Chapter 5: Overhead or Under-Floor Installation?

Cisco Press

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Note - Because a raised floor system is, literally, the foundation of your server environment, it is important to protect it. Here are several recommended practices for maintaining your raised floor system:

  • When lifting floor tiles, limit how many are removed from their normal position at any given time. Also remove panels in a line rather than a block. Removing several tiles or even a small number in a block exposes support pedestals to damage or displacement.

  • Avoid placing heavy items on notched tiles because they are weaker than blank tiles. If you must position equipment on a cut tile, keep concentrated loads, such as the support pegs of a device, as far from the cutout as possible.

  • After removing floor tiles, set them back down carefully upon the raised floor. The panels can be heavy and awkward to handle. With rough edges on the bottom and sharp corners, one tile can easily damage another if dropped or set down roughly.

  • When transporting heavy equipment across the Data Center raised floor, take measures to protect the tile surfaces. Place protective covering along the path of travel and avoid rolling items over cut or perforated tiles.

  • If a floor tile becomes damaged for any reason, replace it immediately. Don't allow minor cracks or gouges to worsen.

If you're concerned about wear and tear on your floor panels, you may want to periodically rotate tiles between high-traffic and rarely traveled areas of the Data Center.

Floor Tiles and Static

The raised floor panels installed in your Data Center should have static-control qualities. Because static can disrupt or damage sensitive electronic equipment, you want to limit how much static your servers are exposed to. Static control tiles help reduce the voltage that is generated from people walking across the surface of the floor.

Floor tiles are commonly either static dissipative, which means that they inhibit the creation of static, or conductive. Raised-floor vendors typically recommend dissipative tiles for areas where various users will walk across the floor, and conductive tiles for rooms where personnel will wear static-control footwear. Choose accordingly based upon what type of foot traffic will occur in your Data Center.

The Electrostatic Discharge Association is a good resource for more information about electrostatic discharge (ESD), including various ESD-related industry standards. The association's website is at

Termination Details

There are several methods and components for routing and terminating Data Center infrastructure under a raised floor. Electrical outlets, for example, can be individually terminated as follows:

  • At the end of a flexible conduit

  • In a cluster on the same conduit

  • Along a floor-mounted guide rail

  • In a stationary raceway, on a vertical power pole

  • Up in a server cabinet.

Equivalent options are available for structured cabling, too. Each approach has its advantages and disadvantages.

Running individual electrical conduits and cable bundles to each server cabinet location is the most flexible configuration, enabling quick and easy redistribution of Data Center infrastructure. This is a tremendous advantage when incoming servers arrive with new or additional requirements for power or connectivity. Infrastructure can be repositioned or altered with minimal disruption to the environment as a whole.

Cable bundles and electrical conduits can be routed along the Data Center subfloor, carefully secured to the raised floor system's vertical pedestals by soft cable ties or fasteners such as Velcro. If you design your Data Center in this manner, include specific instruction in your design package about what paths electrical conduits and data cables are to follow under the raised floor. This configuration involves a significant number of power receptacles and multimedia boxes under the floor, and careful routing enables them to be managed more easily and efficiently.

Note - I recommend against terminating Data Center infrastructure into under-floor raceways or directly into cabinets, although both are common practices. While raceways help organize infrastructure under the floor, they also restrict airflow, inhibit flexibility, and add expense to the price of your server environment. It is much easier to relocate freestanding structured cabling bundles and electrical conduits than those terminated in raceways or cabinets.

If you decide to use cable trays or raceways under the raised floor, place them strategically so that Data Center users can easily access infrastructure. If you intend for people to plug patch cords into under-floor data ports directly under server cabinet locations, make sure that whatever combination of cable trays, wire management, and multimedia panels you use keeps the structured cable bundles out of the way while situating the data ports within easy reach.

Don't restrict access by overlapping or criss-crossing the infrastructure management devices. For example, don't terminate power into a raceway and then run a cable tray directly on top of it, forcing someone to put his or her hand through the lattice of the tray—and between bundles of structured cabling—to reach the electrical outlets. The two distinct raceways should instead be staggered so that one is not directly over the other.

Prepare for additional power and data cables in the future. Choose management systems that have space for growth and that are expandable. If your Data Center cable trays are completely filled by structured cabling the day the room comes online, they are too small. If space under the floor permits, choose deeper or wider trays that enable more cable bundles to be added. It is much better to have large cable trays and raceways that are half-full than small ones that are bulging with infrastructure.

Also, choose infrastructure management components that have a low profile and that can be easily dismantled and reassembled elsewhere. This limits their effect on under-floor airflow and provides an opportunity to reconfigure your Data Center infrastructure.

The last option is terminated power and cabling above the Data Center floor directly into a server cabinet. The primary advantage of this practice is that once infrastructure is in place, there is no need to lift floor tiles to access connections. Servers and networking devices can be installed with no risk of damaging a patch cord or power cable when moving floor panels. This is a very inflexible design, however, because it is very difficult to relocate either infrastructure or individual server cabinets without incurring downtime. This approach should therefore be implemented only in Data Centers that have minimal turnover of equipment.

Figure 5-3 shows power, data, and cooling routed under a raised floor. Electrical conduits and structured data cabling follow parallel but distinct paths under the floor.

Figure 5.3

Figure 5-3

Under-Floor Termination Example

The Subfloor

If you use a raised floor system, make sure that that the subfloor is sealed. This prevents the Data Center air handlers from stirring up minute concrete dust that can in turn damage servers or networking equipment.

Also consider designing troughs into your subfloor. These can provide a degree of protection for your infrastructure in the event that water intrudes into the Data Center. Run the troughs parallel to the server rows, out of the way of under-floor infrastructure. The deeper they can be, the better, because you want them to catch and carry away moisture before it can reach any of the cabling under the floor.

