How to Estimate Motor Load from Measured Current

A motor may draw substantial magnetising current even when shaft torque is low. Power factor and efficiency can vary substantially across the load curve. Dividing measured amps by plate full-load amps gives a current ratio, not a defensible shaft-load percentage on its own.

Different supply conditions, a VSD, harmonics, current imbalance and transient process loads add uncertainty. Observe a representative operating state and record it with the measurement.

For a balanced sinusoidal three-phase supply, a first estimate is P input (kW) ≈ √3 × line voltage (V) × average line current (A) × power factor ÷ 1000. This assumes the PF used applies at the operating point; rated nameplate PF may not.

If you have actual measured three-phase real power, prefer it. Approximate shaft output can then be expressed as P output ≈ P input × efficiency, and estimated load as 100 × P output ÷ rated shaft kW. Rated efficiency is also only an approximation away from rated load.

Assume a fictional 9.2 kW motor operating on 400 V three-phase with average current 12 A, rated PF 0.84 and rated efficiency 89%. The estimated electrical input is √3 × 400 × 12 × 0.84 ÷ 1000 = 6.98 kW.

Estimated output is 6.98 × 0.89 = 6.21 kW, or about 67.5% of rated shaft output. At 4,000 operating hours and £0.22/kWh the indicative annual energy is 27,936 kWh and running cost about £6,146.

The result could differ materially from a true-power measurement if PF and efficiency are lower at this operating point. It does not prove the motor is oversized.

Look for rapidly changing load, unknown power factor, uncertain phase currents or plate connection, VSD output currents, voltage imbalance, and duty profiles that change across shifts. A single reading is a snapshot, not an annual average.

With L1, L2 and L3 current measurements, current imbalance can be screened as (maximum minus minimum) divided by the mean × 100. This is not the same as voltage imbalance and does not identify root cause.

Confirm actual measured kW where practical, log real hours and duty, check the driven equipment and record speed control. If you are assessing a pump or fan, read our VSD payback guide. For correct rating selection, start with reading the nameplate.