EV Charger Circuit Sizing
Enter the EVSE amps or kW, supply and run length to get the breaker, conductor size and voltage drop.
Step-by-step
- Enter valid values to begin.
EVSE is treated as a continuous load: minimum circuit ampacity = 125% of the EVSE rated continuous current (NEC 625.41, 210.19(A)(1), 210.20(A)) and the breaker is the next standard size at or above it. Conductor ampacities are the NEC 310.16 copper and aluminium columns for the chosen terminal rating, and resistances are typical Chapter 9 Table 8 values for uncoated stranded conductors; both are typed from memory, so verify against the adopted code. In kW mode amps = kW × 1000 / V (× √3 for three-phase). Ambient and bundling derating, conduit fill and short-circuit rating are not modelled. Range uses a typical 4 mi/kWh. Check local amendments with your AHJ.
EV Charger Circuit Sizing: Continuous Load, Breaker and Wire
Core Engineering Principles
An EV charger is a continuous load, because it can run at full current for three hours or more. The branch circuit must carry 125% of the EVSE rated continuous current, and the breaker is the next standard size at or above that number. A 40 A charger therefore needs a circuit good for 50 A, and we put a 50 A breaker on it. The 125% is not padding: a breaker run flat out for hours in a warm panel will trip or cook its own lugs. Size the conductor from the same 125% figure, in the terminal-temperature column of NEC 310.16 that the equipment is listed for.
Know which number is which. The EVSE rating is what the charger may deliver, while the vehicle’s on-board charger sets what the car actually takes on AC. Level 1 is 120 V, Level 2 is 208 or 240 V, and DC fast chargers sit far beyond this page. Voltage drop is the second check: on a long run, a cable that passes ampacity can still lose more than 3%, so we step up a size. Aluminium has about 1.6 times the resistance of copper.
P = V × I (1-phase) or √3 × V × I (3-phase)
Vdrop = 2 × L × R × I / 1000 (1-phase), √3 × L × R × I / 1000 (3-phase), L in ft, R in Ω per 1000 ft
NEC & Standard References
NEC Article 625 covers EV power transfer systems: 625.41 sets the 125% rating, 210.19(A)(1) and 210.20(A) apply the continuous-load rule to conductors and breakers, and 625.42 permits listed automatic load management. SAE J1772 defines the North American AC connector and control pilot, and IEC 61851 sets charging-system requirements worldwide. Table values here are typical; verify against NEC 310.16 and Chapter 9 Table 8 and check the adopted edition.1. Minimum circuit ampacity = 1.25 × 40 = 50 A.
2. Breaker = next standard size at or above 50 A = 50 A.
3. 75 °C copper: 10 AWG carries 35 A (too small), 8 AWG carries 50 A, so use 8 AWG Cu.
4. Resistance of 8 AWG is typically 0.778 Ω per 1000 ft, so the drop is 2 × 82.0 × 0.778 / 1000 × 40 = 5.10 V, which is 2.13% of 240 V and under 3%.
5. Power = 9.6 kW, about 38 miles per hour at 4 mi/kWh.
- Breaker at 100%, not 125%. A 40 A breaker on a 40 A charger nuisance-trips on a long summer charge.
- Plug-in units follow the 80% rule. A 40 A EVSE on a NEMA 14-50 receptacle suits a 50 A circuit.
- Check aluminium terminations. Use listed lugs, antioxidant compound and label torque.
- Long runs lose power. Past roughly 30 m, voltage drop, not ampacity, picks the conductor.
- Commercial is often 208 V, not 240 V. The same 40 A then delivers 8.3 kW, not 9.6 kW.