Guideline to Design Electrical Network for Building / Small Area.

 Guideline to Design Electrical Network for Building / Small Area.

 (1)  Calculate Electrical Load:

  • Find out built up area in Sqft.of per flat per House/Dwelling unit.
  • Multiply area in Sqft. by Load/Sqft according to following Table
Type of Load Load/Sqft
Industrial 100 Watt/Sqft
Commercial 30 Watt/Sqft
Domestic 15 Watt/Sqft
  • Apply the diversity factor and Compute the load of all dwelling units in the area.
Type of Load Diversity Factor
Industrial 0.5
Commercial 0.8
Domestic 0.4
  • Add the load of common services such as Auditorium, Street Lights, Lifts and Water Pumps etc. For simplicity purpose 0.5kW/dwelling units may be considered as common load.
  • Compute the “Total Load” of the area by adding load observed at above.
  • Apply the power factor of 0.8 to determine the load in kVA.
  • Compute the Load in kVA= “Total Load”/0.8
  • Take transformer loading of 65% considering the network arrangement Ring Main Circuit.

 (2) Decide voltage grade for Electrical Load:

  • If load is equal to or more than 2.50MVA, the area shall be fed through 33kV feeder. For such loads, the land space for 33/11kV Sub-station shall have to be allocated by builder / Society/ Authority.
  • For load between 1 MVA to 2.5MVA, dedicated 11kV feeder shall be preferred.
  • For load below 1 MVA, existing 11kV feed can be tapped through VCB or RMU.

(3) Decide Size of Transformer:

  • Select T.C Size of 25 KVA,63 KVA,100 KVA,200 KVA or 400 KVA  according to your Load.
  • The maximum capacity of distribution transformer acceptable is 400 kVA as a standard capacity.
  • Only two-no of transformer at one location shall be acceptable. If there is more number of transformers HT shall be required to extend using underground cables to locate additional transformer.

(4) RMU / LT Panel:

  • Either VCB or Ring Main Circuit shall be used to control transformers. There cables should have metering arrangement at 11kV. The protection system at incoming supply shall be using numerical relays.
  • On LT side of transformer, LT main feeder pillar shall be provided. The Incoming shall be protected by MCCB/SFU.
  • The distribution pillar-box shall be connected into Ring Main Unit.
  • The incomer of distribution pillar shall have MCCB / SFU. The outgoing shall have HRC fuses.

(5) The LT cables from T.C to LT panel / Main feeder pillar:

  • Decide Size of LT Cable from T.C to LT Panel as per following Table.

Transformer Size


630kVA transformers 2 no x 1C x 630 Sq mm, Al, XLPE Cable
400kVA transformers 1 no x 1C x 630 Sq mm, Al, XLPE
250kVA transformers 3 ½ C x 400 Sq mm, Al, XLPE
160kVA transformers 3 ½ C x 300 Sq mm, Al, XLPE
100kVA transformers 3 ½ C x 150 Sq mm, Al, XLPE

(6) Considering various Factors & Length of Cable:

  • The factors for cable loading shall be taken as 70%.
  • The factor for multiplicity of cables from same cable trench shall be 80%.
  • The suggested maximum length of LT cable feeder shall be 250 Mtrs.
  • The LT cables shall be connected in ring main circuit.
  • The load on sub-feeder pillar shall be restricted to 150kW.

(7) LT cables from main feeder pillars to distribution pillar boxes:

Load on distribution pillar


LT Cable Size
Up to 50kW 3 ½ C x 150 sqmm, AL, XLPE
Up to 100kW 3 ½ C x 300 sqmm, AL, XLPE
Up to 150 kW 3 ½ C x 400sqmm, AL, XLPE

(8) Calculate Voltage Drop and T&D Losses:

  • The entire system has to be designed for a voltage drop of 2.0% from11kV Side of transformer to metering equipment at end consumer premises.
  • The entire system has to be designed for T&D losses of service maximum 2.0% from 11kV to end consumer meter including of service cable.


  1. NPC Limited.
  2. Electrical code.

About Jignesh.Parmar (B.E,Mtech,MIE,FIE,CEng)
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”,”Smart Energy”, “Industrial Electrix”(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

21 Responses to Guideline to Design Electrical Network for Building / Small Area.

  1. Mukesh Chandra says:

    Dear Jignesh
    Wonderfull & excellent contribution to the engineering community.

  2. MIKE says:

    measure the volts and the amps in the outlets read Audels Handy Book of electricity liked the cable information for the transformer in mm thanks

  3. rizal says:

    nice article Jignesh, whether this can be used to design a high-rise building.

  4. niranjan says:

    Dear Jignesh,
    First of all excellent work,I would like to leave few comments
    1)11kv can feed upto 5MVA load as i read in Rules in Karnataka Electricity Board
    2)I did’t understand cablesizes for 630&400kva transformer.

  5. jaison says:

    could you explain why 1 core using in 630 & 400 kva transformer..

  6. Agbum Ibah benedict says:

    How can i calculate or determine the number of lighting points in a hall?

  7. Ravindra says:

    Hi Sir ,

    I have read your technical blog it’s really informative for who hunger for knowledge .

    Do you have any technical guideline on Data Center design ,operation & Maintenance .If you have please send to me .

    Regards ,
    Ravindra .

  8. kuldip singh says:


  9. Naga Chaitanya says:

    Can you please guide us the design rules for high rise building also

  10. pankajbhai Trivedi says:

    Dear Jigneshbhai
    very useful details given by you.Pl.focus on D.G set selection

  11. siva sankar reddy says:

    sir how can i calculate the LT cables size (in sqmm)depend on load,plse give details

  12. oh Mr.Parmar i appreciate you greatly for re leafing me from this trouble.

  13. comin ewas nig. ltd says:

    I appreciate you. And would like to be In touch with you. Thanks!

  14. Prasad says:

    hi thx. plz mention your reference also in all parts….

  15. fawad says:

    Mr. Jignesh Parmar,
    your work on technical electrical engineering very very good.

  16. right89 says:

    First I would like to offer you a sincere thanks and gratitude to you Secondly, I hope that explains the entire example to determine the electrical loads of the residential tower

  17. Sumit Bobde says:

    Dear Jignesh Sir
    I really feel proud to belong to this electrical comunity who has such a talented person like you. I wish I get to see you and be under guidance of you.If there is any chance of mine to have your guidance plz information.

  18. Mufutau Ibrahim Engn. Techn says:

    Tanx, and Electrical excel tool for this great work

  19. Malyadri says:

    what is the load for sft in Hospitals ( general hospital, super speciality hospital)

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