June 30, 2011

Cable Sizing Calculation - Cable Sizing Design



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, 3.3kV, 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 (during running & motor starting)
  • Current Rating
  • Maximum cable loop impedance for earth fault.
  • Cable Let Through Energy
  • Breaker Let Through Energy

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 40ยบ 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


Cable Sizing Calculation
  • The load current(IL) for static loads and cable site rating ( IZ) are calculated based on the following equations:
Cable Site Rating Calculation
  • The Load current(IL) for motor should be based on ABB(or others) Motor Technical Data.
  • For cable to be acceptable, the value of Iz must be higher than IL and Ib.
  • 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 ( ZT) 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


Fujikura Cable Technical Data

Fujikura Cable Technical Data

Fujikura Cable Technical Data

ABB Motor Technical Data
  




 cable sizing | cable size | cable sizing calculation | cable size calculation | voltage drop calculation | cable size calculator | voltage drop | cable sizing table | cable size table | cable sizing design | cable design

15 comments:

K.Janardhahah said...

Good Presentation

va said...

Great people always share the valuable information

JAMALUDDIN ABU BAKAR said...

Thanks all..:-)

NK said...

It is a good recopilation of useful information. Thank you for sharing.

roby said...

Friends, safety and safeguard is our destination. So, we should think accordingly. It is very nice presentation to protect ourselves. Thanks a lot.

Mohammed Haris Qurashi said...

thnx for sharing a this kind of information.

Madhuri Yadav said...

thanks for valuable information.

Rohan Sharma said...

valuable information..
thanks for sharing!

Hemant said...

Saleem miya thanks

Bryan Lai said...

Very good item but some suggestions for improvement:

This calculation is for AC circuits, not DC circuits as mentioned in Item 2.

Instead of using Fujikura cable data, it's more acceptable to use IEC cable data. Use the international standards as much as possible.

This is a good start, but do consider other things like protection on a case by case basis.

JAMALUDDIN ABU BAKAR said...

Orait..thanks for sharing it Bryan.

fouad said...

Good presentation,

There is an other Cable Selection Criteria: harmonics

shekhar said...

really helpful thanks

S.Ravi said...

Useful details.
Thanks.

S.Ravi

JAMALUDDIN ABU BAKAR said...

Your Welcome Ravi..

Related Posts Plugin for WordPress, Blogger...