Overload Relay Sizing Guide for Motors | Buy Online India
How to Select the Right Overload Relay for Your Motor: A Complete Guide
A practical sizing reference for panel builders, electricians, and procurement teams — covering FLC calculation, trip class selection, and contactor compatibility.
If a motor overload relay is sized wrong, one of two things happens: it trips constantly on normal starting current and stops your line for no reason, or worse, it doesn't trip at all and lets an overloaded motor cook its windings. Getting the setting and trip class right the first time saves callbacks and protects the motor investment. Here's how to do it properly.
1. What Is an Overload Relay?
An overload relay is a protective device mounted directly on, or wired alongside, a contactor to form a complete motor starter. It continuously monitors the current drawn by the motor and opens the control circuit — de-energizing the contactor — if current stays above a safe threshold for too long.
Unlike a fuse or MCB, which protects the cable and reacts to short circuits, an overload relay protects the motor winding itself. It's designed to tolerate brief current surges (like starting inrush) while still catching sustained overload conditions such as mechanical jamming, single-phasing, or a motor working against too much load.
2. Thermal vs Electronic Overload Relay
Thermal (bimetallic) overload relays use current-heated bimetal strips that bend and trip a contact when they've absorbed enough heat to simulate motor winding temperature. They're simple, rugged, and cost-effective — the standard choice for most industrial and commercial motor starters. Good thermal relays include ambient temperature compensation so panel heat doesn't cause nuisance tripping.
Electronic (solid-state) overload relays use current transformers and a microprocessor to calculate motor thermal state in real time. They offer tighter accuracy, and often add protections thermal relays can't — phase loss, phase unbalance, ground fault, and stall/jam detection — plus some models support remote monitoring via communication protocols.
Which to use: thermal relays are the right call for standard-duty pumps, fans, and conveyors where budget matters. Electronic relays earn their premium on critical processes, expensive motors, or sites with unstable power quality where extra protection layers pay for themselves.
3. How to Calculate the Correct Current Setting
The core formula panel builders use:
Step 1 — Find the FLC (Full Load Current): this is printed on the motor nameplate, not calculated from kW alone (nameplate FLC accounts for actual motor efficiency and power factor).
Step 2 — Check the Service Factor (SF): most standard motors are rated 1.0–1.15 SF. This is also on the nameplate.
Step 3 — Multiply and select a relay with an adjustable range spanning that value. For example, a 415V, 5.5 kW motor with a nameplate FLC of 10.5A and SF of 1.15 gives a setting of roughly 12A — you'd pick a relay with an adjustable band that comfortably includes 12A (e.g., 9–13A range) rather than one sitting at the edge of its dial.
Avoid the common mistake of sizing off motor kW using a generic FLC chart — always use the nameplate value for the specific motor in front of you, since actual FLC varies by manufacturer and efficiency class.
4. Trip Class: Class 10, 10A, 20 — What to Use When
Trip class defines how long the relay allows before tripping at 7.2× FLC (roughly locked-rotor current) — in other words, how much starting time it tolerates before assuming something's wrong.
- Class 10A — trips within 10 seconds, tighter tolerance. Fast-response applications and motors with short, quick starts.
- Class 10 — trips within 10 seconds. The default for the majority of general-purpose motors: standard pumps, fans, and compressors with normal starting torque.
- Class 20 — trips within 20 seconds. Needed for high-inertia loads that take longer to reach full speed — large fans, crushers, centrifuges, and heavy conveyors — where a Class 10 relay would nuisance-trip during every normal start.
Rule of thumb: match the trip class to actual starting time. If a motor's normal start takes noticeably longer than a few seconds under load, a Class 10 relay will trip on start-up even though nothing is wrong — that's a sign you need Class 20, not a smaller relay.
5. Compatible Contactors for Each Current Range
An overload relay is only useful if it mounts and coordinates correctly with the contactor it protects. As a general guide:
| Current Range | Typical Contactor Frame | Common Use |
|---|---|---|
| 0.1A – 12A | 9A–12A mini contactors | Small pumps, fractional HP motors |
| 12A – 25A | 18A–25A contactors | Standard 3–7.5 HP motors |
| 25A – 50A | 32A–50A contactors | Mid-size compressors, blowers |
| 50A – 95A+ | 65A–95A+ contactors | Large motors, heavy machinery |
Important: always pair the overload relay with a contactor from the same manufacturer's series (for example, Schneider LRD relays are designed to mount directly onto TeSys LC1D contactors, and L&T MK relays mount onto L&T's MO/MX contactor range). Mixing brands can work electrically but often won't mount mechanically, and you lose the coordinated short-circuit protection rating the manufacturer tested the pair for.
6. Top Brands: Schneider LRD Series & L&T MK Series
Schneider LRD Series is one of the most widely specified thermal overload relay ranges in Indian industrial panels, built to mount directly on TeSys D (LC1D) contactors. It covers a broad current span across the series, with both standard and electronic variants, and is a safe default spec when reliability and easy replacement parts matter.
L&T MK Series is a strong, cost-effective alternative widely used across Indian panel-building shops, built to pair with L&T's own contactor range. It holds up well under local site conditions and is often the preferred choice where budget and easy local availability are priorities.
Both brands offer thermal and electronic options, so the brand decision and the thermal-vs-electronic decision can be made independently based on the application.
7. Shop Overload Relays & Contactors
Voltkart stocks genuine Schneider and L&T overload relays alongside matching contactors, ready for PAN India dispatch with GST invoicing from our Bhagirath Palace warehouse.
Frequently Asked Questions
What happens if the overload relay setting is too high or too low?
Set too high, the relay won't trip in time to protect the motor from a genuine overload, risking winding damage. Set too low, it trips during normal operation or starting, causing nuisance shutdowns. Always set to FLC × service factor from the nameplate.
Can I use any overload relay with any contactor?
Not reliably. For correct mechanical mounting and tested short-circuit coordination, use an overload relay from the same manufacturer's series as the contactor — for example, Schneider LRD with TeSys LC1D, or L&T MK with L&T MO/MX contactors.
Which trip class should I use for pump motors?
Most standard pump motors with normal starting torque work well with Class 10. If the pump has a long or heavy start (large inertia, high starting load), move up to Class 20 to avoid nuisance tripping.
Thermal or electronic overload relay — which is better?
Thermal relays are reliable and economical for standard-duty motors. Electronic relays cost more but add protections like phase loss and stall detection, making them worthwhile for critical or expensive motors.