Dear all there are two parameters for determining the
selection of MCB
1.Type of load-
Resistive(e.g Home circuits)-B curve MCB(operates @
3-5 times the rated current)
Inductive-Motors etc.-C curve MCB(operates @ 5-10
times the rated current)

Highly inductive load-Transformers etc.-D curve MCB
(operates @ 10-15 times the rated current)

2.current in the circuit.

Now you will find that curve along with rated current is
mentioned on the MCB e.g. C6(do not confuse rated current
with the breaking capacity which is the max. current CB can
witstand)
C6 means this MCB is usable for Inductive load where
Circuit can witstand upto 30 Amps and MCB should trip if
Current Reaches 30 amps(5times of 6Amps)-60 Amps(10 Times)

Rated current is mentioned as 'ln' in electrical literature.

Under no load condition it is 20% of rated current of the
device.And in full load condition it is 120% of rated
current.For eg., consider the rated current of a motor is
19ampere.then under no load it is 20% of 19 =3.8ampere
=5amp(approx).for full load it is 120%of 19 = 22.8ampere=20
ampere(approx).

the current rating of the fuse must be equal to the load
because the fuses melting point disconnect the circuit if
the current load becomes greater than the specific rating.
in high ruputer capacity fuse,the fuse able to carry1.2
over the rating.meaning 20% more.MCB rating calculation is
to know how mush the consuption load and according to that
cal.we decide the rate of MCB from the catalog rating of
CB, must be more than the load.DONT FORGET THE SHORT
CIRCUIT CALCULATION

first we have to calculate the total load according that we
can use the formula whether it is single phase I=p/V cos(i)
or 3phase I=(P*L.F)/(1.732*V*cos(i))
for example if we get the value 20Amps, so we should choose
near to that value of rating MCCB(32A)

The MCB rating shall be more than the fuse rating if it is
used for the service to switch off & on while the circuit
is in service.The fuse rating shall be 200% of the rated
curent normally as it is used for short circuit
protection.If the MCB is stay put type(No operation while
in service) it can be selected for rated current or 1.2
times of rated current value.

FUSE RATING CAN BE CALCULATED AS FOLLOWS:
SAY A MOTOR FULL LOAD CURRENT RATING IS 30A,TAKE FUSE OF
RATING 2.5TIMES OF FLC. SO FUSE RATING WILL BE 80A.IF IN
CASE THIS RATING FUSE IS NOT AVAILABLE, WE CAN GO FOR NEXT
HIGHER LEVEL. BUT IF NEXT HIGHER LEVEL FUSE IS SO HIGH THAT
IT CANT BE TAKE THEN GO FOR SOMEWHAT LOWER LEVEL. THIS IS
BASED ON MY EXPERIENCE IN THE INDUSTRY.

SECOND PAART IS FOR MCB:

MCB SELECTION SHOULD BE SOMEWHAT HIGHER THAN FUSE RATING.