Two separate limits determine the cable cross-section, and whichever requires the larger size wins. The first is the current the cable can carry without overheating; this depends on the motor's rated current, how the cable is installed (conduit, tray, underground), how many cables run side by side and the ambient temperature. The second is voltage drop: the longer the cable, the lower the voltage reaching the motor terminals, and the loss becomes most visible at starting, when current climbs to several times the rated value.

Inputs to the calculation:

  • The motor's rated current at 400 V (from the nameplate or product page)
  • The starting method and starting current ratio (Is/In)
  • The actual cable length between panel and motor
  • Copper or aluminium conductor, installation method and ambient temperature
  • For star-delta starting, the two separate cable sets to the motor

The final value must be determined by a qualified electrical engineer according to the applicable wiring regulations; we do not give a fixed size table here, because the same power gives different results at different lengths and installation methods. Rated currents are in the current table, and panel-side checks in the article panel and connection checks when ordering a 30 kW motor. For inverter output cables, see the motor with inverter guide.