How do I size cable runs between Variable Speed Drive and motors?

The cable should be sized for the motor full load current plus overload, without allowance for harmonic content. When using sizing software, most cable sizing programs assume that 600 percent direct-on-line (DOL) starting current will be required. However, such high currents are never encountered in installations using variable-speed drives. When using a cable sizing programme, “start with AC drive” should be selected to avoid over-dimensioning. If this is not available as an alternative, “start with a resistive load” can be used instead.

It is also necessary to work out what the volt drop will be with the length of cable planned for the installation. A motor under AC drive control will draw a current equal to the full load current plus a possible margin due to any over-torque requirement, with an allowable volt drop of perhaps 2 percent.

“When using an AC drive, there is no need to oversize the cable. As there is no inrush current, you do not have the same volt drop as with DOL start. DOL start uses 600 percent start-up current while with an AC drive, you generally only need 100 percent current for 100 percent torque,” explains Brown.

For help with correctly sizing your cable run send application details to

Watch that capacitance

It is also important to bear in mind that capacitance will build up between the conductor and the screen when using long motor cables. In fact, the cable acts as a distributed capacitor and the longer it is, the higher its capacitance. Capacitance coupling in a long cable can cause a substantial current in the earthed screen, due to the high frequency components in the output waveform, which may result in a level that overloads the filter components of the drive. The installation may still work, but it may not perform as expected.

The type of cable is also significant. Good quality steel wire armoured cable to BS 5467 is generally adequate to maintain the Faraday Cage required for EMC compliance. The cable also needs to be correctly glanded and connected to the earth bar in the AC drive. Little difference is seen between three-core and four-core cable up to 10 mm2. Beyond this, however, a three-core cable with separate potential equalizing conductor is recommended.

It is important to use suitable filters to counteract the effect of the long cable’s capacitance – du/dt filters will reduce the rate of change of the voltage pulses and this will reduce the effects of long cables, although there will still be a finite limit which should be outlined in the manufacturers’ documentation. Sine filters, meanwhile, are designed to produce a completely sinusoidal waveform, with no distance limitation.

“With any filter you get a volt drop across the filter, but with a sine filter it delivers no high frequency currents, lower losses, and reduced earth leakage , while du/dt filters reduce the rate of rise and goes some way to decreasing high frequency pick up,” says Brown.

For help with correctly sizing your cable run send application details to

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