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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!

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    FYI, you can most likely (check the specs to be sure) use aluminum for the charger connections. A little more of the 2/2/2/4 AL will cost less than 4 AWG copper. Nov 5, 2021 at 16:11
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    96A/0.8 = 120A minimum (80% derate for continious loads) and upgrading merely to a 125 would barely leave you with power for lights, so think yet bigger panel/feed or smaller chargers.
    – Ecnerwal
    Nov 5, 2021 at 16:21

3 Answers 3

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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).

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Normally, your panel can be way oversubscribed. That's because most loads are either not on at the same time (heating vs. cooling, only some cooking appliances, etc.) or use far less than the breaker rating (lighting circuits, most receptacle circuits unless powering heaters or maxed-out computer servers).

EV charging is a little different, because it is quite plausible that all installed chargers (2 in your case) are in use, at full capacity, at any given time. Which means that if you have, for example, a 100A subpanel with a 60A EV and 60A (@ 240V) or 120A (@ 120V) worth of other circuits (e.g., 120V - 20A garage door, 20A lighting, 20A x 2 receptacles, 240V - 40A welder) there would be no problem at all. But if you have 2 x 60A EV charger = 96A continuous usage, you have gone over the 80A continuous usage rating of your 100A feed.

There are a couple of options:

  • Go even bigger on feed size. The main cost of that will be the larger wire, but you do need to watch out for the total load calculation for the entire house (i.e., including the 2 x 60A EV + all the loads in the main panel).
  • Get chargers that cooperate with each other. I know there has been discussion of this in particular with Tesla chargers, but I don't know if that is readily available or not, or what brand of charger you plan to get. The idea is that the cars and the chargers talk to each other in order to keep the total load to an acceptable level (80A in your case) while allocating in the most useful way - e.g., if one car has a much lower charge state than the other, it gets more of the 80A, and if they are close then they split it evenly.
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    moreover you can have cooperating chargers that also monitor your house's total power draw and throttle charging based on the amount of available headroom. Nov 5, 2021 at 17:28
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    @ratchetfreak In theory that's quite simple. But does anyone have a reasonably priced commercially available product that does that? Nov 5, 2021 at 17:41
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The chargers are "60A" because a 48A continuous load is a 60A load for code purposes (80% derate), so a 120A panel is bare minimum for those and absolutely nothing else.

150A is about where you should be looking (minimum) if you are committing to that size charger. Then again, how often do you expect to need a full charge in the shortest possible time, really? Most cars spend a long time in the garage, so high charging rate is only rarely going to be "needed" for most users.

If you run cable to your EV chargers, you need #4 Copper or #3 Aluminum (likely the more easily available #2) because you are stuck at the 60°C limit by using cable.

If (likely) the EV Charger terminals and (very likely) the breaker teminals are rated 75°C and you use conduit, you can use #6 Copper or #4 Aluminum (the breaker terminals are rated for AL as well as CU, the EV charger terminals would very likely also be, but you have to verify that they are.)

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