How to Select and Set a Thermal Overload Relay for a Motor
How do you select and set a thermal overload relay for a motor? Read the motor nameplate full-load current (FLC), pick a relay whose adjustment range brackets that FLC and whose frame fits the contactor, dial the set point to FLC, and choose a trip class (10A, 10, 20, or 30 per IEC 60947-4-1) that lets the motor accelerate without nuisance tripping. Get any one of those four wrong and the relay either fails to protect the winding or trips the starter on every normal start. This article walks through nameplate reading, range and frame matching, dial setting, trip-class selection, star-delta sizing, and the bimetallic-versus-electronic decision.
Step 1 — Read the Motor Nameplate, Not the Breaker or a Catalog Table
The overload relay dial gets set to the motor's full-load current (FLC) as stamped on its nameplate — not to the motor's horsepower, not to the contactor's continuous current rating, and not to a generic FLC table from a code book. Nameplate FLC reflects the actual winding design of that specific motor; a generic table value is a rough average across a class of motors. A 15 kW, 400 V, 3-phase induction motor might carry a nameplate FLC of 28 A while a table lists 29-30 A for the same frame size, a small gap that matters when the setting already sits near the top of a bimetal's adjustment range. Some nameplates also list a service factor (SF) above 1.0, typically 1.15; the base setting is still FLC. SF widens the margin the motor tolerates before real damage, not the number on the dial.
What we see in the field: technicians sometimes copy the value off the breaker schedule or the drive's rated output current, because that is the number printed largest on the panel documentation. That number belongs to the SCPD or the drive, not the overload relay. Read the actual motor nameplate every time.
Step 2 — Match the Relay's Adjustment Range and Frame to the Contactor
Every bimetallic overload relay has a fixed adjustment range, typically a 1:1.5 ratio such as 9-13 A or 23-32 A, and a frame that mounts on one specific contactor size. Schneider's TeSys LRD line spans roughly 0.10 A to 630 A across a dozen sub-ranges; ABB's TA line runs from TA25DU up through TA200DU and beyond, mounting under the matching A or AF contactor; Siemens SIRIUS 3RU21 comes in frame sizes S00, S0, S2, and S3, each with its own current band. Pick the sub-range that brackets the nameplate FLC with the setting landing in the middle third of the dial, not pinned at either stop — a relay set at the extreme low or high end drifts less predictably, and some manufacturers do not warrant accuracy there.
Frame and range have to agree with the contactor already installed. A relay that electrically covers the FLC but only physically fits a larger contactor frame does not mount. Check the manufacturer's coordination table before ordering, not after the parts arrive on site.
Step 3 — Set the Dial to FLC
With the correct relay in hand, the setting step itself is simple arithmetic; the complexity sits upstream, in nameplate reading and range selection. Rotate the dial to the FLC value and torque the terminals to the printed spec. An under-torqued lug heats under load and drifts the trip point independent of the bimetal's own calibration.
Formula: Overload Relay Dial Setting — Source: IEC 60947-4-1
Iset = FLC
| Symbol | Description | Unit |
|---|---|---|
| I_set | Overload relay dial setting | A |
| FLC | Motor full-load current, from the nameplate | A |
Step 4 — Pick the Trip Class for the Load's Run-Up Time
Trip class defines how long the relay tolerates an overload current before it opens, tested at 7.2x the dial setting from cold per IEC 60947-4-1: Class 10A trips in 2-10 s, Class 10 in 4-10 s, Class 20 in 6-20 s, Class 30 in 9-30 s. A standard pump or fan reaches full speed in 2-5 s, and Class 10 or 10A covers that inrush without tripping. A large fan, centrifuge, or crusher with a high-inertia load can take 15-25 s to reach speed; a Class 10 relay reads that run-up current as a fault and trips before the motor gets there. Move to Class 20 or 30, or to an electronic relay with longer thermal memory, once the calculated run-up time gets within roughly 80% of the trip curve. See overload relay trip classes 10A, 10, 20 and 30 explained for the full curve data.
