Miniature Circuit Breaker

What is Miniature Circuit Breaker?

MCB (Miniature Circuit Breaker ( BS 3871 ) is a mechanical device operate automatic operation designed to connect and disconnect the electrical circuit in the ordinary and extraordinary situation.Suitable for AC and DC voltage.It mean main function is to protected equipment or circuit from overload and short circuit.

The size of MCB is small and usually use for household appliance and domestic.Average fix ampere rating from 6 Amp up to 125 Amp. It can be reset back to the normal condition after trip but cannot adjusted ampere rating.It can be found for single phase 1ø and three phase 3ø voltage power supply.

Basic construction

  • A switching mechanism
  • An automated operation device
  • Arc extinguisher
  • Have a fixed contact and moving contact
  • Comply with IEC 60898
  • Breaking capacity not less than 6 kA ( RMS )
  • The tripping element

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


Description of MCB ( Miniature circuit breaker )

Detail of MCB

1) Actuator piker ( for trip and reset )

2) Actuator Device mechanism

3) Main contact ( flow of current )

4) Terminal Block

5) Bimetallic steel strip

6) Tripping calibration screw

7) Solenoid coil

8) Arc extinguisher

How many pole for Miniature circuit breaker?

There are four designs in the market for MCB

1) One (1) pole

For single phase use ( Live )

2) Two (2) pole

For single phase use ( Live & Neutral )

3) Three (3) pole

For three phase use ( R,S,T ) or TP ( three pole )

4) Four (4) pole

For three phase use ( R,S,T,Neutral) or TPN ( three pole & neutral )

I already discuss in my post about how to sizing the circuit breaker :D feel free to visit :D


  1. Vemula Balaiah says:

    Dear sir,

    This is Mr. Vemula Balaiah
    I am Graduate Electrical Engineer

    Please clarify the below Doubts.
    How we can select the MCB Rating?

    it depends on load? if yes please clarify the below ratings.

    Data is : 3-Phase, 50HZ , 415V .

    1. if Load is 1000 Watts then what is the MCB Rating?
    2. if load is 1500 Watts then What is the MCB Rating?
    3. if Load is 2500 Watts then what is the MCB Rating?
    4. if Load is 2000 Watts then what is the MCB Rating?
    5. If Load is 640 Whats then what is the MCB Rating?

    plz revert to me as early as possible.

    i am waiting for your early kind response.

    Thanking you sir,

    Thanks and regards,
    Mr.Vemula Balaiah

    • Dear mr vemula balaiah
      how to set MCB rating…
      rating for MCB depend on AMPERE
      so,first u must confirm the FULL LOAD AMPERE for your load then u can set the MCB rating.. or u can use this formula for get your ampere I = P / ( V x 1.732 )
      second,u must know what type for your MCB depend on your application..i already expalin detail about type off MCB
      please refer my post…
      thanks for your question…
      sorry for the late reply :D

      • Dear Sir…
        This is Razi Haider.
        I was solved this question but no given me answer correctly. U r Given Formula, I=P/(IxVx1.732). But I Dont Be Understand How will be Solve this Question from this Formula. Plz u do Solve This question and Revert to My Email: Id. Plz Sent me 1 Example.
        Thanking You

        clarify the below ratings. Data is : 3-Phase, 50HZ , 415V .
        1. If Load is 1000 Watts then what
        is the MCB Rating?
        2. if load is 1500 Watts then What
        is the MCB Rating?
        3. if Load is 2500 Watts then what
        is the MCB Rating? 4. if Load is 2000 Watts then what
        is the MCB Rating?
        5. If Load is 640 Whats then what
        is the MCB Rating? plz revert to me as early as
        possible. i am waiting for your early kind

        how to set MCB rating…
        rating for MCB depend on
        so,first u must confirm the
        FULL LOAD AMPERE for your load then u can set the MCB
        rating.. or u can use this
        formula for get your
        ampere I = P / ( I x V x
        1.732 )

        • Hi Razi,

          Sorry for the mistaken formula,Please refer formula P = I x V x 1.732.I solved 1 of your question below :-

          1. If Load is 1000 Watts then what is the MCB Rating?

          P = 1000 watt
          I = Ampere ( unknown)
          V = Voltage ( 415volt)
          1.732 = 3 phase (square root 3)

          P= I x V x 1.732

          I = P / (V x 1.732)
          I = 1000 / (415×1.732)
          I = 1000 / 718.78
          I = 1.4 ampere

          So you can used MCB with 2 @ 3 ampere

      • lemau…hi gm. if our load current is 58 amperes….. so generally we l go for 63 amperes mcb. so if some 60 amperes is flowing in thr circuit how the mcb will operate or trip…. bcoz its rating is 63amperes…. please clarify my doubt ….

