a small subpanel in, with a 50 amp breaker for each outlet in the subpanel.

Then, I'd set each car's current draw at 40% of whatever the main breaker in the subpanel is. That way both cars can charge anytime they want for as long as they want. If you try to play the one will finish before the other starts game, you'll get burned once where the first car takes longer than you thought, the second car trips the breaker, and neither car charges as much as you wanted.

This only gets tricky if you're on a Time Of Use meter, where you'd want to maximize the power draw to have as little park-peak draw as possible.

 

[hcsharp]

My roadster has pulled up to 8 amps at 240v to warm it up in the winter. Not sure if that's the max it will draw when not charging. I've read elsewhere on this site that when hot it will draw a little more than that to cool it off using the AC, front and rear fans. I've never had it hot enough at the end of charging to test that. What's your climate?

 

[jaanton]

Consider also, install two NEMA 14-60 receptacles (yes, they exist, hard to find, but they do exist.) with a 60 amp breaker and appropriate gauge wire . Then you remove the neutral pin on your UMC and it will fit just fine into a 14-60, 14-50 or 14-30 receptacle. With a 60 amp breaker you get 48 amp continuous which can be two 24 amp loads. I think if you compare a 14-50 vs a 14-60 they will not really be different regarding the current they can handle.

 

[Eric from NE]

What's your climate?

Anywhere from sub-zero (F) actual air temps in the winter to above 100 in the summer. Garage remains around 50 on the coldest days and not much higher than 75 on the hottest. COuld be a swing of 140 degrees in a year once you figure wind chill and heat index.

 

[mlascano]

There's a high probability our garage will be the home of a Roadster and an S by the end of 2013. I'm charging 220 exclusively at the office right now, and would like to wire the home garage with a pair of outlets on one 220 line.

Does anyone have any experience charging two cars on the same line simultaneously, particularly overnight? Obviously local codes may dictate otherwise, but is there any actual technical issue here, either with the electrical current or with the Teslas?

My reasoning is I'm in a common garage area in a condo building, and wiring costs aren't going to be cheap, like adding an outlet for a freezer in your private garage adjacent to your kitchen. Literally might have to run 75 feet of conduit across and down from a 25 ft ceiling. If I can cut the install cost in half it would be great.

The obvious workaround is to time one charge, for example, at 6p to 12p and the second from 12p to 6a, but that's not always going to be a possibility.

Eric, any update on what you ended up doing?
I'm in a similar situation (getting a Model S now and I have a Model X reserved) and I want to wire my garage to be able to charge boths cars simultaneously.
From reading the comments on the thread, installing a subpanel (100 amps) with separate circuits (50 amp breakers each) for each NEMA 14-50 outlet seems like the safest bet.

Does anyone know if, with the above set up, an HPC could be added down the line to the same subpanel (on a separate breaker) while still keeping the two NEMA 14-50 outlets (knowing that the max simultaneous pull from any combination of the three would be 100 amps)?