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DISTRIBUTION · SEP 23, 2026
Busway vs Cable Trays in AI Halls
Both options move the same amperes to the same rack row. They differ in where the engineering happens — in a factory under a type test, or in the ceiling under site conditions — and in what a change costs three years after commissioning. This is the comparison, dimension by dimension, with the numbers that decide it.
THE STRUCTURAL DIFFERENCE
Rated assembly versus field installation.
Busway is engineered in a factory
A busway run leaves the plant as a set of straight sections, joints and tap-off boxes with defined ratings: copper U-bar conductors in an open channel, 160 / 250 / 400 / 630 / 800A sections, joints assembled with torque-logged hardware, tap windows cut to an approved layout, and sample runs temperature-rise tested to IEC 61439-6. The rating on the section is the rating of an assembly that was tested as one, with millivolt-drop verification joint by joint and a 100% hi-pot and resistance check on every tap-off box.
Cable ampacity is conditional on the install
A cable's tabulated ampacity assumes a set of conditions — a base ambient temperature, a defined number of current-carrying conductors, a specific installation method. Real halls break all three. Conductors bundle in the tray, ambient in a hot aisle ceiling runs above the table's base, conduit runs get filled, and every one of those conditions applies a correction factor below 1.0. Size the cable from the table alone and the run is under-built before it is energised; size it properly and the conductor set grows.
AMPACITY AND DERATING
What eats the ampacity you paid for.
The derating factors below are the reason two halls with identical load lists can end up with different copper. A busway section's rating already accounts for them by test; a cable installation has to have each one applied on site.
| Factor | Applies to cable in tray or conduit | How busway handles it |
|---|---|---|
| Conductor grouping | Adjustment factor when more than three current-carrying conductors share a raceway or bundle | Fixed conductor geometry inside an engineered channel, rated as an assembly |
| Ambient temperature | Correction factor once ceiling ambient exceeds the tabulated base | Temperature-rise type test to IEC 61439-6 defines the rated current in a defined environment |
| Conduit fill and bundling | Fill limits and heat trapping reduce usable current | Open channel; conductors are separated by slot insulation with defined creepage and clearance |
| Continuous load | Protective device at 125% of the continuous load, sized up to the next standard frame | Same rule — but the section rating is already a continuous rating |
| Installation quality | Termination torque, bend radius and support spacing depend on the crew on the day | Modular joints with torque-logged hardware; telescoping joints absorb building tolerance |
The sizing consequence
Stack those factors and the conductor that arrives on site is frequently a size or two larger than the tabulated number suggested, which is why cable runs in dense halls tend to grow in both copper and tray width as the design matures. A busway selection instead starts from the section class — 400A, 630A, 800A — and the design decision becomes which class the row needs rather than which correction factors apply to it.
TAP-OFF DENSITY
How many racks one run actually carries.
This is where the choice becomes concrete, because the arithmetic is simple and unforgiving: a run has a current rating, and every rack on it draws a share.
| Rack class | Current per rack at 415V 3φ | Racks on an 800A track | Racks on a 400A track |
|---|---|---|---|
| 30 kW AI rack | 41.7 A | 19 | 9 |
| 60 kW high-density rack | 83.5 A | 9 | 4 |
| 132 kW NVL72-class | 183.7 A | 4 | 2 |
| 142 kW NVL72-class ceiling | 197.6 A | 4 | 2 |
Density is set at layout time, changed later for minutes
Tap windows are cut to the layout derived from your rack elevation, so the run arrives ready for the row it was specified against. What makes busway different is what happens afterwards: a tap-off box plugs into the open channel anywhere along the run, tool-free, while the system stays live, with IP2X finger-safe shrouds protecting the conductor path and per-tap metering confirming the new load landed where the layout said it would. You are not limited to the tap positions someone guessed at design time.
The cable answer to a new rack
On a tray installation, adding a rack means a home run or a junction: de-energise the branch, pull the conductor, land it, label it, inspect it, and re-energise inside a maintenance window. It is skilled work done in a ceiling, and it happens every time the row changes. In a hall where racks are refreshed on a two-to-three-year cadence, that is not a one-off cost — it is a recurring project.
