Thermal Overload Relay: How to Size and Set It Right

Key takeaways

• A thermal overload relay protects the motor windings from overheating caused by sustained overcurrent. It does not protect against short circuits; that is the breaker's or the fuses' job.
• The baseline setting is the motor's rated full load current from the nameplate, not the measured running current and not a "bit extra so it stops tripping".
• Trip class (10, 20, 30) decides how long the relay tolerates the starting current. Class 10 suits most standard duties; heavy, long starts need class 20 or 30.
• A relay that keeps tripping is almost always telling the truth: about load, voltage imbalance, or a failing motor. Turning the dial up buys silence and pays with rewinds.
• Every trip is a data point. Logged with current readings and the machine state, trips become one of the earliest cheap wear signals a plant has.

What it protects, and what it does not

Windings die from heat. Sustained overcurrent, a jammed load, a lost phase or chronic undervoltage all push more current through the stator than its insulation can dissipate, and insulation life halves roughly every 10 °C of sustained overtemperature.

The thermal overload relay models that heating: bimetal elements (or an electronic model of them) integrate current over time and open the contactor's control circuit when the accumulated heat says the motor is in danger.

What it does not do: clear short circuits. Fault current protection comes from the upstream breaker or fuses; the overload relay and the short circuit device are a coordinated pair, not alternatives.

Setting the dial: the rules

• Start at the nameplate full load current (FLC) for the actual supply voltage and the actual wiring connection (star or delta figures differ; inside a star delta starter the relay usually sits in the phase path and is set to FLC divided by 1.73, per the starter's diagram).
• Do not compensate for ambient temperature by over-setting. Modern relays are ambient compensated; hot switchrooms are handled by design, not by the dial.
• Never set above what the motor nameplate and the applicable installation code allow. The slack that some codes permit above FLC exists for marginal nuisance tripping cases, not as a default.
• Pick the trip class before arguing with the dial. Class 10 trips within 10 seconds at 7.2 times set current, class 20 within 20 seconds, class 30 within 30 seconds. A high inertia fan or a loaded conveyor with a 15 second start will nuisance trip a class 10 relay at the correct current setting; the fix is class 20, not a higher dial.

A worked example with real numbers

A 15 kW, 400 V conveyor motor, nameplate FLC 29 A, direct on line, measured start time 12 seconds at about 6 times FLC:

• Dial setting: 29 A. Not the 31 A someone measured on a hot day, and not 33 A "for margin".
• Trip class: the 12 second start at 6 × FLC sits too close to a class 10 curve, so specify class 20.
• Commissioning record: dial 29 A, class 20, measured running current 26.5 A on each phase, phase imbalance under 2 percent.
• Six months later the relay trips twice in a week. Measured current: 32 A running, up from 26.5, about 10 percent above the dial setting. The relay is fine; the conveyor bearing that is dragging is not. The trip was the cheapest vibration sensor on the machine.

Reading trips as condition data

Rising running current at constant load means rising mechanical drag or electrical trouble: seized rollers, a dragging brake, a failing bearing, low voltage or phase imbalance. That is why trip events belong in the machine history with the measured currents attached, not in an operator's memory.

Trips correlated with current readings and recent work orders turn "the relay is oversensitive" into "current has crept 20 percent since March", which is a maintenance argument with a date on it, visible in the machine's MTBF trend months before failure.

Electrical checks travel together: when a trip brings you to the starter, the same visit is the moment for a quick insulation resistance test if the motor is suspect (lock out and tag out the starter, verify it dead with a meter, and disconnect the motor leads first), and a look at whether the duty has outgrown a direct start; our comparison of soft starters versus VFDs covers when a gentler start solves both the trips and the mechanical stress.

Recurring trips on one asset should open a work order every time, which is exactly the discipline a preventive maintenance plan in a CMMS helps enforce. To see how Fabrico ties trip events, readings and work orders to one asset, read how Fabrico tracks downtime.

Fabrico is software for OEE, downtime tracking and maintenance. We do not sell parts or repair services.

Common mistakes

• Setting to the measured current instead of the nameplate. A lightly loaded motor measured at 22 A gets a 22 A setting, then trips forever once the process runs at design load.
• Turning the dial up to stop nuisance trips. If the current setting matches the nameplate and it still trips, the cause is the start duty (wrong class), the supply (imbalance, undervoltage) or the machine (drag). The dial is the one place the answer is not.
• Wrong setting inside star delta starters. Set to line current instead of phase current, the relay is effectively blind by a factor of 1.73.
• Endless resetting. Two resets without a measurement is operation by hope. After the second trip, a qualified electrician, wearing the PPE your electrical safety program requires for live work, clamps a meter on each phase and writes the number down.
• Ignoring single phasing. A lost phase pushes the remaining phases far over current. Differential trip mechanisms catch it, but only if the relay is sized and wired per the diagram.

What the number on the dial really means

The setting is not a trip point. Under IEC 60947-4-1 a thermal overload relay must not trip at 1.05 times the set current and must trip at 1.2 times the set current within two hours.

A relay set to 29 A should therefore carry about 30.4 A without tripping and trip within two hours at about 34.8 A. Between those two figures it may or may not trip, which is why a slowly rising running current causes occasional trips long before it causes constant ones.

Choosing the relay range and the star delta position

Pick a relay whose adjustment range contains the setting with some room on both sides. For a 29 A setting that means a range of roughly 23 to 32 A, not one where 29 A sits at the very end of the scale.

In a star delta starter, check where the relay is wired before you set it. Placed in the delta (phase) circuit, it is set to 0.58 times the motor rated current. Placed under the line contactor, as Schneider Electric shows for Type 2 coordination, it is set to the full motor rated current.

Calculating the setting when the nameplate is unreadable

The nameplate current is always the first choice. When it is missing or unreadable, estimate the full load current from the motor power: I = P / (1.73 × U × cos φ × η). For the 15 kW, 400 V conveyor motor, assuming a power factor (cos φ) of 0.82 and an efficiency (η) of 0.91: I = 15,000 / (1.73 × 400 × 0.82 × 0.91) = about 29 A.

Treat that figure as an estimate until you find the motor data sheet, because power factor and efficiency change with motor size, speed and load.

Frequently asked questions

What current should a thermal overload relay be set to?

To the motor's nameplate full load current for the actual voltage and wiring connection. In star delta starters the relay usually measures phase current and is set to FLC divided by 1.73, per the starter diagram.

What is trip class 10, 20 and 30?

The maximum time the relay allows at 7.2 times the set current: about 10, 20 or 30 seconds. It exists to ride through the starting current. Standard duties use class 10; long, heavy starts need 20 or 30.

Why does the overload keep tripping?

In rough order of likelihood: real overload or mechanical drag, phase imbalance or undervoltage, wrong trip class for the start, wrong setting for the wiring connection, and only rarely a defective relay. Have a qualified electrician, wearing the PPE required for live work, measure the running current per phase before anyone touches the dial.

Does a thermal overload relay protect against short circuits?

No. Short circuit protection comes from fuses or the circuit breaker upstream. The overload relay handles sustained overcurrent; the pair together protect the circuit and the motor.

Are electronic overload relays better than bimetal ones?

They hold tighter tolerances, add features like phase loss detection, adjustable trip classes and trip logging outputs, and drift less over time. Bimetal relays remain perfectly serviceable for standard duties when set correctly.