    • Im using 6 drives with 0.5 hp,i want to know the MCB rating for each VFD

    • jeevan yadav says:


  2. Sir,
    We are using capictors for power factor improvement.Please suggest us the
    Rating of MCB for 15Kvar,20Kvar capacitors,.also give us the methode for the calculation of MCB Rating..

    • dear mahesh kulkarni..for the simple calculation for mcb sizing,please refer the FULL LOAD AMPERE for your capacitor,and multiply with 0.25 for safety factor.u can get the rated ampere for your mcb. Example : the full load ampere is 25 ampere,multiply with 0.25 and you get 31.25 ampere…so from the mcb ampere rated from manufacture,you can use mcb 32 ampere rated current…refer to my post about mcb sizing

  3. kunhimon m says:

    dear sir,

    A machine 380v,60hz,its full load amps is 244.
    which amps mcb we can use this circuit.pls reply as soon as possible to my mail id.

    • Dear kunhimon,maximum MCB rated ampere only until 125 ampere.So..from your data,the full load machine is 244 cannot use the MCB…
      I suggest you to use the MCCB ( molded case circuit breaker ) because it have bigger ampere range,from 15 amp until 2000 amp.
      For this machine i suggest you to use 300 ampere of MCCB.

  4. BHARGAV says:


  5. mahesh joshi says:

    sir i need connection diagram of star delta stator motor

  6. mahesh joshi says:

    timer connection of star delta startor

  7. mahesh joshi says:

    sir i don’t know how to connect timer in star delta stator

  8. up to how much kw motor can we use DOL starter?

  9. Vineet says:

    I have two (2) generators of 5.6KW,112v D.C/ 8KVA 220 A.C generator and One (1) generator of 7.5KVA for a complex which has total equipment load of 30 Amps. Can you please suggest where all should I put MCB and of what all rating.

    The set up is like
    8KVA GenNo 1 + 8KVA Gen No 2—- CHANGEOVER 1


    MAIN and Output of Changeover 2 goes to Changeover 3.
    and output of changeover 3 goes to Distribution box. and from distribution box the output goes to various rooms and equipment rooms. can you please suggest broadly where all MCB should be put and of what rating

  10. murugesan says:

    Dear sir
    i want “forward revers” star delta starter connection diagram if use with four contactor only this starter useing from up& down lifting machine

  11. Prodip says:

    witch side is load side, mean where i connect load at top side or bottom side.

  12. deepak says:

    dear sir

    If dc current flowing in 6 amp.AC mcb how much take current to trip 6 mcb

  13. Dear sir
    3 pole MCB 63 A can it trip if it is connected reverse( supply side to Load & vice versa)in case of fault at load?

  14. Rupesh says:

    dear sir,

    can we use AC MCB for DC . please reply

    • AC MCB is not suitable for DC power supply.Actually We can found Breaker for DC.Please refer to your local supplier.
      DC breaker brand : Fuji Electric,Schneider,Allen Bradley,ABB ,ETC

  15. bablesh says:

    I want to fix MCB for a residence (which has 9 tubelights ,4 fans , a frdge , a TV , a geyser and a PC and a few plug points ) and want to know the following :-

    1.What current rating MCB shouls i use ?

    2.Should this be one phase or three phase?

    3.should mcb be 1/2/3/4 pole ?

    4.The residence area has periodic voltage fluctuations , will mcb be able to give protection for the same too?

    Thanks and Regards,

  16. HIMADRI PAUL says:

    I want to know mcb’s load capacity by amp.

  17. sir, i have connected 4 pole 30ma rccb in my portable distribution board (load side) and 4 pole 100 ma rccb at incomer of my pdb. if any fault occurs in the load side both of the rccbs are getting trip. i need to trip 30 ma only. pleas e do the need ful to me.

    • Connect your power supply L1,L2,L3 & N from INCOMING of 4pole 100mA RCCB to your 4 pole 30mA RCCB.
      So if any leakage,only 4 pole 30mA RCCB will trip.

  18. I want to know waht will happen if we connected a 4 pole mcb in this sequence N L1 L2 L3

  19. In my lighting distribution board the neutal terminal block was getting melted every time due to unbalancing current. what should i have to do to avoid this?
    Shall i increase the current rating of the terminal block?

    • Hi,
      Please check your :-
      1) Terminal block current rating.make sure it bigger than your Full load ampere
      2) No connection loose
      3) Please ensure your load is balance between phase to phase and phase to neutral.

