EV Charger: Continuous Load, Terminal Rating, and Service Capacity
A homeowner wants a 7.2 kW, 230 V single-phase EV charger installed in a garage 25 m of cable run from a 100 A, 230/115 V single-phase panel. The unit is fixed, hard-wired, and draws its full rating for hours at a time. Existing calculated demand on the panel is 48 A. Charger and breaker terminals are rated 75 °C. Garage ambient is 38 °C, and the charger circuit will occupy its own PVC conduit. The contractor proposes 6.0 mm² THHN copper protected by a 40 A breaker, arguing that the charger only draws about 32 A so the conductor is 'way oversized.'
1. Determine the minimum circuit ampacity and the correct overcurrent device rating.
Read the worked answer
Load current: 7 200 W ÷ 230 V = 31.3 A.
EV supply equipment is a continuous load, so the branch-circuit conductors and the overcurrent device must be sized at not less than 125 % of the maximum load current:
- 31.3 A × 1.25 = 39.1 A minimum ampacity
- Next standard OCPD at or above 39.1 A = 40 A
So the 40 A breaker is correct - but the breaker is right for a reason unrelated to the contractor's reasoning. The mistake in the proposal is assuming the conductor only has to carry 32 A. It has to carry 32 A continuously, which is a 39.1 A sizing problem.
2. Evaluate the proposed 6.0 mm² conductor against the 75 °C terminal rating and the 38 °C ambient.
Read the worked answer
Two limits apply and you must take the lower:
| Check | Value |
|---|---|
| 6.0 mm², 90 °C column (starting point for derating) | ≈ 40 A |
| Ambient correction at 38 °C (36–40 °C row) | × 0.91 → ≈ 36.4 A |
| 6.0 mm² limited by 75 °C terminals | ≈ 35 A |
| Governing ampacity | ≈ 35 A |
35 A < 39.1 A required. The 6.0 mm² is not acceptable, and the 40 A breaker would be protecting a conductor with a 35 A ampacity.
Move to 8.0 mm²: 90 °C ampacity ≈ 55 A, × 0.91 = 50 A; 75 °C terminal limit ≈ 50 A; governing ≈ 50 A ≥ 39.1 A. Accepted.
Voltage drop over 25 m at 31.3 A with 8.0 mm² (≈2.7 Ω/km): 2 × 31.3 × 0.025 × 2.7 ≈ 4.2 V (1.8 %) - comfortably inside a 3 % branch-circuit target, so length is not the driver here; ampacity is.
3. Can the existing 100 A service carry the new load? State your judgement and what you would require before signing off.
Read the worked answer
Arithmetic first: 48 A existing demand + 31.3 A charger = 79.3 A on a 100 A service. Applying the continuous factor to the charger: 48 + 39.1 = 87.1 A.
Judgement: it fits, but only just - about 13 A of headroom, and a service has to absorb future load, panel diversity, and any error in the existing 48 A figure (which is itself an unverified number supplied by the homeowner). I would not accept the job on that basis.
Before signing off I would require:
- A written demand calculation for the dwelling, not a nameplate wattage count - verify the 48 A figure.
- Confirmation of the main breaker and busbar rating, meter socket, and service-entrance conductor ampacity, not just the panel label.
- Consideration of load management (a charger that throttles when household demand is high) so the charger can be sized to the actual spare capacity rather than forcing a service upgrade.
- If the formal calculation lands above the service rating, the options are a service upgrade or an energy-management device - not a smaller breaker on the same charger, which only causes nuisance trips.