For standard AC induction motors with a 1.15 Service Factor (SF), size the overload protection at exactly 115% of the motor's Full Load Amps (FLA). For motors with a 1.0 SF, the threshold bumps to 125% of FLA. This is the baseline rule of thumb derived from NEC Article 430, designed to let the motor handle brief startup inrush currents while tripping the circuit before the stator windings melt during a sustained mechanical jam.
But motor overload sizing is not a one-size-fits-all calculation. The protection strategy changes radically depending on whether you are driving a 3-phase AC induction motor with a bimetallic relay, a stepper motor with a chopper drive, or a servo with an electronic thermal model. Below, we break down the exact math, the physical wiring of the control circuit, and the failure signatures you need to recognize on the bench.
Motor Types, Load Profiles, and Protection Methods
Before you size a relay, you must match the motor to the mechanical load profile. Treating a stepper and a servo as interchangeable is a common mistake that leads to either massive over-engineering or catastrophic thermal failure. Steppers rely on open-loop current chopping and will silently overheat if stalled at 100% duty cycle, while servos use closed-loop encoders to fault out on position error before thermal damage occurs.
| Motor Type | Torque Curve | Control / Driver Needs | Relative Cost | Overload Protection Method |
|---|---|---|---|---|
| AC Induction (3-Phase) | Constant torque to rated speed; high locked-rotor inrush (600% FLA). | DOL Contactor, Soft Starter, or VFD. | Low | Thermal bimetallic/eutectic relay or VFD I²t algorithm. |
| BLDC (Brushless DC) | High starting torque; flat torque curve through mid-range. | Electronic ESC with Hall sensors or sensorless back-EMF sensing. | Medium | Drive-side current foldback and thermal throttling. |
| Stepper | High holding torque; rapid torque drop-off at high RPM. | Chopper drive (constant current PWM); open-loop. | Low/Medium | Drive current limit (requires external thermal management for stalls). |
| AC Servo | Peak torque (300% rated) for acceleration; precise dynamic control. | Closed-loop servo drive with high-res encoder feedback. | High | Drive-side electronic thermal model + following-error fault. |
Matching the Load and Reading Failure Signatures
Which motor fits this load? If you have a high-inertia load like a centrifugal pump or a conveyor belt that needs to run continuously at a fixed speed, the AC Induction motor is your workhorse. If you need precise positioning at low speeds without a gearbox, use a Stepper. If you need rapid acceleration, high-speed dynamic moves, and exact positional feedback (like a CNC spindle or robotic arm), you must use a Servo.
When things go wrong, the motor tells you what failed if you know how to listen:
- The Hum (AC Induction): A loud 60Hz/120Hz hum without rotation usually indicates single-phasing (one leg of a 3-phase supply is dead) or a locked rotor. The motor is drawing massive current but cannot generate a rotating magnetic field. The overload relay must trip within seconds.
- The Overheat (Stepper/BLDC): Because chopper drives artificially limit current to the rated RMS value, a stalled stepper won't trip a breaker. It will just sit there, converting all that holding current into heat. If the casing exceeds 80°C, you need a drive with idle-current reduction or active cooling.
- The Stall (Servo): A servo drive will detect that the actual encoder position does not match the commanded position. It will throw a 'Following Error' or 'Overload' fault code on the HMI and disable the PWM outputs long before the windings reach thermal failure.
The Math: Sizing Rules and a Worked AC Induction Example
The NFPA 70 (NEC) Article 430.32 dictates the maximum allowable setting for motor overload protection. The core principle is that the overload device must protect the motor at a percentage of its nameplate FLA, determined by its Service Factor (SF) and temperature rise rating.
Worked Example: 5 HP, 3-Phase, 230V Conveyor Motor
Let's size the overload for a real-world scenario. You are wiring a 5 HP, 3-phase, 230V AC induction motor driving a packaging conveyor.
