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.
- Your building has 03 Floors
- 10 Rooms per floor with 2 kW power required in each and 0.5 kW required from backup power from DG
- 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).