Tech Time: When Your Thermal Overload Trips

A tripped thermal overload is one of those events that is an opportunity to avoid unnecessary downtime and repeat calls that not every shop understands. Some techs reset overloads over and over until something breaks. Wiser techs just take a couple extra minutes to find the problem and they touch it once. The difference is sequence and understanding.

Here's an approach that starts where you already are and rules out causes efficiently.

What could have caused it?

Before you do anything, it helps to have the full list of possibilities in your head:

1. The motor was overloaded - too much mechanical demand on the shaft

2. Single-phasing - loss of one inbound power leg, or a worn contactor pole

3. Too many starts in a short window - thermal accumulation in the overload element

4. The overload is set wrong - FLA dial doesn't match the motor

5. A wiring fault - short, break, or high-resistance connection in the circuit

6. High ambient temperature - a cabinet in direct sunlight or a hot press room raises the temperature around the overload element itself, lowering its trip threshold even when the motor is correctly loaded

A quick note on overload types

Traditional bimetallic and melting-alloy overloads work by physically heating up in response to current. Solid-state overloads use current transformers to measure current electronically, there’s no thermal element inside. The troubleshooting sequence in this post applies to both, but if you're working with a solid-state unit, the reset behavior and trip class settings will differ from what you may be used to. Check the manufacturer documentation for your specific unit before assuming a failed reset means a failed overload.

Step 0 — Observe, check supply voltage, then investigate before you reset

When you arrive at a tripped overload, resist the instinct to reset it first. Here's the reality: the line is already down, that clock is already running. A complete electrical and mechanical check takes about 10 minutes, most of which you've already lost the moment the overload tripped. Spending that 10 minutes on a proper evaluation costs you little extra and prevents you getting called back 10 minutes later.

What it buys you is significant. If something is developing, (a loose wire, a worn contactor contact, a winding starting to go, or a mechanical issue in the driven machine), you find it now, while the line is already stopped and while you have time to make a deliberate decision. You reset with confidence, order whatever parts are needed, and schedule the repair on your terms. The alternative is resetting blind, restarting, and finding out what was wrong when it fails completely, likely at the worst possible time and with no parts on hand.

While the cabinet is still energized, check voltage above the contactor across all three legs. You're confirming balanced supply voltage before you lock out. A missing or low leg is your single-phasing answer on the supply side, and you need power in the cabinet to get that reading. Once you have it, lock out the cabinet and stay locked out for everything that follows.

Reset the overload only after you've completed the investigation, identified or ruled out each cause, and made any corrections. Then reset, return to service, and get a clamp meter on it under load to confirm running amps are where they should be.

The electrical check — all from the cabinet

Lock it out first. This eliminates induced voltage that will corrupt your resistance readings and makes everything that follows safe. You can't trust ohmmeter results on an energized or partially energized circuit.

Before you probe anything, read the overload FLA dial. Five seconds. Hold that number in your head or jot it down - you'll confirm it against the motor nameplate when you're standing there. No second trip back to the cabinet.

When you get to the motor, here's how to evaluate the setting: the NEC allows overloads to be set at 125% of FLA for motors with a service factor of 1.15 or higher, and 115% of FLA for motors without one. In practice, the correct dial setting is FLA multiplied by the service factor - both numbers are on the nameplate. A 20A motor with a 1.15 SF has a maximum allowable setting of 23A. If the dial is significantly below that, you may have a nuisance trip. If it's set above it, the motor is underprotected.

One important caveat: service factor is a short-term reserve designed for temporary voltage dips or brief shock loads, not continuous operation. If a motor is running in its service factor all the time, that's a loading problem worth addressing separately. The overload setting should reflect what the motor is designed to run at continuously, not the maximum it can tolerate in a pinch.

Test resistance from the bottom of the overload. This single position lets you read through the entire circuit in one shot. Check the three phase-to-phase pairs, and check line-to-ground on all three.

What the results tell you:

• Good result: All three pairs within 10% of each other, and open-line to ground on any single lug. Electrical system is healthy - continue.

  
  

• OL, OL, and a reading: Two of your phase-to-phase pairs are open or show unusually high resistance, one reads normally. The wire that is common to both abnormal readings is the one with the break or high-resistance connection - a fully broken wire gives you OL on both pairs, a loose or corroded connection may just give you two elevated readings instead. Either way, the pattern is the same: find the wire that shows up in both problem pairs, and that's your suspect.

  
  

• Three numbers more than 10% apart: Uneven resistance across phases - likely a wire-to-wire short or motor winding damage. Before you chase anything, go to the motor junction box, disconnect the motor leads, and retest resistance there. If the readings balance out at the junction box, the motor is healthy and the fault is in the wiring between the cabinet and the motor - chase the outlier leg through the conduit run. If the readings are still unbalanced at the junction box, the fault is in the motor windings themselves - you're looking at a motor replacement. That one retest splits your repair path cleanly and saves you from chasing wiring on a bad motor or pulling a healthy motor on a wiring fault.

  
  

• Resistance to ground on any leg: Insulation breakdown somewhere in the circuit. You don't need to probe every terminal individually, you already have your phase-to-phase readings, so you're simply checking whether any path to ground exists. A regular ohmmeter will catch a clear ground short. If you're getting a real resistance reading to ground instead of OL, something is definitively wrong. The gray zone, where the ohmmeter shows OL but insulation may still be compromised, is where a megger earns its keep. The field rule for returning to service is 1,000 ohms per volt: a 460V motor needs at least 460kΩ to ground. To isolate the fault, disconnect at the motor junction box and retest. If the ground reading clears at the junction box, the fault is in the wiring. If it's still there, it's in the motor, and a motor with a confirmed ground fault needs to come out.

