EVSE Circuit Breaker and Ampacity Sizing Calculator
Calculate the minimum circuit breaker size and conductor ampacity required for EVSE (Electric Vehicle Charging Station) installations in accordance with NEC Article 625 and NEC 210.20.
Formulas Used
1. Total Continuous Load:
Itotal = IEVSE × Nunits
2. Minimum OCPD Size (NEC 210.20(A) & 625.42):
IOCPD ≥ 1.25 × Itotal → rounded up to next standard breaker per NEC 240.6(A)
3. Temperature Correction Factor (NEC Table 310.15(B)(1)):
CFtemp = √[(Trating − Tambient) / (Trating − 30)]
4. Bundling Derating Factor (NEC Table 310.15(C)(1)):
CFbundle = 1.00 (≤3), 0.80 (4–6), 0.70 (7–9), 0.50 (10–20), 0.45 (21–30), 0.40 (31–40), 0.35 (>40)
5. Required Base Conductor Ampacity:
Irequired = (1.25 × Itotal) / (CFtemp × CFbundle)
6. Charging Power:
P = I × V (single-phase) | P = I × V × √3 (three-phase)
Assumptions & References
- NEC Article 625 — Electric Vehicle Power Transfer System: EVSE is classified as a continuous load; branch circuit must be rated at 125% of the EVSE nameplate current.
- NEC 210.20(A) — Branch circuit overcurrent protection must be rated at least 125% of the continuous load.
- NEC 240.6(A) — Standard ampere ratings for fuses and inverse time circuit breakers.
- NEC Table 310.16 — Allowable ampacities for insulated copper conductors in conduit, based on 30°C ambient temperature.
- NEC Table 310.15(B)(1) — Ambient temperature correction factors applied when installation temperature differs from 30°C.
- NEC Table 310.15(C)(1) — Adjustment factors for more than three current-carrying conductors in a raceway or cable.
- NEC Table 250.122 — Minimum size equipment grounding conductors based on OCPD rating.
- All conductor ampacity values are for copper conductors. For aluminum, increase one AWG size.
- Voltage drop calculations are not included; for runs exceeding 100 ft, consult NEC Informational Note to 210.19(A)(1).
- Results assume a single EVSE branch circuit. Load management systems may allow reduced sizing per NEC 625.42 Exception.
- Always verify results with the local Authority Having Jurisdiction (AHJ) and applicable state/local amendments.