Cables in lieu of Bus

[Turk1957]

Adding a vertical secondary section to an existing switchgear lineup... Does it violate ANSI, IEEE or NEC standards and codes to connect the sections of bus with cables in lieu of fabricating a hard bus connection? Based on de-rating for ambient temperatures within the enclosure, the quantity of cables becomes significant (13 single conductor 500 KCMIL per phase for a 3200A. bus) but I would really like to know if this violates the ANSI or IEEE standards?

 

[petersonra]

Adding a vertical secondary section to an existing switchgear lineup... Does it violate ANSI, IEEE or NEC standards and codes to connect the sections of bus with cables in lieu of fabricating a hard bus connection? Based on de-rating for ambient temperatures within the enclosure, the quantity of cables becomes significant (13 single conductor 500 KCMIL per phase for a 3200A. bus) but I would really like to know if this violates the ANSI or IEEE standards?

It would seem simpler to just put a main in the new section and tap off the bus with much smaller conductors.

I don't think it violates any codes to do what you are asking.

There may be equipment standards that it violates. but equipment standards are not enforceable in general by any outside agency.

I am pretty sure that you are not listed by UL to fabricate the bus, but I think you can get terminals from the manufacturer to attach to the bus.

 

[Julius Right]

If I well understood you intend to connect [directly] from the busbar a feeder of a set of cables.
NEC 368.17 Overcurrent Protection. (B) Reduction in Ampacity Size of Busway.
"Overcurrent protection shall be required where busways are reduced in ampacity.
Exception: For industrial establishments only, omission of overcurrent protection shall be permitted at points where busways are reduced in ampacity, provided that the length of the busway having the smaller ampacity does not exceed 15 m (50 ft) and has an ampacity at least equal to one-third the rating or setting of the overcurrent device next back on the line, and provided that such busway is free from contact with combustible material."
There is here an idea to use a cable- bus:
http://www.mphusky.com/cable-bus/overview

 

[Jraef]

Looks to me as though you are not trying to reduce the ampacity of the bus, but that then begs the question of why? You most certainly are not adding 3200A of load to that existing bus, so the suggestion from petersonra makes much more sense.

If you try to do this with cables, you must include the required wire bending space and room for all of those lugs, on both sides, then you must also provide for adequate means of bracing those cables so as to not pull out or whip around during a fault. That extra real estate will not come cheap. If extending the full capacity of the bus is necessary for some reason, I would consider using "Flexi-bus", which is a form of insulated flexible bus bar instead of cables. It can follow bus bar rules instead of cable rules, so the wire bending space issue goes away. It is far more expensive than building cable but probably worth it for this if you must do it in the field and are pressed for time.

Preferrably though, if time allows, I would call up a custom switchgear shop and have them make a transition section into whatever newer version of gear you intend on using. That would be far cleaner.

 

[Fulthrotl]

Adding a vertical secondary section to an existing switchgear lineup... Does it violate ANSI, IEEE or NEC standards and codes to connect the sections of bus with cables in lieu of fabricating a hard bus connection? Based on de-rating for ambient temperatures within the enclosure, the quantity of cables becomes significant (13 single conductor 500 KCMIL per phase for a 3200A. bus) but I would really like to know if this violates the ANSI or IEEE standards?

i've done cablebus installations using fine stranded locomotive cable. it's a listed system,
with tray, spacers, etc. but god, is it expensive. only place i've ever seen it used is LADWP.

http://www.mphusky.com/images/brochures/cablebusbrochure2010.pdf

load up your money gun... this isn't gonna be cheap. but it's listed stuff.


duh. missed the response already referring to this product... so just give it a +1...

 

[petersonra]

I would consider using "Flexi-bus", which is a form of insulated flexible bus bar instead of cables. It can follow bus bar rules instead of cable rules, so the wire bending space issue goes away. It is far more expensive than building cable but probably worth it for this if you must do it in the field and are pressed for time.

having been down the road of flexible bus bar more than once, I would ask these questions.

1. what provision in the NEC allows you to do this? please think carefully before answering. this is not a wire type conductor nor is it a bus bar. where does it fit into the NEC?

2. what ampacity will you use? the ampacity claimed by the manufacturer is far in excess of anything the code would allow for a wire conductor with the same amount of copper.

3. The OP mentioned derating. how would one go about derating such a product given there are no rules in the NEC that cover it.

Preferrably though, if time allows, I would call up a custom switchgear shop and have them make a transition section into whatever newer version of gear you intend on using. That would be far cleaner.

likely your best answer.

 

[Jraef]

having been down the road of flexible bus bar more than once, I would ask these questions.

1. what provision in the NEC allows you to do this? please think carefully before answering. this is not a wire type conductor nor is it a bus bar. where does it fit into the NEC?

It is busbar. Article 408 discusses busbar and switchboards, but I think you are correct anyway in that 408.18 requires that insulated conductors be listed, and Flexi-bus is Recognized, not listed. So it could only be used by a listed assembly shop that was going to add it to their procedure, but not by a contractor in the field. My bad.

2. what ampacity will you use? the ampacity claimed by the manufacturer is far in excess of anything the code would allow for a wire conductor with the same amount of copper.

The ampacity is what it was tested for in the UL recognition process and yes, it is different than an equal cross sectional area of round conductors because of the difference in skin effect. In a round conductor, the current travels primarily on the skin (surface) of the conductor.

3. The OP mentioned derating. how would one go about derating such a product given there are no rules in the NEC that cover it.

You would not derate it, you use it at the rated ampacity.

Still a moot point though for the field guys.

 

[jim dungar]

The ampacity is what it was tested for in the UL recognition process and yes, it is different than an equal cross sectional area of round conductors because of the difference in skin effect. In a round conductor, the current travels primarily on the skin (surface) of the conductor.

Skin effect is negligble at 60hz. Even for round conductors the size of 500kcmil, the factor is less than 20%.

For busbars the ampacity difference has to due with the surface area for heat dissipation in what is basically free air.
Because this product is UL Recognized, its manufacturer's published ratings do not mean much. What is important is the final assembly and the applicable UL Standard.

 

[petersonra]

I would point out that 408.51 requires that busbars be "rigidly" mounted. I don't see how a flexible busbar meets that requirement, short of getting it out from under the requirements of the NEC and into a listed piece of gear.

 

[petersonra]

just to cause more trouble.

the NEC specifically allows one to use busbar, but there is a curious lack of requirements surrounding its use.

there are no requirements for ampacity, temperature limits, derating for more than 3 CCC in a raceway, etc. there is some recognition that it might be insulated or bare, but there don't seem to be any requirements found in the NEC for the insulation.

I don't recall any place where it would prohibit installing insulated or bare bus bar in a conduit either, as odd as that may seem.

there is no place that requires it be listed either.

I don't recall where it is found but someplace there is a reference to ampacity in a specific situation.

I would ask this. Does the lack of specificity in the NEC with respect to busbars mean you can use any copper bar as a busbar? or does the manufacturer have to say it is suitable for that purpose?

UL508a spells bus bar as two words instead of one as the NEC does, but does seem to require some kind of UL approval and uses the commonly seen 1000 A/in^2 rule of thumb for ampacity.

Incidentally the Erico flexibar is recognized as AWM so it can be used in a UL508a panel, although you can't use them with anywhere near the ampacity the manufacturer attributes to them.