How Cable Tray Airflow Affects Data Centre Cooling
Why cable tray airflow is a cooling capacity problem
Cable tray airflow is rarely on anyone’s list when a data centre is designed, and it is frequently on the list when the same facility cannot cool a row it was supposed to handle comfortably.
Cooling capacity on paper is a function of plant. Cooling capacity in practice is a function of whether the air reaches the equipment. Between the cooling unit and the server inlet sits a path, and containment occupies a significant part of that path — under the floor, above the cabinets, or both.
When that path is obstructed, the symptom does not present as a containment problem. It presents as hot spots, as perimeter cabinets running warmer than the row average, or as a computer room air handler working at full output while inlet temperatures drift upward. The cause is easy to miss because containment is not something the thermal model usually accounts for in detail.
What happens under a raised floor
A raised floor plenum is a pressurised air distribution duct. It delivers air to perforated tiles at a rate governed by static pressure, and static pressure depends on how freely air moves through the void.
Put cable containment in that void and the free area drops. Put several layers of containment in it, running in different directions and at different heights, and the void stops behaving like a plenum and starts behaving like a maze.
The effect is not evenly distributed, which is what makes it difficult to diagnose. Air takes the path of least resistance, so tiles near the cooling unit continue to deliver while tiles at the end of a run behind an obstruction deliver less. The row looks identical from above. The inlet temperatures do not match.
Two design responses work. Keep the plenum clear by moving data cabling overhead and reserving the underfloor void for power and cooling. Or accept containment underfloor but size the void and route the containment so the free cross-section is preserved, which means planning containment routes as part of the airflow design rather than after it.
Abandoned cable and the obstruction nobody ordered
The largest underfloor obstruction in most operating data centres was never specified by anyone. It accumulated.
Every decommissioned circuit that was disconnected at both ends but left in the tray is still occupying cross-sectional area and still blocking air. Over ten years of moves, additions and changes, abandoned cable can account for a substantial share of the containment fill in a facility that has never audited it.
This is a containment design issue as much as an operational one. Cable that is difficult to remove tends not to be removed. An open containment system, where a redundant cable can be lifted out rather than pulled through a closed route, makes removal a job somebody will actually do. This is one of the practical arguments for open containment in facilities with a long service life and frequent change, and it connects directly to how the route is sized in the first place, discussed in our article on sizing a scalable cable management system.
Planning containment routes around an existing airflow design? Send us the void depth and route layout and our engineering team will suggest a configuration that preserves free area.
Open mesh is not automatically open
Wire mesh containment has an obvious advantage here. An open grid presents far less resistance than a solid-bottom tray or a closed trunking system, and air moves through it rather than around it.
It is worth being precise about the limits of that advantage, because it is often overstated.
A mesh tray carrying a single layer of cable is genuinely close to transparent to airflow. The same tray carrying a bundle four cables deep is not. The cable is the obstruction, not the containment, and beyond a certain fill depth an open tray and a solid tray present broadly similar resistance.
This means the airflow benefit of open containment is conditional on how it is loaded. A mesh tray specified narrow and filled deep gives up most of the advantage it was chosen for. A mesh tray specified wide and shallow, carrying cable in a single layer, keeps it.
The same logic applies to the cables themselves. A dense bundle runs warmer at its core, and for cabling delivering power over Ethernet that temperature rise directly affects the cable’s performance. Single-layer placement in a wide tray addresses both the plenum airflow and the bundle temperature at once.
Published cross-sectional capacity makes the comparison concrete. A 300 mm tray with a 60 mm side rail offers around 14,050 mm² and encourages single-layer placement. The same area achieved through a narrower, taller profile does the opposite.
Overhead containment and the return air path
Moving containment overhead solves the plenum problem and introduces a different one, which is less often discussed.
In a hot aisle arrangement, heated air rises and returns to the cooling units, often through a ceiling plenum. Containment running above the cabinets sits directly in that return path. Dense, stacked, multi-tier containment can impede return airflow in much the same way underfloor containment impedes supply.
Three things reduce the effect. Adequate clearance between the top of the cabinet and the lowest containment tier. Running containment parallel to the aisles rather than perpendicular where possible. And limiting the number of tiers, which usually means going wider rather than adding another level.
ISO/IEC 11801-5 sets out generic cabling requirements specifically for data centre computer room spaces, and coordinating cabling routes with the thermal design is part of getting that arrangement right rather than an afterthought.
Sizing for airflow, not just for cable
The conventional sizing question is how much cable the tray has to carry. In a data centre there is a second question: how much air has to pass the tray, and in which direction.
