Thermal overload relay: what to set the dial to, and why it trips at normal current

A motor drops out every afternoon. Somebody clamps it, the reading looks fine, and the plan becomes a new thermal overload relay because the old one must be weak. I've pulled plenty of those weak relays, and nearly all of them were doing exactly what they were built to do. The dial was on the wrong number, the clamp went on one leg with the machine half loaded, or the trip class didn't match how long the load takes to come up to speed.

A thermal overload relay uses the motor current to heat bimetal strips, one per phase, and opens a normally closed contact in the contactor coil circuit when that current stays too high for too long. It never switches the motor itself. It tells the contactor to.

What a thermal overload relay covers, and what it leaves to the fuses

Slow trouble is its job: a load that's too heavy, a lost phase, imbalance, a rotor that won't turn.

Device	What it's there for	What it won't do
Thermal overload relay	Current that stays above the setting for seconds to hours: overload, phase loss, imbalance, locked rotor	Clear a short circuit
Fuses or branch breaker	Short circuit and ground fault on the motor branch	Catch a motor running 15% over its rating all shift
Drive's electronic motor overload	One motor on one drive	See one bad motor when several share the drive, since it only sees the total

Three devices, three different jobs on the same motor branch.

An unmarked overload relay also defaults to 5 kA in the panel's short-circuit rating, which is one way a 65 kA panel ends up rated 5 kA.

How it's wired: 95-96 drops the coil, 97-98 reports the trip

The main poles go in series with the motor leads, usually plugged straight into the bottom of the contactor. 95-96 is normally closed and sits in series with the contactor coil, on whichever side of it the drawing puts it. The other pair, 97-98, stays open until a trip and then feeds a PLC input or a pilot light. The relay only opens its own contact. The contactor does the rest.

I land 97-98 on a PLC input on every panel I build. A timestamped "OL tripped" bit ends arguments that otherwise go on for a week.

On a Siemens 3RU21 the TEST slide opens 95-96 and closes 97-98 exactly like a real trip. Use it after wiring, and know what it proves: the control wiring. Nothing gets heated, so it says nothing about calibration.

Motor current runs through the thermal overload relay, but only its 95-96 and 97-98 contacts reach the 24 V DC control side.

What to set the dial to

The motor's full-load current (FLA) off the nameplate, for the voltage you're actually running. The dial is the motor's current, not the trip current.

The margin is already inside the relay. Under IEC 60947-4-1 it must not trip at 1.05 times its setting within two hours, and it has to trip within two hours at 1.2 times. With all three poles loaded evenly, the lowest tripping current sits between 105 and 120% of the setting. A TeSys LRD3363 puts its threshold at 1.14 ±0.06 times the dial.

NEC 430.32 caps how high a separate overload device may be selected to trip: 125% of nameplate full-load current for motors marked with a service factor of 1.15 or more, or a temperature rise of 40 °C or less, and 115% for the rest. If that won't start the motor or carry the load, 430.32(C) allows 140% and 130%.

That's where people double count. Turn a 10 A motor's relay to 12.5 A, and a relay tripping at 1.14 times the dial lets go around 14.3 A, roughly 143% of nameplate. I set FLA and let the relay keep its own margin. Motors that get the 115% limit are where that margin needs a second look: at 1.14 ±0.06 times the dial, FLA can already trip past 115%. On those motors, check where your relay’s ultimate trip actually lands before you leave the dial at FLA.

Situation	Set the dial to
Direct-on-line motor, one voltage	Nameplate FLA
Dual-voltage motor	The current for the connection you have. On 460 V that's the lower figure
Wye-delta, relay in series with the motor windings	0.58 × FLA, because only 1/√3 of the motor current flows through it
Wye-delta, relay in the incoming supply, ahead of the wye-delta contactors	Full FLA
Single-phase or DC motor	FLA, with the current run through all three poles in series
Eaton C306 heater-pack relay on a 1.0 SF motor	Half a position counterclockwise from FLA, taking its ultimate trip from 125% to 115%

Where the thermal overload relay setting is not simply the nameplate number.