THERMAL BEHAVIOUR
Where the heat goes, and who pays for it.
Losses are small; where they land is not
Copper distribution is roughly 99.5% efficient, so a 568 kW row feed — four 142 kW racks — loses on the order of 2.8 kW as conductor heat. That is a modest number until you ask where it goes. An open-channel busway run rejects heat along its length into the ceiling plenum, and its temperature rise was type-tested to IEC 61439-6 limits at rated current. A bundled cable tray traps the same heat inside insulation and tray walls, which is precisely why grouping derating exists: the ampacity table is compensating for a thermal problem, not an electrical one.
Rack heat is a separate, larger load
Do not confuse distribution loss with conversion loss. At 96% Titanium efficiency, a 142 kW rack rejects roughly 5.7 kW inside the enclosure, and that heat is a liquid-cooling or air-handling problem at the rack, not a busway problem. The distribution decision affects a few kilowatts per row; the conversion decision affects tens of kilowatts per rack. Both belong in the same hall thermal model, but only one of them scales with rack density.
COST STRUCTURE
Which line item dominates each option.
The comparison is not a unit price. It is a set of cost lines that behave differently over the life of a hall, and the winner changes with how often the row changes.
| Cost line | Overhead busway | Cable and tray |
|---|---|---|
| Material | Engineered copper sections, joints, tap-off boxes, mounting hardware | Conductor, tray, supports, junction boxes, glands and terminations |
| Installation labour | Modular sections on threaded rod or trapeze sets; telescoping joints absorb building tolerance, so runs slot into weekend windows | Pulling, dressing, terminating and inspecting in the ceiling, with more of the work on site |
| Cost of the first change | One tap-off box inserted live | New home run or junction, inside a maintenance window |
| Metering | Per-tap current monitoring with comms output to DCIM, built into the tap | Current transformers and meters added per point, with their own wiring |
| Engineering effort | Layout service: run and tap map derived from your rack elevation before manufacture | Field routing and coordination with the ceiling, sprinklers and containment |
| Schedule exposure | 45–60 day lead time for the run, ordered against a frozen layout | Conductor availability and site labour scheduling |
| End of life | Sections and tap boxes are relocatable | Conductor is largely scrap |
DECISION GUIDE
Matching the option to the hall.
| Hall characteristic | Better fit | Why |
|---|---|---|
| Rack population changes every refresh cycle | Track busway | Change cost is a tap-off box instead of a branch circuit |
| 132–142 kW racks at four per row | 800A track with 400A-class taps | Matches the row current without paralleling |
| Fixed point-to-point spine from service to distribution | Feeder busway or cable | Feeder sections run 1600A and beyond, to 6300A; no tap points needed |
| Row density above 19 racks at 30 kW | Split rows across parallel tracks | One 800A run carries roughly 19 racks at that class |
| Ceiling congested with containment and sprinklers | Busway with a frozen layout | Rigid, repeatable geometry instead of field-routed bundles |
| Existing tray installation, limited outage windows | Hybrid: track over the rows, cable to the spine | New tap capacity where it is needed without reworking the incoming path |
| Per-tenant billing from the tap | Busway with per-tap metering | Load is measured at the point of change, not inferred upstream |
The hybrid is not a compromise
Most AI halls end up with both: feeder sections or cable for the spine, track busway over the rows where change happens. The mistake is specifying the row distribution as if the hall were static — because the row is the part that changes, and it changes at the speed of a GPU generation, not the speed of a building.
RELATED SYSTEMS
The hardware behind this brief.
Data Center Busway
160–800A open-channel track, UL 857 at 600V, IEC 61439-6 type-tested, tool-free metered tap-offs from 80–400A.
Explore busway →Rack PDU
16–63A inputs, 3φ past 22 kW+, ±1% billing-grade outlet metering — the layer the tap feeds at the rack.
Explore rack PDUs →Design Guides
Ampacity planning, runtime sizing and interface planning guides built from the numbers on this site.
Open the guides →
BUSWAY LAYOUT
