M/E Roundtable: Wiring Underfloor—Not Underfoot
Raised floors offer flexibility and scalability for wiring and cable in facilities with special equipment needs or high staff churn rate
Raised floors are the ideal multi-tasking system, serving both as a plenum for HVAC and conduit for electrical wiring and data cabling. Proper implementation, however, should involve a closely coordinated effort by the design team to guarantee that specifications meet the needs of all systems.
CONSULTING-SPECIFYING ENGINEER (CSE): Many system designers praise the benefits of raised-floor systems for data, telecommunications and power infrastructure? What are the advantages?
LOVORN: Distribution flexibility is definitely the greatest advantage. With raised-floor systems, the designer can locate an outlet anywhere on the raised floor and route any number of conductors to that outlet. When a client has a high churn rate, raised floor is really the only economical type of distribution.
FLICKINGER: Having a defined cable pathway that is flexible and scalable can also provide a flexible air plenum. With computer rooms being totally reconfigured on an average of five years, raised floors facilitate the changes and ease the cost.
LOVORN: I was involved in a design some years ago where every employee moved at least once a year. Plug-in connections for power, telephone and data eliminated the need for an electrician to disconnect and reconnect a floor box every time a workstation was moved. The savings more than pays for the higher first cost.
KOCH: Raised-floor systems are of great benefit in medical facilities as well. For example, radiology equipment may be updated or replaced many times during the life a hospital. Raised floors are common in radiology equipment rooms, such as CAT-scan and MRI rooms, which generally include new control cabinets in different room locations and new cables and routes.
CSE: In addition to higher first cost, what are other disadvantages in using raised-floor systems for wiring and cabling?
FLICKINGER: One possible problem is that it is too easy to fail to establish and maintain disciplined use of the space. Procedures should include capacity planning, cable routing, documentation [according to Electronic Industries Alliance/Telecommunications Industry Association (EIA/TIA) 606] and a plan for cleaning and removing abandoned cable/piping. Failure to institute these procedures will ultimately negate any benefits.
Another disadvantage, when retrofitting an existing space, is the need to build ramps. In this case, a sunken floor is a far superior design.
LOVORN: Without proper planning and design, there are plenty of opportunities to create one's own disadvantages. For instance, if the designer underestimates the cabling area needed and specifies inadequate space below the raised flooring, the cables will not all fit. Also, the type of flooring must be carefully selected. Some of the panels have unpleasant, hollow noises when covered with vinyl tile instead of carpet. Filled panels are not so prone to this problem. Additionally, if the flooring is not properly installed, the panels can rock back and forth as people walk across them.
KOCH: There are many situations where raised floor is not the best choice. The large quantity of communications wiring required for open office spaces is the primary driving force. But in areas where bandwidth to the desktop is adequate—and security is less of an issue—a wireless network may save costs. Special issues, unique designs
CSE: It seems that each type of facility poses unique issues for raised-floor design. How do the needs of various facilities differ?
LOVORN: Due to their limited cabling requirements, most office buildings, libraries and classrooms can use the more compact raised-floor systems—which can vary from two to six inches. This type of raised floor, with its reduced height, is intended only for cable distribution.
But in a data center, the underfloor height may vary from 12 to 36 in. or more, to accommodate the large volume of power and data cables. This would still leave adequate cross-sectional area for air distribution—and for chilled water, condenser water and sprinkler piping.
FLICKINGER: Commercial office buildings that employ raised floors have the added complications of dirt and live loads. Food, coffee and paper clips can all find their way to the subterranean spaces.
KOCH: No matter what type of facility it's for, the decision of where to use a raised floor should be determined early in the job. If the raised-floor room is on grade level and a ramp or step into the room is not desired, then a depressed slab may be required. This work needs to be coordinated with the structural engineer early on, especially if an early structural document package is issued.
CSE: Does a raised-floor system that's used both for HVAC and electrical conduit call for a higher level of integrated design effort?
FLICKINGER: Yes. First of all, defining the depth of the underfloor space must depend on a careful consideration of its uses. Otherwise, the layout of flexible conduit from a power-distribution unit could block the airflow from an HVAC unit. Chilled-water pipe diameter, flanges and insulation must all be taken into consideration in order to facilitate proper system operation and to provide adequate maintenance work space.
LOVORN: Once the dimensions are determined, the architect and structural engineer must make the necessary accommodations to assure that these dimensions are achieved so that there is a smooth transition into the raised-floor area.
Suppose, for example, that the architect believes that the dimension from the slab to the bottom of the raised floor is 12 in., but the structural engineer believes that this dimension is from the slab to the top of the raised floor. There will be a 1-in. transition bump where it abuts the non-raised floor.
KOCH: There's at least one type of project where mechanical and architectural coordination is mandated by code: in a hospital MRI-equipment room. These areas may be deemed by local jurisdictions to fall under the requirements of Article 645 of the National Electrical Code (NEC). In such situations, the HVAC system has to be dedicated to the room and separated from other areas of occupancy. The code's intent is to minimize passage of smoke or fire to other parts of the building, so architectural considerations include the requirement that the room be separated from other occupancies by fire-rated walls with protected openings. Also, any MRI filters penetrating the room may require a rated enclosure. Grounding matters
CSE: What, if any, particular issues of cable and wire type—and grounding—exist for raised-floor designs?
