Setting an Overload Relay: FLC, Service Factor and the Dial

How do you set a thermal overload relay? The dial is set to the motor's full-load current (FLC) read from the nameplate, not the horsepower or kW rating, with the exact margin above FLC governed by the relay's IEC 60947-4-1 setting range and the motor's service factor. Set it too low and the motor nuisance-trips on every start or on a hot afternoon; set it too high and the windings overheat well before the bimetal ever moves. This piece covers where to read FLC, how service factor shifts the margin, the star-delta exception, bimetallic vs electronic setting ranges, and the mistakes that turn a correctly specified relay into a burned motor.

What the Overload Relay Dial Actually Sets

The dial on a thermal overload relay is calibrated in amps, and that number is the current at which the bimetal strips are sized to trip on an inverse-time curve. It is not the motor's horsepower, not the branch-circuit fuse rating, and not the locked-rotor current printed a line or two below FLC on the nameplate. The relay sits between the contactor and the motor and carries the same current the windings do, so the dial is a proxy for the motor's own thermal limit, not for the wiring or the starter feeding it. Confuse the numbers and the relay protects the wrong thing: set to locked-rotor current and it never trips on a real overload; set to breaker rating and it trips on every normal start.

A motor's nameplate carries several current values, and only one is the dial target. Locked-rotor current (or the code letter that bounds it) describes the inrush at start, six to eight times FLC on a typical squirrel-cage motor, and the relay is deliberately slow enough to ride through it without tripping. Service current under partial load is lower than FLC and changes with the driven load, so it is not a fixed reference either. FLC is the one number tied to the nameplate rating, and it is the one the relay's inverse-time curve is built around.

Reading FLC Off the Motor Nameplate, Not a Table

Generic horsepower-to-amps tables exist in wiring handbooks and code appendices, and they are useful for a rough panel layout before a motor is on order. They are the wrong source once the actual motor is on the bench. Nameplate FLC on a specific frame can sit 10% or more away from a generic table value once efficiency class, design letter, and rewind history are factored in — an IE3 motor commonly draws less current than an older IE1 design of the same output, and a rewound motor can drift from its original figure. Set the dial from the table and the relay is calibrated to a motor that is not actually bolted to the panel.

What we see in the field is a mismatched relay left over from a motor swap: the panel builder replaces a tripped or seized motor, reuses the existing overload relay, and leaves the dial where it was. If the replacement motor's FLC differs from the original by more than a percent or two, the setting has to move with it. This is a five-second check during commissioning that gets skipped more often than it should.

Service Factor and How Much Margin to Leave

Service factor (SF) is a separate nameplate figure from FLC, and it changes how much headroom the dial setting is allowed above that current. An SF 1.0 motor has no built-in thermal margin, so the setting sits close to nameplate FLC. A motor marked with a higher service factor, commonly 1.15, is rated to carry that extra output continuously without exceeding its insulation temperature limit, and that extra thermal capacity is why field practice under widely used electrical codes allows a wider overload setting on SF 1.15-or-higher motors than on SF 1.0 motors of the same frame. The relay's own catalog setting range still has the final say — a setting outside the range printed on the relay body or datasheet is not achievable regardless of what the motor allows.

Duty cycle matters here too. A motor on continuous duty gets the setting closest to its calculated margin; a motor on short-time or intermittent duty, where it cools between runs, can sometimes tolerate a setting nearer the top of the allowed range without nuisance tripping, but that depends on how long the off-time actually is relative to the thermal time constant of the winding, and it is a judgment call rather than a fixed rule.

Setting for Star-Delta and Reduced-Voltage Starters

On a straight direct-on-line starter, the overload relay sees the full line current and the dial is set to nameplate FLC as described above. A star-delta starter changes the picture: if the relay sits in the delta leg (the usual location), it carries phase current rather than line current, and phase current works out to line FLC divided by the square root of three, about 0.58 times the nameplate figure. Set the dial to full nameplate FLC on a delta-leg relay and it is over-set by nearly half, meaning a real overload may never trip it. The full derivation and worked numbers are covered in overload relay sizing for star-delta starters and in how to wire a contactor for star-delta motor starting.

Reduced-voltage soft starters are different again: most keep the overload relay in the main line current path, so the dial goes back to nameplate FLC with no square-root correction. Check the specific starter's wiring diagram before assuming either convention — misreading which leg the relay sits in is one of the more common setting errors on multi-contactor starters.

Bimetallic vs Electronic Setting Ranges

A bimetallic relay's setting ratio, the span between its lowest and highest dial position, runs around 1:1.5 — a relay marked 9-13 A only covers that narrow band. FLC has to land inside the frame's printed range, or no dial position gets there; go outside it and the answer is a different frame size, not a different dial setting. That narrow ratio is also why bimetallic lines like Schneider TeSys LRD, ABB TA, and Siemens SIRIUS 3RU2 ship in a long run of overlapping current ranges rather than one part number per contactor size.

An electronic overload relay reads current through current transformers or shunts and models the heating in a microcontroller, which is why its setting ratio opens up to roughly 1:3 or 1:4 on a single part number. Setting is a dial, keypad, or DIP-switch entry in amps or percent, trip class is selectable rather than fixed, and one unit covers motors that would otherwise need three or four bimetallic frame sizes. The comparison between the two families, including where the wider range earns its higher price, is covered in thermal vs electronic overload relays, and trip class selection specifically in overload relay trip classes 10A, 10, 20 and 30.

Common Setting Mistakes That Cause Nuisance Trips or No Protection

Setting to the branch circuit breaker or fuse rating instead of FLC is the most common error on a panel with mixed trades — the overload relay ends up sized to protect against a short circuit it was never meant to clear, while the motor windings sit unprotected against a sustained overload. Setting to a clamp-meter reading taken under partial load rather than nameplate FLC is a close second; that reading moves with whatever the driven equipment happens to be doing that day; and a relay set to it drifts out of calibration every time the load changes.

Leaving the dial at the factory default, usually mid-scale on the frame's range, after swapping in a different-frame motor is another one. So is skipping the reset-mode check: a relay left on auto reset re-closes on its own once the bimetal cools, which can restart a motor into the same fault it just tripped on; hand reset is the safer default for most applications, and the trade-offs are covered in manual vs automatic reset on overload relays. Finally, forgetting the star-delta correction, or applying it to a starter that does not need it, either under-protects or nuisance-trips a perfectly good motor. None of these are exotic failures — they are the setting checks a commissioning walk-through is supposed to catch before the panel is energized.

Conclusion

The dial setting on an overload relay comes down to one number, nameplate FLC, adjusted for service factor within the relay's own catalog range, and corrected for line-versus-phase current on a star-delta starter. Bimetallic relays need the right frame before the dial matters at all; electronic relays trade that constraint for a wider range and a selectable trip class. Get the reset mode and the coordination with the upstream motor protection circuit breaker or fuse right at the same time, and the three-device starter does the job it was specified for. For the full picture of how the relay fits with the contactor and the short-circuit device, see the thermal overload relay engineering guide and how to select and set a thermal overload relay for a motor.