Cables Selection for Motors

When selecting a suitable cable size for a motor, there are more parameters to consider than when selecting cables for static loads like lighting. In this tutorial, the cable calculation is very straight forward.

For motors, the starting condition need to be considered in addition to the normal operating condition. For fractional KW motors, the prevailing parameter is normally the full load current but as the motor size increases, then the starting current becomes the deciding factor.

In this chart - starting and normal operating conditions were considered for the calculation of the maximum cable length.

For purposes of the chart, the following conditions were considered:

1. Starting Power Factor = 0.3
2. Supply voltage remains constant, i.e. 400V
3. Ambient Temperature = 45 deg.C
4. Parallel Cables ampacity = multiples of single cables ampacity
Note: In reality - this is not correct but for purposes of simplifying calculations and in the absence of actual data, no derating factor for parallel cables has been considered.

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