One of the most important is budget. If your budget is tight, an overhead system is the better option. It’s less expensive to buy and much less expensive to install. Also, plenum cables may be required in an underfloor system. In an overhead system where the cabling is installed below the ceiling grade, plenum cables are not required. This results in cost savings.
Next is the size of the room and the cabinets. TIA-942 mandates a minimum ceiling height of 8.5 feet above the finished floor to allow for 7-foot racks and cabinets. In addition, TIA-942 also mandates a clearance of 18 inches above the cable pathways for sprinklers, etc. This could limit overhead cabling in low-ceiling rooms. Don’t forget to measure the cabinet heights as well to allow for these clearances. Also, if there are different size cabinets, you must be take that into consideration as well. If you have different height cabinets, you’ll have to design around that.
Another consideration is the separation of power and telecommunications cables. You need to know where your electrical cables are run, either in the ceiling or below the floor, and plan your data cabling runs accordingly. You’ll need enough distance between the cables to prevent EMI and network performance degradation. CATx cables can actually fail certification tests because of their proximity to high-voltage cables in the data center.
The frequency and volume of expected MACs figures into the equation as well. It’s much easier to add and trace cables in overhead cabling systems than in raised floor systems. Overhead cabling systems also do not constrict airflow in the same way underfloor systems do. Be aware of how much room you have in your basket trays or underfloor systems. Allow enough room so that you can add cables if necessary without going over the 50% fill capacity.
Both systems can become a “showcase” of sorts. Overhead systems with neat bundles of cable can be a point of pride for the data center. Underfloor systems hide the cables and make for a cleaner data center. They are also more secure than overhead systems.
Lastly, be mindful of what types of vertical cable management you have, or need, in your cabinets and racks. Also consider what types of patch panels you have and if they include horizontal cable managers.