Answer / pragnesh

IF we take an Example,

5.5kw Load,

I= Doble than Load = 11A,

MCB Must has More than 11A,

& Fuse Must than 1.5 Tmes more than MCB.

Answer / viraj dua

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.

Answer / abdulsubur

IT CAN BE CALCULATED BY THIS FORMULA ALSO P=V I COS PHI

I=P\VCOSPHI FOR SINGLE PAHSE
I=P\1.732*VCOSPHI

Answer / daher

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

Answer / anil

there is different calculation for three phase and single
phase.
first you maintioned that it is either sigle or three

Answer / vignesh r

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).

Answer / srihari

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)

Answer / balaji venkateswaran.v

Sorry my answer is favour to you bu Your answer for that
question FUSE RATING is rejected by our college professor

Answer / daher

Use this formula for 3 phase: P=3*Vph*Iph*COSphi then
I= P/3*V*COSphi for example p=3000wat, Vph=220vac and
COSphi=0.9 so Iph= 3000/3*220*0.9= 5Amp now you shoose from
the manufacturers cataloque which circuit breakers is
nearest and suitable for your circuit for example 6Amps.

Answer / nitin kaushik

the rating of mcb is calculated by watts divided by
575...575 is the shortest calculation of root 3 cosQ.

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