WiremanWireman · Generator trips under load: troubleshooting for electricians · 2023 NEC ·
Generator trips under load
Overload, a big motor start, an unbalanced leg, or a ground fault on the generator.
Before you start
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How it works
A generator’s breaker protects its windings at its rated current. On a 120/240 V generator, each 120 V leg can carry only half of the rated watts, so piling 120 V loads on one leg trips it long before the total reaches the rating. Motors make it worse: an A/C compressor or well pump draws several times its running current to start, which drags the voltage and frequency down and can trip the breaker or stall the engine.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- When does it trip?
- When a big motor starts (A/C, well pump)Motor starting current too large for the generator
- After running loaded a whileTest: Clamp each generator legExpect: Each leg below its rated current, and the two legs roughly balanced.
- One leg much higher than the otherLoads unbalanced across the two legs
- Both legs at or over the ratingToo much load for the generator
- Both well under the ratingEngine can’t hold speed, or a breaker problem
- GFCI on the generator tripsGround fault, or neutral bonding through the house
- Instantly when one circuit comes onA short or ground fault on that circuit