One size up does not always mean safety. If a motor is loaded well below its rating, the share of losses rises, cos φ falls and unnecessary reactive current is drawn for the same work. As our catalogue notes, a motor running continuously at low load stays in its inefficient range.

An oversized motor has other costs too:

  • Starting current increases; the breaker, contactor and cable size must grow as well.
  • The frame may change: at the same speed, the next power up may move to a different IEC frame and shaft size, so the existing base and coupling may not fit.
  • Excess starting torque may hit belts, couplings or the gearbox as a sudden shock.

The cases where a larger rating is really needed are clear: ambient above +40 °C or altitude above 1000 m, heavy and long starts, frequent starting and stopping, or a measured load very close to the rating.

The right approach is to measure the current the existing motor draws in normal production and compare it with the nameplate current. If it stays near the nameplate value and the motor runs hot, upsizing is considered; if it is clearly below, keeping the same power is considered. The steps are explained in our step-by-step motor selection guide.