How to calculate the size of wire and mcbs for my house

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We received a query from one of our reader on how to calculate the size of MCB for my house. Since, the query is quite common we are here writing the solution.

 

QUERY –

Dear Sir,

 

I am fond of your articles, i study them regularly. Sir i have a query regarding wiring of a 3 floor building.

Each floor has 10.nos of shop, i have given main power to each shop 2000W,

and backup power by 500w by seperate wiring through C/O in main Panel.

 

And at the DB end for Each Power line give 16A of SP MCB and for backup line 6A of MCB.

 

I have distributed all three phase in each floor. Now i want to calculate main incommer size for Power and Backup MCCB for main panel, and C/o Rating.

kindly guidline me. i have Lt connection of 100KVA. and my DG is 15KVA.

 

Thanks & Regards,

Chandramouli.

Vastu Vihar, Hajipur, Bihar.

 

Reply

Dear Chandramouli, thanks for writing to us, as per our understanding to your query.

  1. Your building has 03 Floors
  2. 10 Rooms per floor with 2 kW power required in each and 0.5 kW required from backup power from DG
  3. You want to know the main incomer.

Here is a solution provided on basis of your query by Gopal Singh (Assist. Electrical Engineer), below text lets us calculate the size of mcbs for each circuit and how to calculte the MCB for main incomer.

Ground Floor Layout
Room 1 – Power Ckt – 1, Lighting Ckt -1, Emergency Ckt 1 Room 2 – Power Ckt – 2, Lighting Ckt -1, Emergency Ckt 2 Room 3 – Power Ckt – 3, Lighting Ckt -2, Emergency Ckt 3 Room 4 – Power Ckt – 4, Lighting Ckt -2, Emergency Ckt 1 Room 5 – Power Ckt – 5, Lighting Ckt -3, Emergency Ckt 1
Room 6 – Power Ckt – 6, Lighting Ckt -1, Emergency Ckt 6 Room 7 – Power Ckt – 7, Lighting Ckt -1, Emergency Ckt 7 Room 8 – Power Ckt – 8, Lighting Ckt -2, Emergency Ckt 8 Room 9 – Power Ckt – 9, Lighting Ckt -3, Emergency Ckt 9 Room 10 – Power Ckt – 10, Lighting Ckt -3, Emergency Ckt 10

 

Lighting Load  = 3 Nos 30 W LED per Room  
For ten rooms = 3 X 30 X 10 W  
= 900 W  
 
Let us distribute 900 Watt into three Circuits  
Lighting Circuit L1 = 360 W R  
L2= 270 W Y  
L3= 270 W B  
 
 
Power Load = 2 kW per room  
For ten rooms = 10 X 2 kW  
= 20 kW  
 
Let’s distribute the total power load in 10 power circuit  
P1 2 kW R  
P2 2 kW R  
P3 2 kW R  
P4 2 kW Y  
P5 2 kW Y  
P6 2 kW Y  
P7 2 kW B  
P8 2 kW B  
P9 2 kW B  
P10 2 kW B  
 
Similarly lets distribute emergency load in 10 circuits  
 
E1 0.5 kW R  
E2 0.5 kW R  
E3 0.5 kW R  
E4 0.5 kW Y  
E5 0.5 kW Y  
E6 0.5 kW Y  
E7 0.5 kW Y  
E8 0.5 kW B  
E9 0.5 kW B  
E10 0.5 kW B  
Let us now prepare a DB detail for Light & Power
L & P DB 01 Ground Floor
    MCB   Load Distribution (W)
CKT No Wire in size Amp Type Description R Y B
L1 3 X 2.5 Sq mm Cu Wire 10 SP- B Curve Room 1,2,6 & 7 360    
L2 3 X 2.5 Sq mm Cu Wire 10 SP- B Curve Room 3,4 & 8   270  
L3 3 X 2.5 Sq mm Cu Wire 10 SP- B Curve Room 5,9 & 10     270
P1 3 X 4 sq mm Cu Wire 16 SP- C Curve Room 1 2000    
P2 3 X 4 sq mm Cu Wire 16 SP- C Curve Room 2 2000    
P3 3 X 4 sq mm Cu Wire 16 SP- C Curve Room 3 2000    
P4 3 X 4 sq mm Cu Wire 16 SP- C Curve Room 4 2000    
P5 3 X 4 sq mm Cu Wire 16 SP- C Curve Room 5   2000  
P6 3 X 4 sq mm Cu Wire 16 SP- C Curve Room 6   2000  
P7 3 X 4 sq mm Cu Wire 16 SP- C Curve Room 7   2000  
P8 3 X 4 sq mm Cu Wire 16 SP- C Curve Room 8     2000
P9 3 X 4 sq mm Cu Wire 16 SP- C Curve Room 9     2000
P10 3 X 4 sq mm Cu Wire 16 SP- C Curve Room 10     2000
  Total (in Watts)       8360 6270 6270
Incomer 63 A 4P MCB, 4 X 16 Sq mm Cu. Wire
Let us now prepare a DB detail for Separate Emergency Power
E DB 01 Ground Floor
    MCB   Load Distribution (W)
CKT No Wire in size Amp Type Description R Y B
E1   16 C Curve Room 1 500    
E2   16 C Curve Room 2 500    
E3   16 C Curve Room 3 500    
E4   16 C Curve Room 4   500  
E5   16 C Curve Room 5   500  
E6   16 C Curve Room 6   500  
E7   16 C Curve Room 7   500  
E8   16 C Curve Room 8     500
E9   16 C Curve Room 9     500
E10   16 C Curve Room 10     500
  Total (in Watts)       1500 2000 1500
Incomer 40 A 4P MCB, 4 X 10 Sq mm Cu. Wire

 

 

Since all your floors are identical, let us calculate the total load on incomer

= 78 kW

@ 0.85 PF and @ 0.8 Diversity factor, the total current on Main DB will be = 85 amps

So for the main incomer you can go with a 100 Amp 4P MCCB, 4C X 70 sq mm Al Ar Cable (with 3% voltage drop for 100 m distance from connection).

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