Never set it outside the printed scale. If FLA lands past either end, it's the wrong relay.

Say the plate reads 208-230/460 V and 6.2-5.8/2.9 A on a 480 V panel. The dial goes to 2.9. Put 5.8 on it, because that number catches the eye first or the relay came out of a 230 V panel, and it won't trip until about 6.6 A. More than twice the motor's rated current.

Example nameplate. On a 480 V panel the thermal overload relay goes to 2.9 A, not to the 230 V figure.

Trip class: what 10A, 10, 20 and 30 mean in seconds

The class is how long the relay may take at a heavy overload. IEC checks it at 7.2 times the setting from cold, NEMA at 600% of the dial.

Class	1.05 × setting	1.2 × setting	1.5 × setting	7.2 × setting, from cold (IEC)	600% of dial (NEMA)
10A	No trip within 2 h	Trips within 2 h	Under 2 min	2 to 10 s	No NEMA 10A
10	No trip within 2 h	Trips within 2 h	Under 4 min	4 to 10 s	10 s or less
20	No trip within 2 h	Trips within 2 h	Under 8 min	6 to 20 s	20 s or less
30	No trip within 2 h	Trips within 2 h	Under 12 min	9 to 30 s	30 s or less

Thermal overload relay trip classes, IEC and NEMA side by side.

Class 10A and 10 are for normal starting, 20 for heavy starting, 30 for very long run-ups with a lot of inertia behind them.

An ABB TA75DU class 20 in the 18-25 A range takes about 52 seconds at 3 times its setting from cold, 15 at 6 times and just under 12 at 7.2 times, all ±20%. At 1.2 times the standard only promises a trip within two hours, and below that it may not trip at all.

Those are cold figures. A Siemens 3RU21 that has carried its setting current for a while trips in roughly a quarter of the cold time, which is part of why a motor that tripped once goes again so fast after a reset.

Cold trip times for one class 20 thermal overload relay, against the IEC class windows at 7.2 times the setting.

Why a thermal overload relay trips when the motor seems fine

Before anyone orders parts I want three-phase amps under real load, taken at the relay, written next to the dial setting. Not the nameplate. The dial.

What you see	Usual cause	What to check
Trips after running a while, current "looks normal"	One leg higher than the others, or the reading came off one leg at light load	Clamp all three phases at full load and compare each to the dial
Trips again right after a reset	Relay and motor still hot	Wait out the cooling time, then count starts per hour
Trips while the motor accelerates	Run-up longer than the class allows	Time the start. Heavy loads need class 20 or 30
Trips fast on an ordinary day	Lost phase: a blown fuse or an open connection	Voltage and current on all three phases. Phase-loss-sensitive relays trip faster on purpose
Trips with good amps on all three legs	Dial below FLA, a dual-voltage mixup, or a 0.58 wye-delta setting on a relay that sees full line current	Compare the dial to the setting table above
Trips more in summer	Relay air hotter than its compensated range	Measure the air right next to the relay
Trips behind a VFD	Extra heating in the relay at the drive’s output frequency	Use the drive's motor overload, or raise the relay setting if it has to stay

Thermal overload relay nuisance trips, cheapest check first.

About that summer row: a 100 °F cabinet sits well inside the compensated range, which is -20 to +60 °C on a TeSys LRD, -40 to +60 °C on a 3RU21 and -25 to +55 °C on an ABB TA. If it trips anyway, look past the heat.

If all three legs really are high and the load hasn't changed, suspect the motor. Meg it before you condemn the relay, and split the circuit like you would on any other fault.

And when it doesn't trip

·       The dial got turned up to stop the trips.

·       125% on the dial plus the relay's own margin.

