240V Installation - 48amp vs 72amp

240V Installation - 48amp vs 72amp

First post!

Like many of you, I'm a M3 reservation holder and getting my home ready. I'm trying to decide on installing a 48amp or 72amp 240V outlet in my garage - I know you can add dual chargers to an S or X today and utilize the higher amperage for faster charging.

The 72amp is more expensive - any thoughts/recommendations on this?

dsvick | 9 november 2016

Without knowing if dual chargers will be available on the M3 I'd hold off on a final decision or forgo the 72amp install.

Rocky_H | 9 november 2016

As far as thoughts/recommendations, it may be sometimes useful to have the higher amp charging available in the car, because you may come to places where it would be helpful out on a trip or in a gap between Superchargers to get some faster filling. However, I find that it would be pretty useless at home. You've got overnight to refill it, so 3 hours or 6 hours isn't usually going to make any difference.

So a whole lot of this depends on where the thresholds are as far as what is "more expensive" for your situation. My whole house electrical service is 125A, so there was no way I could put in an extra 100A or 80A circuit. I put in a 50A (40A to the car), and that easily handles overnight charging. Even a wall connector on a 40 or 30 amp circuit would still be OK for overnight charging most of the time.

I should probably mention why the numbers are a little different if you haven't heard about this yet. Electric code requires that electric car charging systems can not run at the maximum of the circuit rating. The breaker has to be at 125% of the current the car uses. Or if it's easier to think of it the other way, you get 80% current from what the breaker is. So the 48A to the car needs a 60A breaker, and 72A needs a 90A breaker.

So I would recommend that if you can get in something like a 30 to 60A breaker without too much expense, that should serve you very well, but I wouldn't take on a lot of extra expense if you have to do major upgrades to get in an 80 or 90.

SUN 2 DRV | 9 november 2016

Rocky +1

Depending on the available extra capacity in your panel it may or may not cost a lot more to install a larger gauge wire, especially if the distance doesn't need to be long.

Are you likely to ever have to Teslas charging at the same time? The new wall connectors are smart enough to share a single circuit and load balance between up to four cars. So a single larger circuit can be more advantageous than two separate but smaller circuits.

raybanp | 9 november 2016

Thanks all for your thoughts.

@Rocky_H: is the 125% breaker spec something unique to your city or is that an electrical standard?

@SUN 2 DRV: the M3 will be my family's only EV / Tesla

Frank99 | 9 november 2016

Well, it's covered somewhere in the National Electric Code which is generally applicable to the United States. Article 625 defines the requirements for EV charging, and section 625.21 defines an EV charger as having to comply with the requirements for "Continuous Duty" equipment, which require a breaker and wiring rated for 125% of the chargers capacity.
So, if you live in the US, you're probably covered by this. | 9 november 2016

@mcadams85 - Electrical codes in most of USA require devices that take continuous current not exceed 80% of the breaker. For example a 100 amp breaker has a maximum of 80 amps for charging. In Canada, this drops to 75%. For the same 100 amp breaker example, that means a maximum of 75 amps.

pnajar | 9 november 2016

Not knowing where you are may make a difference. However if you are in the US go to Home Depot and price a 100A breaker and then price #3 copper cable. That should give you the material cost for a Tesla charger, without the charger. Next price out a 50A breaker and a 14-50 plug and cable appropriate for a 50A circuit. Then get some real bids from an electrician. Expect to pay additionally for conduit connectors and boxes for the installation.

raybanp | 10 november 2016

Great info and advice - thanks everyone.

When Tesla recommends a 240V 48A outlet which are they saying:
- that the car can charge at 48A, so install a ~ 60A?; or
- that the car is only going to charge at ~ 38A, so that's why we recommend a 48A?

Sort of confusing if you are not familiar with electrical codes. I do live in the US, so believe everything that has been shared is applicable to me. | 10 november 2016

It depends on how you plan to connect. If using the Mobile cable, it mates to a NEMA 14-50 connector. This is a 50 amp connection, and would have a 50 amp breaker. It will be limited to 40 amp charging (the car automatically figures this out). This works for all cars (old and new).

If you are connecting a HPWC, you can go up to 80 amps (with a 100 amp breaker). The HPWC can be set for lower amperage too. Older cars with dual chargers could handle 80 amps, and single chargers are limited to 40 amps. New cars have a different charger. The new in-car charger is limited to 48 amps, with an option to go to 72 amps.

It is a bit confusing, but I hope this helps make it a bit more understandable! | 10 november 2016

Also the 72 amp option costs $1500. Most owners do no find the 72 amp option necessary, especially if you're in a high-density supercharger area like California.

Rocky_H | 10 november 2016

@mcadams85, It also sounds like I should clarify some things about where various limits are from these questions I see:

"When Tesla recommends a 240V 48A outlet which are they saying:
- that the car can charge at 48A, so install a ~ 60A?; or
- that the car is only going to charge at ~ 38A, so that's why we recommend a 48A?

Charging goes through a few pieces of equipment, and each of them has some maximum current limit. So if it is limited at any point to a lower level, that will limit how high the whole system can run.

If you are using the "universal mobile connector" (UMC), which plugs into outlets, the highest capacity it can do is 40A continuous from a 50 amp outlet. So if you are using that cable that is included with the car that will be the most current you can use. So it won't matter if the car has the 48 or 72 amp capable charging unit in it. Both can handle at least 40A.

If you want to go higher than 40A, you would need to buy a wall connector. That can be installed on a whole range of circuit sizes, from 15A to 100A. It does not use an outlet, but is hardwired in. If you do that, then the limits of the circuit size and the car's onboard charger come into play. You could get a 60A circuit, providing 48A continuous, and the 48A charger in the car would be fully utilized.

If you want to put in a higher level circuit than that, then you would need to get the upgraded 72A-capable charger in the car to receive more current.

raybanp | 10 november 2016

definitely makes more sense now - thank you everyone!