Three-phase motor current calculator
Estimate the full-load current of a three-phase motor from its power, supply voltage, efficiency and power factor. Use it to sanity-check a nameplate or to size a drive, starter or cable before you ask for a quote.
On the nameplate, or about 85–96 typical
On the nameplate, or about 0.80–0.90 typical
Estimated full-load current
14.2A
Choose a drive or starter rated for at least this current. Always confirm against the nameplate.
Electrical input power
8.33kW
Apparent power
9.8kVA
Started direct on line, this motor would typically draw about 85 to 113 A for a moment. A drive or soft starter limits that.
Need the part?
Send these details to an engineer and get availability, lead time and price.
How it works
The formula is I = P / (√3 × V × efficiency × power factor), where P is the shaft power in watts and V is the line-to-line voltage.
Efficiency and power factor are the two numbers people guess. They are printed on most nameplates as η and cos φ. If yours is missing, typical full-load values for a four-pole motor of 5.5 kW and above are efficiency of about 85 to 96 per cent and power factor of 0.80 to 0.90. Small motors are lower.
The result is an estimate. The current printed on the motor nameplate always wins, and cable and protection sizing must follow SANS 10142-1 and the manufacturer's tables, not this page.
Common questions
Which current should I use to size a variable frequency drive?
Use the nameplate full-load current at your supply voltage, then choose a drive whose rated output current is at least that value for your duty. Many drives list a normal-duty and a heavy-duty rating, and the heavy-duty current is the lower one.
Why does my result differ from the nameplate?
Nameplates reflect the tested motor. Estimates rely on typical efficiency and power factor. A difference of a few per cent is normal. A large gap usually means the voltage, the unit (kW or hp) or the efficiency was entered wrongly.
How much current does a motor draw when it starts direct on line?
Typically six to eight times full-load current for a standard cage motor, for a short time. This is why starters, cables and upstream protection need care. A drive or soft starter limits it.
Send us the part number
Got your answer and need the part? An engineer replies with availability, lead time and price.
More tools
Get a quote
Tell us what you need. An engineer replies with availability, lead time and price.
1/2