Overload Relay Diagram: Wiring the Trip Contact in a Motor Control Circuit

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An overload relay diagram shows how bimetallic or electronic trip elements connect in series with a contactor coil to protect three-phase motors against sustained overcurrent.

A motor overload relay is not simply a large fuse. Its purpose is to detect sustained overcurrent — caused by mechanical overload, single-phasing, or voltage unbalance — and disconnect the motor before the winding insulation is damaged by heat accumulation. Unlike fuses that respond to instantaneous fault currents, an overload relay integrates thermal energy over time, mirroring the actual heating of the motor winding.

In a thermal (bimetallic) overload relay, three bimetallic strips carry current proportional to motor current through heater elements. Heat deflects the bimetal, and when the cumulative deflection exceeds the trip threshold, a mechanical latch releases, opening the relay's normally closed (NC) trip contact and simultaneously closing the normally open (NO) auxiliary contact. The NC contact is wired in series with the main contactor coil in the control circuit. When this NC contact opens, the contactor coil de-energises, dropping the main contacts and disconnecting the motor from the supply.

In an electronic overload relay, current transformers sample each phase current. An internal microprocessor calculates the thermal equivalent using the I²t algorithm, modelling winding temperature with greater accuracy than bimetal. Electronic types offer adjustable trip class (Class 5, 10, 20, 30 — corresponding to trip time at 6× FLA in seconds), phase loss detection, ground fault detection, and remote reset capability.

The overload relay setting dial (or DIP switches on electronic types) must be set to the motor's full-load ampere (FLA) rating as stated on the motor nameplate — not the next higher setting, which is a common and dangerous mistake. IEC 60947-4-1 and NEMA ICS 2 govern overload relay performance standards.

Wiring: the main current path goes from contactor load-side terminals T1, T2, T3 through the overload relay's current-carrying terminals (1/L1, 3/L2, 5/L3 in) and out (2/T1, 4/T2, 6/T3) to the motor. The NC trip contact (terminals 95–96 in most IEC overload relays) is wired in series with the control circuit between the stop button and the contactor coil.

An overload relay wiring diagram shows the thermal or electronic overload relay wired in series with the motor's main contactors so that sustained overcurrent trips the relay and opens the contactor coil circuit, stopping the motor before windings overheat. The relay has a normally-closed (NC) contact in the control circuit and a normally-open (NO) contact available for alarm or indication. The trip current is set by adjusting the full-load current dial on the relay face to match the motor nameplate FLA. In a DOL (direct-on-line) starter diagram, the overload relay is the final protection element between the contactor and the motor terminals — draw and annotate any overload relay configuration for free in the online editor.

How to wire overload relay diagram

Determine motor FLA and relay selection Read the motor nameplate: note the full-load ampere (FLA) at your operating voltage (motors often show values for multiple voltages). Select an overload relay with a current range that includes the FLA — for example, for a 7.5 A FLA motor, a relay adjustable from 5.5 A to 8 A is appropriate. Confirm the relay is rated for the motor starting current and trip class.
Mount the overload relay below the contactor Most IEC overload relays are designed to be direct-mounted (plug-on) to a matching contactor — align the relay's bus bars with the contactor load-side terminals and press firmly until the mechanical locking tab clicks. For separately mounted relays, connect the load-side terminals (T1, T2, T3 of contactor) to the relay's main input terminals (1/L1, 3/L2, 5/L3) using appropriately rated cable.
Connect the main current path through the relay Confirm that motor supply current flows through all three relay heater elements — from relay input terminals (1/L1, 3/L2, 5/L3) to output terminals (2/T1, 4/T2, 6/T3). From the output terminals, run three-phase cable to the motor terminal box, connecting L1/T1 to motor terminal U, L2/T2 to V, and L3/T3 to W.
Wire the NC trip contact into the control circuit Identify the NC trip contact terminals (typically 95 and 96 on an IEC relay). Wire terminal 95 in series with the stop pushbutton output, and wire terminal 96 onward to the contactor coil. When the relay trips, this path opens, de-energising the coil and dropping the main contacts regardless of the start button state.
Connect the NO alarm contact if required Wire terminals 97 and 98 (NO contact) to any alarm circuit, PLC digital input, or pilot lamp that must indicate an overload condition. Use a separate control voltage circuit for alarm purposes — do not mix alarm and trip circuits on the same relay contact.
Set the current adjustment dial Rotate the overload relay adjustment dial (or set electronic relay parameters) to exactly the motor nameplate FLA. Confirm with the relay specification which number on the dial corresponds to which current setting. Photograph or record the setting for maintenance records.
Test the overload relay function Using a test button (most IEC overload relays have a blue test button) or an injection test set, trip the relay and confirm the motor stops. Verify the alarm contact closes by observing the indicator lamp or PLC input. Reset the relay (manual or auto as configured) and restart the motor to confirm normal operation is restored.

