Free tool
Can my panel take an EV charger?
Enter your service, what the house already draws at its busiest moment, and the
charger you have in mind. You get the breaker it needs, whether the service can carry it, and the
largest charger that still fits.
Your highest demand is on your
utility bill if it lists demand, or your utility can supply twelve months of data. No
figure at all? An electrician can log the service for thirty days, which is what the code
asks for when the history is missing.
is the largest charger this service carries
Enter a system
The method. For a house that already exists, the code lets you
work from what the service really draws instead of adding up every appliance. The highest
demand over the past twelve months is taken at 125 percent, the charger is added at 125 percent
because it runs for hours, and the total has to stay inside the service rating. NEC 220.87 and
625.42.
Why the charger is multiplied by 1.25. Anything that runs for
three hours or more is a continuous load, and a car charges for a lot longer than that. A 40
amp charger therefore needs a 50 amp breaker and conductors to match.
If it does not fit. You have three routes before a service
upgrade: turn the charger down in its app, most units let you set a lower current; fit a load
management system that pauses charging when the house draws too much, which the code allows;
or upgrade the service, the expensive option.
What this does not do. It does not size conductors, check the panel
for a free double space, or account for a second charger. It answers one question: does the
service have room.
How long will it take to charge?
The same charger, now in hours. Enter the battery and how far you want to
top it up.
A charger delivers roughly ten
percent less than it draws, and that loss is already in the result.
to go from 20 to 80 percent
The calculation. Charger power is amps times volts. The energy
you need is the battery size times the part you are filling. Divide the one by the other, add
about ten percent for losses, and you have the hours.
Why the last twenty percent is slower. Above roughly eighty
percent the car itself tapers the current to protect the battery, so the final stretch takes
longer than this straight line suggests. That is also why most people set the limit at eighty.
Cold changes everything. In freezing weather part of the power goes
into warming the battery instead of filling it, and charging can take noticeably longer.