Breaker sizing (the 125% rule)
EV charging is a continuous load, so the US electrical code requires the circuit breaker to be rated at least 125% of the charger’s output current: a 40 A charger needs a 50 A breaker, a 48 A charger a 60 A breaker.
Common pairings: 16 A → 20 A breaker, 32 A → 40 A, 40 A → 50 A, 48 A → 60 A, 80 A → 100 A. Most chargers with adjustable amperage can be set lower to fit an existing circuit.
Panel capacity is the usual constraint for 48 A+ installs — an electrician’s load calculation, or a charger with dynamic load management, solves it.
| The rule | Branch circuit rated ≥ 125% of the continuous charging current |
|---|---|
| Continuous load | Current expected to last 3 hours or more (NEC Article 100) |
| Code references | NEC 210.19(A) and 210.20(A); Article 625 for EV equipment |
| Charger → breaker | 16 A → 20 A · 24 A → 30 A · 32 A → 40 A · 40 A → 50 A · 48 A → 60 A · 80 A → 100 A |
| Breaker → maximum charger | 30 A → 24 A · 40 A → 32 A · 50 A → 40 A · 60 A → 48 A · 100 A → 80 A |
| Copper conductors (THHN, 75 °C) | 12 AWG for 20 A · 10 AWG for 30 A · 8 AWG for 40–50 A · 6 AWG for 60 A · 3 AWG for 100 A |
| Service load calculation | NEC 220.57 counts EVSE at ≥ 7,200 VA, at 100%, with no demand factor |
Why 125%, and where the number comes from
NEC Article 100 defines a continuous load as one whose maximum current is expected to last three hours or more. A full charge is five to ten hours at the charger’s maximum current, every night — which makes an EV charger the most continuous load in a typical house, more so than an oven, a dryer or an air conditioner. Sections 210.19(A) and 210.20(A) then require both the conductors and the overcurrent device to be sized at 125% of that load.
The margin is thermal, and it is not really about the wire. A moulded-case breaker is calibrated in free air at 40 °C; inside a full panel with neighbouring breakers running warm, its trip curve drifts and its own terminations heat. Repeated heating and cooling cycles loosen lugs. The 25% headroom keeps the breaker, the lug torque and the terminations in a regime where none of that accumulates.
Read the rule in the other direction and it becomes a shopping rule: on any existing circuit, the largest charger you may run is 80% of the breaker. That is why a 50 A circuit means a 40 A charger, and why a NEMA 14-50 outlet caps a plug-in installation at 9.6 kW.
Both directions, and what to do when the panel is tight
Forward, for a new install: pick the charger from your car’s onboard limit, then multiply by 1.25 and round up to the next standard breaker. A 32 A charger needs 40 A. A 40 A charger needs 50 A. A 48 A charger needs 60 A. An 80 A charger needs 100 A. Breakers do not come in every size, and rounding up to the next standard rating is what the code expects.
Backwards, for an existing circuit: take 80% of the breaker. A 30 A circuit — a typical dryer circuit — supports 24 A, or 5.8 kW. A 40 A circuit supports 32 A. A 50 A circuit supports 40 A. A 60 A circuit supports 48 A. This is how you decide whether a spare circuit is useful before calling anyone.
Nearly every modern charger has an adjustable current setting, either a DIP switch inside the enclosure or a limit in the app, and using it is completely legitimate. Buying a 48 A unit and setting it to 40 A so it lives on an existing 50 A circuit costs 1.9 kW and keeps the option of rewiring later. Two cautions: size the breaker to the setting you will actually use and have the installer document it, and never uprate an existing breaker to fit a bigger charger without confirming the conductors can carry it — the breaker protects the wire, not the other way round.
Wire, voltage drop, and the calculation that actually blocks people
Conductor sizes above assume copper THHN or THWN in conduit with 75 °C terminations. NM-B cable — Romex — is restricted to its 60 °C ampacity column, where 8 AWG only reaches 40 A, so a 50 A circuit in NM-B needs 6 AWG and a 60 A circuit needs 4 AWG. On long runs, voltage drop takes over from ampacity: keep it under about 3%, which on a 48 A circuit starts forcing an upsize somewhere past roughly 80–100 ft. Aluminium conductors are cheaper on long runs and need their own sizing and antioxidant treatment.
The step that most often stops a project is not the branch circuit but the service. NEC 220.57 requires EV supply equipment to be included in the dwelling load calculation at 7,200 VA or the equipment nameplate, whichever is larger, at 100% with no demand factor applied. A 48 A charger enters that calculation at 11,520 VA. On a 100 A service already carrying an electric range, dryer and central air, that is frequently the item that fails.
There are three legitimate outcomes, in increasing cost. Dial the charger down until the calculation passes — a 32 A charger enters at 7,680 VA. Use a charger with a listed automatic load-management system: NEC 625.42 allows the maximum load permitted by that system to be the number used in service and feeder calculations, which can shrink an 11,520 VA entry dramatically. Or upgrade the service. Only a load calculation done by a licensed electrician decides which; a rule of thumb is not a substitute, and an inspector will ask for the calculation.
Frequently asked questions
What size breaker do I need for a 48 amp EV charger?
A 60 A two-pole breaker, on 6 AWG copper THHN with 75 °C terminations (4 AWG if the run is in NM-B cable). The charger must be hardwired — there is no 60 A residential receptacle in common use. 48 A at 240 V is 11.52 kW, which saturates the onboard charger of nearly every current EV.
What size breaker for a 40 amp EV charger?
A 50 A two-pole breaker on 8 AWG copper (6 AWG in NM-B). This is the standard NEMA 14-50 circuit, and for EV use the breaker must be GFCI type under NEC 625.54 if it feeds a receptacle. 40 A at 240 V is 9.6 kW, the ceiling for any plug-in installation.
What size breaker for a 32 amp charger?
A 40 A two-pole breaker on 8 AWG copper. 32 A at 240 V is 7.68 kW, which fully saturates a Nissan Leaf, Nissan Ariya, Toyota bZ4X or Subaru Solterra — for those cars a 40 A circuit is a genuinely cheaper install with no speed penalty at all.
Can I put an EV charger on a 30 amp breaker?
Yes, at 24 A maximum — 80% of 30 A, which is 5.76 kW at 240 V. That is roughly 15–20 miles of range per hour, enough for most daily driving. Many chargers and portable cord sets have a 24 A setting for exactly this reason, and it is a common way to use an existing dryer circuit.
Can I use my dryer outlet for EV charging?
A 14-30 or 10-30 dryer circuit is 30 A, so the charger must be limited to 24 A (5.8 kW) with the manufacturer’s listed adapter. It must be a dedicated use — do not use a splitter that lets the dryer and the charger share the circuit, because EV charging is exactly the continuous load those devices handle worst. Older 10-30 receptacles have no separate ground, which is a further conversation for your electrician.
Do I need a 100 amp breaker for 80 amp charging?
Yes — 80 A × 1.25 = 100 A, hardwired, on 3 AWG copper. That is a large, expensive circuit that most 100–150 A residential services cannot accommodate without an upgrade. Only three current EVs can accept 19.2 kW on AC, so confirm your car actually benefits before committing to it.