EV Charger Derates Only When a Second EVSE Energizes: Decision Tree
Why this matters
When one EV charger throttles its output only when a second EVSE comes online, the behavior is usually intentional load management doing its job, not a fault. Two EVSEs on a service that cannot carry both at full output will, if they share an energy-management system (EMS) or a load-sharing controller, deliberately split the available current so the feeder is not overloaded (NEC 625.42 and 750). Misreading this as a malfunction leads technicians to swap a healthy charger or upsize a breaker that should not be touched. But the same symptom can also come from a real defect: a sagging feeder, a loose lug heating under combined load, a miswired EMS current transformer, or a charger derating on overheat. The job is to determine whether the derate is the system protecting the service as designed, or a fault masquerading as load management.
Symptom presentation
Charger A delivers its full rated current alone. When charger B begins a session, charger A (or both) steps down to a lower amperage, often a clean preset value like half the circuit or a configured cap. The reduction is graceful and repeatable, tied to the second EVSE's state rather than random. If instead the derate is erratic, accompanied by faults, heat, or voltage sag, that points away from designed load sharing toward a defect.
Quick checks
- Determine whether the two EVSEs are intended to share a circuit or service via an EMS, load-sharing pairing, or a separate load controller. If so, derating on the second session is the designed behavior.
- Read each charger's app or display for an active load-management or power-sharing status and the configured maximum.
- Clamp the feeder under both-charging and confirm the combined current sits at or below the breaker and conductor rating; designed sharing holds it there.
- Measure voltage at each EVSE under combined load; significant sag points to an undersized or faulted feeder rather than clean sharing.
- IR-scan the feeder lugs, the EMS current transformers, and the EVSE terminations under combined load for hot spots.
Isolation tree
Branch first on intent. Confirm whether an EMS, a manufacturer load-sharing link, or a standalone load controller governs these EVSEs. If yes, the derate-on-second-charger is the system enforcing the feeder limit, proceed to the commissioning branch to verify it is doing so correctly. If no controller exists and a single charger derates when an unrelated second one energizes, that is a shared-supply or fault condition, proceed to the supply branch.
Commissioning branch: with an EMS or load-sharing setup, verify the configured circuit ampacity, the per-charger caps, and the current-transformer placement and orientation. A CT clamped on the wrong conductor, reversed, or sized wrong makes the EMS think the feeder is more loaded than it is, causing premature or excessive derating. Correct CT placement and the configured limits, then confirm the sharing math matches the actual feeder rating.
Supply branch: if there is no controller, measure the combined draw and the feeder. If two full-rate chargers exceed the conductor or service rating, the only correct outcomes are a properly engineered EMS to share the load (NEC 625.42) or a service/feeder upgrade; do not remove protection. If the feeder is adequate but voltage sags or a charger faults under combined load, look for a high-resistance termination or feeder defect.
Thermal/protection branch: many EVSEs derate when their own internals or the supply overheats. If a charger steps down with heat present at its lugs or in its enclosure, or its log shows a thermal/derate event, the cause is a hot termination or a unit ventilation problem, not load sharing. Resolve the heat source and the derate clears.
Confirming diagnosis
Confirm designed sharing by reading the controller. With an EMS or load-sharing pair, the combined current under both-charging should equal the configured circuit limit, and each charger's reported cap should sum to that limit; if the numbers match the design and the feeder stays within rating, the derate is correct operation, confirmed. Confirm a CT or commissioning error by comparing the EMS's reported feeder current against an independent clamp reading; a mismatch proves the CT or configuration is wrong. Confirm a supply fault by logging voltage at the EVSE during combined charging; a sag that exceeds the acceptable drop, or a charger logging an undervoltage or thermal derate, isolates a feeder or termination defect. Confirm a thermal derate by correlating the step-down with an IR-measured hot lug or an internal over-temperature event in the charger log; clearing the heat and re-testing restores full output.
Never resolve a two-EVSE derate by enlarging the feeder overcurrent device without recalculating and, if needed, upsizing the conductors and verifying service capacity. EV charging is a continuous load and must be sized at 125 percent (NEC 625.41); running two chargers at full output on a feeder that cannot carry the continuous combined demand creates a sustained overheating and fire hazard. Where the service cannot supply both, a listed energy-management system or a service upgrade is required, not removal of protection.
Remediation
If the derate is designed load sharing operating correctly, no repair is needed; document the configured limits for the customer and confirm the EMS matches the feeder rating. For a commissioning error, reposition or re-orient the current transformer, correct the configured circuit ampacity and per-charger caps, and re-verify against an independent clamp. For an inadequate supply, install a listed energy-management system to share the available current within the feeder rating, or engineer a service/feeder upgrade sized for the continuous combined load at 125 percent (NEC 625.41, 625.42). For thermal derating, re-terminate and re-torque hot lugs, ensure charger ventilation and ambient are within spec, and clear any over-temperature condition. After any change, run both chargers simultaneously, log combined current and voltage, and confirm stable operation within the feeder and breaker rating.
References
- NFPA 70 (NEC) 625.41, Continuous-load sizing for EV charging branch circuits at 125 percent
- NFPA 70 (NEC) 625.42, Rating and energy-management-system control of EVSE loads
- NFPA 70 (NEC) 750, Energy Management Systems
- NFPA 70 (NEC) 110.14(D) and 110.3(B), Termination torque and installation per listing
- NFPA 70 (NEC) 220 and 310.15, Service/feeder load calculation and conductor ampacity
- Manufacturer load-sharing and EMS commissioning guides (e.g. ChargePoint, Tesla Wall Connector power sharing, Wallbox)