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.


  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??

  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.

  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

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  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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