Work out the full-load current of a motor and a starting size for the breaker, overload relay and cable. Enter the motor power in HP or kW, the voltage, and how you start it.
Typical rule-of-thumb sizing. Use the nameplate current of the actual motor and your local code for final selection of breaker, overload relay and cable.
Formulas and assumptions
- Input power: output watts divided by (efficiency times power factor). One HP is 745.7 W
- Three phase current: watts divided by (1.732 times volts times efficiency times power factor)
- Single phase current: watts divided by (volts times efficiency times power factor)
- Breaker: full-load current times 2.5 for direct on line, 2 for star-delta, 1.5 for soft starter or VFD, rounded up to a standard size
- Cable: at least 125 percent of full-load current
Example
A 10 HP three phase motor at 400 V, 90 percent efficiency and power factor 0.85 draws about 14.1 A. Started direct on line it needs a 40 A breaker (2.5 times 14.1 is 35.2, rounded up), a 2.5 mm2 copper cable and an overload relay set between 14.1 and 16.2 A.
Frequently asked questions
How do I find the full-load current of a motor?
Read it from the nameplate. If you only know the power, divide the watts by (1.732 times voltage times efficiency times power factor) for three phase.
Why is the motor breaker so much larger than the running current?
A motor draws 5 to 7 times its running current for a moment at start. The breaker must ride through that, while the overload relay protects against long overloads.
What is the difference between a breaker and an overload relay?
The breaker protects against short circuits and heavy faults. The overload relay is set near the full-load current and trips on a slow overload that would overheat the motor.
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