Overload heaters are the small replaceable elements inside a motor starter's overload relay. Each one is sized to a motor's full-load current: when the motor draws too much for too long, the heater warms the relay's trip mechanism and opens the control circuit. They are matched to the starter make and series, so a heater from one maker's line does not drop into another's.
This page carries heater elements for General Electric (CR123 H-series), Allen-Bradley (Type N), Federal Pacific (F-series), Westinghouse (FH and BH series), and a few from Square D, Furnas, Clark and Arrow Hart.
How to pick one: find the heater selection table for your starter (it is usually printed inside the enclosure cover or on the starter's instruction sheet), look up your motor's nameplate full-load amps, apply the table's notes for service factor and ambient temperature, and read off the heater code. Then order that exact code, and use the same code on every overload pole (three on a three-phase starter) so every pole trips alike.
Related: Electromechanical, Relays and Contactors.
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