- Read the Nameplate: The motor nameplate states FLA = 15.2A and SF = 1.15.
- Apply the Multiplier: Because the Service Factor is 1.15 (or greater), the NEC allows a multiplier of 1.15. (If the SF was 1.0, we would use 1.25).
- Calculate the Trip Setting: 15.2A × 1.15 = 17.48A.
- Select the Hardware: You need an adjustable thermal overload relay that spans this value. A standard Schneider Electric TeSys LRD21 or Siemens 3RU2116 (adjustable range 14A to 20A) is the correct physical frame.
- Set the Dial: Use a flathead screwdriver to dial the overload relay exactly to 17.5A.
Edge Case - Ambient Temperature: If this conveyor is in a walk-in freezer or a hot foundry, standard bimetallic relays will trip early or late. You must either buy an overload relay with an ambient temperature compensation probe (like the Eaton PKZM0 series) or manually adjust the dial based on the manufacturer's derating curve.
Wiring the Overload: Terminal Identification and Setup
Sizing the relay is only half the battle; wiring it into the control logic is where most DIYers and junior techs make mistakes. A standalone thermal overload relay does not physically break the 3-phase power lines like a circuit breaker. Instead, it uses a low-current auxiliary contact to drop power to the main contactor's coil.
Power Circuit Terminals (Line and Load)
The main power flows directly through the overload relay's power poles, which sit physically between the contactor and the motor.
- L1, L2, L3 (Line Side): These terminals connect to the bottom of the main contactor (T1, T2, T3 of the contactor). Power flows from the disconnect, through the contactor, into the L terminals of the overload.
- T1, T2, T3 (Load Side): These terminals connect directly to the motor's U, V, W winding leads. The internal bimetallic strips sit between the L and T terminals, heating up proportionally to the current draw.
Control Circuit Terminals (The Logic Drop)
This is the critical safety path. When the bimetallic strip bends from excess heat, it pushes a mechanical trip bar that opens a set of dry contacts.
- 95 and 96 (Normally Closed - NC): This is your trip circuit. You wire the 24VDC or 120VAC control power through terminals 95 and 96, and then into the contactor's coil (A1). If the overload trips, 95-96 opens, de-energizing the contactor coil, and dropping the main 3-phase power. Never bypass this.
- 97 and 98 (Normally Open - NO): This is your fault indication circuit. These contacts close only when the overload has tripped. Wire these to a PLC digital input or a red fault indicator light on the panel door so the operator knows the motor faulted, rather than assuming the stop button was pressed.
Electronic Drives vs. Standalone Relays
If you are using a Variable Frequency Drive (VFD) instead of a mechanical contactor, you generally do not need a physical thermal overload relay. Modern VFDs (like the Yaskawa GA800 or Allen-Bradley PowerFlex 525) calculate motor heating internally using an I²t (Current-squared-time) electronic thermal model.
You simply program the VFD with the motor's nameplate FLA, SF, and the number of poles. The drive's microprocessor tracks the thermal mass of the motor in real-time. If you command a heavy load at low RPM (where the motor's internal shaft fan isn't moving enough air to cool the casing), the VFD will mathematically predict the overheat and fold back the output frequency before damage occurs.
However, if your VFD is driving multiple motors in parallel, or if the VFD is bypassed via a contactor for 'across-the-line' fallback operation, NEC code requires physical overload relays on each individual motor branch. Furthermore, as Fluke's motor troubleshooting guidelines emphasize, electronic drive protection does not eliminate the need for periodic thermal imaging of the physical connections; a loose T1 terminal will cause localized resistive heating that an internal VFD algorithm cannot see.
Ultimately, correct motor overload sizing bridges the gap between theoretical nameplate data and the harsh reality of mechanical jams, voltage sags, and ambient heat. Dial the relay to the exact calculated FLA multiplier, wire the 95/96 NC contacts into your safety chain, and respect the distinct thermal limits of steppers and servos.