  20. Dear Sir..
    This is Razi Haider. And My question is:
    Why we used value of squar root 3 which is 1.732. in this formula Formula, I=P/(Vx1:732). What is this ?and why we take of this value.
    Plz Revert to Me. As Soon As Posible. Plz.
    Thanking You.
    Razi Haider.

    • Square root 3 used when your power supply are 3 phase..

      WHY used square root 3 in calculation –> this is the answer,please read discussion on this forum :

      FROM JON :

      Step 1 is to remember that voltage is _always_ measured between two points. So when you say that a particular circuit is 277/480V wye, you provide the following information: Measured phase to phase the voltage is 480V. Measured phase to neutral the voltage is 277V. The source is wye connected, eg. a transformer bank with 277V secondaries.

      Step 2 remember that for AC circuits the voltage is a constantly changing value, and when we give a single number, it is a form of average voltage, called the RMS voltage. The RMS voltage of an AC waveform corresponds to the DC voltage which would deliver the same power to a resistive load. If you take AC at 480V RMS and apply it to a 480 ohm resistor, the average power delivered to that resistor would be 480W. If you take DC at 480V and apply it to a 480 ohm resistor, the power delivered to that resistor would be 480W.

      So now we have the two important facts: voltage is measured between two points, and an AC voltage is an average over time.

      The ‘voltage is measured between two points’ concept can be expanded with the idea of a ‘zero reference’. If you pick one point on a system and _arbitrarily_ call it ‘0V’, then you can reference every other point on that system to that zero reference. In electrical systems, it is most common to pick ‘earth’ as the zero reference. Once you have a zero reference, you can speak of any other single point as having a voltage. In reality we are still measuring voltage between two points, but one of those points is assumed, so we don’t have to mention it.

      Now for something nifty: the _instantaneous_ voltages between points must always add up. If I measure 5V (DC) from A to B, and 6V (DC) from B to C then I will get 11V from A to B. If I measure 5V from X to Y and 6V from X to Z, then I will measure 1V from Y to Z. Finally , if I measure 5V from A to B, I will measure -5V from B to A. But this is all DC, or instantaneous voltage.

      To see how AC voltages add up, we have to remember that the AC voltage is a form of average, and we have to look at the instantaneous values, get an instantaneous sum, and take the average of that.

      So now consider phase A in our 277/480V wye system. We can plot the voltage relative to our earth reference as a function of time, and get a graph, ideally a nice sine curve. At time zero the voltage will be zero. At 1/240 of a second, the voltage will be +392V (277V * 1.414, the square root of 2). Then at 2/240 second the voltage will again be zero. At 3/240 of a second the voltage will be -392V, and at 4/240 second (1/60 second) the voltage will again be zero. This cycle will repeat.

      Now imagine another point in the system, also connected to phase A. If we measure the voltage at this point, we will get the exact same curve. Finally, try to measure the voltage between these two locations. If you look at each instant in time, the voltage _difference_ will be zero. The average of zero is still zero. Net result is that if you measure the voltage between two points, both phase A, you will get zero volts, as expected.

      Now I’ll set up a nifty little tool; a special transformer that lets me produce output of arbitrary phase angle. The output is still 277V RMS relative to our zero reference, but I can shift its phase relative to phase A. Lets call the output of this transformer ‘phase W’. I adjust phase W so that it is very slightly delayed from phase A, and graph the two phases.

      At time zero, phase A is zero, but phase W is negative, say by a couple of volts. Then at time 1/240 phase A is at its positive peak, but phase W is slightly below. A teensy bit later, phase A is on its way down, and phase W hits its peak. And so on through the cycle. Both phases have the same amplitude, but hit their peaks at different times. There is usually a slightly different voltage between the two, but since sometimes A is more positive and sometimes W is more positive, the two graphs _must_ cross.

      Make a graph of the difference between A and W, and you will find out that it is a sine wave with low amplitude. In fact, it is a law of mathematics that the sum or difference of two sine waves of the same frequency (but possibly different amplitudes or phases) is another sine wave of the same frequency, again with different frequency or phase.

      Now, imagine that I start delaying phase W more and more. As I do so, it gets more and more different from phase A. The sine wave that is the difference between phase A and phase W gets larger and larger. Eventually this resultant sine wave is actually _greater_ than phase A. This difference sine wave reaches its peak when phase W is exactly 1/2 cycle from phase A, meaning that the +peak of A corresponds to the -peak of W. At this point, the amplitude of the _difference_ between the two phases is twice the amplitude of each phase alone. This is to say that if phase A is 277V relative to neutral, and phase W is 277V relative to neutral, and 180degrees out of phase with phase A, then the voltage A to W is 534V.

