Miniature Circuit Breaker Sizing

When we design or build a electrical installation,we must know how to choose the suitable circuit breaker for the any application that we design.For electrician,this is compulsory knowledge.So here i want share my little knowledge about circuit breaker sizing method.

This calculation for sizing the miniature circuit breaker.For other circuit breaker likes MCCB,ACB,VCB,OCB and etc,it have a additional factor and calculation that we must consider.

For that why i will discuss it in my next post…so this time i want explain the simple way how to sizing the miniature circuit breaker for our application.

 How to sizing a miniature circuit breaker?



Why it so important? have many reason as below :

  • For electrical safety purpose
  • To not over cost to purchase circuit breaker when we over sizing it. or not suitable
  • To prevent the equipment from damage before it can use it.
  • To easy when we want purchase the mcb,because we have all detail  information about it.
  • To make the design matter more effective and faster to complete.

How to select a suitable  MCB for the application?

we must focus in 3 main points to choose a right MCB for electrical circuit :-

1) Ampere rating ( In )

2) Short circuit breaking capacity ( If )

3) Application type

Calculation for Miniature circuit breaker ( MCB ) sizing

Example : We want install 1 unit single phase motor.full load ampere is 20 ampere. Power supply 240VAC,50 Hz

1) Ampere Rating ( In )

Formula : In = Iz + (0.25 x Iz)

In = ampere rating      Iz = Load ampere

Calculation :

In = 20 ampere + (0.25 x 20 ampere)

In = 25 ampere@ 32 ampere

* in market we cannot found 25 ampere of MCB,so we can used 32 ampere for our application.

Refer the ampere rated in technical data for MCB from manufacture

From the technical data,we selected the ampere rated for MCB is  : 32 ampere

2) Short circuit breaking capacity ( If )

Formula : If = V / 0.276 (R cable)

If = short circuit breaking capacity

V = Voltage

R cable = resistance cable ( 0.276 )

Calculation :-

If = 240 / 0.276

If = 870 ampere

From the answer,we can decide to select rating for one ( 1 ) kA


3) Application type.

From application,it state the single phase motor,so we can choose TYPE C for MCB.It for general application and for small motor.Please refer the table below for MCB type for the application.

* For more detail,please refer the manufacture Technical data / user manual or contact the supplier.

ABB MCB Technical data

MCB Type,Instant trip and Application table


1                                  2.7 – 4 In                   Domestic and commercial

B                                  3 – 5 In


2                                  4 – 7 In

C                                  5 – 10 In                     General use,small motor

3                                 7 – 10 In


D                                10 – 20 In                    High in rush equipment

4                                10 – 50 In


K                                                                       Frequent short duration


Z                                                                       Semiconductor devices


So…the conclusion from the calculation,we get the specification for MCB to install on our application :-

1) Rated ampere is : 32 ampere

2) Short circuit breaking capacity is : 1 kA

3) Type of MCB is : Type C

For more detail,please contact the electrical engineer for their suggestion.This is a simple way how miniature circuit breaker sizing .Please refer the IEE manual for electrical guideline.


Lemau is Owner of .Involved in Electrical & Automation Engineering since 2001. Passionate to shared all experience & knowledge about Electrical.

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  1. Please give an example for selection of MCB(type C or D)for a 3phase induction motor,say 10HP,15Amp- if started using an across the line(DOL)
    as well as wye-delta starter.

  2. from your example :-
    10 hp;15 ampere

    the calculation is :

    15 ampere x 0.25 = 3.75 + 15 ampere = 18.75 ampere
    So we can choose the MCB ampere rating = 20 ampere
    For induction motor with DOL stater,the suitable type is C or D

    • As the starting current is not the same for DOL and star-delta starters
      can we use smaller MCB for star-delta starting?. Is it correct if I say 20A for star-delta and 25A for DOL starting

      • U can use a smaller MCB but make sure it not below from the full load ampere rating.One more important thing is the TYPES of MCB must suitable with your application.Example MCB type C or D for motor stater.For more safety about overload protection,we still have the overload relay :D

  3. For frequent start/stop or DOL starting, OL relay won’t protect. MCB will trip for inrush current.So we used MCB type C rated 2xFLA for DOL and 1.5xFLA for star delta starting. Another important aspect is the rating and type of MCB should match the duty of motor -AC1,2,3 etc. For frequent start/stop like pressure switch operated water boosting pumps should be use type C duty AC3 or type D AC1?. Therefore the formula 1.25xFLA is for which duty class and will the tripping and contact wear problem will be solved for some class of duty by using class D MCB?. Please elaborate.Manufacturer’s literature will be a useful guide.

