Home Improvement → Electrical question

I'm working on a plan and I need to know if there are any code or other issues with my idea.

Currently, I own an electric car (Nissan Leaf) and have a home charging station installed on the front of my house, installed to code and with permits. The current charging station is rated for 30 amps and is connected to a 40 amp 240V breaker in a sub-panel. The wiring goes from the sub at the back of the house by conduit on the outside of my house (house is two story on slab so running it inside would have been messy) to a disconnect box and then by flex conduit around the corner to the station.

I am looking at buying a second EV (actually, a plug-in hybrid) and installing a second charging station.

One thing I should note is that both my current car and the planned purchase car actually cannot draw more than 16 amps at 240V.

My plan is to replace the disconnect box with a small surface-mount sub panel with two breakers, and run a second conduit to a new station on the other side of the garage door on the front of the house.

I will leave the 40A breaker in the original sub. In the new sub, I would put a 40A breaker for the existing station and a 20A breaker for a new 16A max station. Since my car will only draw 16 amps, this should not be a problem. The only problem would be if the existing station were used with a car that draws higher and the new station was used at the same time, in which case, the breaker in the old sub would blow.

If necessary, I could replace the existing station with a 16 amp max station and use two 20A breakers in the new sub.

So I have two specific questions:
(1) Is there any issue with a sub panel being fed from another sub panel?
(2) The current circuit, as well as both stations are pure 240V (L1, L2 and ground, no neutral wire). Is there any rule that requires a neutral wire in a sub if none of the loads have a neutral?

Any other issues?

I'm in California, USA, by the way.

Thanks

You need to verify what size wire is run between the breaker and the existing charging station. By code it should be sufficient to handle 40 A because that's the size of the breaker protecting it but you certainly need to verify that. Depending on the size of conduit run for the original installation it might be very easy to upsize the wiring so you could use a 60 amp breaker in the box and have 30 A available for each charging station to cover any future needs. You're going to be very close to the code limits with 20 A providing each charging station however, since under NEC code the continuous load can only be percent of the rating for the circuit and 16 A which you are quoting exactly 80% of 20A which each circuit will have available. Overall your solution seems like a good idea however.

As I said, everything was done to code and inspected and signed off by the local inspector so I'm confident that the wiring size is sufficient for 40A. It is standard to put 40A breakers on 30 amp charging stations and 20A breakers on 16 amp charging stations, due to the 80% rule.

I'm concerned about upgrading the entire run to 60 amps as I don't want to get involved in questions of total load. I'd rather leave the existing breakers as is.

Total load is not an issue. If you total the breaker load in most breaker boxes it exceeds the supply to that box by a significant amount. As long as the conduit is of sufficient size for the larger wires supplying it with a 60 amp breaker and then 2 30 amp ones with future proof the entire installation. I don't see any problem doing what you are proposing but depending on your future needs you may need to be redone when you buy your next electric automobile.

If you're going to be doing the work yourself it's one thing but if you going to hire electrician anyway you might just as well go all the way.

Thanks