Overload Relay Nuisance Tripping on Motor Start
Why does an overload relay trip on motor start? A nuisance trip on start means the relay's inverse-time thermal curve is crossing the motor's actual run-up current before the motor reaches full speed, even though no real overload exists. The consequence is a stalled startup, a nuisance-trip callback, and — if someone raises the dial to stop the trips instead of fixing the cause — a motor that is no longer protected against a genuine overload. The usual culprits are trip class mismatch on a high-inertia load, an undersized setting on a star-delta motor starter, ambient heat inside the panel, voltage sag during start, and back-to-back starts that do not let the bimetal cool.
What Counts as a Nuisance Trip vs a Real Overload?
A real overload trip happens because the motor is drawing more current than its full-load current (FLC) for longer than the relay's set trip time — a jammed pump, a seized bearing, a mechanically overloaded conveyor. A nuisance trip happens during a start that the motor would complete normally if the relay let it run 2-5 more seconds. The tell: the relay trips at a repeatable point in the start sequence, every time, on a motor that runs fine once it reaches speed. If the trip point moves around, or the motor is audibly straining, that is not nuisance tripping — that is a real fault and the relay is doing its job.
Trip Class Mismatch: The Most Common Cause
Every overload relay is built to a trip class tested at 7.2x the current setting, from cold. 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 2-4 seconds and never comes close to the Class 10 curve. A large fan, centrifuge, or crusher with a heavy rotating mass can take 10-15 seconds to run up, and a Class 10 relay set for that motor's FLC will trip partway through the start — every single time — because the relay reads the run-up current as a sustained overload. Set it wrong and the motor cooks; set the class too low and the motor never gets to start at all.
Formula: Trip Class Test Point — Source: IEC 60947-4-1, Clause 7.2.1.4 (Table 8)
ttrip at I = 7.2 × Iset (from cold)
| Symbol | Description | Unit |
|---|---|---|
| Iset | Overload relay dial setting (= motor FLC) | A |
| ttrip | Class 10A: 2-10 s | Class 10: 4-10 s | Class 20: 6-20 s | Class 30: 9-30 s | s |
Fix: match the class to the motor's real run-up time, not to whatever class shipped in the panel. See overload relay trip classes 10A, 10, 20 and 30 for the full breakdown, and read the run-up time off the motor's datasheet or a start-current trace before picking the class.
Star-Delta Starting and Undersized Relay Settings
On a star-delta starter, the overload relay normally sits in the delta leg and sees phase current, which is line FLC divided by the square root of 3 — about 0.58x the nameplate line current. If the relay was set to the full line FLC by mistake, it is now roughly 1.7x too sensitive, and it will nuisance-trip on the star-to-delta transition, where current spikes briefly as the contactors switch. This is one of the most common wiring-desk errors on star-delta motor starters, and it produces a trip pattern that looks identical to a trip-class problem — a trip during the start sequence, motor otherwise fine.
Formula: Overload Relay Setting — Star-Delta (Delta-Leg Mounting)
IOL = FLC / √3
| Symbol | Description | Unit |
|---|---|---|
| FLC | Motor nameplate full-load current (line) | A |
| IOL | Overload relay dial setting (delta-leg / phase current) | A |
Full method in overload relay sizing for star-delta starters. Confirm which leg the relay is actually wired into before recalculating — some panels mount it on the line side instead of the delta leg, which changes the math back to plain FLC.
Ambient Temperature Inside the Panel
A bimetallic overload relay includes a compensating bimetal so the trip point does not drift much between roughly -5 and +55/60 C ambient. That compensation has limits. A relay mounted above a drive, next to a resistor bank, or in a panel with no ventilation on a summer afternoon can sit well above the compensated range, and the trip point shifts down — the relay reads the motor as hotter than it is and trips on a start that would pass in a cooler enclosure. Electronic overload relays are less sensitive to their own ambient temperature than a bimetal element, because the trip decision is computed rather than driven by a heated strip, which is one reason panel builders move to electronic units in hot cabinets.
What we see in the field: relays that nuisance-trip only in summer, or only on the top row of a densely packed MCC, almost always trace back to panel heat rather than a bad relay or a wrong setting.
