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Using overhead cables to tidy your data center: Ask the Expert podcast

As data centers move away from tangled messes under the floor, overhead cables usher in a new era of visibility and tidiness.

Should you keep power and communications cables overhead or put them under your floor? That's the question asked by data center build teams across the board. Where you choose to run your cables can have far-reaching effects in your data center.

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Overhead cabling with Robert McFarlane

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I'm Erin Watkins, assistant site editor for, and today I've got facilities expert Robert McFarlane here to shed some light on the subject of overhead cables in the data center. Bob is a principal with Shen Milsom and Wilke, a consulting and international technology design firm in New York City. He also teaches the data center facilities course for Marist College.

Erin Watkins: Hello, Bob, and welcome to the podcast.

Robert McFarlane: Glad to be with you!

Watkins: So, what are some of the benefits to using overhead cables for power and communications in the data center?

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McFarlane: When I ask my students and clients this question, the answer I usually get is that it keeps cabling visible and therefore hopefully neater. So I guess it's the old "out of sight, out of mind" syndrome. It would appear that data center managers have grown tired of dealing with the mess of accumulated cable under the raised floor. Most of this is data cable, of course, but power cabling can get out of control as well.

One of the major benefits of getting all of the "stuff" out of the floor is the elimination of under-floor air obstacles — cooling usually improves. And since every barrier you remove from the air path also reduces the amount of fan power that's needed to move air under that floor there's also potential gain in energy efficiency.

Lastly, I have to point out the advantage of avoiding the need to comply with article 645 of the National Electrical Code (NEC) and the dangerous Emergency Power Off (EPO) button that article requires.

Stories of data centers being instantly brought crashing down by someone pushing this button run rampant. If you can get the cable overhead — particularly the power circuits — you have no reason at all to put this big, scary red button in your data center.

Watkins: Who benefits most from overhead cables and why would a data center choose it over more common under-floor options?

McFarlane: I'd have to say that the data center manager probably thinks that he or she is the person that will benefit most. The comments I get from those who've done it or want to do it are mainly that it makes it easier to manage the facility. If you run conventional circuits overhead using flexible wiring — I'm talking about power circuits now — the electricians usually find it easier to make adds and changes. And if you use the newer busway systems you may not even need electricians to make power changes. And of course if everything is labeled properly it's also easy to see at a glance what circuits are in use and what's connected to them. When things are overhead, they're visible instead of hidden under the raised floor. Again, simplified management for everyone involved is probably the answer.

On the cabling side, it's easier to string cable overhead through accessible power trays than it is to remove and replace floor tiles and pull cables through the floor. Anybody who's actually done this can certainly attest to that. This is true for both data and communications cable. People think "Oh I can run cable under the floor easily just by pulling up a whole bunch of tiles," but in order to avoid destabilizing the floor — very expensive to have realigned and re-leveled — you should never remove more than two adjacent tiles and you should leave at least four tiles before removing the next two. In reality, properly installing cable under a raised floor takes a bit of effort.

Then there's the fact that cable is changing dramatically in data centers. Permanent cable infrastructures have been common for years now. That greatly reduces the amount of ad-hoc cable that needs to be run and the associated mess that goes with it, so that's lessening a bit. We're moving more heavily into fiber interties, which means far less cable volume, but it's still a concern.

There's a growing trend toward pre-terminated cable that comes from the factory cut to correct length, and that's available for both power and communications cable. It can be purchased this way pre-tested and well-marked, which is a real advantage. It's generally easier to lay this cable into overhead trays than it is to fish it under the floor and up through the floor just as we talked about a moment ago. Since bundled pre-terminated cable greatly reduces over-cabling, there'll be more need to add cable bundles over time as they are needed.

What we used to do in many data centers was just put in all the cable that might be needed and much of it ended up not used and people were concerned about the extra cost. Now with pre-terminated cable, you can put in just what you're really going to need day one. It's easy to buy another bundle from the manufacturer, stick it in the tray, bring it in and just screw it into the rack. Using pre-terminated cable also eliminates that mess of wire cuttings in your data center every time more cable needs to be added. So we specify it as the norm these days and as a result, we're putting a lot more overhead cable in the data centers. It's easier!

