Cable Sizing for Building Services

Cable Sizing for Building Services


To perform a basic cable sizing calculation for Building Services by a simple and presentable worksheet for internal checking and submission.


  1. Check current carrying capacity of supply cable
  2. Check voltage drop
  3. Confirm supply and earthing cable are protected by the protective device in normal and fault conditions
  4. Check whether earth fault protection can be achieved for 0.4s or 5s disconnection
  5. Calculate maximum fault level at downstream


  1. Activate the functions for complex numbers in Excel. (Activate Analysis ToolPak under Tools -> Add-Ins)
  2. In the worksheet, you shall enter
    • Source Fault Level (MVA)
    • Voltage drop limit (%)
    • Connected Load (A)
    • No. of Phase ( 1 or 3 )
    • Cable Type ( A íK. M )
    • No. of Cable per phase ( 1 as initial trial )
    • Length of Cable (m)
    • Rating of Protective Device ( The rating shall be larger than the Connected Load )
    • Type of Protective Device ( B / 3 / Fuse )
    • Type of Earthing ( Cable / Armour / Tape )
    • Size of Earthing ( ie. Size of Cable / N/A for Armour / Size of Tape (25x3 / 25x6 / 50x6 )
  3. Then, you shall increase the size of cable until all cells under Remark are blank. If this cannot be achieved with the largest cable size, enter a higher value in No. of Cable per phase.


Performance of overall distribution (i.e. from LVSB to sub-panels to final circuits) cannot be evaluated in a single worksheet.

Comment and Recommendation

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  1. Low voltage electrical installation handbook by Johnny C.F. Wong
  2. Code of Practice for the Electricity (Wiring) Regulations 1997 Edition by EMSD


  1. The worksheet shall not be used for any commercial purposes.
  2. We shall not hold any liabilities on the accuracy of the worksheet.

Special Thanks to Roger who tackles all the I.T. issues

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