Technical Foundation
What Is a Overload Relay?
An overload relay is a motor protection device wired in series with a contactor that monitors motor current and trips to open the circuit if current stays above a set threshold for too long, protecting the motor windings from thermal damage during a stall, jam or single-phasing condition. Thermal bimetallic relays sense heat generated by current flow through a strip that bends and trips a mechanism; electronic relays measure current directly via current transformers and calculate a thermal model in firmware, giving tighter accuracy and features like phase-loss and ground-fault detection.
Trip class (10, 20 or 30) defines how many seconds the relay allows at 6x rated current before tripping, and must be matched to the motor's locked-rotor withstand time — a class 10 relay trips faster and suits general-purpose motors, while class 30 is specified for high-inertia loads like large fans or centrifuges that take longer to reach running speed. The relay's current adjustment range must bracket the motor's full-load amps with the dial set to the actual nameplate FLA, not the relay's maximum.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Trip Class | Class 10, 20, 30 (per IEC 60947-4-1) | Must match motor starting characteristics — too fast trips nuisance-trip a high-inertia load |
| Current Adjustment Range | e.g. 9–13A, 17–25A | Set to actual motor nameplate FLA, not relay maximum, for correct protection |
| Mounting Standard | IEC direct-mount vs. NEMA melting-alloy | Determines physical compatibility with the paired contactor line |
| Sensing Type | Bimetallic thermal vs. electronic/CT-based | Electronic types add phase-loss, ground-fault and communication capability |
| Auto/Manual Reset | Selectable reset mode after trip | Manual reset required by code in some applications to prevent unattended restart |
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Applications
Where Overload Relays Get Used
Overload relays are specified anywhere a three-phase motor needs code-compliant protection against sustained overcurrent.
Industrial Motor Control Panels
Paired with a contactor in a motor starter to provide code-required overload protection on three-phase motors.
HVAC & Chiller Plant Equipment
Protecting compressor and fan motors from sustained overcurrent during fouled-coil or bearing-drag conditions.
Water & Wastewater Treatment
Protecting pump motors in lift stations and treatment plants where nuisance tripping causes process downtime.
Material Handling & Conveyors
Protecting conveyor drive motors from jam-induced overcurrent without nuisance-tripping on normal startup inrush.
Oil & Gas and Process Industries
Electronic overload relays with communication used for remote motor health monitoring in process facilities.
OEM Machine Building
Specified as a standard motor-protection component in machine control panels shipped to code.
FAQ
Overload Relays Questions, Answered
What's the difference between a thermal and an electronic overload relay?
Thermal (bimetallic) relays sense heat from current flow through a bending strip and are simpler and lower cost. Electronic relays measure current via current transformers and use a firmware thermal model, giving tighter trip accuracy, adjustable trip classes, and added features like phase-loss, ground-fault and current monitoring output.
What does trip class mean and how do I choose one?
Trip class (10, 20, or 30) specifies the maximum seconds a relay allows at 6x rated current before tripping. Class 10 suits general-purpose motors with normal starting; class 30 suits high-inertia loads like large fans, centrifuges or crushers that take longer to accelerate and would nuisance-trip a faster class.
How do I size an overload relay's current range?
Select a relay whose adjustable current range brackets the motor's full-load amps (FLA) from the nameplate, then set the dial to the actual FLA value — not the top of the relay's range. An oversized range set too high won't protect the motor; too low causes nuisance tripping on normal starting current.
Are IEC and NEMA overload relays interchangeable?
No. IEC relays direct-mount onto IEC-standard contactors of matching frame size, while NEMA melting-alloy relays use a different mechanism and mount in NEMA-rated starters. The two families aren't physically or electrically interchangeable — match the relay to the contactor standard already in use.
What's the difference between auto-reset and manual-reset overload relays?
Auto-reset relays re-close automatically once the bimetallic element cools after a trip, which can restart a motor unattended. Manual-reset requires a physical reset action and is often required by code or safety procedure on equipment where unexpected restart poses a hazard.
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