03 — Tools
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.
A guide, not a design to standard. 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.
Contact
Tell us what it is about.
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