UPS cable routing: overhead trays vs underfloor
UPS cable routing: overhead trays or underfloor, and how to choose
Most UPS installations end up choosing between two routes for the power cables: overhead trays or the void under a raised floor. Both work. The difference shows up years later, when someone has to find one cable inside a bundle of twenty and replace it on a Sunday night.
Three questions settle it, and none of them are about appearance. Can a technician reach the terminations without dismantling something else? Can one damaged cable come out without disturbing its neighbours? Is there room left for the UPS that gets added in year five?
Answer those first, then pick the route.
1. Start with the technician, not the tray
The UPS, battery cabinets, trays and distribution boards belong to one layout, not four separate ones drawn by four different people.
Set the equipment out from the maintenance task: what does a technician have to reach, from where, with which tools, and with the system still running. Panel opening clearance, cable bending radius, battery drawer travel and the route a replacement cable has to take all come out of that one question.
The cable route follows the layout. Reverse the order — fix the route, then squeeze the maintenance space around it — and the compromise stays with the equipment for its whole service life.
Where the installation is one room in a larger build, the same thinking sits inside the wider UPS selection guide for hospitals, banks and factories: access, monitoring and earthing arrangements change with the application.
2. What overhead routing gives you
Overhead trays suit rooms with no raised floor, and they are usually the practical answer on retrofit work. The floor stays as it is, cables stay visible and reachable, and power and control or communication runs can be kept on separate trays rather than sharing one trench.
Two things catch projects out.
Battery cable weight.
A 240 mm² copper battery cable weighs more than two kilograms per metre — conductor stranding, insulation and sheath all add to the bare copper figure, so the number moves with the construction. For several parallel strings, take the weight per metre off the cable datasheet, multiply by the support spacing, and size the tray against that total. Put that load on a long span and it stops being a footnote. IEC 61537:2023 is the standard that governs this: it specifies requirements and tests for cable tray and ladder systems, and the figure that matters is the safe working load (SWL) — the maximum uniformly distributed load the system is declared to carry, tested at defined support spans and orientations.
Specify the tray from the calculated cable weight and your actual support spacing. Picking a tray because it is the width on the shelf is how a run sags.
Working height.
A tray counts as accessible only if a technician can inspect it without booking a ladder and a second person every time. Set the tray height against the lighting, HVAC ducts, sprinkler mains and cable baskets already occupying the ceiling zone — the tray usually arrives last and loses the argument.
Overhead routing tends to win where a raised floor is impractical, the ceiling zone has room, and the battery bank sits close enough to keep the DC runs short.
3. What underfloor routing gives you
Underfloor routing is the common choice in new data centres and purpose-built UPS rooms. The room reads clean, the path between UPS, battery cabinets and downstream distribution stays direct, and lifting a panel is a cheap way to inspect a run or add a circuit.
The void is not unlimited space, and treating it as such is the usual mistake.
Bending radius eats the room.
Large battery cables have a minimum bending radius in the order of six to eight times the overall cable diameter — the exact figure comes off the cable datasheet, not a rule of thumb. Several parallel strings turning the same corner fill a void faster than anyone expects from the drawings.
Height.
A void carrying cables only is typically built around 250 mm of clear height; one that also serves as a supply-air plenum, around 500 mm. Both are figures from common practice rather than a number you can point to in a standard, and neither is the depth your site actually needs. That comes from the cable count, the bending radius those cables carry, and whether a person has to get in there to work. On a bank of several parallel strings, all three push the requirement past 250 mm long before anyone argues about the plenum.
What else lives down there.
Dust collects, water from an upstream leak finds the lowest point, and rodents treat a warm void as habitat. When twenty cables share one trench, pulling the damaged one out means lifting everything on top of it.
Underfloor routing wins when the void is genuinely sized for the cable count and a person, and loses the moment it is sized for the cables alone.
4. Overhead or underfloor: let the site decide
Neither route is better in the abstract. The site decides.
| Site condition | Overhead | Underfloor |
|---|---|---|
| Maintenance access | Direct, if the tray sits at a height a technician can reach without a tower | Requires a void deep enough to enter, with panels that lift clear of the equipment |
| Retrofit, no raised floor | Usually the practical option | Not available |
| New build with designed floor | Suitable | Common choice |
| Large parallel battery strings | Check tray SWL against the calculated cable load and the support spacing | Check void depth, bending radius and whether a person still fits |
| Tight ceiling zone | Difficult | Usually better |
| Shallow floor void | Unaffected | Risky |
| Water ingress risk above the room | Exposed, and easy to inspect | Needs protection and drainage thinking |
| Frequent future additions | Easier while spare tray width and SWL remain | Easier while spare void capacity remains |
| Cables must stay out of sight | Needs a ceiling or shroud | Clean by default |
One item has to be settled before either route is set out: which way the cables leave the cabinets. On TAFENG battery cabinets the entry direction is fixed by the model — top or bottom, whichever it is built for — and that has to reach the layout drawing before the tray hangers or the floor grid are set out. Changing it after the cabinet is landed means re-terminating the string.
