A couple of people have asked me about what I've had to do to wire my garage up for my EV. Now having done it twice, I have a better idea of what to do/what not to do.
If you know what you're doing(TM), feel free to skip ahead to my TL;DR (why are you even here?). First off, if the main electrical panel to your house is rated for under 150A service, you will probably have to upgrade it. The alternative, and it may be a good one for you, but it's not a route I've taken yet for various reasons (especially installation cost), is to have an additional service run to your house with a Meter on your local utility's EV-only time-of-day rate plan.
This plan on DTEEnergy has off-peak hours from 11pm to 9am. It's slightly cheaper than the standard time-of-day rate plan, but has more restrictive hours. Standard Time-of-Day is off peak from 7pm-11am. It's actually been cheaper for me to be on this rate plan for my whole house even when excluding EV power usage because my usage tends to be pretty flat throughout the day, and it gives me more flexibility on when to charge. The longest my Chevy Bolt has taken to charge was when I used up the entire battery in the dead of winter. It was around 8.5 hours. My wife's 2019 Volt with the upgraded 30A charger takes about 2.5-2.75 hours.
Before I go into the specifics of this, I should note that I am not an electrician by trade, but have spent lots of time reading through the NEC book and haven't had any issue getting my wiring or installations permitted and approved. You also may have special requirements that are above and beyond what the NEC details based on your municipality. Ultimately your goal is to get your installation approved- even if requirements from your municipality contradict something in the NEC, you may have problems trying to get that final approval if you don't follow them. My municipality has a higher permit cost for "EV Chargers" but since I am installing generic outlets and not hardwiring the charger, this extra cost was avoided.
If you main panel is in or near your garage, it's not necessary to run a sub-panel for the purpose of EV charging unless your box is out of available circuits. My recommendation is to install a sub-panel fed from your main panel, on a 2-pole 100A breaker. Get a sub panel that is rated for at least 125A. You get more room for circuits and, while not strictly necessary here, it follows the 80% rule for continuous loads. The sub-panel itself does not need to have a main breaker, and I'd actually recommend against installing one. In that sub panel, install two 2-pole 50A breakers, and feed each of those circuits out to 14-50 outlet. On putting my sub-panel in, I additionally ran more circuits for my garage and some higher amperage circuits as well for special equipment (30A 240V welder specifically). If you have a generator transfer panel, you may have to be a bit choosey about how you go about this. I have not had to deal with it.
Running wires
The big pain in the butt here is that the NEC is hard to navigate when it comes to sizable power runs in residential situations. I'm guessing it's because it's not all that common and they just expect you to follow the general rules here regarding ampacities, temperatures, wire-fill, and protection. Romex (NM-B) is really only available up to an ampacity rating of 95A @ 60C. 60C is generally what you will need to use here as that is the limitation on Romex, and everything that is connected to the wire needs to be rated for those temperatures. 75C breakers and outlets can be found but I didn't find it worth the effort for the minor cost savings that may be there. I chose to run 1/0 AWG SER rated 3+1 Aluminum wire to my sub-panel. 1/0 Aluminum is the minimum size for 100A, and it was quite a bit cheaper than a 1 AWG copper equivalent would have been. The run was about 45 feet from the Main. If it was over 50ft I probably would have sized up one. If this wire is exposed in your garage, it may need to be protected from damage by running it in conduit since residential code doesn't make any specific accommodations for using SER this way.
In the Main panel, I ended up using #4-2/0 grounding lugs to land my ground wire and to land my neutral onto the neutral bar. The 100A breaker and sub-panel should accommodate a 1/0 wire no-problem. For wires of this size, if you are running it perpendicular to joists in your basement, you do NOT want to drill holes in the joists to run this wire through it. Code specifies it's fine to use cable straps secured to the bottom of the joists, and this wire is essentially impossible to bend enough to feed it through restrictive spaces anyway. If you are used to wiring things with additional length as a "service loop", don't bother to do that with these wires. They only need to be as long as necessary and you're going to have a hard time closing your panel with the extra length of wire, if you can even bend it in to fit.
Your sub panel should not be mounted directly to a concrete wall. Recent code requires that it be on plywood or similar. Something to insulate it from the cold walls of the basement since in high-humidity environments, it can encourage condensation and eventual rust/corrosion inside your panel. Your inspector may want to see a grounding rod run from your sub-panel. Personally I don't think it's necessary and my inspector didn't even mention it. I put the two 2-pole 50A breakers in my sub panel, as well as some other circuits that total another 100A. Since these others are occasional loads, it shouldn't be a problem.
Into the garage
Since my garage is already drywalled, I opted to put in surface mount 14-50 receptacles that I could feed directly into the back of through the wall. Your setup may require running wire somewhat differently and within protective conduit and dedicated outlet boxes. I had to do this for my last installation and it was kind of annoying. Following the 80% rule for continuous loads, you should never exceed 40A on these circuits, so purchasing an EVSE that is rated higher than that isn't necessary (it's also going to be expected to be hard-wired in, which is an approach I personally don't care for here). Romex is fine to run to these circuits, but it will have to have an ampacity rating of at least 50A @ 60C. I ran 6/3 copper romex for these outlets. I would definitely avoid using aluminum here as you may need to bend the wire a bit more for the outlet/receptacle boxes, so any 4/3 aluminum should be avoided unless you already have it available. If it is run through conduit, you may have to argue with your inspector about the wire-fill rules. Romex should be counted as a single wire as it cannot be run as individual strands, thus wire-fill calculations should go off of this. If you run two through the same conduit it's no longer a "single wire", however. If you cannot convince your inspector, you will have to tear it out and install larger conduit.
Mistakes I've made (or almost made)
- If you are installing metal outlet boxes and emt conduit, each individual box needs to have a ground tied to it that eventually leads back to the main. By code, the conduit can't be relied on to be a grounding path. You also cannot rely on the outlet itself to ground to the box, there has to be a ground wire tied down in the box.
- Don't run just enough power for one EV if you are putting in a sub-panel. My first installation was a 60A sub-panel that would be totally inadequate today. Originally my household only had a single EV, but now my wife also has a plugin hybrid. While some people make do by swapping cars after they charge, it can be completely avoided by planning for two in the first place.
- When you measure the length of SER cable you need, make sure you are checking that there isn't anything you have to route around. Add an extra 10% or so to your measurement. Throwing away 5 feet of expensive cable is better than having to run that stuff and finding out you're only a few inches short. My wire that I purchased 5 feet longer than I measured ended up only being about 2.5 feet too long because of having to run it a different way than I expected.
If you read the above, there's no new information to glean here, but it may serve as a reminder. My preference is to run a 100A sub panel, fed from the main on 1/0 Al 3+1 SER. From there, run two 14-50 outlets on 50A 2-pole breakers using 6/3 Cu Romex, even if you are only planning a single EV. If your Main panel is in or close to your garage, the Sub-panel is unnecessary (but you might want it for extra garage circuits if yours is currently lacking). Make sure you check with your municipality's building department if they have strange code requirements that don't follow NEC recommendations.
