AC Charge Tapers From 32 A to 16 A at the New House
A customer moved into a house with an existing 240 V, 40 A branch circuit feeding a 32 A J1772 EVSE on a 14-50 receptacle. Her EV charged at 32 A at her old condo. At the new house it ramps to 32 A and falls back to 16 A after about ten minutes. Panel voltage is 240 V; at the EVSE line terminals under load it is 232 V. After 40 minutes the J1772 connector body measures 61 °C on an IR camera and two pins look discolored; the vehicle inlet is 45 °C. The vehicle reports no DTCs. The EVSE is a plug-in unit on the receptacle.
1. Where is the derate coming from? Separate the vehicle, the EVSE, and the supply.
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The EVSE connector is the prime suspect, and the vehicle is behaving correctly.
- EVSE: 61 °C at the connector body with discolored pins is localized resistance heating at the contact interface. Discoloration means it has already been hot enough to oxidize, which raises resistance further - a runaway. This is a replace-the-connector/charge-cable-assembly finding, not a clean-and-retighten finding. The EVSE's own thermal sensor or the vehicle's inlet sensor then commands the taper to 16 A.
- Vehicle: 45 °C at the inlet is a normal consequence of the heat coming up the connector; the car has no DTC because derating is a protective strategy working as designed. Replacing the onboard charger would be wrong and expensive.
- Supply: 240 V to 232 V under roughly 7.7 kW is a 3.3% drop - unremarkable. A supply problem would look like a much larger sag, a warm breaker, or a tripped protective device.
Also inspect the 14-50 receptacle and the breaker terminations for heat and torque. A plug-in EVSE on a receptacle is a known hot spot, and a 32 A continuous load on a 40 A circuit is already at the 125% continuous-load limit.
2. How would you confirm that the pilot signal is not simply advertising 16 A and that this is genuinely a thermal derate?
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Decode the pilot, then compare it to the taper behavior.
- J1772 encodes available current in the pilot duty cycle: amps ≈ duty% × 0.6, valid over roughly 10–85%. 32 A is about 53% duty; 16 A is about 27%.
- Read it with a pilot/EVSE tester at the handle, or read the vehicle's PID for EVSE-available current on the scan tool.
- If it reads 16 A from the first second, the EVSE is configured for 16 A (DIP switches or installer settings) and the fix is configuration - but that does not match a unit that ramps to 32 A and then falls back.
- If it reads 32 A initially and the vehicle tapers, the EVSE advertised full current, the vehicle accepted it, and the reduction came from a temperature limit. The 61 °C connector and discolored pins explain it.
- Log inlet temperature, requested current, and actual current together over a 30-minute session. A taper that follows connector temperature is conclusive; a taper that follows nothing is a vehicle-side control question.
3. What are the safety and liability implications, and what do you release to the customer?
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Safety first.
- A 61 °C connector with discolored pins is a fire and arc hazard at a connection carrying 32 A continuously. Take the EVSE out of service, tag it, and tell the customer not to use it until it is replaced - not "use it and watch it."
- Do not treat turning the current down as a permanent repair. It reduces the heat but leaves a damaged contact interface in service.
Circuit verification before anything is re-energized:
- Confirm conductor ampacity and breaker rating support a 32 A continuous load (40 A circuit, 125% rule).
- Verify the receptacle rating and torque the terminations to the manufacturer's specification.
- Thermal-scan the breaker, receptacle, and new connector under full load after the repair.
Release: replace the connector/charge cable or the EVSE, document the before/after thermal images and the pilot readings, and recommend a hardwired EVSE if the receptacle shows any heat damage. Give the customer the photographs. If the receptacle or breaker shows damage, that becomes an electrician's scope and should be stated in writing rather than absorbed into the vehicle repair order.