Updating Installation Methods for Floor Boxes

Apr 29, 2004 6:29 PM, By Jim Pfefferkorn, Clark Nexsen Architecture and Engineering

With Cat. 5e or Cat. 6 cabling now being installed in most new construction projects, some changes in installation methods are in order for slab-on-grade floor boxes with communication outlets

Construction on the Advanced Technology Center in Virginia Beach, Va., was completed in late 2002. Final testing of the Cat. 5e cable plant was proceeding smoothly with the exception of the cables to the slab-on-grade floor boxes. The cables to the slab-on-grade floor boxes failed testing. An investigation revealed that the conduit run under the slab contained water.

Prior to installation of the cables, the telecom contractor, who was a sub to the electrical contractor, verified that the conduits had been blown clear to remove any water that may have been present. Yet two months later the PVC conduit contained water and the cable cover was discolored where it had been in contact with the water. The conduit joints were glued, which prevented any water infiltration from the ground. So where was the water coming from?

Condensation was the culprit.. The mechanical contractor on this project had installed a plenum-return HVAC system for the building. Accordingly, the telecom contractor installed plenum-rated Cat. 5e cable throughout the building. In a plenum-return HVAC system, rooms are slightly more pressurized than the ceiling plenum. Air moves from the rooms to the ceiling plenum and then to the HVAC unit.

Holding one’s hand near the floor box conduit revealed a slight air movement. The conduit was acting as a miniature HVAC duct. Air was moving from the room to the lower pressure ceiling plenum through the conduit. During humid summer conditions the air moving through the conduit was cooled below its dew point where the conduit was in direct contact with the soil under the slab, causing condensation to accumulate in the conduit.

Compliance with codes and standards. Research into the various codes and design methods for under-slab conduit construction reveals that 300.5(D)(5) of the 2002 NEC requires that “cables and insulated conductors installed in enclosures or raceways in underground installations shall be listed for use in wet locations.” In addition, BICSI’s Telecommunications Distribution Methods Manual, Chapter 4, Section 1, page 4-5, under the heading ‘Wet Locations’ states, “…slab-on-grade construction where pathways are installed underground or in concrete slabs that are in direct contact with soil (e.g., sand, gravel, etc.) are considered to be ‘wet locations’.”

However, plenum-rated cable isn’t suitable for installation in wet locations. It has microscopic holes in its outer covering, which allows the water to wick past the cable sheath and change the electrical properties of the cable. And experience has shown that water degrades the performance of Cat. 5e and Cat. 6 cable to a much greater extent than is the case with Cat. 3 and Cat. 5 cable.

In the absence of a dual-rated plenum/wet location cable, a transition point was installed in the wall between a plenum-rated cable to a wet location-rated Cat. 5e cable for that portion of the cable that runs under the slab. This type of cable has a waterproof sheath suitable for outdoor use and contains a gel fill to prevent water wicking. This same solution can be used on Cat. 6 cabling where all components are Cat. 6-rated.

The situation at the Advanced Technology Center points to the need to make some design and installation changes from common industry practice for slab-on-grade floor boxes. The methods in use for Cat. 3 and Cat. 5 cables won’t work for Cat. 5e and Cat. 6 cables. Where slab-on-grade conditions exist, floor boxes with communication outlets should be avoided when possible. If required, they should be designed with a transition point and outdoor-, or wet location-rated cable for the portion of the cable that runs under the slab and to the floor boxes.

Pfefferkorn, Jr., P.E., RCDD/OSP is an electrical engineer with Clark Nexsen Architecture and Engineering in Norfolk, Virginia.