Cable Sizing Calculation

Objectives

1. To determine the size of electrical cable from the main source (Generator/Power Grid) / Distribution Panel to the terminal of end user equipment.
2. To determine the type of electrical cable to be used for Power and Control Cables (6.6kV, 415V, 240V, 220V DC), Lighting Cables and Grounding Cables

For the sizing of cables, the following aspects shall be taken into consideration:

• Thermal short circuit capacity
• Voltage Drop
• Current Rating
• Maximum cable loop impedance for earth fault.

Cable Selection Criteria and Basis

1. Selected cable site rating is greater than the equipment full load current
2. Nominal current setting for the protective device shall be greater than the design full load current plus a safety margin but does not exceeds the cable site rating
3. Running voltage drop does not exceed more than the permissible voltage drop of its rated voltage. Voltage drop from the main power generation to the terminal of end user shall not exceed 5%.
4. From IEE Wiring Regulation,the 17th edition (in force from 1 July 2008) limits allowable voltage drop to: (3% for lighting) (5% drop for other loads) (but 6% and 8% respectively are permitted for private supplies)
5. Motor starting voltage drop are less than 20% of rated voltage
6. Cable let through energy is greater than protective device’s let-through energy
7. For selection of breaker/protective device current setting, a safety margin of 10% and 15% of the full load current is considered for static load and motor load respectively. 
8. Motor starting power factor is assumed to be 0.3 lagging (worst case)
9. The full load current, locked rotor current and running power factor for motor loads are based on ABB motor technical data 
   
10. For cable laid in air and laid sub-sea at maximum temperature of 40º C, the correction factor of cooling air temperature is 1.0 and 0.88 respectively based on Fujikura Power Cable Data. 
11. Cable group de-rating factor is assumed based on Fujikura Cable Data as below:  

• For cables laid sub-sea, the group de-rating factor is taken as 0.9 with an assumption that less than three (3) circuits are laid together. 
• For cables laid together on a metal cable ladder, the group de-rating factor is taken as 0.7 with an assumption that ten (10) circuits are bunched together 

Sample of allowable voltage drop (click to enlarge)

Cable Sizing Calculation

• The load current(I_L) and cable site rating ( I_Z) are calculated based on the following equations:

Click to enlarge

• For cable to be acceptable, the value of I_z must be higher than I_L and I_b.
• The motor starting voltage drop and running voltage drop are calculated based on the equation below:

• Therefore, the motor starting voltage drop and running voltage drop must be lower than the respective allowable running and starting voltage drop, in order for the cable size to be selected.
• To check for let through energy, equation below can be used:

•  Total impedance ( Z_T) are calculated by using equation below:

for 415V AC

for 240V AC and 220V DC

• And the selected cable let-through energy must be greater than the breaker let-through energy such that when the fault occur, the breaker will open and clear the fault before the cable breaks.

References

1. IEE Wiring Regulation 16th Edition
2. ABB Motor Technical Data
3. Power Cables IEC Standard by Fujikura

  

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