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Three-Phase Motor Power Calculator

Clamp-on amps and a voltage reading tell you a lot about a motor. Use these to estimate real power, shaft horsepower and yearly energy cost, or to estimate the current a motor of a given size should draw.

Three-Phase Power, Horsepower and Energy Cost

Formula

kVA = 1.732 x V x I / 1,000
kW in = kVA x PF = 1.732 x V x I x PF / 1,000
Shaft hp = kW in x Efficiency / 0.7457
kWh per year = kW x hours;   Cost = kWh x $/kWh
Inputs used
  • Line-to-line voltage (V)
  • Average line current (A)
  • Power factor
  • Motor efficiency (%)
  • Run hours per year (optional) (hours/yr)
  • Power cost (optional) ($/kWh)

Worked example (fictional plant data)

  1. Aeration blower motor at 460 V drawing 52 A, PF 0.86, efficiency 93.6%, running all year at $0.10/kWh
  2. kVA = 1.732 x 460 x 52 / 1,000 = 41.43 kVA
  3. kW = 41.43 x 0.86 = 35.63 kW
  4. Shaft hp = 35.63 x 0.936 / 0.7457 = 44.72 hp
  5. Energy = 35.63 x 8,760 = 312,122 kWh/yr; cost = 31,212 $/yr

Full-Load Amps from Horsepower (Estimate)

Formula

I = hp x 745.7 / (1.732 x V x PF x Efficiency)
Inputs used
  • Motor output (hp)
  • Line-to-line voltage (V)
  • Power factor at full load
  • Efficiency at full load (%)

Worked example (fictional plant data)

  1. 50 hp motor, 460 V, PF 0.86, efficiency 93.6%
  2. kW in = 50 x 0.7457 / 0.936 = 39.83 kW
  3. I = 39.83 x 1,000 / (1.732 x 460 x 0.86) = 58.1 A

Cautions

  • Only qualified electrical workers should take readings inside energized equipment. Follow NFPA 70E and your lockout and arc flash rules.
  • Power factor and efficiency fall at light load. Nameplate values apply at full load.
  • On a VFD, take readings on the input side for energy. Output current and voltage are not sine waves and need a true RMS meter.
  • For conductor and breaker sizing use NEC Table 430.250 currents; for overload settings use nameplate FLA.

References

  • NFPA 70, National Electrical Code, Article 430 and Table 430.250 (full-load current, three-phase motors).
  • NEMA MG 1, Motors and Generators (efficiency and nameplate data).

Results are estimates for operator and technician use. Equations follow the references above; always check against the instrument manual, your calibration procedure and plant data.