# Guideline to Design Electrical Network for Building (Small Area)

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## (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) 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 Cable 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

###### References:
1. NPC Limited
2. Electrical code

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#### Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control) ,B.E(Electrical). He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 13 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", "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.

1. Melvin A. Delostrico
Aug 24, 2016

hi.

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?

thank you.

2. shahi
Jun 19, 2016

thanks

3. 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

4. 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?
Thanks.

5. Pankaj Desai
Oct 20, 2015

Hi,,
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.

6. Vijay
Feb 20, 2015

hi Jignesh,
Industrial100 Watt/Sqft
Commercial30 Watt/Sqft
Domestic15 Watt/Sqft

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

7. Engr Essien
Jun 20, 2014

I realy appreciate this blog

8. 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?

Regards,
Srinivas