Calculation for Miniature circuit breaker ( MCB ) sizingExample : 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

Fom 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 forgeneral 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

Determine current in amps. Divide the wattage by the voltage of

the circuit, in North America usually 120 or 240, ie: An electric

water heater 4500w / 240v = 18.75 amps. This is the approximate

amperage the device requires.

Derate the circuit. Multiply by 1.25, the National Electrical code

requires protection at %125, ie: 18.75 x 1.25 = 23.43 amp circuit.

continuously by multiplying the circuit breaker's rating by .8

to determine 80% of the rating (a 15 amp breaker can pass 15

x .8 = 12 amps and a 20 amp is 20 x .8 = 16 amps). Do not

connect loads that exceed this 80% calculated value.

Be sure the wiring you are attaching to this circuit breaker is

the right size to carry the amperage. Minimum 14 gauge for 15

amp circuits and minimum 12 gauge for 20 amp circuits. This is

important. The NEC (National Electrical Code) is published by the

NFPA (National Fire Prevention Association) and that's not a

coincidence!

Be sure you use a circuit breaker that is the same brand as

the panel box you are installing, otherwise no warranty.

NEWTECH MCB SELECTION CHART

S.no Appliances Capacity/Approx. Wattage at 240 V Single Phase

Current Rating of MCB (Amps.)

Type of MCB

1. Air i) 0.5Ton-775 W 5 C

Conditioners

ii) 0.75Ton-upto 1200 W 7.5 C

iii) 1 Ton-upto 1.5 kW 10 C

iv) 1.5 Ton-upto 2.5 kW 16 C

v) 2 Ton-split unit upto 3.5 kW 20 C

2. Refrigerator

i) 165 Ltrs. 1.5 C

ii) 285 Ltrs. 2 C

iii) 400 Ltrs. 3 C

iv) 581 Ltrs. 5 C

3. Washing Machine

i) 300 Watts 2 C

ii) 1300 Watts (with heater) 7.5 C

iii) 1800 Watts (with heater) 10 C

iv) 2200 Watts (with heater) 16 C

4. Domestic Pump Set

i) 0.5 H.P. 6 C

ii) 1 H.P. 10 C

ii) 2 H.P. 16 C

5.

Water Heater (Storage or Instaneous Geyser)

i) 1 KW 6 C

ii) 2 KW 10 B

iii) 3 KW 16 B

iii) 6 KW 32 B

6. Cooking Range 4500 Watts 25 B

7. Oven Cum Griller 1750 Watts 10 B

8. Oven Only 750 Watts 6 B

9. Microwave Oven 1000 Watts 6 B

10. Electric Kettle 1500 Watts 10 B

11. Room Heater

i) 1000 Watts 6 B

ii) 2000 Watts 10 B

12. Ironi) 750 Watts 5 B

ii)1250 Watts 7.5 B

APPLICATIONS:

B Type MCB

• For protection of Resistive loads such as bulbs, heaters etc

,generally suitable for domestic applications. They may also be

used in light commercial applications.

C type MCB

• For protection of Inductive loads such as motors, air conditioners

etc devices are the normal choice for commercial and industrial

applications.

MCB Selection: The first characteristic is the overload which is intended to prevent the accidental overloading of the

cable in a no fault situation. The speed of the MCB tripping will vary with the degree of the overload. This is

usually achieved by the use of a thermal device in the MCB.

The second characteristic is the magnetic fault protection, which is intended to operate when the fault

reaches a predetermined level and to trip the MCB within one tenth of a second. The level of this magnetic

trip gives the MCB its type characteristic as follows: – ·

Type               Tripping Current                                      Operating Time Type B            3 To 5 time full load current                    0.04 To 13 Sec

Type C             5 To 10 times full load current               0.04 To 5 Sec

Type D            10 To 20 times full load current              0.04 To 3 Sec

The third characteristic is the short circuit protection, which is intended to protect against heavy faults

maybe in thousands of amps caused by short circuit faults.

The capability of the MCB to operate under these conditions gives its short circuit rating in Kilo amps

(KA). In general for consumer units a 6KA fault level is adequate whereas for industrial boards 10KA fault

capabilities or above may be required.

D type MCB

• For protection of Cables and highly inductive loads which

have high starting current such as examples include large

battery charging systems, winding motors, transformers,

X-ray machines and some types of discharge lighting.