·       A dual-voltage motor on 460 V with the relay set to the 230 V current.

·       A wye-delta relay in series with the windings set to full FLA, which is 1.73 times too high.

·       The motor runs between 1.05 and 1.2 times the setting, where the relay can take hours to trip, or never trip at all below its threshold.

·       Relay tripped, motor still running: 95-96 isn't in the coil circuit, somebody jumpered it, or the contactor tips welded.

H and A: the reset switch nobody reads

On a TeSys LRD, H is manual (hand) reset and A is automatic. A Siemens 3RU21 marks the same choice M and A on the blue RESET button, which you push down and turn. Neither position turns protection off. A thermal overload relay trips the same way in both, and the switch only decides what happens once the bimetals cool.

They have to cool first, too. A Siemens 3RU2136 typically needs 10 minutes to recover after an overload trip, and it won't take a reset before then.

Automatic reset is fine with 3-wire control, where a button or the PLC has to issue a fresh start. On 2-wire control, a maintained switch or a PLC bit that just stays on, the motor restarts by itself the moment the relay resets. NEC 430.43 is plain about it: an overload device that can restart a motor automatically after tripping can't be installed where that restart can injure someone. The red STOP button on a 3RU21 has the same trap: with maintained control, the motor comes back when you let go.

Relays usually ship on manual. A TeSys LRD01 to LRD38 built since June 2005 switches back and forth. Older ones and TeSys K LR2K relays are one-way, so once they're on A, they stay on A.

I leave them on H unless somebody shows me why not. An operator who has to walk up and press reset is also an operator who notices the motor is too hot to touch.

H for manual, A for automatic. The thermal overload relay trips the same either way; only the reset changes.

Swapping one: what has to match

·       A setting range that covers your FLA with room on both sides.

·       The same trip class. A class 20 spec takes a class 20 relay.

·       The contactor family: LRD on TeSys D, LR2K on TeSys K, a 3RU21 on the same-size 3RT2, an ABB TA25DU on A9 to A40.

·       Contacts. Most have 1 NO and 1 NC, so check which ones the drawing uses.

·       Reset mode, and whether that switch is one-way.

·       The short-circuit rating with the fuse or breaker already in the panel.

Thermal overload relay questions people ask most

What should a thermal overload relay be set at?

The motor's nameplate full-load current at the voltage it's connected for. The relay already trips 5 to 20% above its dial, so adding the NEC 125% or 115% on top stacks the two margins.

What is the difference between class 10 and class 20?

How long it may take on a big overload. At 7.2 times the setting from cold, class 10 trips in 4 to 10 seconds and class 20 in 6 to 20. Use 20 when the load is slow to accelerate.

Why does my thermal overload relay keep tripping?

Usually the real current is higher than anyone measured. Next most likely is a dial set below FLA or a hot restart, and then a lost phase. Measure all three phases under load before replacing anything.

Does a thermal overload relay protect against a short circuit?

No. The fuses or breaker on the branch clear the short. The relay handles sustained overcurrent.

Can you use a thermal overload relay with a VFD?

Yes, though on a single motor the drive's own motor overload usually covers it. With several motors on one drive each needs its own relay, because the drive only sees the total. Same story on HVAC fan arrays.

When the thermal overload relay really is the part that failed

If the readings point at the relay and not the motor, match the catalog number and setting range off the old one. IQElectro keeps surplus relays on the shelf: Schneider TeSys D LRD08 (2.5-4 A), LRD10 (4-6 A) and LRD16 (9-13 A), TeSys K LR2K0312 (3.7-5.5 A) and LR2K0314 (5.5-8 A), the Eaton C306DN3B, and for older Allen-Bradley NEMA starters the 42185-800-01 overload relay for Size 0-1, plus Type W heater elements such as the W68 for the 38.5-51 A range. Everything ships with a two-year warranty. Send the catalog number off the side of the old relay and we'll confirm the match before you order.