How to calculate the size of MCBs while designing a circuit

Posted on Posted in Electrical, Study Material

To decide the number and types of MCB/ ELCBs in your office/ home, you must note down the number of electrical equipment used in your premises. Then start dividing them into different electrical branches (on the basis of your wire size calculations and cost effectiveness).

Example 02 — Click here

 

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Example 01

For example, suppose one building has 8 rooms with following equipment in each room.

Room-1

3 X 40 W Bulb (NC)

1 X 60 W F an (NC)

Room-2

2 X 40 W Bulb (NC)
1 X 60 W Fan (NC)

Room-3

2 X 40 W Bulb (NC)

2 X 160 W Computers (NC)

Room-4

1 X 40 W Bulb (NC)

1 X 60 W Fan (NC)

1 X 200 W Fridge (C)

Room-5

1 X 20 W Bulb (NC)

1 X 60 W Fan (NC)

1 X 1.7 kW Geyser (NC)

Room-6

1 X 20 W Bulb (NC)

1 No of 1Ph, 3KW, A.C (NC)

Room-7

1 X 20 W Bulb (NC)

1 No of 3Ph, 1HP, Motor-Pump (NC)

Room-8

1 X 20 W Bulb (NC)

 

 

 

Let’s break the above requirement in 6 branch circuits (How to calculate the wire/ conduit / switches is already discussed in earlier posts)

 

  • Branch Circuit-1: 4 No of 1Phase, 40W Bulb of Non Continues Load + 3 No of 60W Fan of Non Continues Load + 2 No of 160W Computers of Non Continuous load
  • Branch Circuit-2: 4 No of 1Phase, 40W, Bulb of Non Continues Load + 1 No of 60W Fan of Non Continues Load + 4 No of 1Phase, 20W, Lamp of Non Continues Load
  • Branch Circuit-3: 1 No of 1Ph, 200W, Fridge of Continues Load.
  • Branch Circuit-4: 1 No of 1P, 1.7KW, Geyser of Non Continues Load.
  • Branch Circuit-5: 1 No of 1Ph, 3KW, A.C of Non Continues Load.
  • Branch Circuit-6: 1 No of 3Ph, 1HP, Motor-Pump of Non Continues Load.

 

 

Calculation
Branch 1 —

Calculate the load currents of Bulbs, fan and computers individually

Current of 4 bulbs – (4 X Wattage X Demand factor) / 230 volts

= 4 X 40 X 0.6 / 230 = 0.417 Amps

Current of Fans = 3 x 60 X 0.6 / 230 = 0.47 Apms

Current of computers = 2 X 160 X 0.6 / 230 = 0.83 Amps

Total = 0.417 + 0.47 + 0.83 = 1.717 Amps

Now the branch has current less than 6 amps, MCB required will be 6 amp. (refer table 3) Class B Type (Load is lighting load) Single pole.

(NC- Non continuous load -Loads which are switched not on for full day

C – Continuous load – Loads which are in work full day like fridge, ac etc.)

Branch 2 —

Current for 40 W Bulbs = 0.417 A

Current for 60 Fan = 0.15 A

Current for 20 W Bulbs = 0.78 A

Branch 3 —

Current for branch 3 = 1.25 X 0.52 = 0.65 A

Note –  For continuous load, as per NEC the current is to be multiplied by a factor of 1.25

Branch 4 —

Current for branch 4 = 4.43 A

Branch 5 —

Current for branch 5 = 7.82A

Branch 6 —

Current for branch 6 = 0.6 A

Note –  For three phase loads current = Wattage/ 1.732 X Voltage

 

Now we have calculated the load current for each branch, let’s make a sheet for the MCBs to be used.

For MCB class, refer table 2.

Result :-

S.No Branch No Load current (A) MCB Class No of poles MCB (A)
1 Branch 1 1.717 B SP 6A / 6 kA
2 Branch  2 0.78 B SP 6A / 6 kA
3 Branch 3 0.65 B SP 6A / 6 kA
4 Branch 4 4.43 C SP 16 A / 6 kA
5 Branch 5 7.82 C SP 16 A / 6 kA
6 Branch 6 0.6 C TP 16 A / 10 kA

 

****If you have a building/premises or project and you need to calculate/ design the low side electrical (internal wiring), mail us your requirement at pratham6138@gmail.com and we will revert with any information you require. It is not chargeable.****

 

Table 1

Fault Current
Voltage
Fault Current
230V
6KA
430V
10KA
11KV
25KA

 

 

Table 2

Class of MCB/ELCB/RCCB
Type of Load
Class
Sensitivity
Lighting
B Class
I∆n:30ma
Heater
B Class
I∆n:30ma
Drive
C Class
I∆n:100ma
A.C
C Class
I∆n:30ma
Motor
C Class
I∆n:100ma
Ballast
C Class
I∆n:30ma
Induction Load
C Class
I∆n:100ma
Transformer
D Class
I∆n:100ma

 

 

Table 3

Size of MCB/ELCB
Current (Amp)
Lighting Load MCB/ELCB (Amp)
Heating/Cooling/Motor-Pump Load MCB/ELCB (Amp)
1.0 to 4.0
6
16
6.0
10
16
10.0
16
16
16.0
20
20
20.0
25
25
25.0
32
32
32.0
40
40
40.0
45
45
45.0
50
50
50.0
63
63
63.0
80
80
80.0
100
100
100.0
125
125
125.0
225
225
225.0
600
600
600.0
800
800
800.0
1600
1600
1600.0
2000
2000
2000.0
3000
3000
3000.0
3200
3200
3200.0
4000
4000
4000.0
5000
5000
5000.0
6000
6000
6000.0
6000
6000

 

13 thoughts on “How to calculate the size of MCBs while designing a circuit

      1. Gopiraj
        For AC system .. please do not forget to keep the PF along.

        i.e. W=V*I*(PF)
        Less the power factor, more will be load current.

  1. I have a question to ask that to prepare a complete line diagram is main db is fix with mcb and from there to rccb is required?

  2. Hi! Thanks for the info, it is really helpful.
    Quick question, what size MCB will be required for 36KW Water Heater?
    Is a 225 A MCB Enough?

    1. Is it a 3 Phase Hot Water Generator?
      See, the current rating changes abruptly for three phase system. For 3 phase 36 kW, you can use a 100 A 4P MCCB or 225 Amp DP MCB for 36 kW single phase hot water generator.

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