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Question
how to calculate cable size, olr rating, contactor rating 
and mccb rating,to below given details three phase motor
75 kw,415 v,.85 pf,50 hz?
 Question Submitted By :: Electrical-Engineering
I also faced this Question!!     Answer Posted By  
 
Answer
# 1
motor rating 75kw,so it taken full load p=1.732*v*I*cosfy
i= p/1.732*v*cosfy
=123 amp. so we requared a cable to carry 123 amp
contonious rating. so require to run motor in star delta
starter . in star delta connection load current is 58% of
line current. we require 58% of 123 amp carring capasity
cable.it is around 71 amp.generly copper cable taking 4
times current of cable rating and alluminium cable taken
1.5 times of cable ratting .so for 71 amp carring alu.
cable 50mm2 suffisent but due to high starting curent its
take 70 mm2 cable ib best.
its overload relay ratting is in range to 71 amp. it to be
60-80 amp,and mccb rating is 2.5 times of full load =200
amp
is best.
 
Is This Answer Correct ?    33 Yes 19 No
Brakadeesh
 
Answer
# 2
75 Kw motor takes 134 amps full load current. But for
selecting its Cable, Contactor, OLR and MCCB, we have to
consider many other factors such as need of its operation.
Place of the Motor Control Centre and possibility of laying
the cable Wheter inside the duct, underground or in the
air. For selection of the Contactor enclosure of the
panel, its environments etc are to be taken into
consideration. So giving answer at a stretch basing on
some thumb rule is not technically qualified.

Other particulars are required.
 
Is This Answer Correct ?    6 Yes 0 No
Pvr.parthasarathi
 
 
 
Answer
# 3
motor rating 75kw,so it taken full load p=1.732*v*I*cosfy
i= p/1.732*v*cosfy
=123 amp. so we requared a cable to carry 123 amp
contonious rating. so require to run motor in star delta
starter . in star delta connection load current is 58% of
line current. we require 58% of 123 amp carring capasity
cable.it is around 71 amp.generly copper cable taking 4
times current of cable rating and alluminium cable taken
1.5 times of cable ratting .so for 71 amp carring alu.
cable 50mm2 suffisent but due to high starting curent its
take 70 mm2 cable ib best.
its overload relay ratting is in range to 71 amp. it to be
60-80 amp,and mccb rating is 2.5 times of full load =300amp
is best.
 
Is This Answer Correct ?    43 Yes 39 No
Rsbangra
 

 
 
 
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