Common Problems

Routing the hundreds or even thousands of cable runs and electrical conduits in your Data Center can be very complicated, so errors of one type or another are likely to occur. The sections that follow discuss some of the more common errors to look out for.

Tile Cut-outs Are Poorly Sized or in the Wrong Location

While a contractor may misread the instructions in your design package, these errors most often occur during the Data Center planning stage. It is easy to miscalculate sizes for tile cut-outs, by either not providing a big-enough opening at cabinet locations where several cable bundles pass through or else making the notches so large that air escapes wherever only a handful of patch cords is routed. One solution to this problem is use of two cut-out sizes for your notched tiles and strategic placement of them. Perhaps use an 8-inch (20.3-centimeter) square opening for panels near cabinets packed with cables and a 4-inch (10.2-centimeter) square cut-out for tiles near sparsely cabled cabinets.

Still more potential for error exists when cutting notched tiles and orienting their cutouts in relation to servers in the Data Center. If cabinets in your Data Center don't have the same width as your floor tiles—2 feet (61 centimeters)—pay careful attention to how they align with your room's notched floor tiles. Different widths can cause tile cutouts and the equipment cables that are to pass through them to drift apart.

Note - Many of the Data Centers I manage use server cabinets that are 22.5 inches (57.2 centimeters) wide. While only slightly narrower than the panels on the raised floor, the cabinets are typically situated flush with one another. Over the span of an entire server row, each cabinet drifts a bit more from the center of the floor tile it sits upon. At the end of a server row with 13 cabinets, that is a gap of 19.5 inches (49.5 centimeters). I can—and have—still run cables through cutouts that are offset by this much, but it is awkward, and I have to use longer patch cords and electrical cables. The same problem occurs when cabinets in a row are wider than the floor tile. The drift between servers and tile cutouts just occurs in the opposite direction.

There are three ways to address drifting cabinets:

  1. Don't let cabinets become misaligned from floor tiles. Space out cabinets, narrow or wide, so that that they are always centered on your Data Center floor tiles. For wider cabinets, this may mean skipping over tile locations and potentially wasting floor space.

  2. Adjust the placement of the notched tile cutouts as you progress down the row, so that they are always centered near the equipment intended to use them. This requires additional customization and may not meet your needs when new equipment arrives for the space.

  3. Make sure that your server rows are interspersed with wide and narrow cabinets. This dovetails with the approach of organizing servers by function and getting a heterogeneous mix of equipment, as mentioned in Chapter 4.

Figure 5-4 illustrates how cabinets narrower than Data Center floor tiles can be separated from the notched tiles that their cabling is intended to pass through. Row A shows cabinets becoming misaligned with cut-out tiles, while rows B, C, and D show the possible measures to realign them.

Figure 5.4

Figure 5-4

Preventing the "Drift" of Floor Tile Cutouts

Cabling Installed in Plenum Spaces Aren't Properly Rated

Clearly specify the type of structured cabling you want installed in your Data Center and double-check that you receive the correct materials. Some manufacturers use similar part numbers for their plenum and non-plenum cables, identical except for a single digit. Mistakes can occur, and you want to quickly catch any errors so there is adequate time to receive replacement supplies. Review the items as soon as possible. Even if you have experience with non-plenum, plenum, and low smoke/zero halogen cables, you may not immediately recognize an incorrect item without close scrutiny.

The Raised Floor System Isn't Strong Enough to Accommodate Equipment

It is easy to plan your Data Center for today's servers, but hard to make accurate predictions for future equipment. Remember: If future devices become larger, they are likely to add weight onto your Data Center floor; if future devices become smaller, they are also likely to add weight, because Data Center users are going to install more of them in each server cabinet. For example, a floor-standing machine that occupies one floor tile and weighs 750 pounds (340.2 kilograms) might be replaced by a device that occupies two floor tiles and weighs 1800 pounds (816.5 kilograms), or by a server cabinet filled with 20 narrow servers weighing 75 pounds (34 kilograms) each. That third configuration may seem lighter at first glance, but when you do the math it works out to the heaviest load of all—1500 pounds (680.4 kilograms) in one tile location, with extra space still left in the cabinet! Make your building as strong as physical conditions and your budget allowenable, and then design the Data Center floor to match the weight-bearing ability of the building.


Power, data cabling, and cooling may be routed above a Data Center's false ceiling or below a raised floor.

An overhead installation is less expensive and takes up less space that an under-floor one. Patch cables and power cords all connect above your Data Center server cabinets when infrastructure is installed overhead. This makes them unlikely to be snagged in passing but requires Data Center users to climb a stepladder every time they want to connect to or disconnect from the infrastructure, which is inconvenient and carries its own snagging risks. If you install power and data overhead, use twist-lock plugs and receptacles and patch cords with sturdy connector tabs to reduce the risk of accidental unplugs.

Most Data Centers have a raised floor system despite their cost and space needs. Chilled air can be channeled through the under-floor and directed with greater precision to server cabinet locations by way of perforated and solid tiles better than from overhead vents. Routing infrastructure under a raised floor also makes it easy to access but out of sight, therefore protecting it from accidental damage or disconnects.

Electrical wiring generates electromagnetic interference, so keep it physically separate from any structured data cabling. Do this by routing them along separate paths in the Data Center and, if using an overhead installation, terminating them in distinct raceways.

You must use specially rated cabling and associated materials in the spaces that distribute air through your Data Center, typically located above the ceiling or below the raised floor. These are plenum-rated cabling in the United States or low smoke/zero halogen in most European countries. Non-plenum cabling is acceptable for use in other areas of the server environment.

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