Calculating cable tray weights and support requirements

[mull982]

I recently came across a situation where there were several large cables (42 500MCM cables) being run in a single cable tray. Just prior to installation there became a concern over the amount of cable weight in the tray (aprox 78lbs/ft) and how to best support the cable tray for the total amount of weight.

The design for this project specified the size and quantify of the feeders to be routed overhead and left the actual routing of the feeders up to the contractor.

In a situation such as this who's responsibility would it be for determining how to properly support the cable stray for the given weight? Would it be the responsibility of the contractor to either provide adequate support or break it up into multiple cable trays, or is the engineer responsible for providing a support detail that will adequately support the cable weight?

I would think that this would fall under the responsibility of the contractor, if the engineer only specified cable size/quantity and left the routing up to the contractor? Perhaps maybe if the engineer specified the number of trays and the exact routing of those trays then he would be responsible for a details but otherwise it would seem to fall on the contractor?

There was a concern with hanging tray from the ceiling (roof joists) so it will likely be supported along building columns.

 

[Smart $]

With 42 500kcmil conductors involved, it sounds like a civil engineer should have already been involved.

 

[mull982]

With 42 500kcmil conductors involved, it sounds like a civil engineer should have already been involved.

At this point a civil engineer is now involved.

However it was not specified anywhere that all of the cables should be run in a single tray. The design drawings simply showed the feeder sizes and said they should be field routed.

The contractor could have broken these cables up into multiple cable trays to lessen the weight however he chose to run them all in one, perhaps to save on cable tray cost. If that's the case shouldn't the responsibility fall on the contractor for having and adequate support designed?

Or should the engineering documents specify how many trays, etc....?

 

[dkidd]

Have there been any calculations submitted on cable tray fill, and conductor ampacity, stating the configuration and spacing?

 

[Smart $]

Hmmm... this is bordering on legal advice. IMO the entity responsible is the designer.

 

[don_resqcapt19]

Running the cables in multiple trays does not really change anything as far as the supports from the structure to the tray. The structure still has to support the total weight which even goes up a bit with multiple trays as opposed to a single tray. You also have to look a the design loads for the tray itself. There is plenty of tray available that is designed to support in excess of 100 pounds per foot.

For that type of load on the building structure, I would want to see the supports specified by a structural engineer.

 

[steve66]

I don't see this as any different than any other run. The contractor is normally responsible for supporting all the feeder runs on a project.

If the weight is enough to bring the building structure into question, then I would think an RFI would be appropriate.

I'm assuming no seismic bracing is required.

 

[Tony S]

A cautionary story

https://www.youtube.com/watch?v=h7Ab70eYMT8

 

[mull982]

Running the cables in multiple trays does not really change anything as far as the supports from the structure to the tray. The structure still has to support the total weight which even goes up a bit with multiple trays as opposed to a single tray. You also have to look a the design loads for the tray itself. There is plenty of tray available that is designed to support in excess of 100 pounds per foot.

For that type of load on the building structure, I would want to see the supports specified by a structural engineer.

This is a good point in that even with multiple trays the load on a building or structure to support these trays does not change. I agree 100% that the application should be reviewed/specified by a structural engineer but I guess I'm trying to figure out who's responsibility it would typically be to solicit the review/support of a structural engineer. Is this something that the engineer should do as part of the electrical design or would this responsibility fall on the contractor upon going to install the tray and realizing the heavy cable weight?

I've seen a lot of designs that simply give a typical "hanger detail" or "support detail" for cable tray. At what point would there be a need to future investigate support requirements above and beyond a typical detail (I.E. have it reviewed by structural engineer) Is there a certain lb/ft limit where anything above that value should be further reviewed?

 

[gadfly56]

Hmmm... this is bordering on legal advice. IMO the entity responsible is the designer.

I would have said "Send an RFI", but it looks like the contractor is a day late and a dollar short on that request. I suspect the original design firm was trying to save on engineering time and preserve their fee and let it slide, even though anyone "skilled in the art" should have realized these conductors are going to be quite a load somewhere, especially if they are required to be overhead.

 

[Smart $]

I would have said "Send an RFI", but it looks like the contractor is a day late and a dollar short on that request. I suspect the original design firm was trying to save on engineering time and preserve their fee and let it slide, even though anyone "skilled in the art" should have realized these conductors are going to be quite a load somewhere, especially if they are required to be overhead.

Yeah, I thought RFI by contractor also if it was at the pre-buildout stage... but it's not.

 

[Julius Right]

See[ for instance]:
B-Line Cable Tray Systems[NEMA]:
http://www.cooperindustries.com/con...e_tray_systems/all_products/Series1-Steel.pdf
http://www.cooperindustries.com/con...tray_systems/all_products/Manual-Appendix.pdf
or:
Support Load Calculation Per IEC 61537[or BS EN 61537]
http://www.sourceiec.com/Catalogs/Chapter 14 Cable Support Systems.pdf
https://www.legrand.co.uk/media/4377/beama_tray_and_ladder_best_practice_guide.pdf