So You Are Thinking About an Electric Vehicle (EV), Solid Advice on the Charging Setup

OK here is some free consulting.   LOL you get what you pay for.

In selecting a vehicle and charging system, one formula is very basic and very important.   But don't turn your brain off even if you don't want to do calcs, you don't have to, just follow along.

 W=V*A

Voltage will always be 240V on a 2 Phase (which uses a 2 pole breaker), in your area that might be 230V no big difference in the calcs.

 

The Average peak charging rate shown in the spreadsheet is 6.6 kW, plug that in and solve for Amps

6600 W=240 *A

AMP= 27.5

The other main max charging rates are 10kW, and for the Tesla an option for 20kW

10,000 =240 * A    solves A for 41.67A

20,000 = 240 * A   solves A for  83.33A

Now a very important observation from over a decade of solar PV work, and the NEC (National Electric Code) has been slowly adopting more conservative codes to reflect  that fact that solar circuit often operate near peak load, and for a long time.   This means the wires heat up over time, and over time the insulation takes a beating, as well as the connections whether they are wire nutted or bolted.    

The main effect of Amperage is in selecting wire size, at least in conventional systems.    But PV electric systems, and car charging system are similar in one important way, and so the knowledge gleaned from PV systems is directly applicable to EV Charging systems, and that similarity is that they both run near peak amperage for long periods of time, many hours.   This creates heat, and also losses, and the higher the heat, the higher the losses, so it is like a cat chasing it's tail. 

One other aspect of PV, because of the heat issue, is that the most recent codes require that any wire that is in a run of conduit, that is situated where it can be directly lit by the sun must be further “derated” and this simply means you must jump up one wire size from the other calculations which also have some conservatism built into them.

I will introduce quickly one other concept that is affected by wire and breaker sizing – the amount of electricity lost while running through the wire (it’s lost as heat).    The smaller the wire the bigger the loss.   In most modern electrical systems, it is almost always a no-brainer to upsize the wire as the additional cost will be more than paid for by lower losses. 

The minimal “derate” on solar PV is done by multiplying the expected Amps time 125%, and sometimes this is also multiplied by another 125%.    125% *125% = 156%

If you take the 27.5A Charging amp above and use the 156% multiplier you get 42.9A, which would use a 50A 2 Pole breaker.  

So yes, a 50A 240V (2 Pole) circuit would be pretty comfortable for all the currently available electric cars.

How about the Volkswagen at 7.2kW charging rate?

7200 W = 240 *A = 30 A

30A *156% = 46.8A

You work out the numbers for the BMW at 7.4kW --- try it, there is no rocket science here.   You will leave your EV or PV “Salesman” in the dust though, with just this amount of knowledge.

The Tesla at 20kW

20,000 = 240 * A   solves for 83.33 A times 156% would be 130A in this conservative calc, and even using a 125% multiplier would be 104A. 

It should be clear if you are following that a 50A 2 Pole service (or breaker if you will) would be definitely pushing the limit to charge the Tesla at the 10kW rate, and may even violate local codes.   The manufacturers of the car and charger may also have their own requirements, which you can never go under, but you can definitely, and probably should, go over in picking breaker and wire size.

Electricians will probably want to “bid” the smallest breaker and wire size, not for your long term benefit, but because they want a better chance of winning bid, and if they upsize your wire and breaker to get you the safer system and the one with less power losses over time, they will lose the bid.     So be aware, they will convincingly try to assure you the smaller wire and breaker is “fine” and meets code. 

And back to the concept of electrical losses, the bigger wire and breaker will provide less electricity losses, especially on system that run often and for many hours.     So I wouldn’t necessarily rip out a 40A charging circuit, and replace it with a 50A because that project cost would probably not be justified.    But if designing fresh, I would almost always choose the 50A as the upgraded cost is minimal.

One final thought.    You might be designing a charging system for a Chevy Volt at 3.3kW.   That only gives you 11 Miles for each hour of charging.   That might be really inconvenient at times, you may want a quicker charge rate.      You might “put up with that” on your first EV, but I strongly feel that as time goes on, people are going to insist on faster charging rates, and future vehicles and chargers will require a fast, strong shot of electricity --- big Amps.    So doing it right, once, and on the first system, makes a lot of sense.    Doing it over can cost A LOT more, and with less electricity losses, doing the larger system gives you benefits right out of the gate, even if your second EV is many years down the road.  

Stock out

Model	Max Charge	~Miles Added Per Hour	100% Electric or PHEV
Audi A3 e-tron	3.3 kW	11	PHEV
BMW i3	7.4 kW	25	100% Electric / REx
Cadillac ELR	3.3 kW	11	PHEV
Chevy Spark EV	3.3 kW	11	100% Electric
Chevy Volt	3.3 kW	11	PHEV
Fiat 500e	6.6 kW	22	100% Electric
Ford C-Max Energi	3.3 kW	11	PHEV
Ford Fusion Energi	3.3 kW	11	PHEV
Ford Focus Electric	6.6 kW	22	100% Electric
Honda Accord Plug-In Hybrid	6.6 kW	22	PHEV
Hyundai Sonata Plug-in Hybrid	3.3 kW	11	PHEV
Kia Soul EV	6.6 kW	22	100% Electric
Mercedes B-Class Electric	10 kW	29	100% Electric
Mercedes S550 Plug-in Hybrid	3.3 kW	11	PHEV
Mercedes C350 Plug-in Hybrid	3.3 kW	11	PHEV
Mitsubishi i-MiEV	3.3 kW	11	100% Electric
Nissan LEAF	3.3 kW / 6.6 kW	11 / 22	100% Electric
Porsche Cayenne S E-Hybrid	3.6 kW / 7.2 kW	12 / 24	PHEV
Porsche Panamera S E-Hybrid	3 kW	10	PHEV
Smart Electric Drive	3.3 kW	11	100% Electric
Tesla Model S	10 kW / 20 kW	29 / 58	100% Electric
Tesla Model X	10 kW / 20 kW	29 / 58	100% Electric
Toyota Prius Plug-In	3.3 kW	11	PHEV
Volkswagen e-Golf	3.6 kW / 7.2 kW	12 / 24	100% Electric