WiremanWireman · Motor trips the breaker on start: troubleshooting for electricians · 2023 NEC ·
Motor trips the breaker on start
Faults, jammed loads, long starts and breakers that are too small or set too low.
Before you start
How much electrical experience do you have?
This sets how much detail each step shows. You can change it any time.
Showing detail
How it works
A starting motor draws locked-rotor current, commonly around 6 times its full-load amps, until it picks up speed, plus a brief extra spike in the first half cycle. The motor branch breaker or fuse is sized well above full-load current, using the NEC table current and Table 430.52, so it rides through the start, while the overload relay handles running overloads. A breaker that trips on start means a fault, a start that takes too long, too much current, or a protective device too small or set too low.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- When exactly does it trip?
- Instantly, the moment the contactor closesTest: Test the motor and cableExpect: High insulation resistance (IEEE 43 recommends at least 5 MΩ for motors under 1 kV) and balanced winding readings.
- Low insulation to groundGround fault in the motor or cable
- Windings unbalanced or near 0 ΩShorted winding
- Tests goodWhat’s protecting the circuit, and is it sized per Table 430.52?
- Smaller than the table allowsBreaker or fuse too small
- Standard breaker, sized rightWorn breaker or a hard-starting motor
- Instantaneous-trip or adjustable electronic breakerInstantaneous setting too low
- After a few seconds of trying to startWith power locked out, does the motor and its load turn freely?
- Stiff or jammedJammed load or seized bearings
- Turns freelyTest: Measure voltage during the startExpect: No lower than about 90% of nameplate during the start.
- Drops below about 90%Voltage sags during start
- Holds upWhat’s protecting the circuit, and is it sized per Table 430.52?
- Smaller than the table allowsBreaker or fuse too small
- Standard breaker, sized rightWorn breaker or a hard-starting motor
- Instantaneous-trip or adjustable electronic breakerInstantaneous setting too low
- Only now and thenWhat’s protecting the circuit, and is it sized per Table 430.52?
- Smaller than the table allowsBreaker or fuse too small
- Standard breaker, sized rightWorn breaker or a hard-starting motor
- Instantaneous-trip or adjustable electronic breakerInstantaneous setting too low
- Instantly, the moment the contactor closesTest: Test the motor and cableExpect: High insulation resistance (IEEE 43 recommends at least 5 MΩ for motors under 1 kV) and balanced winding readings.