In short
- Busbar trunking is a prefabricated, type-tested way to distribute low-voltage power in bars instead of cables.
- Ratings run from about 25 A lighting tracks to 6,300 A copper feeders.
- Plug-in tap-off units take power off the run wherever a load is, without a new circuit back to the board.
- Verified as an assembly under IEC 61439-6 (UL 857 in North America).
- In data centers it feeds main switchboards, UPS systems and, as track busway, the racks themselves.
What busbar trunking is
A busbar trunking system is a factory-built run of insulated conductor bars in a steel or aluminium housing. Straight lengths are bolted together on site to form a route from a transformer or switchboard to the loads. Because each element is built and tested in the factory, the run behaves as one tested assembly rather than as a collection of cables, glands and terminations.
The same idea goes by several names: busway in North America, busbar trunking system (BTS) in IEC markets, and track busway for the light overhead versions used above server racks.
The parts of a run
| Element | What it does |
|---|---|
| Straight length | The conductor bars and housing, typically 1 to 4 m long, with or without tap-off outlets. |
| Feed unit | Connects the run to a transformer, switchboard or cable at the start (end feed) or in the middle (centre feed). |
| Tap-off unit | A plug-in box with a breaker or fuses that takes power off the run to a load. |
| Fittings | Elbows, tees, offsets and reducers that follow the route around the building. |
| Expansion unit | Absorbs thermal movement on long straight runs and across building joints. |
| Fire barrier | Keeps the fire rating where the run passes through a fire-rated wall or floor. |
| End cap | Closes and insulates the last length. |
Four types, by job
| Type | Typical rating | Use |
|---|---|---|
| Feeder (transport) | 800 – 6,300 A | Transformer to main switchboard, switchboard to UPS or to floors. No outlets. |
| Plug-in distribution | 160 – 1,600 A | Rising mains and production floors, with tap-off outlets along the run. |
| Track busway | 250 – 1,250 A | Overhead above data center white space; continuous slot, tap-offs anywhere. |
| Lighting track | 25 – 63 A | Light fittings and small loads in halls and warehouses. |
Ranges are typical of current product lines; check the maker's catalogue for exact steps.
Copper or aluminium, and how big a neutral
Bars are made of copper or aluminium. Aluminium conducts about 61 % as well as copper, so it needs roughly 1.6 times the cross-section for the same resistance, but it weighs about half as much and costs far less per tonne. Copper is more compact and more forgiving at joints.
- 3P + N + PE: three phases, neutral and a separate protective earth, the most common layout.
- Housing as PE: some systems use the metal housing as the protective conductor.
- 200 % neutral: IT equipment and LED lighting create third-harmonic currents that add up in the neutral. A double neutral keeps it from overheating.
Compare both metals for a real run in the copper desk.
The ratings to read on a datasheet
| Rating | Meaning |
|---|---|
| Rated current (Inc) | Continuous current at the reference ambient temperature, usually 35 °C average over 24 hours. |
| Short-time withstand (Icw) | RMS fault current the run survives for 1 second. |
| Peak withstand (Ipk) | The highest instantaneous fault current it withstands. |
| Resistance and reactance | Per metre, at operating temperature; used for voltage drop and losses. |
| IP and IK | Ingress and impact protection; IP54 to IP55 is typical for distribution runs. |
| Fire performance | Fire resistance of the barriers and, where specified, of the run itself. |
The standards behind it
- IEC 61439-6: design verification of busbar trunking systems, read with IEC 61439-1.
- UL 857: busways in North America, installed under NEC Article 368.
- IEC 60364: how the installation is protected, selected and erected, including voltage drop.
Because the run is a verified assembly, compare datasheets only between products tested to the same standard.
Where it sits in a data center
- Transformer to main switchboard: heavy copper feeders of 4,000 to 6,300 A over short distances.
- Switchboard to UPS and back: feeders sized for the UPS input and output.
- UPS to the white space: feeders or plug-in runs to the hall distribution.
- Above the racks: track busway with a tap-off per rack, so racks can be added or changed without new cables.
Racks of 100 kW and more, and the move to 800 V DC for AI, push more current through each run; see the 800 V DC shift.
Questions about busbar trunking
Is busway the same as busbar trunking?
Yes. Busway is the North American term, busbar trunking the IEC term. Track busway is the light overhead version used above racks.
Can tap-off units be added while the run is live?
Many plug-in systems are designed for it, but only where the maker rates the system for live insertion and your safety rules allow it. Check both before planning work on an energised run.
Does busbar trunking need maintenance?
Little, but not none: periodic infrared thermography of joints and tap-offs and torque checks on bolted joints as the maker specifies.
Why use a 200 % neutral?
Single-phase IT loads and LED drivers produce third-harmonic currents that add in the neutral instead of cancelling. A double neutral carries them without overheating.
Sources
- IEC 61439-1 and IEC 61439-6, low-voltage switchgear and controlgear assemblies.
- UL 857, Busways; NFPA 70 (NEC), Article 368.
- IEC 60364 series, low-voltage electrical installations.
- Manufacturer catalogues for typical ratings (e.g. Siemens SIVACON 8PS, Legrand Starline).