How to size an EV charger circuit (breaker, wire, and the 125% rule)
Updated · Reviewed against the current ChargerAtlas dataset
The rule: an EV charger is a continuous load, so the circuit must be rated at least 125% of the charging current. Flip it around for shopping: on any existing circuit, the biggest charger you may run is 80% of the breaker — 40 A on a 50 A breaker, 48 A on a 60 A breaker.
Why 125%?
Breakers and wire heat up under sustained current. The National Electrical Code defines any load lasting 3+ hours as continuous and requires the circuit to carry it at no more than 80% of its rating — and a full EV charge is hours of maximum draw, the most continuous load in a house. Every certified charger enforces its own maximum, but the circuit behind it is your electrician's responsibility — and yours to budget for.
Sizing table
| Charging current | Power at 240 V | Minimum breaker | Typical copper wire* |
|---|---|---|---|
| 16 A | 3.8 kW | 20 A | 12 AWG |
| 24 A | 5.8 kW | 30 A | 10 AWG |
| 32 A | 7.7 kW | 40 A | 8 AWG |
| 40 A | 9.6 kW | 50 A | 8 AWG |
| 48 A | 11.5 kW | 60 A | 6 AWG |
| 80 A | 19.2 kW | 100 A | 3 AWG |
*THHN/THWN copper in conduit with 75°C terminations. NM-B ("Romex") cable is limited to its 60°C rating and often needs one size larger — a 60 A circuit in NM-B typically means 4 AWG. Long runs may also need upsizing for voltage drop. Wire sizing is exactly the kind of detail a licensed electrician settles; this table is for budgeting, not installation.
Will your panel take it?
The breaker is the cheap part; the question is whether your service can carry the new load on top of everything else. Rough heuristics before you call an electrician:
- 200 A service: a 50–60 A EV circuit almost always fits unless you already run electric heat plus a pool or hot tub.
- 100 A service: often tight. A formal NEC load calculation decides; common outcomes are stepping down to a 32 A or 40 A charger, or using a charger with adjustable current limits or load management, which throttles charging when the rest of the house draws heavily — many networked chargers support this. If a water-heater replacement is also on the electrification plan, compare the very different panel demands of 120 V and 240 V heat-pump water heaters before allocating the remaining capacity.
- Anything involving 80 A charging: a 100 A dedicated circuit is a service-upgrade conversation for most homes — see whether 80 A is even worth it first.
Right-sizing: start from the car, not the panel
There's no prize for the biggest circuit your panel survives. Your car's onboard AC limit caps real charging speed — a Nissan Ariya (7.2 kW) gets nothing from a circuit beyond 40 A. Look your vehicle up in the onboard-limit guide, size the charger to saturate it, then size the circuit to the charger with the table above. One sensible exception: if the wire run is short and cheap, wiring for 60 A while installing a 40 A charger costs little and leaves headroom for the next car.
Related: 40 A vs 48 A · hardwired vs NEMA 14-50 · breaker sizing in the glossary