Motor Overload Relay Pakistan 2026: Sizing Guide
Motor Overload Relay Pakistan 2026 — Thermal Overload Sizing & Selection
Last verified: 2026-05-29 · Author: CNC Electric Pakistan Engineering Team · Reading time: 7 min
The motor overload relay (thermal overload, OL relay) is the device that prevents a motor from burning out when it draws more current than its nameplate FLA for too long. Sized correctly, it gives years of trouble-free operation. Sized wrong, you'll either trip every time the motor starts, OR you'll let the motor cook itself to death over weeks of slight overload. This guide tells you exactly how to size and wire one.
Quick Answer — Motor to Overload Sizing Chart
| Motor HP | 3-phase FLA @ 400V | Overload Range Setting | CNC OL Model | Price (PKR) |
|---|---|---|---|---|
| 1 HP / 0.75 kW | 2.1 A | 1.6 – 2.5 A | JR36-20 / 2.5 | Rs. 1,400 |
| 3 HP / 2.2 kW | 5.4 A | 4 – 6.3 A | JR36-20 / 6.3 | Rs. 1,400 |
| 5 HP / 3.7 kW | 8.5 A | 6.3 – 10 A | JR36-20 / 10 | Rs. 1,500 |
| 7.5 HP / 5.5 kW | 12.4 A | 10 – 16 A | JR36-20 / 16 | Rs. 1,600 |
| 10 HP / 7.5 kW | 16.6 A | 14 – 22 A | JR36-32 / 22 | Rs. 2,200 |
| 15 HP / 11 kW | 24.1 A | 20 – 32 A | JR36-32 / 32 | Rs. 2,400 |
| 20 HP / 15 kW | 31.7 A | 25 – 40 A | JR36-63 / 40 | Rs. 3,600 |
| 30 HP / 22 kW | 44.7 A | 40 – 63 A | JR36-63 / 63 | Rs. 4,200 |
| 50 HP / 37 kW | 73.1 A | 63 – 100 A | JR36-160 / 100 | Rs. 7,500 |
| 75 HP / 55 kW | 108 A | 100 – 160 A | JR36-160 / 160 | Rs. 9,500 |
Sizing rule: Set overload at the motor's nameplate FLA (Full Load Amps) — not at the breaker rating. The overload protects the motor windings; the breaker protects the cable.
Class 10, 20, 30 — Trip Time Curves
| Class | Trip Time @ 720% FLA | Use |
|---|---|---|
| Class 10A | 2-10 sec | Quick-start motors (compressors, hermetic pumps) |
| Class 10 | 4-10 sec | Standard motors (default — 90% of Pakistani applications) |
| Class 20 | 6-20 sec | Heavy-start motors (mixers, conveyors with full load) |
| Class 30 | 9-30 sec | Very-heavy-start (centrifuges, large fans, crushers) |
Match the overload class to the motor's locked-rotor time (typically 6-12 sec for normal induction motors). Class 10 is the safe default for Pakistani SME workshops.
Wiring — Overload Goes Between Contactor and Motor
3-phase supply L1 L2 L3
┃ ┃ ┃
┃ ┃ ┃
[Contactor Main: 1-2 / 3-4 / 5-6]
┃ ┃ ┃
┃ ┃ ┃
[Overload Relay: T1-T2-T3 carries motor current]
┃ ┃ ┃
┃ ┃ ┃
U V W ─→ Motor windings
Aux contacts on overload:
95-96 (NC): closes back when reset; OPENS when overload trips
→ wire in SERIES with contactor coil A1-A2
97-98 (NO): closes when overload trips
→ wire to a red "FAULT" indicator lamp
Critical wiring rule: The 95-96 NC contact of the overload MUST be in the contactor coil circuit. When overload trips, this contact opens, the coil de-energises, contactor drops out, motor stops. Without this, the overload relay does nothing useful.
Reset — Manual or Auto?
- Manual reset (default, recommended): after overload trip, an operator must press the blue reset button on the relay before the motor can restart. Forces a human to investigate why it tripped before re-energising.
- Auto reset (set with screwdriver): relay resets itself after cooling 1-5 min. ONLY use on unattended automated processes where lockout is impossible. NEVER on workshop motors operators can be near.
Common Mistakes
- Setting overload to MCB amp rating instead of motor FLA: e.g. 16A MCB but motor is 5.4A — set overload to 5.4A NOT 16A. The MCB protects the cable; overload protects the motor.
- Auto-reset on a workshop motor: motor auto-restarts while someone is repairing it = fatality risk. Always manual reset on operator-accessible motors.
- Class 30 used as "won't nuisance trip" default: a Class 30 overload allows damaging current for 30 seconds — the motor windings reach 200°C+ and insulation degrades. Don't oversize the class to mask a real problem.
- Single-phase load on a 3-phase overload: the overload has 3 heating elements; single-phasing only heats 1 of 3. The trip threshold is wrong. Use single-phase overload or differential single-phase-sensing relay.
Electronic vs Thermal Overload
| Type | Trip Mechanism | Pros | Price Δ |
|---|---|---|---|
| Thermal (bi-metal) | 3 heating coils bend a bi-metal strip | Cheap, proven, no power needed | baseline |
| Electronic | CT-based current sense + microprocessor | Phase-loss detection, ground fault, jam, stall | +150% to 300% |
For 95% of Pakistani SME applications, thermal bi-metal is correct. Electronic overload is worth the premium for hard-start expensive motors (pumps, mixers, escalators).
FAQs
What is the price of a motor overload relay in Pakistan?
CNC overload relays: 1.6-2.5A from Rs.1,400; 6.3-10A Rs.1,500; 10-16A Rs.1,600; 25-40A Rs.3,600; 63-100A Rs.7,500; 100-160A Rs.9,500. Electronic overload adds 150-300% premium.
How do I size an overload for my motor?
Set overload to the motor's nameplate FLA (Full Load Amps). Example: 5.4A FLA motor → set overload dial to 5.4A (within a 4-6.3A range model). Do NOT set to MCB amp rating — that protects the cable, not the motor.
Why does my overload trip during motor start?
Either (a) wrong overload class — try Class 20 instead of Class 10 for heavy-start loads; (b) overload set too low; (c) genuine starting overload from a stuck load; (d) low-voltage start (under-voltage causes higher inrush). Check FLA setting first.
Class 10 or Class 20?
Class 10 (default) for general workshop motors, fans, pumps with light starting loads. Class 20 for mixers, conveyors loaded at start, larger pumps. Class 30 for centrifuges and crushers only. Match to motor locked-rotor time.
Can I use a 3-phase overload on a single-phase motor?
No — the trip threshold will be wrong because only 1 of 3 heating elements gets current. Use a single-phase overload or a differential phase-loss-detecting overload designed for the application.
Related CNC Categories
- Magnetic Contactors — paired with overload in starters
- Circuit Breakers — protect the cable upstream of overload
- Timer Relays — for star-delta + reversing starters
- Panel Indicator Lights — fault indication when overload trips
- 3-Phase Contactor Wiring — DOL / Star-Delta