      The last bit is to ask ‘how do I calculate the voltage difference between two phases with some other phase angle?’ Clearly this is some function of the amplitude of the phases, and also a function of the phase angle between them. Simplify the question by stating that both phase A and phase W have the same phase to neutral RMS voltage V. Call the phase angle between these two phases T. The voltage between the phases is then given by V * 2 * sin(T/2). So for the 180 degree phase difference we get V * 2 * sin(180/2) = V *2

      And for a 120 degree phase difference we get V * 2 * sin(120/2). Quick high school trig question: what is sin(60) { or sin(pi/3) if you are into radians). Taaa-daaa.

      You can use this to figure voltages when you get strange phase angles, say in a 6 phase system for a rectifier, where you have two transformer secondaries wound to give different phase angles on the output. If you know the voltage and the phase angle, then you can calculate the voltage difference between any of the two phases.

  21. Dear Sir
    This is Razi Haider.
    Sir.. u said 25 ampere MCB not available in market but ABB Company is Provided 25 ampere MCB It is Right.

  22. kabeer says:

    Dear Sir
    i have one transformer of 5000kva we need MCB for this ?

  23. dasharath reddy says:


    Please can any one of u clarify my doubts:

    Suppose I have 10A MCB(Miniature circuit breaker) which used for lighting purpose in residences, if i have each bulb of 36 watts, what is the maximum lights and upto what load (in watts) can be connected to 10A MCB, for 1- phase supply???

    • First,you need to know FLA for 1 unit bulb 36 watt- use clamp meter.
      Example FLA for 1 unit bulb 36watt = 1.5 ampere,so you divide with 10 ampere

      10 ampere /1.5 ampere = 6.66 unit
      So you can install 6 unit bulb max for 10 ampere breaker.

  24. bhavin says:

    Can i connected 45 amp load with 63 amp rated MCB? I already connected 60 amp load with this MCB

  25. Arshad saeedi says:

    Dear sir,
    This is a arshad saeedi
    P/z solve this questions.
    1 kv= ? Volts
    1 kv= ? watts
    Plz give answer as soon as possible
    Regard &thanks
    Arshad saeedi

  26. Sir, Please clarify if MCB is bidirectional in nature? whether it can trip if overcurrent flows in either direction?

  27. please help me I want to know the device I can use to check the mcb even if the labelling of the mcb h
    as been removed. ….because I hav a problem with customers who remove the mcb sizes so I want to know device which I can use that can tell me the size of the mcb in terms of readings and sinewaves.

  28. rukmender.kotam says:

    Dear sir,
    pls tell me how much capacity 2 pole mcb we can use in home for main incoming power supply with 2 wire.(phase and neutral).4 fans,2 tv,6 lights,1freeze,1cooler,1heater,1exhaust fan.

    • Hi,
      You can calculate for total ampere of electrical equipment that you used and estimate suitable size for your breaker. Standard practice for home application for main breaker is 40 A or 62 A

  29. Subbaiah says:

    How much amps phase to phase and phase to neutral can we take from a 315 amps change over switch?

  30. I want to know per phase ampere capacity of each phase of Say 24A TPN mcb. Is it 8a per phase or in between 8a to 24a per phase? Also what maximum current each phasd can take Say in 25a TPN MCB.

  31. mahendra tajne says:

    Sir can i loop 3 phase 6 amp mcb into 12 amp rated current for single phase induction motor
    Looping is in parallel or in series

  32. mahendra tajne says:

    MCB is 6 amp 3 phase then what is the per phase current

  33. mahendra tajne says:

    If the motor rating is 7 amp single phase can i use 3 phase mcb for it

  34. bharath says:

    How to choose RCCB rating for a connected MCB’s. For example if there are 4 no.s of MCB 16A for single phase AC load. what will be the RCCB rating.

    • Kalidass says:

      If there is an over current in the circuit means RCCB can Trip? if not means why we need to consider the current rating for the RCCB?

  35. simone zimmer says:

    Interesting post ! I am thankful for the insight . Does anyone know if my business might get access to a sample CA FTB 3805V document to work with ?


  1. […] circuit.For Low voltage circuit breaker,i already explain on my last post.Please read my post about miniature circuit breaker (MCB),molded case circuit breaker (MCCB) and residual current circuit breaker (RCCB) for more […]

  2. […] kali ini akan dijelaskan cara sederhana bagaimana menentukan kapasitas MCB (Miniature Circuit Breaker) untuk aplikasi […]

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