220: Two outlets, two cars, one line

[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)?