(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|
Apply the diversity factor and Compute the load of all dwelling units in the area.
|Type of Load||Diversity 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
(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:
|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
|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% 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
- NPC Limited
- Electrical code