Motor Overload Relay Tripping: Causes, Diagnosis, and Lasting Fixes

Key Takeaways: An overload relay trips because the motor drew more current than its safe thermal limit for too long.

That is a symptom with only a handful of real causes: a mechanical overload on the driven equipment, low or imbalanced supply voltage, single-phasing, a wrong relay setting or class, or excessive ambient heat. Diagnosis starts with a clamp meter on all three phases, compared against the motor's nameplate full-load amps, before anything gets replaced.

What the overload relay is protecting

The overload relay models the motor's winding temperature from the current flowing through it. Sustained current above the dial setting means the windings are heating toward insulation damage, so the relay opens the contactor circuit. It is slow by design (thermal), unlike a breaker or fuse that reacts to short-circuit current instantly.

A tripping overload relay is therefore telling you the motor has been working too hard for too long, not that a momentary spike occurred.

The real causes, in order to check

• Mechanical overload. Failing bearings in the motor or driven machine, an over-tightened belt, product jams, a pump running far off its curve, material buildup. This is the most common cause and the one most often "fixed" by upsizing the relay instead of repairing the machine.
• Voltage problems. Low voltage raises current for the same load; phase imbalance raises heating dramatically (a small voltage imbalance produces a much larger current imbalance).
• Single-phasing. One phase lost (blown fuse, failed contact): the remaining phases carry heavily increased current. A three-phase current measurement catches it immediately.
• Wrong relay setting or trip class. The dial should match the motor's nameplate full-load current (adjusted per the wiring configuration), and the trip class (10, 20, 30) should match the starting duty. A class 10 relay on a high-inertia load trips on every hard start.
• Heat. High ambient in the enclosure, blocked ventilation, or sun-load on outdoor panels shifts the relay's thermal model.

Measurement-first diagnosis

Safety first: steps 1 and 3 are measurements inside an energized panel. They are work for a qualified person with a properly rated meter and the arc flash PPE your site requires. Anything hands-on, from turning the driven machine by hand to checking lugs and contactor poles, starts with lockout, tagout and a verified zero-voltage check.

• 1. Measure running current on all three phases and compare to nameplate FLA and to each other.
• 2. Balanced but high current: the load is real, look at the driven equipment (turn it by hand during lockout, check bearings, belts, product path).
• 3. Imbalanced current: measure phase-to-phase voltage and chase supply issues, then lock out and verify zero voltage before you check for loose lugs or a failing contactor pole.
• 4. Normal current but still tripping: verify the relay's setting and class, then suspect the relay itself or panel heat.
• 5. If a VFD drives the motor, its trip codes contain the same story with more detail: see our guide to VFD overcurrent faults.

Fixes that last

The lasting fix addresses the current draw, not the trip. Repair the mechanical cause, correct the supply problem, set the relay to nameplate values, and document each trip in your CMMS with the measured currents, so the third trip on the same conveyor becomes a work order for a bearing, not another reset.

Trending motor current through condition monitoring catches the load creeping up weeks before the trip.

And because operators often just reset and restart, these events vanish from manual logs: Fabrico's computer-vision-verified OEE records every one of these short stops with true duration, which is how repeated "small" trips finally show up as the capacity loss they are.

Related drive-fault guides: VFD overcurrent fault, VFD overvoltage fault, Schneider Altivar and Danfoss VLT.

Overload is only one of several reasons a motor runs hot. For the full set of causes and how to tell them apart, see electric motor overheating.

How to reset an overload relay safely

Before you press reset, take the three-phase readings above so you know why it tripped. Then check the relay's reset mode: many overload relays can be set for manual or automatic reset. Keep manual reset wherever an automatic reset could restart a conveyor or machine while someone is working near it, because EU machinery rules require that a machine can only be started by a deliberate action on a control. If it trips again straight after a reset, stop and diagnose rather than resetting it again.

What the trip class number means

IEC 60947-4-1 defines the trip class by the maximum time the relay takes to trip at 7.2 times its current setting: class 10A trips within 2 to 10 seconds, class 10 within 4 to 10, class 20 within 6 to 20 and class 30 within 9 to 30. A motor that takes longer than about 10 seconds to reach speed will usually trip a class 10 relay during the start, which is what class 20 and class 30 relays are for.

Put a number on voltage imbalance

Voltage unbalance is the largest deviation of any phase-to-phase reading from the average of the three, divided by that average, times 100. For example, 398 V, 402 V and 390 V average 396.7 V, the largest deviation is 6.7 V, and the unbalance is 1.7 percent. NEMA MG 1 recommends keeping unbalance at the motor terminals under 1 percent and derating the motor above that, and a 1 percent voltage unbalance can produce 6 to 10 percent current unbalance. That is why a small supply problem shows up as an overload trip.

FAQ

Can I just press reset and keep running?
Once, while you plan the measurement. Repeated resets without diagnosis cook motor insulation and convert a bearing-sized repair into a motor-sized one.

Is it ever correct to increase the relay setting?
Only up to the motor nameplate full-load current per the wiring scheme. Setting a relay above nameplate to stop nuisance trips removes the motor's protection.

The motor trips only on hot afternoons. Why?
Panel or ambient heat is shifting the thermal model, or the supply sags at peak load times. Check enclosure ventilation and afternoon voltage.

To see how automatic capture of every stop, even the two-minute resets nobody logs, changes your view of real capacity, book a demo.

Conveyor drives are frequent overload offenders: belt mistracking and material buildup raise running load, as covered in conveyor belt tracking problems.