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Contactor Sizing Calculator

Work out contactor sizing instantly with clear inputs, formula shown and shareable results.

About Contactor Sizing Calculator

Contactor selection starts from motor full load current, then applies the utilisation category: AC-3 for normal starting, AC-4 for plugging and jogging where the contacts break locked-rotor current and need roughly double the rating. High starting frequency adds a further factor.

Formula

Contactor sizing

required rating = motor FLC x category factor x starting frequency factor

Worked example

With the default values below, Contactor Sizing Calculator returns Contactor rating required of 17. Change any input above and every figure here is recalculated in your browser.

Inputs used

Motor rating
7.5 kW
Supply voltage
400 V
Utilisation categoryone of AC-1 resistive, AC-3 squirrel cage motor, AC-4 plugging and jogging
AC-3 squirrel cage motor
Starts per hour
20 starts/h

Results

Contactor rating required
17 A
Motor full load current
13.99 A
Category factor
1.250 x
Recommended frame size
18 A

Step by step

  1. 1Values used
  2. 2Contactor sizing
  3. 3Contactor rating required= 17 A
  4. 4Motor full load current= 13.99 A
  5. 5Category factor= 1.250 x
  6. 6Recommended frame size= 18 A

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What is the difference between AC-3 and AC-4?

AC-3 breaks running current; AC-4 breaks starting current, which is six times higher. AC-4 duty therefore demands a much larger contactor for the same motor.

Does the contactor also protect the motor?

No. A contactor only switches. Overload protection comes from a thermal overload relay or electronic protection relay in series with it.

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