June 10, 2002
Overload relays provide motor overload protection. If an overcurrent
condition persists that can cause motor damage by overheating, the overload relay
responds to clear the overcurrent. Thermal overload relays detect and respond to
motor overcurrent by converting the line current to heat by a resistive element. Solid-
state overload relays can also be used and have programmed response characteristics.
Overload relays are designed to protect against an overload condition; other protective
devices provide short circuit current protection.
Two types of thermal overload relays are available:
Bimetallic thermal overload relays in which a bimetallic element bends as it heats,
eventually causing a set of contacts to open. Bimetallic relays automatically reset as
they cool; however, the design frequently includes a manual reset switch.
Melting alloy overload relays in which the heat generated by the current melts a
metallic alloy. These relays are usually reset after a few minutes when the alloy
solidifies again. Melting alloy overload relays are not the preferred type for use.
Standard, slow, and fast response relays are available. Standard units
should be used for motor starting times up to 7 seconds. Slow units should be used for
motor starting times in the 8 to 12 second range. Fast units should be applied only to
special purpose applications with very fast starting times.
Thermal overload relays are sensitive to ambient temperature; they trip
sooner in a high temperature and longer at a low temperature. Use ambient
temperature-compensated overload relays if the motor is located in a nearly constant
ambient temperature environment and the thermal overload device is located in a
Magnetic overload relays are solenoids that respond magnetically to an
overcurrent. Magnetic overload relays are used only for unusual applications and
should not normally be considered an alternative to solid-state or thermal overload
Provide motor overload protection in accordance with NEC Article 430, Part III
(2002 Edition). Apply overload protection to each motor 0.125 horsepower (93.25
watts) and larger. Provide three phase motors with overload protection in each
ungrounded conductor. The following explains the principal NEC requirements:
NEC Article 430.32 (2002 Edition) requires continuous duty motors rated
above one horsepower having a marked service factor of not less than 1.15 or a
temperature rise not over 40 °C (104 °F) to have overload protection rated for no more
than 125 percent of the motor nameplate full-load rating. All other continuous duty
motors above one horsepower must have overload protection rated for no more than
115 percent of the motor nameplate full load rating.