KOCH: Cables other than those run in raceway—as defined by the NEC—shall be MI [mineral insulated], MC [metal clad], AC [aluminum coated] or, more typically, DP [data pair]. Type DP cable has fire-resistant characteristics adequate for use under raised floors in an IT equipment room, and fire-resistant cable may be tested by the "vertical flame test."
LOVORN: For the non-air-handling, limited space floors of offices, libraries and classrooms, the cabling and grounding issues are no different from other wiring methods. In data facilities, however, there are significant issues about the types of cable and grounding. Because the space under raised floors is used for air handling, the cables must be suitable for installation in a plenum application. For situations where there is a chance of water under raised floors, the cables should also meet the requirements of wet or damp locations.
FLICKINGER: There is a definite need to provide adequate grounding for the raised-floor system, and a grounding bus bar for the above-floor equipment.
LOVORN: As for grounding, the limited space raised floors are, many times, non-metallic, so there's really nothing in the floor assembly to ground.
But for data floors, the pedestals and panels are typically metal and are required, both by the NEC and the computer equipment manufacturers, to be grounded. The NEC is concerned about grounding the floor from a safety viewpoint, and the code states that if there is an incidental contact between the metallic floor system and an energized conductor, the floor must be grounded to the distribution-system ground point to assure that the overcurrent device protecting the circuit trips. The computer equipment manufacturer, on the other hand, wants a continuous, equipotential grounding surface to reduce the opportunity for static charge buildup and a fixed-signal reference ground for all equipment connected to the same system.
CSE: Have innovations in data cable and electrical wiring been the incentive for greater use of raised-floor systems?
FLICKINGER: Not really. We have generally used raised floor in data-center applications and have not seen significant innovation in the last five years. One improvement that has allowed some routing flexibility is cable-tray mounting blocks, which allows setting cable tray directly on the floor.
LOVORN: A more notable incentive for raised floors has been the increasing use of personal computers and the desire to aggregate personnel into working groups.
CSE: Describe how your design teams have used innovative approaches to address concerns and solve problems with raised-floor systems.
FLICKINGER: We have found that segregation of systems—and hence, of the building trades—optimizes installation time and maintenance. In data centers, for example, HVAC, power wiring and plumbing go underfloor, while
data and telecom are installed overhead. The data cabling in a server farm is in constant flux, so accessibility from overhead is ideal to accommodate the necessary change. This leaves the space below the floor to the mechanic, electrician and plumber who have traditionally worked together and understand each other's need for space coordination.
KOCH: One client desired the flexibility of a raised floor in a classroom where the rooms were to be rearranged from traditional classrooms into computer labs and a video conferencing center. The cost for a raised floor was prohibitive. The resultant design was a square figure-eight pattern of in-floor two-channel duct, flush with the floor. Each channel is 1.25 in. x 6.25 in., with one used for power cable, the other for communications cable. Removable carpet sections provide access to the 18-in. on center channel openings. The channels are located approximately 4 ft. off the wall so that the 8-ft. tables may be arranged in the room to exit down the center of the room or along the walls.
M/E Roundtable Participants
Thomas F. Flickinger, P.E., managing principal, Durrant Engineers, Madison, Wis.
Tim Koch, P.E., electrical project engineer, HDR Inc., Omaha
Kenneth L. Lovorn, P.E., president, Lovorn Engineering Associates, Pittsburgh
Scott Siddens , moderator
In data centers, the multiple piping systems and the condensate draining from the on-floor air-conditioning systems creates the perfect opportunity for water pooling under raised floors, from clogged drains or broken pipes. Various types of water sensors are available to monitor the problem.
With flat slabs, however, a significant amount of water can collect before a water sensor is tripped. Coordination among all disciplines is critical in developing a system that will detect water occurrence as soon as possible in order to prevent damage to equipment, either from water pools directly, or from the elevated humidity levels that result from undetected pools of water.
National Electrical Code: Changes in Store
The 2002 edition of the National Electrical Code (NEC) includes more than 300 significant changes. One change, in particular, addresses an issue with raised-floor systems.
The biggest change, according to Kenneth Lovorn, P.E., president of Pittsburgh-based Lovorn Engineering Associates, is "an addition in 645.5(d) that states, 'Abandoned cables shall not be permitted to remain unless contained in metal raceways.' This is one issue that has always been a problem in underfloor spaces."
Lovorn describes a scenario where an equipment installer puts in a computer system, linking all of the equipment with cables that are longer than necessary. Then, when the system is upgraded, the installer cables new equipment into the system—without removing the old cable. After four or five years of upgrades, the underfloor space is jammed with signal cables, resulting in air-distribution problems and consequent equipment overheating.
"No one ever looks at the underfloor area and says, 'I would like to know how many of those cables I could take out and still have the computer system completely functional,'" explains Lovorn.
Otherwise, the NEC 2002 revisions will have little impact on underfloor systems. Thomas Flickinger, a managing principal of Durrant Engineers, Madison, Wis., feels that most of the significant NEC code changes already occurred in the 1999 code cycle and were incorporated in Article 645.
"The changes have not presented any application problems," says Flickinger. "None of the proposed changes has a major impact on our typical design, either with respect to cost or basis of design."