I have 200amp service for the house and I'm looking to potentially add capacity for two EV chargers in the garage.
I'm hoping to install two EV chargers that can charge up to 48 amps each. I know that in order to fit into the 125% continuous use rule, that I'll need to have 60 amp breakers and appropriately sized cable (I believe #4 copper?) going to each charger.
I'm wondering if I run a 100 amp subpanel (using 2-2-2-4 Al SER) from the main panel to the garage, can I put the two 60 amp breakers in the subpanel for the 2 EV chargers?
That is, does the subpanel need the capacity for the breaker rating (120 amps total across both chargers) or the continuous use rating (96 amps total across both chargers)?
My understanding is that the load calc would be based on the actual usage and not the breaker ratings, but I wanted to double check – thanks!
Best Answer
I'm going to focus on the modern way to do this, since it's so very correct that I'd almost call other ways wrong.
The newest EVSEs provide for that. Use them.
I.E. don't make the thing harder than it is "for tradition's sake".
The latest EVSE's are designed exactly for your use-case, since "multiple EVs" is not a surprise. These share a power allocation and divide the power to multiple EV's on the fly, based on their immediate needs.
Really.
During commissioning, you make one the "master" and the other(s) the "remote". You tell the master EVSE how much the group is allowed to draw. A popular share size is 64A actual, using 80A of provisioning at the subpanel.
They communicate with each other to split the load dynamically.
If two cars are plugged in but only one is drawing power, the drawing one is given the whole shebang.
The concept of this may shock traditional thinkers (not many of those drive EVs lol) but it is valid. We are too accustomed to having very stupid electrical devices that don't know how to manage load. That is no longer valid.
Examples are Sun Country's Share2, or Tesla's Wall Connector rev 3. (See page 23 and on in this manual, which granted was UL approved before the power sharing feature was finalized, and there may be changes since.
Note that Teslas charge just fine on "everyone else's" charger, we have a Tesla down the street with a ClipperCreek charger. And non-Teslas charge fine on Tesla Wall Connectors, which are very competitively priced. You simply need an adapter to match up the incompatible plug shapes. (The signals on the wires are the same).
How does this work?
The EV is not a dumb charger (and the EVSE is not a charger at all). To understand the EV charging protocols, look at this video, which covers all the necessary details.
https://www.youtube.com/watch?v=RMxB7zA-e4Y
The sharing software is rated to UL 1998 (number, not year) standards for safety-rated software.
The EVSE has a current detector, so it knows how many amps are actually being drawn by the EV. This allows artful things, like seeing that a car is only drawing 16A, so down-authorize it for 18A and give its remaining share to the other car(s). We know the Tesla chargers have some ability to do this, because (page 30 in that manual) when the power sharing feature fails due to loss of communication, each unit downgrades to its raw share of the provisioned power. (E.g. if four EVSE's are sharing 64A actual, each one will go to 16A because it can't negotiate more with the others).