Breaker trips under load

Separate overload, fault, connection, equipment, and coordination causes.

A breaker that trips after the load has been running may be responding correctly to overload, heat, ground fault, or short circuit—or the circuit may have a connection, equipment, application, or coordination problem. Preserve timing and load evidence before resetting or changing anything.

Record first: Trip time, operating load, breaker indication, ambient conditions, and recent changes
Compare: Measured current and duration against the actual device curve and equipment duty
Never assume: A larger breaker is safe without revalidating conductors, equipment, and protection

1. Preserve the event

Record the elapsed run time, process state, connected loads, breaker handle or trip-unit indication, weather or room temperature, and whether the trip is repeatable.
Retrieve electronic trip-unit event data, drive faults, controller alarms, and load trends before they are overwritten.
Identify breaker manufacturer, catalog number, rating plug or setting, poles, interrupting rating, trip functions, enclosure, and installation orientation.
Record recent conductor, equipment, programming, maintenance, or load changes; a delayed trip that began after a change is valuable evidence.

2. Verify the protected circuit

Trace every load and conductor actually supplied by the breaker, including intermittent heaters, compressors, defrost, charging, or process stages.
Confirm conductor material, size, insulation, terminal temperature ratings, grouping, ambient correction, neutral loading, wiring method, and equipment nameplates.
Check that breaker type and settings match the design, equipment listing, available fault current, selective-coordination requirements, and manufacturer instructions.
Do not infer circuit capacity from the breaker handle number alone.

3. Inspect de-energized conditions

Use the approved energy-control procedure and verify the isolated condition before opening, tightening, or resistance testing the circuit.
Look for discoloration, insulation damage, moisture, contamination, loose conductors, damaged bus connections, poor conductor preparation, incompatible lugs, and evidence of arcing.
Check mechanical operation and termination torque using the device instructions; tightening an already damaged joint is not a repair.
Isolate sensitive electronics before insulation-resistance or other tests that could damage connected equipment.

4. Measure the operating load safely

Qualified persons working under an approved diagnostic procedure may measure current on every phase and the neutral through a representative operating cycle.
Capture starting, cycling, duty, phase imbalance, harmonic-rich loads, and simultaneous operation rather than relying on a single steady-state reading.
Where justified, thermal inspection can compare phases and connection points, but temperature must be interpreted with load, ambient, emissivity, distance, and equipment condition.
Compare observations with the breaker time-current curve and trip-unit records for the exact catalog number and settings.

5. Distinguish likely causes

Current above the circuit design for sufficient time supports a true overload or an incorrect application/load calculation.
Normal measured current with localized heating suggests a high-resistance connection, damaged breaker contact, enclosure heat issue, or another thermal influence.
Phase imbalance can indicate a supply, connection, motor, or load problem even when average current appears acceptable.
Intermittent ground-fault or instantaneous indications require a fault-isolation plan; repeated resets are not a diagnostic method.

6. Correct and prove the repair

Correct the confirmed load, connection, conductor, equipment, environment, breaker, or settings issue through the responsible designer or qualified person.
Do not upsize or bypass protection until conductor ampacity, terminal limits, equipment protection, fault current, coordination, and adopted-code requirements have been revalidated.
Repeat the representative load cycle while logging current, time, temperature, and protective indications.
Document final settings, measurements, replacement parts, torque records, and the evidence linking the repair to the original trip.

Worked field example

Diagnostic example: breaker opens 18 minutes after a process starts

Preserve the exact trip indication and trend which process stages were active during the 18-minute interval.
Identify the exact breaker and obtain its manufacturer time-current information and current settings.
Verify the circuit conductors, terminations, enclosure, load schedule, and equipment ratings under the approved safe-work procedure.
Trend all phase and neutral currents through the full process cycle and compare the recorded time/current point to the device curve.
Correct the proven overload, connection, equipment, or application issue and repeat the same cycle with measurements.

What this proves: Trip timing plus measured current and the exact device curve can separate a protective device operating as designed from a heat, connection, equipment, or setting problem.

Verify before use

Official sources

Source set reviewed September 2026. Recheck the current adopted requirements before design, purchase, permit, or installation.

NFPA 70 — National Electrical Code ↗
Use the adopted edition for overcurrent protection, conductor ampacity, equipment, and installation requirements.
OSHA — Control of Hazardous Energy ↗
Official hazardous-energy control guidance for inspection and service work.