EV Charger Wire and Breaker Size

A practical guide to choosing the correct cable and circuit breaker for a Level 2 home EV charger -- covering NEC continuous-load rules, AWG selection, voltage drop, and common charger sizes.

Why correct sizing matters

An EV charger is one of the highest continuous loads a domestic circuit will ever carry. Unlike a kettle that runs for two minutes, a Level 2 charger may draw full current for six to ten hours overnight, every night. The NEC classifies any load running for three hours or more as a continuous load, and that classification has direct consequences for both the wire and the breaker.

Undersized wiring overheats inside the wall -- insulation degrades silently before a breaker trips. An undersized breaker nuisance-trips and may eventually fail to trip at all once its contacts have been thermally stressed. Either failure can cause a fire. Getting the numbers right the first time is not merely good practice; in most jurisdictions it is a code requirement.

The continuous-load rule (NEC 210.20 and 625.42)

Under the NEC, a branch circuit feeding a continuous load must be rated at 125% of the load's full-load current. NEC Article 625 (Electric Vehicle Power Transfer Systems) adds that the branch circuit supplying an EV charger must be rated at no less than 125% of the EVSE's maximum ampere rating.

Worked example: 7.2 kW (32 A) charger on 240 V

• EVSE maximum draw: 32 A
• 125% continuous-load multiplier: 32 × 1.25 = 40 A minimum circuit rating
• Required breaker: 50 A (next standard size up from 40 A per NEC 240.6)
• Required wire: ampacity must cover 40 A continuous -- see the AWG table below

Use our breaker / fuse sizing calculator to check standard breaker sizes and confirm the 125% rule for any charger amperage.

Choosing the wire gauge

Wire must be selected to match the breaker rating, not just the charger draw. The cable must also be suitable for its installation method (in conduit, direct burial, free air) and ambient temperature. The table below summarises the most common EV charger configurations using 60 degC/75 degC rated THWN-2 or NM-B conductors in a conduit at 30 degC ambient.

For runs longer than about 30 metres (100 ft), or when the cable passes through a warm attic or conduit bundle, these values should be verified with a full calculation. Use our wire size calculator to find the correct AWG for your specific run length and installation method.

Checking voltage drop

The NEC recommends keeping voltage drop to 3% or less on branch circuits (informational note to 210.19). At 240 V that is a maximum of 7.2 V. For EV chargers this matters both for charging efficiency and because some EVSE units will de-rate or disconnect if the incoming voltage falls too far.

How to calculate it

For a single-phase 240 V circuit, voltage drop = (2 × conductor resistance per metre × run length × current). The factor of 2 accounts for the outgoing and return conductors. At 32 A over a 25-metre run on 8 AWG copper (resistance ≈ 2.1 mΩ/m): VD = 2 × 0.0021 × 25 × 32 = 3.36 V (1.4%, well within limit).

For longer garage-to-house runs or sub-panel feeds, use our voltage drop calculator to confirm you remain within the 3% recommendation.

Practical installation notes

Conduit vs direct NM cable

Metal conduit (EMT or rigid) protects the cable from physical damage and is mandatory in garages in many jurisdictions -- check your local authority having jurisdiction (AHJ). It also allows a future upgrade to a larger wire without opening walls. NM-B (Romex) is only permitted where the cable is protected from physical damage and local codes allow it for EV circuits.

GFCI and AFCI requirements

The 2023 NEC requires GFCI protection for EV outlets and hardwired EVSE in garages and outdoors (210.8(A)(2)). Many dedicated EVSE units incorporate internal GFCI, but confirm with the unit's listing. AFCI is generally not required on EV circuits above 150 V to ground.

Panel capacity check

Before adding a 50 A or 60 A breaker, verify your main panel has the physical space for a two-pole breaker and that total calculated load (NEC 220) does not exceed the service rating. A 100 A service may be borderline with an EV charger added; a 200 A service typically handles it comfortably. Your electrician will perform a load calculation to confirm.

Frequently Asked Questions