On a large installation the cable count drives everything downstream, including how the strings are tapped and monitored. Where a battery bank runs to several strings, the wiring gets specific fast: our guide on wiring a multi-string battery bank and its midpoint covers that side, and the 120 kVA medical UPS installation shows what a full string layout looks like on site.
5. Three details that decide whether it still works in year five
Label for the person who did not install it
Identification matters most exactly where UPS rooms get complicated: multiple battery strings, a bypass line, parallel units, distribution circuits feeding different rooms.
Labels go where a technician can read them without tracing the cable back through the whole route, and they survive the environment they are in. For the planning and installation practice around pathways and the separation of power and information cabling, EN 50174-2 is the reference — note that its scope covers information technology cabling and excludes power supply cabling, so it sets the pathway and segregation expectations, not the rules for the battery conductors themselves.
TAFENG ships a terminal schedule with every battery cabinet. Site labels should match that schedule. A string marked "String 2" in the cabinet and "BC-02" on the tray is how a replacement cable gets landed in the wrong place.
Design the replacement, not the installation
A route always looks fine on handover day. The test arrives years later:
- Can one battery cable come out without lifting the other eleven?
- Is there a spare way for the second UPS, or does the new cable cross the live one?
- Where does a 3 m replacement length actually enter the tray?
Answering those on the drawing costs an hour. Answering them on site costs a shutdown. The same logic applies to capacity: sizing from rated, actual and peak power tells you how much headroom the electrical design is leaving, and the cable route has to be able to grow into it.
Bond the metalwork as you go
Trays, cabinets, battery racks and every other exposed conductive part get bonded according to the electrical design. IEC 61537:2023 includes an annex on using the tray system as a protective earth conductor — which is only legitimate when the system is specified and installed for it, with the electrical continuity the standard tests for.
Earthing belongs in the routing design, not in the snag list. Once the cables are in, reaching the tray joints to add bonds is the job nobody wants.
6. Isolation needs a point in the route
UPS cabinets and battery banks hold hazardous AC and DC energy after the utility supply is gone. The DC side is the one that surprises people: a 480 V battery string stays live with the UPS switched off and the input breaker open.
So the route has to leave somewhere to prove it. Isolation, voltage verification and lockout/tagout follow the UPS manufacturer's procedure, the battery manufacturer's service instructions and the site's electrical safety rules — and all three need physical access to both ends of the conductor. A battery cable buried under nineteen others in a sealed void has no test point, and the technician is left choosing between a risky measurement and a shutdown. IEC 60364-5-52 covers the selection and erection of wiring systems; it does not replace the procedure written for the machine in front of you.
Route the cable for the person who replaces it
There is no universal winner between overhead trays and underfloor routing. A retrofit without a raised floor pushes you overhead; a new room with a properly sized void makes underfloor attractive. What holds in both cases is the order of decisions: maintenance access first, cable route second.
A tray chosen on width instead of safe working load, a void sized for cables with no room for a person, a bundle with no spare way — all three pass handover. All three come back years later as a cable that cannot be pulled without taking the load down.
Design the route for the technician who will inspect it, trace it and eventually replace the cable in it. Not for the installer putting it in today.
FAQ
Is overhead or underfloor cable routing better for a UPS?
Neither wins in the abstract — the building decides. Overhead trays suit retrofits and rooms without a raised floor because the floor stays untouched and cables stay reachable. Underfloor routing suits new builds with a void deep enough for both the cables and the people who service them, and gives a more direct path between the UPS, battery cabinets and distribution.
How much floor void does a UPS installation need?
No universal figure exists. Cable-only voids often land near 250 mm and plenum voids near 500 mm, but both shift with cable size, quantity, bending radius and whether a technician has to work inside. Size the void from the layout, not from a published number.
Can UPS power cables and data cables share the same tray?
Power and information technology cabling are separated, and EN 50174-2 sets out the pathway and segregation expectations for the IT side. Its scope excludes power supply cabling, so the battery and AC conductors follow the electrical design and IEC 60364-5-52 instead. Where separation cannot be achieved by distance, a divider or a separate tray does it.
How much spare capacity should a UPS cable route leave?
Enough for the next machine, not just the current one. On trays, that means spare width and a support span that still meets the safe working load once the cables are added; in a floor void, it means depth and cross-section that a second string can share without burying the first. Decide it on the drawing — adding a way after handover usually means working around live cables.
