Proper 3 phase motor overload protection is typically sized at 115% to 125% of the motor’s Full Load Amps (FLA), depending on the nameplate service factor. For a motor with a 1.15 service factor, set the thermal overload relay to 115% of FLA; for motors without this rating, use 125%. This protection prevents insulation burnout during sustained mechanical overloads or phase loss, acting independently of the branch circuit short-circuit breaker. While the breaker protects the wire from catastrophic short circuits, the overload relay protects the motor windings from thermal degradation.
Failure Signatures and Overload Trip Classes
Before selecting a relay, you must identify the failure signature your specific application is most likely to encounter. Motors fail in distinct ways depending on the mechanical load and power quality:
- Humming and Vibration: Usually indicates single-phasing (one phase lost) or a locked rotor. The motor draws massive current on the remaining two phases, rapidly heating the windings. Phase-loss sensitive overload relays are required here.
- Sustained Overheat: Caused by continuous operation above rated torque, poor ventilation, or high ambient temperatures. The winding insulation breaks down over time (the 10-degree rule: insulation life halves for every 10°C rise above rating).
- Stall: A mechanical jam or severe voltage sag prevents the rotor from reaching synchronous speed. The motor draws locked-rotor current (typically 6x to 8x FLA) indefinitely until the overload trips.
Overload relays are categorized by NEMA and IEC standards into 'Trip Classes', which define how long the relay takes to trip at 600% of the FLA setting (simulating a locked rotor or across-the-line start). Choosing the wrong class results in nuisance tripping during startup or burned windings during a stall.
| Trip Class | Trip Time at 6x FLA | Typical Application | Load Inertia Profile |
|---|---|---|---|
| Class 10 | 10 seconds (max) | Submersible pumps, rapid-cycling compressors | Low inertia, fast acceleration required |
| Class 20 | 20 seconds (max) | Centrifugal pumps, fans, general machine tools | Medium inertia, standard acceleration |
| Class 30 | 30 seconds (max) | Crushers, large blowers, conveyors | High inertia, long acceleration times |
| Class 40 | 40 seconds (max) | Large centrifuges, induced draft fans | Extreme inertia, very slow run-up |
Motor Types, Load Profiles, and Protection Demands
The type of motor dictates both the control topology and the specific demands placed on the 3 phase motor overload protection. You cannot blindly apply a standard thermal relay to every motor type without considering the drive method and torque curve.
| Motor Type | Torque Curve & Starting | Required Driver/Controller | Overload Protection Strategy | Relative Cost |
|---|---|---|---|---|
| TEFC Squirrel Cage (Induction) | High starting current (6x FLA), standard breakdown torque. | DOL (Direct-On-Line) contactor, Soft Starter, or VFD. | Bimetallic or solid-state thermal relay (Class 10/20). Bypassed if VFD handles protection. | Low |
| Wound Rotor (Slip Ring) | Low starting current, extremely high starting torque via external resistance. | Rotor resistance bank controller, specialized contactor arrays. | Stator overload relay (Class 20/30) plus rotor circuit thermal monitoring. | High |
| Permanent Magnet Synchronous (PMSM) | High torque at zero speed, precise dynamic control. | VFD with Field Oriented Control (FOC) or servo drive. | Electronic protection internal to the drive (PTC/PT100 stator sensors required). | Medium-High |
If you are driving a standard squirrel cage motor with a modern Variable Frequency Drive (VFD), the VFD's internal electronic thermal model (often conforming to IEC 60947-4-1) usually replaces the need for a physical hardware overload relay. However, if the VFD is configured to bypass and run the motor DOL after starting, or if local code requires a physical disconnect and trip mechanism, you must still wire a physical overload relay in the bypass path.
Sizing the Overload Relay: Rules of Thumb and a Worked Example
Sizing 3 phase motor overload protection relies on the motor nameplate data, specifically the Full Load Amps (FLA) and the Service Factor (SF). According to NEC Article 430.32, the maximum setting for a thermal overload relay is governed by these parameters to ensure the motor can handle brief, harmless overloads without tripping, while still protecting the windings from sustained thermal damage.
