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? Ok..it 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.

**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.

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.

why multiply with 0.25 every time with the full load current of the component?

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

TYPESof 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 :DFor 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.

stl

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 Ampere..it ok if ampere size is bigger.It spare for future additional of mcb

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.

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

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.

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??

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.

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

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

Those fumula are very useful for electrician thanks

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

hello,

why we the 0.25 factor..

and i am using the fan motor rating is 3.8 amps can u tell me appropriate MCB

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…..

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You have done a wonderful job!

your above calculation of the mcb sizing is wrong

how do u arrive at 25@32ampere,the working is confusing.

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.