Contactor Size Calculator
Enter the motor power, the supply voltage, single or three phase and the duty. The calculator gives the motor's full load current, the contactor rating to buy and the overload relay setting.
- Contactor rating
- Motor current
- Overload setting
Use the nameplate current if you have it.
Size the contactor's AC-3 current at or above the motor's full load current. A 7.5 kW motor on 400 V three phase draws about 14.2 A, so it needs an 18 A AC-3 contactor. For inching or reversing duty (AC-4), pick at least 1.6 × the current, a 25 A contactor.
Contactor size chart
400 V three phase. Search for a motor.
| Motor (kW) | Current (A) | AC-3 contactor (A) | AC-4 contactor (A) |
|---|---|---|---|
| 0.75 | 1.4 | 9 | 9 |
| 1.5 | 2.8 | 9 | 9 |
| 2.2 | 4.2 | 9 | 9 |
| 4 | 7.5 | 9 | 18 |
| 5.5 | 10.4 | 12 | 18 |
| 7.5 | 14.2 | 18 | 25 |
| 11 | 20.8 | 25 | 40 |
| 15 | 28.3 | 32 | 50 |
| 22 | 41.5 | 50 | 80 |
| 30 | 56.6 | 65 | 95 |
| 45 | 84.9 | 95 | 150 |
| 55 | 103.8 | 115 | 185 |
| 75 | 141.5 | 150 | 265 |
Amps.
Motor and contactor
400 V three phase. Slide the motor power.
The contactor grows with the current.
How to size a contactor
Motor to rating. Step through it.
Power
kW.
Match the coil voltage to your control circuit.
AC-1, AC-3 or AC-4
Utilisation categories. Compare them.
Resistive.
Motors.
Hard duty.
The same contactor carries far less current in AC-4 duty.
Before you wire
Contactor checks. Tick each item.
Tick each item as you go.
Fit an overload relay set to the motor's full load current.
What you get
Contactor sizing. Each tile explains one result.
Amps.
Full load.
Category.
Phase.
Setting.
Contactors switch the motor, overload relays protect it.
Frequently Asked Questions
No question matches that search.
What size contactor do I need?
One whose AC-3 rating is at or above the motor's full load current.
What size contactor for a seven and a half kilowatt motor?
An eighteen amp AC-3 contactor at four hundred volts three phase.
How do I work out motor current?
Divide the power by root three times voltage, power factor and efficiency.
What does the AC three rating mean?
The rating for starting and stopping squirrel cage motors.
Do I need a bigger contactor for reversing?
Yes. AC-4 duty needs about one and a half times the rating.
What coil voltage should I choose?
Match your control circuit, often twenty four or two hundred and thirty volts.
Does a contactor protect the motor?
No. An overload relay does that.
Switch your motor safely
Size the contactor to the motor's current and duty and it switches cleanly for years.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.