Tray Cable - Part I
When the electrical industry thinks of tray cable they think industrial strength robust cable assembly designed to meet that tough rugged work environment. When I think of tray cable I see a simple nonmetallic-sheathed cable assembly that is constructed almost identically to service entrance cable with a much thicker nonmetallic jacket. I see inner electrical wires of the same type found in metal-clad cable, armored cable and the same as pulled in raceways on nearly every construction site in the country. So why is tray cable limited to specific uses in the National Electrical Code* if it's so similar to those other cable products. Well the answer is easier to find than you would ever have conceived.
Tray Cable is constructed under UL 1277. This UL-ANSI Standard is used by manufacturers to construct tray cable to specific cable construction specifications. The standard (UL 1277) also tells us the type of inner electrical wires that need to be used as well. Ironically those wires are exactly the same as the other cable assemblies, like Type MC, AC and SE Cables. So why limit tray cables use?.
The answer stems in the fact that tray cable itself directly competes with other products manufactured by companies who do not actually produce tray cable products.
We all know that service entrance cable is permitted to be installed in a wide variety of residential and commercial applications where deemed compliant with section 338.10 of the NEC*. The UL Standard for service entrance cable is UL 854. Ironically, the inner wires specified within service entrance cable are the same found in tray cable. However, with tray cable, the outer nonmetallic jacket is nearly double the thickness of any other nonmetallic cable jackets.
During the 2017 NEC Development Process the code making panel recognized that tray cable is just like any other nonmetallic cable assembly and should be treated as such. The code making panel expanded the use of tray cable in one and two family dwelling units for applications where the tray cable contains BOTH power and control conductors. While this author doesn't agree they went far enough in the expanded use of this product and some of the language was poorly written, like the term "both"which dramatically limits the expanded use which is likely the intended response. However, I am sure they might say the same thing about this article so i totally understand.
There are individuals who argue that tray cable is designed for industrial applications only. However, code making panel #7 disagrees with that logic. The code making panel summary agreed that tray cable is no different than any other nonmetallic cable. If the inner wires are the same THHN/THWN-2 or XHHW-2 wires that are used in service entrance cable, metal-clad cable, armored cable, and pulled in raceways then why treat is so differently. Is it possible that tray cable is a direct competitor to other wiring methods a company may already produce and could harm their bottom line?
So here is just one of my many arguments to expand the use of tray cable. As a master electrician in Virginia and electrical inspector for 30 years I have seen my share of installs where pulling service entrance cables over trusses and joists have ripped the nonmetallic jacket requiring corrective measures, with tray cable the jacket material is twice as thick in most cases and pulls easier without the fear of damage to the nonmetallic jacket.
In fact, on some construction job sites the extra robustness to the tray cable jacket itself may be something engineers and design professionals desire to specify in order to reduce potential damage to the nonmetallic jacket and ultimately the inner wires yet are handcuffed due to tray cables limitations currently within the NEC. Let's not forget that tray cable can be direct buried as well (where marked) while service entrance cables are not permitted for such an installation. Just imagine the possibilities.
This author has personally asked all the major manufacturers of tray cable what their concerns are for not expanding the use of tray cable in the same locations as service entrance cables and metal-clad cables are permitted. I have yet to get a legitimate response. Is it because tray cable is not required to pass a "joist pull test"? The 2017 NEC has established that within an informational note, unenforceable but does provide direction. However, it appears that UL has published a CRD on June 6, 2016 for tray cable manufacturers which added this test procedure to UL 1277. The test process is documented in the CRD and is aligned with those found in UL 719.
Let's also not forget that tray cable with the -ER designation has passed the same crush and impact tests as metal-clad cable, so why continue to limit it's expanded use? It is this authors opinion that section 336.10 needs a re-write to permit the expanded use of tray cable in all equivalent applications where other cable assemblies are already permitted.
There are some that will say tray cable is not tested to the standards of UL 854 so why compare it to service entrance cable, which could be conceived as a true statement. However, tray cable is tested to a higher crush and impact standard than service entrance cable, offers a much thicker nonmetallic jacket than service entrance cable yet both are permitted in cable trays per 392.10(A). Clearly tray cable can meet all the requirements within UL 854 and then some. In fact, tray cable passes all the joist pull testing requirements as specified in UL 854 for service entrance cables or UL 719 for nonmetallic-sheathed cables with flying colors.
We need to give electricians and design professionals more options which allows more robust nonmetallic cable assembly to be specified in applications where a more rugged cable assembly is needed. Tray cable is the perfect solution.
Remember, tray cable is constructed no differently than service entrance cable with the same inner wires and voltage ratings with the exception of the nonmetallic jacket, which is much thicker and more robust for that extra level of cable assembly protection.
By : Paul W. Abernathy,CMI
Manager of Codes and Standards
Encore Wire Corporation
* NEC or National Electrical Code are registered trademarks of the National Fire Protection Association. There use is for educational purposes only.
