New 3 car detached garage: how big should the wire be

[Roger Feeley]

We are building a 3 car garage with living quarters above. I’m trying to think ahead to the day when cars will be electric and we might want to charge three at the same time. Really, the only decision I have to make now is the size of the wire from the main panel to the garage. If the wire is big enough, changing breakers at either end would be relatively cheap.

one thing I did ask the builder for is a 2” pipe from house to garage with a pull cord so we can add other kinds of wire in the future. Optical fiber, maybe. Who knows?

Suggestions?

[Tom M King]

How far, and direct bury or in conduit? Wire has to be derated when in conduit.

[Alan Lightstone]

Assuming that you are using NEMA 14-50 outlets for the cars (or the equivalent), 3 cars could be pulling 40 amps x 3 = 120 amps at 220V charging. Not a trivial amount of power. I don't think these cars have a current surge when starting a charge (my Tesla doesn't), so that's probably a reasonable steady state of charging current. But that's going with the 80% rule per outlet. So 3 NEMA 14-50 circuits at 50 amp rating a piece = 150 amp rated wire at 220V. An awful lot of current. Electric meters gonna spin like crazy with that one.

Not sure how big wire that would require. I've leave that up to more knowledgeable people. Gonna be big.

[Kendall Scheier]

Do 3 separate circuits so you don’t need 1 gauge wire. 3 40 or 50 amp circuits are better than one 120 amp circuit.

[Mike Henderson]

I put in a subpanel in my garage with two 50 amp NEMA 14-50 outlets. If I recall correctly, I use 6 gauge from the subpanel to the outlets but I don't remember what I used from the main panel to the subpanel.

Mike

[Kev Williams]

I would think 6 gauge should be plenty, but what you might also consider doing is upgrading your incoming service by 100 amps...

[Charlie Velasquez]

Three ev’s, figure 100-150 amps,
living quarters above… temps in Falls Church ranges from single digits to 90s so figure both heating and cooling will be necessary, refrigeration, cooking…

Even a 200 amp sub may be a stretch if you plan to charge all at once.

Looks like maybe 2/0 or 3/0 copper or 4/0 or 250mcm aluminum (my choice; and 4/0 is the max you can fit in 2” conduit if that is your plan.)

edit: the smaller of each of the copper and aluminum is only if your ahj lets you to use the 80% rule. It is not really applicable in this situation, but inspectors have been known to allow it.
? Is your main sufficient to handle this load?

[Mike Henderson]

Remember that an EV will only pull 80% of the outlet value, so on a 50 amp outlet, the max a car will pull is 40 amps. Most people do not charge their EV every night, and you can program the car to only charge at a certain time, so even if you have two EVs, you can arrange so that only one is charging at a time (if that's a concern).

Mike

[Bruce Volden]

]We are building a 3 car garage with living quarters above.[/B] I’m trying to think ahead to the day when cars will be electric and we might want to charge three at the same time. Really, the only decision I have to make now is the size of the wire from the main panel to the garage. If the wire is big enough, changing breakers at either end would be relatively cheap.

one thing I did ask the builder for is a 2” pipe from house to garage with a pull cord so we can add other kinds of wire in the future. Optical fiber, maybe. Who knows?

Suggestions?

Many fires from EV's!
I wouldn't even consider such an undertaking. Nothing against EV's.

Bruce

[Jim Becker]

Vehicle chargers typically utilize a 50 amp circuit, although they don't typically draw more than 40 amps or so. So if you want to support two chargers, you'll want a 100 amp subpanel minimum in the garage. That's if it's not being used as a shop, too. The additional living quarters are in addition to that. If you're putting in additional conduit and it will carry coms, it needs to be separated from the conduit that the electric service will pass through. I just pulled my Ethernet between the house and the new shop building today, as a matter of fact. (pre-terminated direct contact rated Cat 6 cable...even in a conduit, it needs to be rated for wet conditions. Conduits leak)

You may be able to get a tax benefit from preparing to support EV charging that will help with the cost of the subpanel, etc.

[Lee Schierer]

I would give the building separate metered service from the power provider into at least a 200 amp panel. That lets you charge the cars from separate outlets and still leaves some power for the living quaters, assuming it is not a 3 bedroom house.

[Roger Feeley]

Remember that an EV will only pull 80% of the outlet value, so on a 50 amp outlet, the max a car will pull is 40 amps. Most people do not charge their EV every night, and you can program the car to only charge at a certain time, so even if you have two EVs, you can arrange so that only one is charging at a time (if that's a concern).

Mike

Im betting that as electrics take over the chargers will get smarter about balancing multiple cars.

[Alex Zeller]

My biggest problem with EVs is charging time. But that's while traveling. For home charging speed isn't that important most of the time. For a 3 car set up I would do one dedicated level 2 charger and a second power sharing (also called dual charging) level 2 charger. That'll be two 50 amp circuits, not 3. So the question is how often do you plan on driving 200 miles a day in all 3 cars in multiple days in a row? Most likely never.

[Jim Becker]

I would give the building separate metered service from the power provider into at least a 200 amp panel. That lets you charge the cars from separate outlets and still leaves some power for the living quaters, assuming it is not a 3 bedroom house.

He can do things the same way I just did here and the billing is simpler...upgrade the service to a 320 amp meter and split just past the meter...200 amps to the house and 200 amps to the outbuilding. The power company here considers a separate meter for an outbuilding to be a commercial account and the extra charges for the privilege get put on the bill. The exception to my advice relative to the living quarters is that if it's going to be a rental, one has to decide whether to include electric in the rent (or have a private meter to track it) or have separate service and separate billing. The latter complicates having the garage with EV charging on the same meter as the rental living quarters.

[Mike Henderson]

My biggest problem with EVs is charging time. But that's while traveling. For home charging speed isn't that important most of the time. For a 3 car set up I would do one dedicated level 2 charger and a second power sharing (also called dual charging) level 2 charger. That'll be two 50 amp circuits, not 3. So the question is how often do you plan on driving 200 miles a day in all 3 cars in multiple days in a row? Most likely never.

It's better that that. Just like with gasoline, you don't (often) run it all the way down before you recharge. I'd expect that most people would recharge when they get down to about half charge. So if you had a 90kWh battery, you'd need to put about 45kWhs into it. At 40 amps and 240 volts, that's a bit less than 5 hours. If your electricity was $0.30/kWh, it would cost you about $13.50 to "fill the tank". That's quite a bit less than for gasoline.

Mike