Level 2 Charge Stops After 40 Minutes
A 2022 EV with a 77 kWh pack and 11 kW onboard charger intermittently stops charging at home after about 40 minutes. The customer's EVSE is on a 40 A breaker with 6 AWG conductors and a 25 ft run; the EVSE pilot indicates 32 A available. At idle, L1-to-L2 is 239 V; during charging, L1-to-L2 drops to 221 V at 30 A, and the breaker is 52 C with warm conduit. The vehicle logs an AC inlet over-temperature DTC. A second EVSE at the dealer charges the same car at 30 A for two hours with no fault.
1. Compute the voltage drop and percent drop at 30 A, compare with common branch-circuit guidance, and decide whether the vehicle or the premises wiring is the more likely cause.
Read the worked answer
Drop: 239 V - 221 V = 18 V. Percent: 18/239 = 7.5%.
Common guidance is no more than 3% on a branch circuit and 5% total. At 30 A, 18 V implies about 0.6 ohm of loop resistance. A 25 ft run of 6 AWG copper should be roughly 0.02-0.03 ohm loop, producing well under 1 V. The measured drop is many times higher than the conductor alone can explain.
Because the dealer EVSE charges the same car at 30 A without fault, the vehicle is less likely to be the root cause. The premises wiring, breaker terminations, receptacle/EVSE terminals, or a damaged conductor is the more likely cause. The warm conduit and 52 C breaker support a connection-resistance problem. Stop using the circuit, inspect and torque terminations, check the breaker and wire, and thermal-scan under load before returning it to service.
2. The EVSE pilot says 32 A. The car draws 30 A. The customer wants to set the car to 40 A to charge faster. Explain why that is not possible and what the 125% continuous-load rule means here.
Read the worked answer
The pilot duty cycle is the EVSE's advertised maximum; the vehicle must not draw more than 32 A. Setting the car to 40 A would exceed the EVSE limit and the branch-circuit design.
The 125% rule treats EV charging as a continuous load: the circuit and breaker must be sized at 125% of the continuous current. A 40 A circuit therefore supports 40/1.25 = 32 A continuous. That is why the EVSE advertises 32 A on a 40 A breaker. The car's 30 A draw is already within the limit; the problem is the 18 V drop and heating, not the current setting. Raising the setting would increase the drop and the heat.
3. After the wiring is repaired, the owner asks how long 20-90% will take at 11 kW. Estimate with assumptions and state what would make the actual time longer.
Read the worked answer
Usable energy = 77 kWh x (0.90 - 0.20) = 53.9 kWh. If the onboard charger delivers about 90% of the 11 kW to the pack, the rate is about 9.9 kW. Time = 53.9/9.9 = about 5.4 hours.
Actual time will be longer because charging tapers near 90%, the pack may thermally derate, and the last portion includes balancing. If the onboard charger derates to 7.2 kW, the estimate becomes 53.9/(7.2 x 0.9) = about 8.3 hours. Treat the 5.4-hour figure as a best-case estimate, not a promise.