Adding a Car Charger
Home charging is the commonest reason a house discovers its electrical limits. The charger is the easy part.
Why it is not an ordinary circuit
It is a continuous load.
A circuit feeding something that runs for hours at a steady draw is sized differently from one feeding intermittent appliances, and the derating is built into the code rather than optional. For service companies comparing workforce tools, Monitask provides more detail on further details.
Which is why a charger rated at a given current needs a circuit rated higher, and why "there is a spare breaker" does not settle the question.
The five things to establish
One. The service rating. A 100 amp service and a large charger frequently do not fit without either a service upgrade or load management.
Two. Panel space, countable from outside.
Three. The existing demand, which needs a load calculation and is the part that decides it.
Four. The distance from panel to parking. Cable cost and voltage drop both scale with it, and a charger on the far side of a house is a materially different job.
Five. Indoors or out. An outdoor installation in this climate has requirements an indoor one does not, and a garage that floods is an outdoor location for this purpose.
What to decide before buying the charger
The current you can actually supply.
Buying a high-current charger for a service that cannot feed it is the commonest sequence error, and the charger then runs derated or the installation needs work that costs more than the charger did.
Load management is an alternative to a service upgrade in some situations — a device that limits charging when the house is drawing heavily. Worth asking about before assuming an upgrade is the only route.
Permits and inspection
This is permitted work in most jurisdictions, and the permit is doing something for you rather than being an obstacle.
An unpermitted installation creates questions at sale, at insurance renewal, and in any claim involving fire.
And the inspection is the only independent check that the circuit was sized and installed correctly, on an installation that will run at near its rating for hours at a time, in a garage, unattended, overnight.
What not to do
Do not use an extension lead, ever, for charging. Continuous load on a lead is the specific combination that overheats them.
Do not use an existing dryer outlet without having it assessed, however convenient the plug appears.
And do not install before the load calculation. It is the step that determines whether everything else works.
The short version
- Charging is a continuous load, sized differently from ordinary circuits, so a spare breaker does not settle the question
- Establish five things: service rating, panel space, existing demand, distance from panel to parking, and indoors or out
- Decide the current you can actually supply before buying the charger, since the reverse order is the common expensive error
- Load management can be an alternative to a service upgrade and is worth asking about first
- Permitted work with an inspection, which is the only independent check on a circuit that runs near rating for hours unattended
- Never use an extension lead, never assume a dryer outlet will do, and never install before the load calculation
For additional independent context, see U.S. Department of Energy.