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Cable High and Low

Members of the ISP-Colo list delve into the ductwork and floorways of data centers, debating the relative merits of raised floors and overhead wiring for webhosting data centers.

[September 30, 2002]

On the ISP-Colo list in September, AP asked,

"What do you think about raised floor versus overhead wiring for a data center? My thoughts:

Raised floors were often built to facilitate forced-air cooling, but cooling is more effective if done from above, since cold air stays below. On the other hand, with overhead cooling, you need to have a lot of large sheet metal conductors, which are expensive.

Raised floor does look a lot neater. Of course, underneath, I've seen some horrible things at some places. With overhead wiring, its all out in the open: if there's a cabling mess, it'll be immediately obvious.

Electrical wiring is an issue. If the code requires circuits to be in a steel conduit, I don't see how one can use overhead wiring cost-effectively: building overhead steel conduits to each rack is too expensive. And having power come through raceways without steel enclosure might affect Cat-5 if it's nearby-or at least so I was told by an electrician when I asked if I could run Cat-5 and power in the same conduit."

AD suggested that efficiency of cooling isn't really a differentiator:

"The higher air pressure in the raised floor overwhelms any tendency for the cooler air to stay low. Older computer equipment was designed to accept cool air from below, and raised computer flooring was required. Current equipment is no longer designed that way, and the key is simply good air circulation: raised floor facilitates this, but is certainly not required. You do not need large amounts of air ducts: just separate the intakes and outlets sufficiently so the air flows across the equipment."

Others looked at the pros and cons of the two options:

[JP offered] "I would imagine raised floor data centers to be more expensive, because you'll need higher ceilings, which limits your real estate options unless you are building new. We use overhead wiring, and we recently built our data center new. The sheet metal work for cooling was not expensive: most of the ducting is in the 'attic' area above, insulated, and pokes down through the ceiling where we want the air distributed. This leaves us maximum vertical room for tall racks."

[PI added] "We have a data center using overhead wiring run on cable trays that resemble ladders. With the racks in rows, you can just run the network and power cables across the ladders and down to each set of servers."

[AD agreed] "Whether your wires are above or below, they must be properly organized. It is much, much easier to organize on ladder racks above, and it does not have to look bad. In my opinion, raised floor actually looks old-fashioned, while ladder racks look more high-tech. A more serious issue, though, is securing the racks. The old mainframe equipment was meant to simply rest on four legs on the floor. Now, most equipment is rack-mounted, and the racks must be positively secured. Since you can't secure to raised floor, you either have to extend the bolts to the actual floor below, or use racks that rest on legs."

Others considered the legal issues regarding the use of conduit:

[JM advised] "There are certain cities that do require the power to be in conduit or closed raceway all the way to the receptacle at the rack: Chicago is one such city. I would suggest checking with the city, or with a local engineering firm. Regardless, you should try and keep a minimum of six inches' separation between the power and the copper—this distance is always debated quite a bit. In my experience, though, if the conduits or raceways are grounded, then a six inch separation should be adequate."

[AD agreed] "Electrical codes are very location-specific. Some locations require electrical service to be in conduit, while others do not. You certainly should not run network cables in the same conduit as power. We run power through the rack at floor level, and data cables above the racks on ladders, but I would actually recommend conduits instead because the conduit shields the noise."

[JP added] "We have an enclosed raceway from the electrical panel down the middle of the building. Then we run the cable left or right in the ceiling and have the outlets in the ceiling, one circuit per rack. If it drops from the ceiling, it has to be in conduit. Our UPS cords are long enough to reach the outlets in the ceiling, so very little wiring actually has to be in its own conduits."

[PI countered] "I don't believe there's any code requiring steel conduit to the racks: that's likely just oversell from the electrician. Running Cat-5 in the same conduit as electrical, though, would definitely be a bad idea."

—End