16 — Calculators
Calculate motor current
The nameplate states shaft power but sizing cable, contactor and protective device needs the current. Efficiency belongs in the calculation: it is the difference between what the motor delivers and what it draws.
Result
- Rated current
- —A
- Apparent power
- —kVA
- Electrical active power
- —kW
- Starting current, direct on lineAssumed at 6 times rated current.
- —A
Please fill all fields with valid numbers.
- Opens the print dialog. Choose “Save as PDF” as the destination.
- Opens your email program with the values from this calculation. Nothing is sent to us until you send it yourself.
A guide, not a design to standard. We give no warranty for the correctness of the results, the applicable standards and case-by-case verification govern.
Worked example
The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.
Inputs
- System
- Three-phase (3~)
- Shaft power
- 7.5 kW
- Nominal voltage
- 400 V
- Power factor cos φ
- 0.86
- Efficiency η
- 90 %
Result
- Rated current
- 14 A
- Apparent power
- 9.69 kVA
- Electrical active power
- 8.33 kW
- Starting current, direct on line
- 83.9 A
Formula
- Three-phase: I = (P / η) / (√3 · U · cos φ)
- Single-phase: I = (P / η) / (U · cos φ)
- S = P / (η · cos φ)
Assumptions and standards
- P is the shaft power (mechanical output) from the nameplate.
- Starting current is assumed at 6 times rated current. A common guide value for direct-on-line starting. The actual figure is in the data sheet and lies between 4 and 8 times.
- Sizing the protective device also depends on the starting time.
Frequently asked
Why is the current higher than the rule of thumb “2 × kW”?
The 400 V rule of thumb ignores efficiency and power factor. For a 7.5 kW motor with cos φ 0.86 and 90 % efficiency the rated current is around 14 A rather than 15 A. So the rule of thumb lands above or below depending on the motor.
Which cos φ should I use?
The nameplate value at rated load. At partial load it drops considerably. A motor running permanently at half load has a markedly worse power factor.
Calculating is the easy part.
A formula gives you a number. Designing a plant also demands installation method, grouping, discrimination, the standards in force and a look at the installed base. That is what we take on.
More tools
- 01Cable cross-section
- 02Voltage drop
- 03Cable resistance
- 04Cable losses
- 05Maximum cable length
- 06Current-carrying capacity
- 07Parallel cables
- 08Conductor temperature
- 09Cable capacitance
- 10Short-circuit current
- 11Cable impedance
- 12Network impedance
- 13Short-circuit current at the far end
- 14Thermal short-circuit withstand
- 15Length and disconnection
- 17Torque
- 18Starting current
- 19Star-delta starting
- 20Soft starting
- 21Motor efficiency
- 22Speed control instead of throttling
- 23Speed and slip
- 24Setting the motor protection
- 25Single-phase motor
- 26Motor feeder
- 27Required motor rating
- 28PFD and SIL
- 29PFH and SIL
- 30Protective conductor size
- 31Earth rod
- 32Touch voltage
- 33Residual current protection
- 34Check discrimination
- 35Connecting a surge arrester
- 36Enclosure cooling
- 37Reference designation
- 38Reactive power compensation
Contact
Tell us what it is about.
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