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 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) 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 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 boxes:

Load on distribution pillar


LT 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% 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.