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?

mcb current capacity is 125% of rated capacity. that why we r adding 25% of rated

Please advice me the book reference for the study of cable sizing calculation

Hi,

Please read my last post about cable sizing –> http://www.electricneutron.com/wiring-techniques/electrical-cablewire-sizing/

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.

sir plz tel me

63 amprs load

i have 4.a/c 1`5 ten

1000wts light

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??

Your calculation 100% right.

Pls ….mcb class selection

Ex= class B for Light

Class C for soficticated

Class L for ( B+C)

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

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.

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

15 amp motor

In=Iz+(0.25×Iz)

Amp rating =15 +(0.25×15)

=15+3.75

=18.75 amp.

Because…market avaleble MCB is 20amp

=20 amp, c class.

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

Your motor watt is =1000

Because mcb rating is =4.75amp

Your appropriate MCB is 6amp C class

Great…nice sharing

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.

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.

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?

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.

how many sizes start mcb and and last size

Hi,You can refer Manufacturing Catalong

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

350 Hp =350×746

=261100 watt

Amp =593.40

Rating =741.75 amp

Appropriate MCCB is= 800 amp,4 pole ISLCX0028 .

(standardelectrical.net/pdf/mccb.pdf)

Dear Sir

what is . Selection method for MCCB,

How much rating of circuit breaker for 1500kva?plz answer with formula.

How can i calculate PFI rating?plz answer with formula and example.

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.

Lights 100w fan 50 w + one 5a socket

What is the range of mcb

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

How to size MCB for DC load

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

excuse me sir…

Why you chooses resistance cable with 0.276 it depend on table or calculate?

can you saw me plz

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!!

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?

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

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

Hi,

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.

How to test mcb for its tripping without much resource

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

Dear sir,

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

I want a 3 phase mcb which can trip whenever any phase is absent…

Please tell me which mcb is suitable….

Must use phase failure device

Please use Phase protection relay

A 6A type B mcb is used as a combined overload,short-circuit and earth fault protective device in circuit.

(a)what is the maximum operating current as,

(1)An overload protection device?

(2)a short-circuit protection device?

(b)what is the maximum allowable earth fault loop impedance of the circuit?

Dear Sir, Im pleased u that… where did you get this formula In=Iz+(0.25*Iz)

Sir what is the formula for mcb for om service entrance