20th September 2020

Motor Cable sizing

Motor cable sizing
Motor cable sizing

The Proper cable sizing is required for the proper working of the equipment in full load, withstand short circuit when occurred and to provide desired voltage to the equipment(limiting voltage drop). The improper cable sizing can be over-size of cable or under-size of cable, hence proper care to be taken for Motor Cable sizing.

The effects of over-sizing of cable

  1. The loss of capital
  2. More Space required in the cable trestle or cable trench.

The effects of under-sizing of cable

  1. Heating of Cable
  2. Chances of Cable to burn out
  3. Undesired voltage drop from the source to the load
  4. Ac motor may have starting problem due to the large voltage drop at the starting of the motor.

Criteria on which the sizing of cable depend

1. Selection Of Cable Size Based On Ampacity (Current Carrying Capacity) Of The Cable

Selected cable should be such that the current carrying capacity of the cable should be equal or more than the current rating of the feeder. The current carrying capacity of cable depends on the various parameters.
Cable Current Capacity IDerated > Ifeeder
Where,
IDerated   =  IRated x Derating Factor

Where,  IDerated   =  De-Rated Current Capacity of Cable (Amp)
IRated      =  Rated Current Capacity in Air/Duct/Ground as per the Cable Catalog.

Derating Factor of cable depends on various parameters like :-

  • Ambient temperature
  • Cable laid in Trench/Duct/Tray
  • Cable is laid Single or in Group
  • No of Cables in group(if Laid in group)
  • Depth of buried cable(If cable is buried)
  • Distance between the Cables
  • Temperature of Ground (if cable is buried)
  • No. of Trays arranged in a tier and distance between trays (if cable is laid in tray)

Derating factor explained in detail in example calculation at the end of the article.

2. Selection Of Cable Size Based On Voltage Drop

Voltage drop is critical while cable sizing for motor, to make it simple and define what is voltage drop “Voltage on receiving end that is equipment(motor) is less than sending end voltage(feeder) from distribution panel(industrial terms called as MCC- motor control center)“.

The Voltage drop occurs due to the resistance and reactant of the cable. Voltage drop is to estimated for motor while it is in normal running condition and in starting condition.

If the voltage drop for a motor is large then the motor will not be able to reach its rated Speed (rpm) and rated Torque (N-m). The Voltage Drop should be less than that is mentioned in IS/IEC/BIS Standards.

  • Voltage Drop During Running Condition

    The voltage drop due to the Resistance and Reactance of Cable should be such that it should not affect the Load.

    For 3-Phase feeder
    %Running Voltage Drop = √3  × (IR Cosɸ +IX Sinɸ) × 100/V
    For 1-Phase feeder
    %Running Voltage Drop = 2 × (IR Cosɸ +IX Sinɸ) × 100/V
    V= Line voltage (V)
    I = Rated Feeder Current under normal running condition (Amp)
    R = AC resistance of cable. (ohm)
    X =Reactance of cable. (ohm)
    Cosɸ = Power factor of Load during the normal running condition.

  • Voltage Drop During Starting Condition

    For motors, the starting Current and Power factor is different than normal running condition. On Starting  6 to 7.2 times of the normal rated current and the power factor is very low, it can be upto 0.2. Starting current many vary based on rating of the motor, IE3 energy efficient motors with higher starting current and type of motor starter.

    So there will be large  starting voltage drop compared to normal condition. The Voltage Drop should be less than that is mentioned in IS/IEC/BIS Standards. If the starting voltage drop is huge than the motor can go in stall condition and will not start which may cause overheating of motor and make it go burn.
    %Starting Voltage Drop = √3  × (ISTR CosɸST +ISTX SinɸST) × 100/V
    For 1-Phase feeder
    %Starting Voltage Drop = 2 × (ISTR CosɸST +ISTX SinɸST) × 100/V
    V                = Line voltage (V)
    IST              = Starting current of  (Amp)
    R              = AC resistance of cable. (ohm)
    X              = Reactance of cable. (ohm)
    CosɸST    = Power factor of Load during the normal running condition.

3. Selection Of Cable Size Based On Fault Current Withstand Capacity (Only For ACB/VCB Controlled Feeders)

For the feeder which is controlled by the ACB (Air Circuit Breaker) or VCB (Vacuum Circuit Breaker), the cable should be sized so that it can withstand the short circuit current for the time that the breaker takes to break the circuit. The Short circuit capacity is calculated with the following formula as per the manufacturer catalog.
Isc  = IT × √(t)
Where,
ISC    = Short Circuit Current to be withstand by cable for 1 second
IT      = Short Circuit Current to be withstand by cable for t second
T       = Time taken by breaker to break the circuit or duration of the fault (in second)

Inputs required for Motor Cable sizing

  1. Motor Rating (From Motor Datasheet or From Motor Nameplate).
  2. Rated voltage (System Voltage).
  3. Rated Current of Motor (From Motor Datasheet or can be calculated using motor rating and efficiency).
  4. Efficiency of the motor (Take 0.9 if datasheet is not available).
  5. Running Power Factor of Motor (Take 0.85 if datasheet is not available).
  6. Starting Current of Motor (Generally 7.2 Times of Rated current or from Motor Datasheet).
  7. Starting Power Factor of Motor (Generally 0.2 can be taken if datasheet is not available).
  8. Ampacity of cable (It can be taken from IS-3961 or from any cable manufacture catalog ).
  9. Resistance and Reactance of Cable (These parameters can be taken from IS-3961 or from any cable manufacture catalog ).
  10. De-rating factors (It can be taken from IS-3961 or from any cable manufacture catalog ).

Note:-

  1. All the parameters required for cabling sizing are available in the motor Datasheet.
  2. All parameters related cable can be find in IS-3961 or from the catalog of cable manufacture

Relavant article : LV Switchgear or Low Voltage SwitchgearDirect On-line(DOL) Motor Starter

In our next article we will provide detailed sample calculation and explaining each step of calculation and consideration.

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