How do you calculate overload relay setting?

Use the motor nameplate full-load current and multiply it by the selected setting percentage. For a relay in the line, Setting = FLA × percentage / 100. For a relay inside the delta branch of a star-delta starter, use approximately FLA / √3 before applying the percentage.

Should overload relay be set to nameplate current or running current?

Normally start from the motor nameplate full-load current, not the no-load or lightly loaded running current. Check the motor, relay and local code requirements before final commissioning.

How do you calculate overload relay setting with CT ratio?

First calculate the required primary current. Then convert it to relay secondary current using Relay Setting = Primary Setting × CT Secondary / CT Primary.

Overload Relay Setting Calculator | Motor FLA Thermal Overload

Overload Relay Setting Calculator

Calculate thermal overload relay dial setting from motor nameplate FLA, estimated motor current, star-delta relay location and CT ratio.

⚠️ Field Rule: Use the motor nameplate FLA first whenever available. This calculator gives a practical starting point; final setting must follow the motor nameplate, overload relay manual, starter wiring, local code and site commissioning test. Use with Motor Current Calculator, Contactor Size Calculator, Breaker Size Calculator and Voltage Drop Calculator.

🛡️ Motor Starter Protection: Supply → Contactor → Overload Relay → Motor

Motor Nameplate FLA

Relay Location / Starter

Setting Percentage

Relay Dial Minimum

Relay Dial Maximum

Trip Class

For most line-mounted thermal overload relays, start from motor nameplate FLA. For star-delta relays placed inside the delta branch, the relay sees about 58% of line current.

Presets:

Motor Power

Phase

Line Voltage

Power Factor

Efficiency

Setting Percentage

Relay Location

Use this only when nameplate FLA is not available. Real nameplate current can differ by motor design, voltage, efficiency and service factor.

Examples:

Motor Line FLA

Relay Connection

CT Primary

CT Secondary

Setting Percentage

Relay Dial Min

Relay Dial Max

For a CT-operated overload, the relay dial is the CT secondary current. Example: 90A motor with 100/5 CT gives 90 × 5 ÷ 100 = 4.5A.

Presets:

📐 Formula Reference

Line Relay Setting

Relay A = Motor FLA × Setting % ÷ 100

Star-Delta Delta Branch

Relay A ≈ Motor FLA ÷ √3 × Setting % ÷ 100

CT Secondary Relay

Relay A = Primary setting × CT secondary ÷ CT primary

3-Phase FLA Estimate

I = Pout ÷ (√3 × V × PF × η)

📋 Quick Reference

Relay Location

Line / DOL1.00 × FLA

Star-delta line1.00 × FLA

Inside delta0.58 × FLA

Trip Class Guide

Class 10normal start

Class 20heavy start

Class 30long start

Do Not Confuse

Overload relaythermal

Breaker / fuseshort-circuit

MPCBboth functions

📚 Field Notes

Use nameplate FLA firstCalculated current is useful for planning, but overload commissioning should normally start from the motor nameplate current.

Star-delta placement mattersIf the overload relay is inside the delta branch, it sees winding current, roughly line current divided by √3.

Do not hide a real overloadIf the motor trips at correct setting, check load, voltage unbalance, phase loss, bearing, fan/pump blockage, ambient temperature and starting time.

VFD applications are differentMany drives have electronic motor overload protection. Use the drive motor data parameters and manual before adding output-side relays.

What is an Overload Relay Setting Calculator?

An overload relay setting calculator helps technicians set the thermal overload dial for an electric motor. It converts motor full-load current, starter wiring and CT ratio into the relay amp setting.

How to calculate overload relay setting

For a direct-on-line or line-mounted relay, use Relay setting = motor FLA × setting percentage / 100. For a star-delta starter where the overload is inside the delta branch, use about FLA / √3 before applying the percentage.

Overload relay vs breaker

An overload relay protects the motor from sustained overload, phase loss and overheating conditions. It is not a replacement for short-circuit protection from a breaker, fuse or properly selected MPCB.

❓ Frequently Asked Questions

For many line-mounted thermal overload relays, the usual starting point is the motor nameplate full-load current. Final setting should follow the motor nameplate, relay manual and local electrical rules.

If the relay is on the line side, use line FLA. If the relay is inside the delta branch, set it to approximately motor FLA divided by √3, which is about 58% of line current.

Calculate the required primary current first, then multiply by CT secondary and divide by CT primary. Example: 90A with 100/5 CT gives 4.5A relay setting.

Do not simply increase the setting. First check actual load current, phase voltage, voltage unbalance, cooling, bearing condition, blocked load, starting time and relay class.

Class 10 is common for normal starting motors. Class 20 or 30 may be used for heavier starting loads, but it must match the motor and application limits.

Many VFDs include electronic motor thermal protection. Enter the correct motor nameplate data in the VFD and follow the drive manual before adding external output-side protection.