Contactor Size Calculator

Calculate recommended contactor rating, AC-3 motor contactor size, AC-1 load contactor size, coil voltage notes and overload relay range for electrical starter panels.

⚡ Field Rule: A contactor is not selected only by amp rating. Check utilization category like AC-3 for motors and AC-1 for resistive loads. Use this with the Motor Current Calculator, Motor Starting Current Calculator, Breaker Size Calculator, Voltage Unbalance Calculator and future Cable Derating Calculator.

🔌 Contactor Selection — Load Current + Utilization Category

Motor Rating

Line Voltage

Power Factor

Efficiency

Utilization Category

Sizing Factor

Coil Voltage

Auxiliary Contacts Needed

For motors, select contactor by AC-3 rating at the motor voltage and kW/current. Always verify manufacturer contactor table.

Presets:5.5kW motor15kW pump55kW motor

Load Current

Load Type

Sizing Factor

Voltage

Available Contactor Rating to Check

Duty / Switching Frequency

Use this tab when load current is already known from nameplate, measured current or another calculator.

Motor Full Load Current

Overload Setting

Relay Range Step

Starter Type

Overload relay is normally set near motor nameplate FLA. Follow motor nameplate, starter wiring and local standard.

📐 Formula Reference

3-Phase Motor Current

I = kW × 1000 ÷ (√3 × V × PF × η)

Recommended Contactor

Ie ≥ load current × sizing factor

HP to kW

kW = HP × 0.746

Overload Setting

Setting ≈ motor nameplate FLA × setting %

📋 Quick Reference

Utilization Category

AC-1resistive

AC-3motors

AC-4inching

Check Before Selection

Main poles3P/4P

Coil voltagecontrol

Aux contactslogic

Starter Parts

Contactorswitching

Overload relaymotor protection

Breaker/fuseshort circuit

📚 Engineering Notes

AC-3 is for motor switchingA contactor may have a high AC-1 amp rating but a lower AC-3 motor rating. Use the correct category.

Coil voltage must match control circuitCheck whether the panel control voltage is 24VDC, 110VAC, 230VAC or another value before ordering.

Overload relay is separateThe overload relay protects the motor from overload. Use motor nameplate FLA for setting.

Check starter systemUse the Motor Starting Current Calculator, Breaker Size Calculator and Voltage Unbalance Calculator.

What is a Contactor Size Calculator?

A contactor size calculator helps select the correct contactor rating for motors and electrical loads. It considers load current, motor kW/HP, voltage, power factor, utilization category and safety factor.

How contactor size is calculated

The calculator estimates motor full-load current or uses known load current, then applies a sizing factor based on the load type. The selected contactor should have an equal or higher rated operational current for the correct utilization category.

Contactor sizing workflow

Start with the Motor Current Calculator or motor nameplate current. Check starting current with the Motor Starting Current Calculator. Then select contactor size, overload relay range and short-circuit protection with the Breaker Size Calculator.

Important limitation

This calculator is a planning aid. Final contactor selection must follow the manufacturer’s AC-1/AC-3/AC-4 tables, motor kW rating at voltage, ambient temperature, enclosure conditions, switching frequency, coordination type, overload relay compatibility and local standards.

❓ Frequently Asked Questions

AC-3 is the utilization category commonly used for squirrel-cage induction motors, where the contactor makes motor starting current and breaks running current.

No. AC-1 is mainly for non-inductive or slightly inductive loads. Motor applications should use AC-3 or the category specified by the manufacturer.

No. The contactor switches the load. The overload relay protects the motor and is normally set according to motor nameplate full-load current.