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Home / Technical Articles / Guideline to Design Electrical Network for Building (Small Area)
Industrial zone
Industrial zone

(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 LoadLoad/Sqft
Industrial100 Watt/Sqft
Commercial30 Watt/Sqft
Domestic15 Watt/Sqft

Apply the diversity factor and Compute the load of all dwelling units in the area.

Type of LoadDiversity Factor

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) 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 SizeCable
630kVA transformers2 no x 1C x 630 Sq mm, Al, XLPE Cable
400kVA transformers1 no x 1C x 630 Sq mm, Al, XLPE
250kVA transformers3 ½ C x 400 Sq mm, Al, XLPE
160kVA transformers3 ½ C x 300 Sq mm, Al, XLPE
100kVA transformers3 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

Load on distribution pillarLT Cable Size
Up to 50kW3 ½ C x 150 sqmm, AL, XLPE
Up to 100kW3 ½ C x 300 sqmm, AL, XLPE
Up to 150 kW3 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% from 11kV 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.

Originally published at Electrical Notes & Articles

  1. NPC Limited
  2. Electrical code

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More Information

Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


  1. adeyemi alaba
    Aug 12, 2021

    Please can I get book on electrical building design and load calculation with numbers of fittings and sockets in a room. I will be grateful if you can help. Am writing a paper on hand book on electrical building designs.
    Thank you

    Mar 08, 2018

    dear sir
    please guide me how to design 100kw LT PANEL FOR SOLAR SYSTEM.

  3. Melvin A. Delostrico
    Aug 24, 2016


    Why 33kva and 167Kva transformer is decided KVA rating of a transformer? why not used 35Kva or 160Kva/165Kva/170Kva instead of usiing those two ratings?

    please answer me in clear explaination.

    thank you.

  4. shahi
    Jun 19, 2016


  5. Enumah Ugochukwu
    Dec 03, 2015

    Thanks you sir for this very informative article. I would like to know what is the maximum length a 33KV and 11KV feeder should go

  6. Sonny
    Oct 29, 2015

    Hi Mr. Parmar, two questions 1) Is there any requirement in NEC about spare breakers in a panel (5% or 10%) ?
    2) Do I need a lockable c.b. for supplying emergency lights(reg fluorescent with built in inverter) for a small office in Manhattan?

  7. Pankaj Desai
    Oct 20, 2015

    Greetings from Pankaj Desai! I am really impressed of your work for electrical engineering and excel sheet you made.

    Can we connect 4 Nos of 200 KVA TC to one no of 125 KVA DG? If yes, how? Pl share with drawing.Each 200 KVA TC for each building and 125 KVA DG set all four building. DG set for common lighting and Lift. Each building common electrical load around 25 KVA.

  8. Vijay
    Feb 20, 2015

    hi Jignesh,
    Type of Load Load/Sqft
    Industrial 100 Watt/Sqft
    Commercial 30 Watt/Sqft
    Domestic 15 Watt/Sqft

    Regd this NBC states 1.5 w/sq.ft.
    Could you suggest which factor should be used.

  9. Engr Essien
    Jun 20, 2014

    I realy appreciate this blog

  10. m
    Jun 25, 2012

    Hi Friends,

    Below queries are w.r.t LV distribution.
    We need to distribute 3 phase power to 3 phase loads and 1 ph loads. As per our requirement we are supposed to use insulation monitoring devices for input and MCB’s with RCB’s for loads control. Pl. suggest the use of both, application, why both of them are required in circuit.

    Also let me know use of phse sequence relay, where, why should we use?


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