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Over a short distance, the effect of the cable between panel and motor is negligible; as the distance grows, cable resistance lowers the voltage the motor sees. This drop is felt most at starting. If the motor is fed by a drive, a long cable can also cause voltage peaks at the motor terminals. In these two cases, distance becomes part of the motor decision.
Three situations where distance affects the choice
Loads with heavy starting
The torque of an induction motor varies with the square of the voltage. Because current is high at starting, the drop in the cable also increases and the torque available to the motor falls. With loads such as a full screw conveyor or a sand pump, this difference can decide whether the motor runs up or not.
Motors running at their limit
A long line means a permanent voltage loss. A motor operating below its rated voltage draws more current to deliver the same shaft power and heats up. If the load is already close to the rated power, the next size up can be considered; however, the real solution is usually the correct cable cross-section.
Supply from a frequency inverter
The fast switching pulses at the drive output can be reflected along a long cable and rise at the motor terminals. In this case, the cable length permitted by the drive manufacturer and the filters it recommends are decisive.
Questions to ask before ordering
- The actual length of the cable route between panel and motor,
- The cross-section and conductor material of the existing cable,
- The starting method and the load condition at starting,
- Whether there will be a local isolator or stop button next to the motor.
Cable cross-section and voltage drop are determined from the motor nameplate, the cable manufacturer's data and the calculation of qualified electrical personnel. For a first estimate you can use the motor cable cross-section calculator; for pump applications at remote points, see the pump motors page.