Special Case — Setting for Star-Delta Starters
A star-delta (wye-delta) starter changes the setting math, because the overload relay usually sits in the delta leg, not the line. In that leg it sees phase current, not full line current — roughly line FLC divided by the square root of 3, about 0.58x the nameplate value. Set the dial to that reduced number, not to the nameplate FLC directly, or the relay is oversized for the current it actually carries and offers no real protection margin. See overload relay sizing for star-delta starters for the full worked example, and wiring a contactor for star-delta starting for the three-contactor circuit itself.
Bimetallic or Electronic: Which One to Select
Bimetallic relays cover most direct-on-line motor starters and cost less per pole. Electronic (solid-state) relays earn their premium on wide setting ratio, selectable trip class, and diagnostics a bimetal element cannot provide. The choice comes down to the setting ratio the application needs, whether ground-fault or true phase-loss detection matters, and whether the relay has to keep its thermal memory of a hot motor through a power cycle.
| Criteria | Bimetallic | Electronic |
|---|---|---|
| Setting ratio | ~1:1.5 | ~1:3 to 1:4 |
| Trip class | Fixed, usually 10A or 10 | Selectable, e.g. 5/10/20/30 |
| Phase-loss detection | Differential trip bar (mechanical) | Electronic, often faster |
| Extra protections | None | Ground-fault, stall/locked-rotor, thermal memory |
| Typical cost position | Lower | Higher |
For the full breakdown, see thermal vs electronic overload relays.
Setting Mistakes That Cause Nuisance Trips or No Real Protection
Three mistakes account for most of the overload-relay complaints we see: dialing to the service-factor current instead of FLC, which oversizes the setting and lets a real overload run too long; dialing to line current on a star-delta motor instead of the reduced delta-leg current, which also oversizes it; and mounting the relay in a hot cabinet corner where ambient compensation cannot fully offset the extra heat, which undersizes the effective setting and trips a healthy motor early. None of these show up on a quick check of the dial position. Only a nameplate cross-check and a look at where the relay physically sits catches them.
This depends on the panel's ventilation and where in the cabinet the starter sits: a relay next to a drive's heatsink runs hotter than one on an outer door panel, even with the same nameplate ambient rating.
Frequently Asked Questions
What current do I use to set an overload relay?
Use the motor nameplate full-load current (FLC), not the breaker rating, the drive's rated output current, or a generic horsepower table value.
Do I set the overload relay to the service factor current or the FLC?
Set the dial to FLC. A service factor above 1.0 widens the margin the motor can tolerate before real damage, but it is not the number dialed in.
How do I set an overload relay on a star-delta starter?
Divide the nameplate line FLC by the square root of 3, about 0.58x, and set the dial to that phase current, because the relay sits in the delta leg and sees phase current, not line current.
What happens if the overload relay setting is too high?
The motor can run in a real overload condition, current above FLC for an extended time, without the relay tripping in time, and the windings overheat before protection engages.
Which trip class should I pick for a fan or pump motor?
Class 10 or 10A covers most pumps and fans, which reach full speed in a few seconds. Longer-accelerating loads need Class 20 or 30 so the relay does not read the start-up current as a fault.
Conclusion
Selecting and setting a thermal overload relay comes down to four decisions: read the FLC off the actual nameplate, pick the range and frame that bracket it and fit the contactor, dial the set point to that FLC (or the corrected delta-leg value on a star-delta starter), and choose the trip class that matches how long the load takes to accelerate. Get the nameplate and the trip class right, and the relay does its one job: protect the winding on a sustained overload without shutting the motor down on every normal start. For deeper detail on the surrounding starter design, see the thermal overload relay engineering guide and Type 1 vs Type 2 coordination. Stock for thermal overload relays and matching motor protection circuit breakers is available at Stoklink.
