Motor Cable Selection Chart Explained

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I am receiving a significant number of enquiries on how to use the cable sizing chart for 400V motor feeders. With this reason, I want to provide some explanations on the assumptions used on the chart and on how to use the chart in selecting cable lengths for motors.

  1. Assumptions
    1. Allowable Voltage Drops
      1. Running Condition: 5%
      2. Starting Condition: 15%
    2. Starting Condition Power Factor: 0.3
    3. Maximum Cable Length: 3000 m
    4. Derating Factors are based on IEC
    5. Cable Data: Draka Cable
    6. Motor Data: Brook Crompton
  2. Other Conditions
    1. If the cable length exceeds 3000m, the cable length will display blank instead of the cable length. To display the values beyond 3000m, increase the maximum cable length to a high value.
    2. If the motor full load current is higher than the cable current carrying capacity, the chart will display ":". This indicates that the cable is not suitable for the motor.
    3. Cable size constraints - normal and italic fonts. The normal fonts indicate that the motor full load current is the deciding factor in the cable selection else it will be the motor starting current.
  3. Examples
    1. 0.75kW motor at 300m from Motor Control Center (MCC).
      1. There are 2 possible cable selections, 2.5mm² and 4mm². Both are possible but a 2.5mm² cable is cheaper than a 4mm² so for the purposes of economy, select 2.5mm².
      2. As an additional note, some projects require that the minimum cable size for power is 4mm² thus a 4mm² need to be selected.
    2. 11kW motor at 200m from Motor Control Center (MCC).
      1. The motor full load current exceeds the current carrying capacity of a 2.5mm² cable, thus the ":" which means that the cable is not possible to be used for the purpose.
      2. As the expected cable length is 200m, a 4mm² will be selected as it has a maximum cable length of 234m which is greater than the required length.
See also  IEEE C57.12.00 - Preferred Continuous kVA Ratings of Transformers

This chart is very useful when you are selecting cables for hundreds of motors in a project. You can not afford to calculate the cable size for each motor as you will incur a significant amount of time to complete it.

This motor selection chart is commonly called waterfall chart as it looks like a waterfall. It is an acceptable method worldwide of selecting cables for motors.

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