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kailasamvv
Associate

Join Date: Aug 2008
Posts: 39

Cable Selection for 350KVAR Capacitor

12/03/2011 9:27 AM

Dear Engineers,

I want to select & lay a cable for 350 KVAR capacitor bank for a 900KW motor load. I considered the reactive power cos(phi) = 0.8. The load current is 583A.

I = 350000/(1.732 x 433 x 0.8) = 583.36 A.

The selection of cable for the capacitor bank is 4Cx300Sqmm (1 run)

And the selection of Circuit Breaker is 583 x 1.5 times = 875A => 800A breaker (approx)

I request you kindly clarify whether the above method is correct or wrong? Instead of 800A breaker, if I use 630A breaker, is it works without any problem?

Kindly clarify or advice me the correct method.

Thankyou.
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wareagle
Guru
United States - Member - New Member

Join Date: Jun 2008
Location: Alabama
Posts: 1604
Good Answers: 63
#1

Re: Cable Selection for 350KVAR Capacitor

12/03/2011 5:20 PM

I = 350000/(1.73 x 433) = 467 amps. No PF included in this calculation.

According to this http://www.elsewedyelectric.com/Catalogs/Doha%20Cables.pdf

300 CU M2 = 440 amps in free air 70C. So what should the ampacity be?

Breaker 1.5 x 467 = 700 amps.
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SolarEagle
Guru

Join Date: Mar 2007
Location: by the beach in Florida
Posts: 33392
Good Answers: 1817
#2

Re: Cable Selection for 350KVAR Capacitor

12/03/2011 5:25 PM

This might help...

http://www.engineeringtoolbox.com/electrical-systems-t_33.html

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pnaban
Guru

Join Date: Jun 2010
Location: srilanka
Posts: 2725
Good Answers: 5
#3

Re: Cable Selection for 350KVAR Capacitor

12/03/2011 11:14 PM

The manufacturer of capacitor can guide you in selecting cable as well as CB for their product.

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kvsridhar
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India - Member - New Member

Join Date: Sep 2009
Location: Temporarily at Ashburn, VA
Posts: 2744
Good Answers: 164
#4

Re: Cable Selection for 350KVAR Capacitor

12/04/2011 12:30 AM

Switching on a capacitor is a severe duty. Remember that an uncharged capacitor acts as a short-circuit when switched ON. There are special contactors to take care of the high frequency inrush current which can cause welding of contacts, nuisance tripping of protective devices, etc. You may get some idea if you study this document 1sbc101140c0202.pdf

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PWSlack
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 32175
Good Answers: 839
#5

Re: Cable Selection for 350KVAR Capacitor

12/05/2011 3:35 AM

Doesn't anyone read British Standard 7671 any more? Such a shame....

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