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.
Room1
3 X 40 W Bulb (NC) 1 X 60 W F an (NC) 
Room2
2 X 40 W Bulb (NC) 
Room3
2 X 40 W Bulb (NC) 2 X 160 W Computers (NC) 
Room4
1 X 40 W Bulb (NC) 1 X 60 W Fan (NC) 1 X 200 W Fridge (C) 
Room5
1 X 20 W Bulb (NC) 1 X 60 W Fan (NC) 1 X 1.7 kW Geyser (NC) 
Room6
1 X 20 W Bulb (NC) 1 No of 1Ph, 3KW, A.C (NC) 
Room7
1 X 20 W Bulb (NC) 1 No of 3Ph, 1HP, MotorPump (NC) 
Room8
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 Circuit1: 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 Circuit2: 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 Circuit3: 1 No of 1Ph, 200W, Fridge of Continues Load.
 Branch Circuit4: 1 No of 1P, 1.7KW, Geyser of Non Continues Load.
 Branch Circuit5: 1 No of 1Ph, 3KW, A.C of Non Continues Load.
 Branch Circuit6: 1 No of 3Ph, 1HP, MotorPump 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 
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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/MotorPump 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”
mcb chart
16amp ke sweech pr hum kitne kilowatt load de sakte he or 16amp mcb pr kitna load de sakte he
3 kW at Par, keep it 1.52 kW
6amp per kitna watt load de sakte hai
6amp MCB per kitna watt load de sakte hai
1.2 kW at par, keep 800 Watt max
1380 watts tak de sakte hain?
Put the formula
We know that W=V*I
W=watt, v=voltage, I=current.
I=6amps,v=230.
Then, 6*230=1380 watts
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.
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?
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?
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.
I love this and i have learnt much. thanks
Nice