The Sizing Rule of Thumb:
- If the motor nameplate Service Factor is 1.15 or greater, set the overload relay to 115% of the FLA.
- If the motor nameplate Service Factor is less than 1.15 (or not marked, defaulting to 1.0), set the relay to 125% of the FLA.
Never size the overload based on the motor's horsepower or kilowatt rating alone. A 10 HP motor driving a high-inertia flywheel will draw different starting currents and require different thermal mass handling than a 10 HP motor driving a lightly loaded conveyor. The FLA and SF account for the specific thermal design of that exact stator.
Worked Load Example: Centrifugal Pump Application
Let’s size the overload for a specific jobsite scenario. You are installing a 15 HP, 460V AC, 3-phase TEFC motor to drive a centrifugal water pump.
• Horsepower: 15 HP
• Voltage: 460V / 3-Phase
• Full Load Amps (FLA): 19.6A
• Service Factor (SF): 1.15
• NEMA Code Letter: F (Locked Rotor kVA/HP = 5.6 - 6.29)
Step 1: Calculate the Trip Setting
Because the Service Factor is 1.15, we use the 115% multiplier.
19.6A (FLA) × 1.15 = 22.54A.
Step 2: Select the Relay Range
You need a relay whose adjustment range encompasses 22.54A. Looking at a standard catalog like the Schneider Electric TeSys LRD series, the LRD22 model covers a range of 16A to 24A.
Step 3: Set the Dial and Verify Class
Set the physical dial on the LRD22 to exactly 22.5A. Because a centrifugal pump has a medium-inertia load profile (torque increases with the square of the speed), a standard Class 20 trip curve is appropriate. The motor will accelerate to full speed in roughly 4 to 8 seconds, well below the 20-second maximum trip time at 6x FLA, preventing nuisance trips during startup.
Wiring the Control Circuit: Terminal Identification and Interlocks
Physical installation requires correct terminal identification to ensure the overload relay interrupts the contactor coil circuit during a fault while simultaneously signaling the control system. Modern relays, such as the Eaton XTCE or ABB AF series compatible thermal overloads, follow standardized IEC terminal numbering.
Working inside a motor control center or starter enclosure exposes you to lethal 460V/480V AC potentials. De-energize the main breaker, apply lockout/tagout (LOTO), and verify the absence of voltage using a CAT III or CAT IV rated multimeter tested on a known live source before and after verification. Local codes may require a licensed electrician for this work.
Main Power and Load Terminals
The overload relay is mounted directly below the main 3-phase contactor. Power flows from the contactor's output, through the overload's heating elements, and out to the motor.
- Line In (from Contactor): Terminals marked L1, L2, L3 (or 1, 3, 5 on some legacy NEMA blocks). Connect the contactor's T1, T2, T3 outputs here.
- Load Out (to Motor): Terminals marked T1, T2, T3 (or 2, 4, 6). These connect directly to the motor peckerhead (U, V, W).
Control Circuit Terminals (The Logic Interlock)
The overload relay contains an internal trip mechanism linked to a set of dry contacts. These contacts control the contactor coil and provide fault feedback.
- 95 and 96 (Normally Closed - NC): This is the critical safety interlock. Wire these terminals in series with the contactor coil (A1/A2). Under normal operation, current flows through 95-96 to hold the contactor closed. If the bimetallic strips bend from excess heat, the mechanism forces 95-96 open, dropping power to the coil and opening the main power contacts.
- 97 and 98 (Normally Open - NO): These close only when the relay trips. Wire these to a PLC input, an indicator lamp, or a SCADA alarm to signal 'Motor Fault' to the operator.
Testing the Trip Mechanism
After wiring, do not rely solely on the math to verify safety. Use the physical 'Test' button located on the face of the overload relay. With the motor running at no-load (or the control circuit energized with the main breaker locked out), press the Test button. This mechanically simulates the tripping action, forcing the 95-96 NC contacts open. The contactor must immediately drop out. If it does not, your control wiring is bypassing the overload interlock, rendering the 3 phase motor overload protection completely useless.
