When EV novices start down the level 2 home charging path, they tend to fall into a certain series of errors. Let's head you off.
Charging rate, and unexpected headaches.
Cars need more power than level 1 (1.4kW). They don't need that much, though.
The universal novice misconception is "cars need 50A". Only during travel. It's wild overkill for daily at-home charging. I consider 10A per "hour per day you spend driving" is sensible. (when you're commuting, you're not going 70 mph in town.) Technology Connections has an excellent video on this and I've cued it up. Feel free to rewind for more basics. Really, check this out.
Any 2-car installation should contemplate Power Sharing. This gives you get the best of both worlds: Sensible space in the Load Calculation and bonkers overkill speed-charging when you want it. How does that work? The group of "chargers" (EVSEs) is given a current allocation to share. When one car is actively charging, it gets all of it. When a second car plugs in, they split the allocation 50/50, etc. etc.
The first car plugs in, it gets the full 40A. The second car plugs in, now they're splitting 20A until one finishes, then the remaining car gets 40. How? This is not your father's battery charger. EVSE's ("chargers" but not really) send a signal to the EV telling how much power they can draw. The EV obeys that signal. So it's easy for multiple EVSE's to coordinate - they are simply linked by a cable or even radio. This must be Safety Rated and approved by UL, and it is.
The NEC Article 220 Load Calculation
A common error is to hear about a Load Calculation and then "freestyle it". That's incorrect: there are only 3 acceptable ways that will comply with code, avoid a fire, and avoid main breaker trips.
- The long form NEC Article 220 method
- For dwellings, the alternate method in NEC 220.82, which usually calculates out better.
- A worksheet your municipality has developed.
You get to take the most favorable; the municipality can't refuse an Article 220 method.
If you are using Power Sharing, you can simply program the master unit with the "spare room" figure in the Load Calc.
If you are not satisfied with that power level available to you on the Load Calc, that can be beaten using EVSEs with a grid/service limiting feature. These attach sensors to your supply wires, and limit charging if the panel or service would otherwise overload. Nothing prevents EVSEs from using this method and Power Sharing also, but I couldn't name such a model off the top of my head.
A hardwired EVSE should never, ever be on a GFCI breaker. Why? Because an EVSE is not a charger; it #1 tells the car the safe amps, #2 energizes the connector on the car's request, and #3 is a smart GFCI. A hardwired EVSE is a GFCI receptacle, and you don't need a GFCI breaker on a GFCI receptacle.
Once, playing around on Apple's retail site, I found it was possible to configure an iMac to cost over $20,000, and I thought to myself, "who does that?"
Copper heavy feeder is like that Mac. In the 70s a mistake was made using aluminum AA-1350 "transmission line" wire for small branch circuits, and that's morphed into a general sense that copper is "deluxe", like that Mac. Well it's not. #1, the lugs you'll be landing on are made of aluminum, so the concept is wasted: you're not avoiding a dissimilar metal splice, you're creating one. And #2 the "safety" money should be spent on what actually matters.
So yeah, #1 aluminum for honest 100A, or #1/0 aluminum for honest 120A. Both are easier to work than #2 copper (lighter metal, less stiff). The distance isn't far enough for voltage drop to matter, remember it's 240V.
The 100A-bus-rating subpanel is running at "safety red-line" and has too few breaker spaces, so it paints you into a corner - you'll run out of breaker spaces soon and are limited to 20A of solar. With some of that wire budget, get a panel with lots of spaces, at least 125A internal busing (which you would need to exploit the capacity of #2 Cu or #1/0 Al) or even 200A internal busing.
How about AFCIs and GFCIs retrofitted in the main house? Code requires them on new builds for a reason. Ground rods tip top?
You're DIYing, so are you using a torque wrench/driver to tighten ALL connections which specify a torque, per 2014 NEC 110.14? Untorqued copper < torqued aluminum.
Will you be trenching to proper depth? Will the feeder be outdoor rated wire?
How about EMT conduit? Because if "deluxe" is your thing, EMT is the very definition of it. And 3/4" EMT is very DIYable (and error-forgiving, unlike PVC conduit), and will support any foreseeable charge rate, since the wires are easily pulled out and replaced. It also carries ground for you. You have to use copper the last few feet from disconnect/subpanel to EVSE, because EVSE makers don't bother to use Al-rated lugs. (which isn't hard and they should really do it).
In any conduit you get to use far cheaper THHN wires, which has a better thermal rating. 60A needs only #6 Cu THHN ($1.72/ft for both wires) instead of 4/3 Cu NM ($9.73/ft largely because nobody uses it). If you get an F150 Lightning and want honest 100A, you can't do that with Romex at all (SER maybe), but in that same 3/4" conduit you can fit two #3 THHN and a #8 bare ground (the ground just being there to be a 3rd wire, due to a glitch in conduit fill rules).
Two months ago everything exploded. The signal protocols on the charging plugs are absolutely first-rate - a fourth generation design from SAE, who is in the business of selling cars, not service upgrades. However there was squabble over the physical shape of the plug, particularly the DC fast charge pins... and Tesla/NACS had basically lost to the J1772 "Fetch" connector.
And then a week in May. Ford switched to Tesla/NACS. (whaaa? April fools joke???) GM switched a week later. We held our breath for a month while the foreign makers hemmed and hawed, but they all came over. The clincher was Volkswagen, which controls Electrify America.
2025 models will have NACS/Tesla. J1772/"Fetch" is not going to happen.
So... back around to our EVSE selection... you may be constrained into needing certain models to support grid service limiting or Power Sharing. But if you can live without grid service limiting, The Tesla wall units (fairly cheap at $475) do support Power Sharing and they will have the future connector. Use an adapter for this EV and your next EV won't need it.