  4. stl

  5. Hi guys,

    Please advice me. if i have 5 unit 10 Amp MCB type and 1 incoming fuse isolator. what would be the rating of fuse inside isolator..example:total all MCB = 50A?

    • You can used isolator switch 40 ampere or 62 ok if ampere size is bigger.It spare for future additional of mcb

  6. 15 ampere x 0.25 = 3.75 + 15 ampere = 18.75 ampere

    Dear sir , in above example to calculate ampere rating why you addaed 15 , plz explain

    • Dear sid…
      first we multiply the full load ampere value with 25%.After that we add back with total ampere value to get total ampere for MCB rating.

  7. Michel says:

    Hi guys
    Please someone can confirm if I can use 3 phases breaker on a 1 phase load. It may sounds weird, but I do have this spare and dont wont to buy one new 1 phase breaker.
    Believe is ok, but nor sure. Thanks

  8. can you update me about this post: The calculation for sizing the miniature circuit breaker.For other circuit breaker likes MCCB,ACB,VCB,OCB and etc,it have a additional factor and calculation that we must consider.

  9. ankita says:

    dear sir,

    can you plz tell me why you are multiplying total load current (Iz) with 125%?

    i have been trying this but i was multiplying the load current with 120% and getting the right ans for the ratings which are given in watts but couldn’t got the ans for the ratings which are given in hp.

    can you plz solve this for me??

    mcb rating for 0.5 hp.
    my sol. is—
    total load current will be Iz = 0.5*746/220 = 1.69A

    In = 1.69(120\100) = 2.03A

    according to this we can use mcb of rating 2.5A.

    but the ans is 6A.

    can you solve this??

    • Jayanta says:

      Your calculation 100% right.
      Pls ….mcb class selection
      Ex= class B for Light
      Class C for soficticated
      Class L for ( B+C)

  10. dharma says:

    hello sir,
    I am having problem with 40hp 3phase motor 63amps MCB trips
    after running for an hour,when i bypass the MCB and run for an hour
    it blows off the incoming 100A fuse.checked motor meggar,good more than 20Momhs,checked phase to phase coil winding-good.phase to phase
    voltage also good-415v.What could be the reason for the above problem
    thanks.pls help.

    • Please check your phase either is completed 3 phase or not.

    • balan says:

      you need 125 MCCB/Fuse
      40 x 746 = 29840 watt
      FLA = 29840/611
      FLA = 49 Amps
      Assume your are using star-delta starter:
      49 x 2.5 = 123 Amps
      So the nearest MCCB available in market is 125 Amps

      • asanka says:

        1) If This is VFD what is the breaker size?
        2) If this is soft Starter What is the Breaker size?
        3) Assume if this is DOL starter what is the breaker size?
        4) For selecting of cable we have to use the rated current or breaker selected current.

  11. vsskh says:

    How do we know R Cable ? or 0.276 is a default value ?

  12. Those fumula are very useful for electrician thanks

  13. I have 15 amps motor but am using mcb 63 amps this is not suitable how to calculate mcb amps I dnt no guide me

    • Jayanta says:

      15 amp motor
      Amp rating =15 +(0.25×15)
      =18.75 amp.
      Because…market avaleble MCB is 20amp
      =20 amp, c class.

  14. pavan says:


    why we the 0.25 factor..
    and i am using the fan motor rating is 3.8 amps can u tell me appropriate MCB

    • Jayanta says:

      Your motor watt is =1000
      Because mcb rating is =4.75amp
      Your appropriate MCB is 6amp C class

  15. mohammad says:

    why are used the constant values to rated current and what is the relation between resistance cable and the MCP circuit breaker ….. and your equation i can u used it in all times and for all difference value of rated current ……
    ABB more price and less clear compare with Schneider…..

  16. Hi! This is my first visit to your blog! We are a team of volunteers and starting a new initiative in a community in the same
    niche. Your blog provided us beneficial information to work
    You have done a wonderful job!