That changes the answer in a specific way. A route sized purely on cable area will tend toward the narrowest profile that delivers the required cross-section, because that is the cheapest option and the easiest to coordinate. A route sized with airflow in mind tends wider and shallower, because a single cable layer is the condition that preserves both air movement and thermal performance.
The cost difference between those two options is one size step on the material line. Support count, joint count and installation labour are broadly unchanged, since the route length is the same either way.
Support spacing remains governed by load rather than airflow. IEC 61537, the international standard for cable tray and cable ladder systems, defines safe working load and tests lengths, fittings, supports and cantilever brackets separately, and the figure for a specific configuration should come from the manufacturer’s published load data.
Where the underfloor route still makes sense
Overhead containment is not the universal answer, and there are cases where the underfloor route remains correct.
- Power distribution. Busbar and power cabling underfloor keeps heavy services out of the overhead space and away from the data routes.
- Deep plenums with low containment density. Where the void is generous and the containment occupies a small fraction of it, the obstruction is not material.
- Facilities with overhead constraints. Where ceiling height is limited, forcing all containment overhead can compromise clearance above the cabinets, which creates a return air problem in place of a supply air one.
- Retrofits into occupied space. Where overhead installation would require work above live equipment, the disruption may outweigh the thermal gain.
The decision is a trade between supply path and return path, and it should be made with the thermal design rather than independently of it.
A representative retrofit scenario
The scenario below is illustrative rather than drawn from a specific project.
A facility commissioned twelve years ago runs all cabling in an underfloor plenum. Cooling was adequate at commissioning. Following three refresh cycles, two rows at the far end of the hall run consistently warmer than the rest, and the operator is considering additional cooling capacity.
An underfloor survey finds three containment tiers crossing in the affected area, a significant proportion of the cable carried in them no longer terminated at either end, and tray sections filled several cables deep. The free cross-section in that part of the plenum is a fraction of the design value.
Removing the abandoned cable and consolidating the remaining live circuits into wider, shallower containment restores most of the free area. The additional cooling plant is deferred. The intervention cost a containment package and a cable audit rather than a capacity upgrade.
This pattern is common in facilities that have been through several equipment generations, and it is the reason containment routing belongs in the thermal review rather than only in the electrical one. Similar considerations apply across data centre and critical infrastructure environments, where cable density grows steadily and the plenum rarely gets larger.
Frequently asked questions
How much does cable tray airflow actually affect cooling performance?
The effect depends entirely on how much of the plenum or return path the containment occupies and how densely it is filled. A lightly loaded open tray in a deep void has minimal impact; multiple tiers of densely filled containment in a shallow void can materially reduce the air delivered to the tiles behind them.
Is wire mesh tray better for airflow than solid bottom tray?
Yes, but conditionally. An open grid carrying a single layer of cable presents far less resistance than a solid tray. Once the tray is filled several cables deep, the cable itself becomes the obstruction and the difference narrows considerably.
Should data cabling go overhead or under the floor?
Where the raised floor is used as a supply plenum, moving data cabling overhead generally protects airflow and leaves the void for power and cooling. Where ceiling clearance is limited, overhead containment can obstruct the return path instead, so the decision should follow the thermal design rather than a general rule.
Does abandoned cable really affect cooling?
It occupies cross-sectional area and blocks air exactly as live cable does. In facilities that have been through several equipment refreshes without a cable audit, it is frequently the single largest obstruction in the plenum.
How much clearance is needed above cabinets for overhead containment?
There is no single figure, because it depends on the return air arrangement, cabinet height and containment depth. The principle is that the lowest containment tier should not sit in the path of hot air leaving the cabinets, and tier count should be minimised by going wider rather than stacking.
Design the air path, not just the cable route
Containment is part of the thermal design whether or not it was treated as such. Free area in the plenum, single-layer cable placement, clearance above the cabinets and a discipline for removing dead cable are four decisions that together determine how much of the installed cooling capacity actually reaches the equipment.
Sizing for a single cable layer also supports link performance, which is covered in our article on choosing cable tray for structured data cabling.
CK manufactures wire mesh containment at a fully automated facility in Kemalpaşa, İzmir, publishing cross-sectional capacity, weight and coating data for every configuration across the standard wire mesh cable tray range, in EG, pre-galvanized, hot dip galvanized and stainless finishes.
Send us your plenum depth, cabinet layout and cable schedule. We will return a containment configuration sized to preserve free area, with support spacing from our load data.
