We are installing conductors into a large switch gear unit at work. They are terminating into 2 of 600 amp switches and 1 of 800 amp switch. The wires are 500 kcmil 4 conductor cable parallel feeds( 28 conductors total. We are tie wrapping all 28 conductors together for approximately 1.5 meters together(all touching) in the switchgear along the side of the unit.
The debate we are having is: Is there a derating factor b/c of the bundling of the 28 conductors along 1.5 meters?
1) 12-108(2) manadates that the spacing and alignment of conductors in parallel are maintained in such a way as to reduce inductive reactance between conductors to ensure the proper division of currents between the cables. See Appendix B for suggested cable arrangements.
A byproduct of ensuring that alignment of conductors is maintained is that the effect of induced heating in the structural steel of the switchgear will be minimized. See 4-008.
2) The correction factors of Table 5C are mandated in this instance by rules 4-004 (1c) or (2c) and (10).
I would reject this install as you described it if it came my way. Having said that, check with the AHJ in your area for a way to do this job in a way that meets the code's requirements.
You may have to think about bringing the cables into the switchgear from different directions.
[This message has been edited by Rick Kelly (edited 06-14-2006).]
Of course they apply. Because a conductor is inside a switchgear enclosure or in a splitter does not mean it is safe from the effects of inductive reactance or mutual heating.
I would expect that all such installs would be planned in such a way as to ensure that proper cable spacing, as much as possible, is maintained, as mandated by 12-102(2).
We are only talking about two cable diameters between bundles of conductors, as in...
The diameter of 500 kcm is (as per Table 10) is 24mm, so 48mm or 2 inches between a bundle of phase conductors is what is required. Might look like this...
A A (B C) 2" (B C) | 2" A A (B C) 2" (B C) | 2" A A A (B C) 2" (B C) 2" (B C)
(The "A" conductors are supposed to be over the "BC" pairs but the site won't show them that way)
You might have to construct racks to support the cables.
Neutrals can be placed where convenient.
If you can not fit this into the existing switchgear enclosure, in this manner, then the switch gear is to small and someone needs to do some engineering on it.
The code does take into consideration the reality that sometimes cables will be bundled together over short disctances. Hence 4-004(10), where contact between multiple cables for less then 600mm is considered acceptable. Over 600mm you need to derate the ampacity of the cables as per Table 5C. So... your 500kcm cables get derated to 237 amps (395 * .60).
In the example given... 28 conductors bundled together for 1.5 meters without derating them... no way, at least IMHO no way.
The above mentioned conditions are part of the reality of conductors in parallel and bundled together that can not be avoided.
Don't want to do the above... bring out 1250 kcm for the 600 amp feeds and 2 parallel runs of 600kcm or bus duct for the 800 amp feed.
For what it is worth... I would recommend having an Electrical Engineer bless this setup then have your AHJ nod approval as well.
Why not run this idea by the your local AHJ and let us know what was said by them about this install. I am curious to see if they raise the same points I did.
These are not trivial issues. Please do not ignore them. Or... have your liability insurance up to date and current.
[This message has been edited by Rick Kelly (edited 06-15-2006).]
Re: Conductors inside switchgear#102585 06/15/0611:57 AM06/15/0611:57 AM
Join a forum and jump in I always say. The wireing to the switch board is governed by part 1 of the CEC. Cable spacing and derating are covered there. Inside the cabinet is part 2 of the CEC and the code is pretty loose here. Manufactured products are approved from Canadian Standards, where one exists, and the internal wiring of the feeder cables is prescribed by those standards. As an inspector I am aware of the mutual heating of 1 cable by another where spacing is not maintained and will require a contractor to maintain cable separations until the wires get into the cabinet. Now I don't have any rules to enforce except those supplied from the manufacturer or if I did field product evaluations I might have some authority or expertise to qualify how the internal wiring is done. The internal cable spacing is important but the CEC part 1 does not prescribe the internal wiring of panels. Don't believe that means I'll tolerate a mess. Hope that confuses the situation.
The common sense answer is try the best you can to maintain spacing but without air circulation there reaches a point where all the wires are at or near the same temperature.
Re: Conductors inside switchgear#102587 06/16/0609:12 AM06/16/0609:12 AM
Under our act and regulations, in my region of operation, we have jurisdiction on all installed equipment other than equipment owned by the supply authority and used in their function as a "Utility".
As to the Part 2 issue, OEM equipment from a manufacturer would have had the required internal supports needed to ensure that proper cable spacing was maintained. That the manufacturer observed proper cable support and spacing would be a requirement of them before they were granted certification on the equipment in the first place.
Given the info that Cinner offered, it appears that there are no internal supports for his outgoing conductors to the down stream equipment. As such, he needs to provide support for them in a manner that meets the engineering need. The fact that the required cable supports were missing does not allow him to ignore the need for them.
Your point about air temperature is valid, though switchgear manufacturers do take those issues into consideration when they design the "heat loading" of the structure.
I can pretty well guarantee, however, that none of the switchgear manufacturers would bundle 28 conductors together for 1.5 meters.
[This message has been edited by Rick Kelly (edited 06-16-2006).]
Re: Conductors inside switchgear#102588 06/16/0610:54 AM06/16/0610:54 AM
Rick I agree with all you points and we do on occasion reject approved equipment if it appears that there is a mis application or someone is using it inapropriately. Lets look at a simple 42 circuit house panel and how an electrician usually finishes the internal wiring. Separate all the bonds and route them along the outside of the can to the bonding terminals. Then the neutrals (grounded circuit conductors) are grouped and terminated last the hots to the o/c devices. all these wires typically get grouped together for neatness and visual appeal. Generally the only thing I like to prevent here is the use of ty raps to tightly bundle these groups of wires. So that is a typical panelboard with a lot of #14, and #12, We dont have any fault bracing nor I suspect need any but once we get to 500mcm we want spacing? and Bracing. OK the bracing I understand but I cannot remember seeing bracing installed in any panel for load circuits. I have seen a lot of feeders braced but never have I seen braces installed for load conductors. I have seen lots of load conductors ty wraped to frame members of the switchboards but never a bar or rack or frame specfically installed to attach the outgoing cables. So I am wondering what you have in mind for internal wire management of a disribution panel? Can you tell I am a S*** Disturber? Again I agree that all you say is logical and a good idea. I would ask an electricain to respect spacing or even demand it inside the switchboard but I really don't believe that the internal wiring of a panel is governed by part 1 of the CEC but part 2 which we don't have direct authority. Yes we always have authority to challenge or reject anything that our experience would judge a hazard or substandard. I am just offering that it might not actually be a violation to allow intimate contact of the conductors within the panel.
Rick I hope we didn't offend you with some comments that were made on the chat about northern lifestyles I thank you for pointing out that it wasn't polite to mention some folk tales about our northern Canadian friends.