  17. Sanusi Adebola Taofik says:

    your above calculation of the mcb sizing is wrong
    how do u arrive at 25@32ampere,the working is confusing.

  18. deepak says:

    Hi sir, In our machine we are using 4 servo motor and 2 induction motor, servo motor ratings are total 2300w, induction motor loads are 2000w, how i can select the main MCB for this machine.

    • Hi,
      Please calculate total full load ampere for your system ( 2300W + 2000w).Use this formula to get FLA –> P = I x V x 1.732.
      After you get the FLA than you can sizing the MCB.

  19. Rajiv Aggarwal says:

    The incoming neutral of two sources of supply should not be mixed up ? Why?

    For three phase input and three phase and single phase output , TPN MCB is preffered over FP MCB .Why?

  20. as per the above example,i came understood that in an small non ac house each and every equipment (right from small water pump to geyser or water heater) of different amp rating should have different,different mcb’s rather than traditional 16amp or 20amp single mcb for the whole small house,
    Is it right sir,replay me soon.

  21. mohammed sailani says:

    how many sizes start mcb and and last size

  22. How we can calculate mccb for 350hp motor

    • Hi,
      Please get the full load ampere for 350hp motor,and used formula to get suitable breaker size

      • Jayanta says:

        350 Hp =350×746
        =261100 watt
        Amp =593.40
        Rating =741.75 amp
        Appropriate MCCB is= 800 amp,4 pole ISLCX0028 .

  23. Dear Sir
    what is . Selection method for MCCB,

  24. Rashedul Haque says:

    How much rating of circuit breaker for 1500kva?plz answer with formula.
    How can i calculate PFI rating?plz answer with formula and example.

  25. Nagesh says:

    I am starting a fitness centre. I am taking a supply of 40 kW to cater to all my requirement. Supply is 3 phase. What should be the mcb capacity at my distribution box. What type of mcb, how many poles and current capacity. Builder has installed a 63 amp mcb. Should I change it.

  26. JEYASANKAR says:

    Lights 100w fan 50 w + one 5a socket
    What is the range of mcb

  27. Hi
    Sir i have 6 stright light one have pole having 6 bulb one bulb for 2000watts and i am using each pole one breaker separat separate 32 amps breaker how should be main breaker size

  28. Aashiesh says:

    How to size MCB for DC load

  29. Aashiesh says:

    1.If unit power is 300 watts and current is 3.13 why current safety factor is 125% considered whichi s 3.91 amps.

  30. excuse me sir…
    Why you chooses resistance cable with 0.276 it depend on table or calculate?
    can you saw me plz

  31. Please help:

    I will start a project in U.S and my incoming source is 480V/3P/60Hz/100A. What size mccb should i use that is good. How about 690V/3P/50-60Hz/100Amp/?KA?? Need help to size up the KA!!

  32. Mi Mi says:

    hello Sir,

    I want to use 13A socket 6 nos for onl circuit so what size of breaker can I use? how can I calculate the breaker size?

  33. Suhanul says:

    Hi , I have fitted 6 emergency lights with 8 watts each light, now Iam not sure what right rate of mcb I should used in consumer unit, please can anyone help me calculate the mcb amps !? Thank you all

  34. akshay sahu says:

    Sir pls tell which ampere rating mcb should we used for different different load ??

  35. Imran says:


    I have 3 phase 10 hp hydraulic pump motor and 2 nos. 15 kw single phase heater in my panel.

    Please help me to get the rating of main incomer MCB and power cable for this.
    And pls tell me the type of mcb also

    Thanks in advance.

  36. Arunya says:

    How to test mcb for its tripping without much resource

  37. SYED ASIF PASHA says:

    I have 63 kV transformer and I need to run 100 HP motor shall I run in the same transformer or have to go for 100 kV transformer.pls reply

    And also if I use 100 HP motor with 1440 rpm than can I use 125 amps capacity MCB and star Delta fully automatic relay and contactors. Pls reply

  38. Vinod kumar says:

    Dear sir,
    Let me know, which circuit breaker can be use for fixing 2 nos 3 phase 12kW heaters( 2 nos)

  39. waquar husain says:

    I want a 3 phase mcb which can trip whenever any phase is absent…
    Please tell me which mcb is suitable….


  1. […] should done a proper sizing calculation for fuse overload relay and circuit breaker to suit with starting current for motor […]

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