Specifications

IEC standard governing overload relays	IEC 60947-4-1 (Low-voltage switchgear: AC motor starters)
NC trip contact current rating (IEC typical)	6 A – 10 A at rated control voltage
Trip class (IEC) — Class 10 trip time at 6× FLA	≤ 10 seconds from cold
Typical current setting range (relay adjustable)	0.1 A – 100 A (various relay sizes; confirm model range)
Ambient temperature rating (standard thermal relay)	−20 °C to +60 °C (derating applies above 40 °C)
IEC trip contact terminal numbering (NC)	95 (common) – 96 (NC output)
IEC trip contact terminal numbering (NO alarm)	97 (common) – 98 (NO output)

Safety warnings

ALL work on motor control circuits must be performed by a qualified and authorised electrician in accordance with applicable electrical codes: IEC 60364, NEC/NFPA 70 (Article 430 for motor circuits), BS 7671, or AS/NZS 3000. Isolation, lockout/tagout (LOTO) procedures must be applied before any panel work.
Verify the circuit is dead using an approved voltage indicator or multimeter before touching any terminal. Three-phase supplies can deliver lethal energy. Isolate all sources including control voltage and any backup/UPS supplies.
Never bypass or short-circuit the overload relay NC contact to allow a motor to run after a trip. Investigate and resolve the cause of the trip. Bypassing the overload relay removes the primary motor protection and risks fire, equipment destruction, and injury.
Capacitor banks and variable-speed drives retain dangerous charge after isolation. Follow the manufacturer's specified discharge time (commonly 5 minutes minimum for VSD DC link capacitors) before touching terminals.
After setting the overload relay dial, document the setting. An overload relay set above motor FLA provides inadequate protection — the motor can overheat, the winding insulation can fail, and a fire can result.

Tools needed

Insulated screwdrivers (flathead and Pozidriv/Phillips, 1000 V rated)
Digital multimeter with AC/DC voltage, resistance, and continuity (1000 V CAT III minimum)
Approved voltage indicator (non-contact type for safe isolation verification)
Lockout/tagout (LOTO) kit with padlocks and hasp
Torque screwdriver or wrench (for terminal connections per manufacturer torque specification)
Wire ferrule crimping tool and insulated ferrules (for stranded cable terminations)
Current clamp meter (for verifying motor running current against FLA after commissioning)

Common mistakes

Setting the overload relay above motor FLA to stop nuisance trips: this is the most dangerous mistake. A motor drawing 8 A FLA protected by a relay set to 12 A can run at dangerous overtemperature for extended periods before tripping. Fix nuisance trips by diagnosing the root cause.
Omitting the self-holding (sealing) contact on the contactor: without the auxiliary NO sealing contact wired in parallel with the start button, releasing the start button immediately de-energises the contactor. The motor runs only while the button is held. The sealing contact is not optional.
Connecting the motor supply wires to the relay input (upstream) side instead of the output (downstream) side: current still flows through the main contactor but bypasses the overload relay heater elements entirely — the motor has no overcurrent protection.
Wiring the NC trip contact in the motor supply circuit instead of the control circuit: the NC contact is a low-current auxiliary contact rated for control circuits (typically 6 A to 10 A). Wiring motor supply current through it will destroy it.
Ignoring phase sequence when connecting the motor: a motor wired with swapped phases will rotate in the wrong direction. For pumps and compressors this can cause immediate mechanical damage. Check rotation direction with a brief no-load jog before full commissioning.

