capacitor banks

[stew]

Have been asked to qoute on an installation of some fairly large cap banks. The total kvar is 400 and the consultant has recommended 2 ea 200 kvar units to accomplish this. The resultant current is 481 ampsx1.35 for a total of 650 amperes according to his numbers. He is asking for parallel 500 mcms for this and I think 400 mcm is sufficient. The other question is code related as it applies to the disconnecting means. Code state s that all the ungrounded conductors shall be simultaneously disconnected which stands to reason. This unit is to be fed from a Square D DS206E breaker in the main swithcgear(800A) set at thier reccomended long time pick up of 560 amps. Is it ok to have the discconect be the breaker.? It is located about 15 ft away on the same side of the gear as the cap bank location . Also does 560 amps sound correct??? The consultant is also specking rsc as the conduit. Is there any reason it needs to be rsc instead of emt? It also seems to me that if I bring the main conductors into a gutter or cabinet above the cap units that I could then slpice in that enclosure and feed each individual unit with a single 400 mcm conductor correct?

[ June 01, 2005, 12:43 AM: Message edited by: stew ]

 

[rcwilson]

Re: capacitor banks

I assume these are 480V capacitors. 400 KVA @ 480V = 481 A. x 135% = 650 A. Two/phase = 325A per conductor without any allowance for unequal sharing of current. I add 5% imbalance allowance for paralleled circuits (> 400A) based on many actual measurements on operating circuits. +5% = 341 amps per conductor.

341 amps requires 500 kcmil (75 C) assuming no ambient temperature correction. Without my margin for paralelling, 400 kcmil is OK. Maybe the engineer wants the margin provided by 500 kcmil incase there are some problems after teh caps are on line. (A regular occurance).

With the breaker set at 560 A, parallel 400?s are protected. But the setting might have to be increased in the future due to harmonics or other nuisance trips. 500 kcmil wire would allow the setting to be increased above 670 A if needed.

Yes, the switchgear breaker can be the disconnect, make sure it is well labeled.

My reading of the tap rules says running only one 400 to each cap group is not per code, unless you put a 350 A breaker on each capacitor. The single conductor could handle the load but it is not protected per Article 250.

 

[bob]

Re: capacitor banks

You have left several questions to be answered in your post.

The total kvar is 400 and the consultant has recommended 2 ea 200 kvar units to accomplish this.

I think this is a 3 phase installation. If so do you mean 3 200 kvar units?
Is the voltage 480 volts?
481 amps x 480 v x 1.73 = 400 kvar. Thats 133.3 kvar per phase. Which is it?

 

[stew]

Re: capacitor banks

yes this is 480 v 3 phase. They are recommending 2 ea 200 kvar @480 v 3 phase. Also I did not think this out too well either. the other postewr is correct I cannot use just single 400 mcm per phase to feed each unit as my conductor would then be overprotected by the main disc. duh.The cap banks are I think self contained 3 phase units. I am getting the cut sheets on them this week.

 

[stew]

Re: capacitor banks

re read the tap rule because if you use only a 10 ft tap and follow all the other rules it is perfectly fine to do. I am going to use parralel 2/0 cable for each phase to each 200 cap per the manufactrres directions. I will also run these thru a 400 a non fused disc. to each. the banks each have thier own fuses.You scared me off with your first post until i reread the rule.

[ June 08, 2005, 12:04 AM: Message edited by: stew ]

 

[rcwilson]

Re: capacitor banks

My interpretation of 240.21 (B)(1)(2) is that a capacitor is not a "switchboard,
panelboard, disconnecting means, or control devices" mentioned in parts (2) & (3).

Despite this interpretaion, IMO tapping works. The cap bank is probably in an enclosure that could be considered a switchboard-like assembly.

Good Luck.