Check the contactor. You're already locked out. Push the contactor in manually and check resistance through each of the three contacts top-to-bottom. You're looking for consistent low resistance on all three poles. High resistance or an open on any one contact is a single-phasing mechanism. Worn or pitted contacts can pass a no-load voltage check and still drop badly under current. This failure mode is easy to miss if you go straight to mechanical.

It's good to go ahead and reset the overload at this point too and check across its lugs. Make sure it doesn't have resistance across its contacts.

Now tighten every lug in that circuit. The resistance test just told you where to look, maybe it found something, maybe it didn't. Now torque everything in that circuit down while you're in there. Overload terminals, contactor terminals, any junction points in the run to the motor. Then take it one step further: if you found a loose lug in one power circuit in this cabinet, the rest of the cabinet deserves a look too. Loose connections don't usually happen in isolation, heat cycles, vibration, and age affect every terminal in the enclosure equally. Spending an extra few minutes going through the other circuits in that cabinet while you already have it open will save future calls that would otherwise look like unrelated problems.

The mechanical check

Now walk to the motor. Before you do anything else, check the motor temperature. As you approach, hold your hand near the motor frame and feel for radiant heat. A motor that was working hard will throw heat you can feel before you touch it. If you feel nothing, carefully make contact with the back of your hand to check for warmth. Never place your palm flat on a motor you haven't verified. A frame that's been running hot under a fault condition can cause a burn on contact.

If the motor is hot, that narrows the field immediately. A hot motor frame means the windings were generating excess heat before the overload tripped. That points directly at two causes: a mechanical overload pushing the motor beyond its rated capacity, or single-phasing forcing the remaining windings to carry more than their share of the current. Both cook the motor. If the motor is at or near ambient temperature, the trip was likely caused by something other than winding heat such as a cabinet ambient issue, a mis-set overload, or thermal accumulation from too many starts.

Lock out at the local disconnect, remove the bell housing, and spin the fan/shaft by hand. It should rotate freely with no binding, grinding, or unusual resistance. Compare the nameplate FLA (and service factor) to the overload setting you already noted at the cabinet.

Also check the cabinet environment on your way back. Is the starter cabinet in direct sunlight or in a poorly ventilated space? The overload element is running hot before the motor ever draws a single amp in those conditions. That alone can cause nuisance trips on an otherwise healthy system and points to a shading or ventilation fix rather than an electrical one.

If the shaft spins freely and the numbers match, your electrical and mechanical systems are both sound.

The operational check

Talk to the operator. Has this machine been started and stopped repeatedly in a short window? This is more common than most shops realize and easier to do than most people think. NEMA MG-1 specifies that a motor should only be started twice in succession from cold, or once from operating temperature, before stopping to investigate. Standard duty rating for most industrial applications is no more than five starts per hour. An operator who cycles power three or four times trying to get a stubborn line running has already exceeded what the overload was designed to absorb. Each start pulls six to eight times FLA through the motor for several seconds. That heat accumulates in the overload element with every attempt.

Here's the tell: if the motor was cool to the touch when you checked it during the mechanical inspection, repetitive starts becomes your primary suspect. The overload element saw the inrush current spike on every restart attempt and built up heat each time, but the motor itself never ran long enough under sustained load to get hot. Cool motor, tripped overload, operator who was cycling the start button, that’s a pattern, not a mystery. It points to a conversation about start limits rather than a parts order.

Running amps — the final confirmation

After you've completed the investigation, made any corrections, reset the overload, and returned the line to service, get a clamp meter on the motor under normal load and confirm actual running amps against the nameplate FLA. This is the check that catches what everything else misses. A motor that is electrically sound, mechanically free, and correctly set can still be running overloaded if the driven equipment has changed, process conditions have shifted, or the load is simply more than the motor was sized for. Running amps under real operating conditions is the ground truth. If they're high, you now have the information to make a real decision: adjust the load, resize the motor, or accept the risk with eyes open.

If nothing pans out

Watch for a pattern. If it trips a second time without a clear cause, go back through every connection (overload terminals, contactor, junction boxes, motor leads) and torque everything down. Intermittent connection problems are notoriously hard to catch with a meter under no-load. A terminal that reads fine cold can develop enough resistance under heat and load current to trip the overload reliably, every single time.

Why this matters more than it looks

There's a pattern that runs through a lot of shops: the overload trips, someone resets it, it runs for a while, it trips again, someone resets it again. This continues until the motor fails completely and then the shop goes into reactive mode, dealing with a damaged winding or a seized motor, expediting parts, losing production, and absorbing the full cost of an unplanned outage. In manufacturing, even a brief forced stoppage is expensive. A motor replacement plus downtime plus emergency labor plus lost production can run into the thousands before anyone has written a single work order.

What makes this especially costly is that some of the causes on this list would take seconds to fix if someone had just looked. A loose wire that's causing single-phasing will destroy a motor if you keep resetting it. That same wire, found during a proper investigation, gets tightened in seconds and the line is back up. The difference between a five-second repair and a motor replacement is whether someone pulled out a meter before hitting the reset button.

A single thorough check, done once, will always outperform a dozen sloppy resets without diagnosis. The line is already down. The clock is already running. Spend the 10 minutes.

Do the check. Do it thoroughly. Do it every time.