Watkins: What are some of the technical and logistical challenges to implementing an overhead cabling system in an existing data center?

McFarlane: In an existing data center it's really all about overhead space. If your ceiling is very low or if you're in a seismic zone and have overhead cabinet bracing then pathways for overhead tray or busway can be a problem. And the most common obstacle is columns in the middle of cabinet rows. Those can get messy to get around with this stuff.

But speaking of messes, what you really want to avoid is suspending the tray from overhead in an existing facility. Drilling into overhead concrete or removing insulation from beams so you can attach anchors creates dust and contaminates; you just don't want that stuff in your equipment. So in an existing facility you're usually better off mounting the tray to the tops of the cabinets and racks. Many are made to accept the accessory stand-offs that are made for this purpose.

(A)t some point in the future, frankly, I think we're likely to see most of the power and cabling overhead with the raised floor used mainly for water piping to serve cooling needs of close-coupled cooling systems.

Robert McFarlane

Watkins: What options are available for businesses that want to incorporate overhead cabling in a new data center build?

McFarlane: The real decision there is on power. Are you going to use conventional flexible power whips in a cable tray, or are you going to go for power busway? That decision will probably be based on a combination of cost and flexibility.

Busway is probably going to be more expensive initially, but if you're going to be making a lot of changes in your data center or can't predict what you're going to need, where and when, then busway is without question the ultimate in flexibility.

If you're not familiar with power busway, think of it as a giant version of track lighting. You buy modules rated for the circuit amperage that you want, and then have the receptacle type you need to connect to specific cabinets. You just plug it into the module in the overhead busway just like you'd put a light fixture into a light track.

Some systems will let you connect anywhere you want along the length of the bus while others have connection points at fixed intervals. They're close enough that it probably doesn't make a lot of difference. All of them allow you to do this while it's hot — that is without shutting off the power — so it's really pretty neat.

But in my opinion, a very important thing to consider if you're going to use busway is if the connection points — the infeed to the bus — can be IR scanned without shutting off the power. Mechanical connections of electrical wiring can loosen over time. They should be scanned at least yearly with an infrared camera to make sure they're not starting to overheat. This should be done for every part of the data center power system and the busway should be no exception. If you're going to use overhead busway, try to find one with an IR scanning window in the infeed — I consider that very important.

If you're not using a busway for communications cabling and power whips, try a basket-type cable tray. It's lightweight, easy to install and junctions and corners can be radiused without special fittings. That's important to getting maximum capacity out of the tray and to avoid over-bending the cables. Cable can be fed through the openings in these trays, although I always want to see the proper spill-out fittings used. Power boxes can easily be attached to them with adapters that are made for the purpose. Lastly, because it's wide open, air moves through the basket easily and you can always see what's in it.

Watkins: So energy efficiency is a buzzword these days. How large an impact does overhead power distribution and data cabling have on energy efficiency and cooling?

McFarlane: Well, by itself it really doesn't have any. The impact it may have is the removal of air blockages from under the raised floor, which we mentioned at the beginning. But I do have to emphasize that if under-floor cabling — power and communications — are designed and maintained correctly, there shouldn't be any real difference. It's more a matter of preference in managing the data center.

Watkins: Based on your experience with data center facilities, what will be the most popular method going forward: overhead or under-floor?

McFarlane: I think that in the relatively near future, we're probably going to see about 50/50. A number of those will not even use raised floor. Some still will, but the raised floor will be used for cooling with all power and communications overhead. In others, communications will be overhead, but power will still be under the floor and in those cases the floor may even be low and not even used for cooling. There's a lot more overhead cooling these days.

But at some point in the future, frankly, I think we're likely to see most of the power and cabling overhead with the raised floor used mainly for water piping to serve cooling needs of close-coupled cooling systems.

With all the high-density stuff we're getting now, we're going to use more water-cooled cabinets, in-row coolers and rear door coolers — things that require water piping under the floor. And there's always such a paranoia about water in the vicinity of cable so that along will probably move an awful lot of cabling overhead.

Watkins: Well, thank you very much for joining us today, Bob. I appreciate your input and we'll see you all next time.

McFarlane: Thank you very much!

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