I have a standby generator with an automatic transfer switch. It is set up to power all the circuits in my house. It does not have high enough wattage to power an EV charger in addition to the current load. If I installed a level 2 EV charger is there a way to insure it will not draw power while the standby generator is running?
You could split your electric service into two panels: a main and a sub.
The main panel would hold all the devices you don't wish to run on generator:
- EV charger
- Electric water heater
- Electric dryer
- Electric stove
- Outdoor A/C unit
The subpanel would be everything else:
- Regular outlets
- Gas furnace
- Well pump
- Sump pump
Now put the automatic transfer switch in the subpanel.
Large enough to carry the load to be served
This is the requirement on generator size for ANY installation with an automatic transfer switch.
"The Load to be Served" is decided by a NEC Article 220 Load Calculation on the loads served.
You are specifically not allowed to go "ah, well, my loads rarely exceed 10kW so I'll just use a 10kW generator and not worry about an unlikely case. I can just turn off circuits if it's a problem". That is not allowed. (not least, you might not be there!) That would be allowed with a manual transfer switch, since you are there.
However, Load Shed loads do not count
There are technologies which attach to a load and listen for the generator starting to bog down, and will drop out the load when that happens. These are called "Load Shed" - they work the same as a power company doing rolling blackouts to relieve stress on generating capacity. Load Sheds are appropriate for - well, any potential load, but commonly water heaters, dryers, A/C units and certainly electric vehicle supply equipment.
So you can get your house's loads (inclusive of EVSE) within spec simply by attaching Load Shed devices to them.
Otherwise, consider a Critical Loads Subpanel
You have your electric meter, main breaker, and main panel. Then, in the main panel, you have a breaker that feeds a subpanel.
This subpanel has a transfer switch on it. All the loads you want to be accessible to generator are placed in the critical-loads subpanel.
There is no limit to the size of the critical loads subpanel. You can use a 40-space subpanel and put most of your loads in there if you want to. However, the more loads you install (less any on Load Shed), the larger your generator must be in order to use an ATS.
If you don't want to move almost every circuit from your main panel to a critical loads sub, you could add a new main panel elsewhere and convert this panel to the critical loads subpanel.
Note that EV charge rate is variable
EV novices tend to be swayed into the belief that all level 2 EV charging is 50A. This is because every car comes with a free TRAVEL charger" emphasis travel! that is to be kept in the trunk and used for opportunity charging on the road. Naturally they provide the two sockets most often found on the road - the normal one and the RV socket NEMA 14-50. People get their cars home and take one look at that socket, and go "OH! Level 2 must be 50A!" No. Not at all.
Actually level 2 charging can happen at any speed from 15A (70-90 miles in 10 hours) to 100A (500-600 miles in 10 hours, WTH, or more realistic, 200 miles in an MRAP). The car can work with any of these; thanks to the miracle of 1980s microcontroller tech, the EVSE (charge unit) tells the car the safe amp rate. On the travel units, this is a microchip inside the plug itself, which also reduces amp rate in case of a hot plug. Note that you're not married to just the two speeds; 240V/15A, 240V/20A, and 240V/30A plug dongles are available.
Or EVSEs can be hardwired. This removes the NEC 2020 requirement for a $150 GFCI breaker.
Certain hardwired EVSEs can adjust charging amps dynamically and this can act as a Load Shed, keeping system power draw within limits by turning down the EV charge rate.
To talk more about that last option, the EVSE places a Current Transformer inside your main panel that clamps the utility supply wire(s). You then tell it "don't let service draw exceed 80A inclusive of your load". So if the water heater kicks on and suddenly it's 90A, the EVSE will cut the car from 24 to 14 amps. Now, if you want to do that on generator also, and that needs to happen at a lower ampacity from utility, you can cheat a bit. CT's operate on ampere-turns. Most wires go once through the CT and counts as accurate amps. But if you loop a wire through a CT twice, it will "see" twice as much current. So if you have 100A service and 25A generator, route the generator wire through the CT 4 times (3 loops) so the EVSE will read the generator current as 4x what it actually is.
Some great suggestions here to get the load off the generator by using a subpanel or even a contactor.
Another option: replace the EVSE breaker with a shunt trip circuit breaker. Connect the shunt coil to a 120v output from the generator. When the generator starts up and starts outputting voltage, it will trip the EVSE breaker and stop the charging. If they are available for your panel it could potentially save a bunch of rewiring work. One downside, if you go on generator and it trips charging won't start again until you reset the breaker. This isn't a big deal unless you both regularly have power outages and also drive many miles on a daily basis.
Does your charger have a remote shutdown or remote enable input? If so, that could be as simple as wiring a relay to an outlet wired from the generator side on the transfer switch.
Similarly, you could get a suitably sized normally closed contactor to turn power to the charger off. If you can get a contactor with 120/240 v coil inputs, that would be easiest from a wiring perspective. Just connect the generator output to the coil inputs. Contactors that take 24 v AC on the coil input are more common. In that case you would get a door bell transformer and connect that to the generator outputs, and the 24 v output of the transformer to the coil input of the contactor.
If you go with the contactor, make sure you get a normally closed contactor. That's less common, so make double sure you're getting the right one.