Voltage Sag, Frequent Starts, and Thermal Memory
Across-the-line starting on a weak supply — long cable run, undersized transformer, several motors starting close together — pulls the bus voltage down during inrush. Lower voltage on an induction motor stretches the run-up time because torque falls with the square of voltage, so the motor takes longer to reach speed and the relay sees the elevated inrush current for longer than the trip class was sized for. This looks like a trip-class problem but the fix is different: firm up the supply, stagger starts, or move to soft starting rather than upgrading the trip class alone.
Repeated starts in a short window compound the issue. A bimetal element that has not fully cooled from the last start begins the next one already partway up its heating curve, and electronic relays with thermal memory intentionally carry that history across a power cycle so the protection stays accurate — which means a motor that is jogged, plugged, or restarted quickly after a trip will trip sooner on the next attempt, correctly, not because the relay is faulty.
How to Diagnose Nuisance Tripping Before Changing the Relay
Work through this order rather than guessing:
1. Confirm the setting matches the actual current the relay sees
Check nameplate FLC against the dial, and confirm whether the relay is in the line or the delta leg on a star-delta starter. A setting error is the fastest fix and the most common cause.
2. Time the start
A clamp meter with inrush capture, or a motor starter's built-in trip log if it has one, tells you how long the motor actually takes to reach speed. Compare that time against the relay's trip class curve at the observed multiple of FLC.
3. Check the panel temperature where the relay sits
A thermometer reading at the relay body during a hot-day trip separates an ambient problem from a class problem.
4. Log the supply voltage during start
A sag of 10-15% during inrush on a motor already near its trip-class limit is enough to push a borderline setup into nuisance-trip territory.
If all four check out and the relay still trips, the trip class is the wrong one for the load — move up to the next class rather than raising the dial past the motor's FLC, which removes real overload protection instead of fixing the start problem. See how to select and set an overload relay for a motor for the full sizing checklist, and confirm the upstream contactors and motor protection circuit breakers are still coordinated to the same fault rating after any relay change.
Frequently Asked Questions
Why does my overload relay trip every time the motor starts, but the motor runs fine afterward?
The relay's trip class is almost certainly too fast for the motor's run-up time, or the setting is wrong for where the relay is wired (line vs delta leg on a star-delta starter). Time the start and compare it against the Class 10/20/30 curve before changing the dial.
What trip class should I use for a high-inertia load like a large fan or centrifuge?
Class 20 or Class 30, depending on measured run-up time. Class 10/10A suits standard pumps and fans that reach speed in a few seconds; anything with a run-up longer than about 10 seconds needs the slower curve.
Can voltage sag during start cause an overload relay to trip?
Yes. Lower voltage reduces motor torque and stretches run-up time, which extends the period of elevated current the relay sees. A relay already close to its trip-class limit can nuisance-trip under a sagging supply even though the motor and relay setting are both correct on paper.
Why does my star-delta overload relay trip on the star-to-delta transition?
Usually the relay is set to the full line FLC instead of the delta-leg current (FLC divided by the square root of 3). Recalculate the setting for the leg the relay is actually wired into.
Does mounting position or panel heat affect nuisance tripping?
Yes. Ambient compensation on a bimetallic relay covers roughly -5 to +55/60 C; a relay mounted above a hot drive or in a poorly ventilated cabinet can sit outside that range and trip sooner than its rated curve predicts.
Is it safe to just raise the overload relay's dial to stop nuisance tripping?
No. Raising the dial above the motor's nameplate FLC removes protection against a genuine overload. Fix the actual cause — trip class, setting, ambient, or supply voltage — instead of masking the symptom.
Conclusion
Nuisance tripping on motor start is a curve-matching problem, not a defective-relay problem, in the large majority of cases. Confirm the setting is correct for where the relay is wired, time the actual run-up against the trip class, check panel temperature, and log supply voltage during start before assuming the relay needs replacing. For the underlying trip-class math and star-delta sizing, see the thermal overload relay engineering guide; for available bimetallic and electronic units across TeSys, TA, and SIRIUS ranges, browse thermal overload relays.
