EVSE Circuit Breaker Sizing Calculator
Calculate the minimum circuit breaker size required for Electric Vehicle Supply Equipment (EVSE) installations per NEC Article 625.42 and NEC 210.20(A). EVSE loads are treated as continuous loads (3+ hours), requiring the circuit breaker to be sized at 125% of the EVSE rated current.
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Use 100% for NEC 625.42 compliance. Reduced demand factors may apply per NEC 625.42(B) for managed charging systems.
Formulas Used
NEC 625.42 — EVSE as Continuous Load (125% Rule):
Minimum Circuit Ampacity = (IEVSE × N × Df) × 1.25
Where:
- IEVSE = Rated current of each EVSE unit (A)
- N = Number of EVSE units on the circuit
- Df = Demand factor (1.00 = 100% per NEC 625.42; reduced per NEC 625.42(B) for managed charging)
- 1.25 = Continuous load multiplier per NEC 210.20(A)
Power Calculations:
- Single-phase: P (kW) = V × I / 1000
- Three-phase: P (kW) = √3 × V × I / 1000
Standard Breaker Selection: Per NEC 240.6(A), select the next standard size at or above the minimum circuit ampacity: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400 A.
Assumptions & References
- NEC 625.42 — Branch circuit requirements for EVSE; EVSE loads are classified as continuous (operating 3+ hours).
- NEC 210.20(A) — Branch circuits supplying continuous loads must be sized at 125% of the continuous load current.
- NEC 240.6(A) — Standard ampere ratings for fuses and inverse time circuit breakers.
- NEC 310.16 — Conductor ampacity table; wire sizes based on copper THWN-2 conductors at 75°C in conduit.
- NEC 625.42(B) — Demand factors below 100% are only permitted with listed managed/smart charging systems; verify with AHJ.
- NEC 625.54 — GFCI protection is required for EVSE in certain locations (garages, outdoors, etc.).
- Wire sizing assumes copper conductors; aluminum conductors require upsizing (typically 1–2 AWG sizes larger).
- Ambient temperature corrections (NEC 310.15(B)) and conduit fill derating (NEC 310.15(C)) are not applied here — consult a licensed electrician for final design.
- This calculator is for estimation purposes only. All EVSE installations must be designed and inspected by a licensed electrical contractor per local codes.