Cable Size & Motor Breaker Selector
Enter a motor's power, supply and distance from the distribution board to get the full-load current, the cable size in mm², and the breaker and contactor to fit.
Cable size & breaker
Cable, breaker and contactor for a motor or CNC machine.
One-way cable run.
Enter the motor power to size the cable and breaker.
How this is calculated
- 1
Full-load current
Three phase: P ÷ (√3 × V × efficiency × power factor), using 88% and 0.85. Single phase: P ÷ (V × efficiency × power factor), using 75% and 0.8 for small motors.
- 2
Breaker and contactor
The breaker is the next standard size at or above 125% of full-load current, D curve for motor inrush. The contactor is the next AC-3 rating at or above full-load current.
- 3
Cable for the current
The smallest PVC cable whose current rating, in conduit at 30°C (IEC 60364 method B1), is at least the breaker rating. Aluminium is rated at 78% of copper.
- 4
Voltage drop check
Long runs lose voltage. If the drop at full load is over 5%, the cable is increased until it is within limits.
Frequently asked questions
What cable size for a 10 HP motor?
A 10 HP (7.5 kW) three-phase motor draws about 14 to 15 A at 400 V. With a 20 A breaker, 2.5 mm² copper is the minimum on a short run, and many electricians use 4 mm² for margin and longer runs.
Which breaker for a 5 HP motor?
A 5 HP three-phase motor draws about 7 A at 400 V, so a 10 A D-curve breaker with a 9 A contactor and the overload set to the nameplate current. On 220 V single phase it draws far more, around 28 A.
Copper or aluminium cable?
Copper carries more current in a smaller size and is easier to terminate. Aluminium is cheaper and lighter for large feeders, but needs a bigger size and proper bimetallic lugs.
Is this enough for my installation?
It is a guide for planning and quoting. Final cable and protection must be selected and installed by a qualified electrician to the applicable wiring regulations.