Motor Overload Protection

Motor protection is a two-part system: short-circuit protection up front handles faults and inrush, while the overload relay watches the running current and trips on a sustained overload the breaker would ignore.

I size the overload from the nameplate full-load amps and service factor, not the breaker. Getting these two mixed up is the classic motor-circuit mistake.

When it applies

Motor overload protection is sized to the motor's marked full-load current, and it is a different animal from the short-circuit and ground-fault protection sitting upstream. Overload guards against a motor slowly drawing too much; the upstream device handles faults.

Common mistakes

• Assuming the branch-circuit breaker also protects against overload. It usually does not react to a gradual overcurrent.
• Sizing overloads to the nameplate horsepower instead of the actual full-load amps and service factor.
• Oversizing heaters or dialing up an electronic overload to stop nuisance trips, defeating the protection.

In the field

A conveyor motor in a packaging line kept overheating and nobody could explain it. I found the overload relay set well above the motor's full-load current because a previous tech bumped it to stop trips. The motor was cooking under a jammed load with nothing to catch it. I set the overload to the nameplate value and the relay started doing its job. Overload and short-circuit protection are two separate jobs; never let one masquerade as the other.