Troubleshooting

Motor will not start; contactor hums or clicks but drops out immediately: Cause: Overload relay NC contact (95–96) is open — relay has tripped or contacts are worn open Fix: Check whether the overload relay trip indicator (flag or LED) is showing a trip. If tripped, investigate the motor load and supply before resetting. Measure continuity across terminals 95–96 with the relay in the reset state — if open-circuit, the relay mechanism is faulty; replace the relay.
Overload relay trips within seconds of start even with no mechanical load: Cause: Relay set below motor FLA, single-phase supply (phase loss), or motor fault (winding short) Fix: Verify relay setting against motor nameplate FLA. Measure supply voltage on all three phases — if one phase is absent, the relay correctly trips to prevent motor damage. Test motor winding resistance: all three windings should read equal resistance to 0.1 Ω tolerance. Unequal resistance indicates a winding fault.
Overload relay trips on heavy start but motor and load are normal: Cause: Trip class too low for load inertia, or ambient temperature above relay rating Fix: Review motor starting time — if the motor takes more than 5 seconds to reach full speed, Class 10 may trip during start. Switch to Class 20 and verify start time. Check ambient temperature at relay location — thermal overload relays require derating above 40 °C; apply the derating factor or switch to an electronic relay with temperature compensation.

Frequently asked questions

What is the difference between an overload relay and a circuit breaker?

A circuit breaker provides short-circuit protection by responding to instantaneous fault current — it trips in milliseconds. An overload relay provides motor protection by integrating thermal energy over time, tripping in seconds to minutes under sustained overcurrent. Most installations require both: a breaker or fuses for short-circuit protection and an overload relay for motor protection. A motor circuit breaker (MPCB) combines both functions.

How do I set the overload relay current correctly?

Read the motor nameplate full-load ampere (FLA) rating at the voltage you are operating. Set the overload relay adjustment dial to this exact value. Never set it to a higher value to prevent nuisance tripping — correct any underlying cause of nuisance trips (voltage imbalance, high ambient temperature, or a failing motor) instead. Setting above FLA removes the protection the relay exists to provide.

What is trip class and which should I select?

Trip class defines how long the relay takes to trip at six times FLA: Class 10 trips in ≤10 seconds, Class 20 in ≤20 seconds, Class 30 in ≤30 seconds. Most standard squirrel-cage induction motors suit Class 10. Class 20 or 30 suits high-inertia loads (fans, centrifuges, compressors with long start ramps) where the motor legitimately runs at high current during starting.

Why does the overload relay have both NC and NO contacts?

The normally closed (NC) contact (95–96) carries the motor trip function — it is in series with the contactor coil so that when it opens the motor stops. The normally open (NO) contact (97–98) is used for alarm, fault indication, or remote monitoring circuits — it closes when the relay trips, signalling a fault to a PLC input, annunciator panel, or supervisory control system.

How long must I wait before resetting a tripped overload relay?

A thermal (bimetallic) overload relay requires a cooling period before reset — typically 1 to 2 minutes for a manual reset type, though the relay body and motor must also cool. An electronic relay may allow immediate reset if the thermal model shows the motor is within safe limits. NEVER repeatedly reset and restart without investigating the cause of the trip — repeated thermal cycling degrades winding insulation.

How is an overload relay wired in a motor control circuit diagram?

The overload relay's main current-carrying elements are connected in series in each motor phase, sitting between the contactor output terminals and the motor terminals, so all motor current passes through the bimetallic or electronic sensing elements. The relay's normally-closed (NC) auxiliary contact is wired in series in the contactor coil circuit: when the relay trips, this NC contact opens, de-energising the coil and dropping out the contactor to disconnect the motor. A manual reset button on the relay face must be pressed after a trip and after the motor has cooled, ensuring the fault is investigated before the motor is restarted.

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