The short answer
In other words, the rating of the charger is not the first thing you choose. It follows from what your installation can already carry.
Why 40 amps and not 32
An EV charger is what the Code calls a continuous load: it draws current for hours without interruption. The Code requires sizing the breaker and the wire at 125% of that continuous load.
A charger set to 32 amps therefore needs a 40 amp circuit. A 40 amp charger needs a 50 amp circuit. A 48 amp charger needs a 60 amp circuit.
That is why a charger sold as a “40 A” model does not take up 40 amps in your panel, but 50. The difference counts when there is not much room left.
What your electrical service can already take
Many homes in Montreal’s Southwest still have a 100 amp service. That was plenty before the house filled up with electrical appliances.
What counts is not the free space in the panel, but the load already connected to it. You can have six empty slots and no capacity available.
- An electric range: often 40 amps
- A dryer: 30 amps
- A water heater: 30 amps
- A heat pump or a central air conditioner: variable, often 30 to 40
- Electric heating, if there is any: the biggest load in the house
The calculation an electrician runs
The Code sets out a load calculation method for a single family dwelling. It adds up the living area, the heating, the fixed appliances and the special loads, with demand factors.
It is that calculation, and not an estimate by eye, that says whether a charger fits in your current service.
We record what is already there
Rating of the service, rating of the main breaker, list of the circuits and the fixed appliances.
We apply the demand factors
Appliances never all run at the same time, and the Code accounts for that. It is what keeps a service from being oversized for nothing.
We add the charger as a continuous load
At 125% of its rated current, like any load that runs more than three hours at a stretch.
We compare with the rating of the service
If the total goes over, you either upgrade the service or manage the load. Both options are valid, and they do not cost the same.
When you have to move up to 200 amps
On a 100 amp service that is already well loaded, adding a charger often tips the calculation. Upgrading to 200 amps then becomes the simplest solution, and often the one that lasts.
That is not bad news in itself. A 200 amp service leaves room for what comes next: a second charger, a hot tub, radiant floor heating, a heat pump.
We say it before the work, never halfway through. That is exactly why we look at the panel first.
Load management, the other option
There are load management devices that watch the consumption of the house and reduce or interrupt charging when the rest of the house draws heavily.
The Code recognizes them, and in some cases they allow a charger to be added without touching the service. The trade-off: charging can slow down during peak hours.
For someone who plugs in at night and leaves in the morning, it is almost never noticeable. For a household with two electric vehicles, that is another conversation.
What to send us to get an answer
Two things are enough to size up the project remotely, before anyone even comes out.
- The model of your vehicle, or of the charger if it is already chosen
- A photo of the electrical panel with the door open, with the circuit labels readable
Master electrician, RBQ licence 5855-0849-01 and CMEQ member 17468. About ten years in the trade across Montreal’s Southwest and the South Shore.
Who does the work