Busway and bus duct distribute power between switchboards. Compare air-insulated vs sandwich types and size ampacity, IP and short-circuit ratings.
A busway—often called a bus duct—is a prefabricated, enclosed system that carries large electrical currents between switchboards, distribution boards, and major loads. Instead of running many individual cables, a busway uses solid copper or aluminium busbars housed inside a grounded metal enclosure. Tap-off points let you draw power at almost any interval without cutting or de-rating the main run. In low-voltage installations the system normally operates at up to 1000 V AC and is built and tested to recognised standards such as IEC 61439-6 and UL 857.
For panel builders and procurement teams, busway is the standard answer when a single feeder must move 250 A to several thousand amps across a machine hall, a data-centre floor, or between a transformer and the main distribution board. The busbars are factory-tested as a complete assembly, so field wiring is reduced to bolting flanges together. That single fact cuts installation time and removes the most common source of cable-termination faults.
The two dominant construction types differ in how the conductors are separated and cooled:
| Feature | Air-Insulated (AIS) | Sandwich (Compact) |
|---|---|---|
| Conductor spacing | Air gaps between phases | Phase layers pressed together with insulation foil |
| Width / footprint | Wider, bulkier | Narrow, space-saving |
| Heat dissipation | Better natural convection | Relies on conduction through layers |
| Typical use | Vertical risers, high short-circuit sites | Horizontal runs, dense plant rooms |
| Cost profile | Lower at very high ratings | Competitive in mid ratings |
Air-insulated busway stays cooler under sustained load and is easier to inspect because each phase is visible; sandwich busway wins where wall cavities and ceilings are tight, packing the same rating into roughly half the width. Neither type is “better” in the abstract—the choice follows the route and the rating.
Feeder busway is a continuous, sealed conductor with no intermediate tap-offs—ideal for a single large load such as a transformer-to-switchboard link or a generator tie. Plug-in busway has standardised tap-off boxes (typically every 0.5–1 m) so machines, UPS units, or sub-boards can be connected or relocated without shutting down the whole run. Choose plug-in when your layout changes; choose feeder when the route is fixed and the load is singular.
Ampacity is the continuous current a busway can carry without exceeding its temperature-rise limit (usually 65–85 °C above ambient at the joints). Size it from the load, not from the cable you are replacing:
| Load (A) | Typical rating to specify | Application note |
|---|---|---|
| 250–400 | 400 A busway | Common floor distribution |
| 500–800 | 800 A busway | Small plant main |
| 1000–1600 | 1600 A busway | Data-centre / large MCC |
| 2000–3200 | 2500–3200 A busway | Transformer secondary |
Add about 25 % headroom for future expansion and de-rate for ambient above 35 °C or enclosed routes. As a rule of thumb, every 10 °C of extra ambient above the rated condition removes roughly 5 % of usable current. Always match the busway’s rated current to the protective-device setting, not to the nameplate peak, and confirm the joint temperature rise rather than only the conductor rating.
Modern loads are rarely purely linear. Drives, servers, and LED lighting inject harmonic current that flows through the neutral, so specify an oversized or separate neutral conductor (often 100–200 % of phase size) for IT and commercial loads. A dedicated protective-earth bar must run the full length and be bonded at every joint and at both ends; do not rely on the enclosure alone. For medical, data-centre, or hazardous-area sites, confirm the insulation-coordination and touch-voltage limits with the supplier before purchase.
Beyond current, specify these points before issuing a purchase order:
Ask the supplier for the type-test certificate covering short-circuit withstand, temperature rise, and dielectric strength, plus the IP and fire-rating evidence. For exported projects, confirm the marking scheme your market accepts (CE for Europe, UL for North America, CCC for China, EAC for the customs union). A credible supplier provides the test report reference, not just a catalogue claim.
| Factor | Busway | Cable in tray |
|---|---|---|
| Installation speed | Fast, plug-together | Slower, per-cable pulling |
| Flexibility | Tap-offs anywhere | Rewire for changes |
| Space | Compact sandwich option | Wide tray footprint |
| Very high current | Up to 6000 A | Many parallel cables |
| Cost at low current | Higher | Lower |
Below roughly 250 A, cable tray is usually cheaper. From 400 A upward, busway’s speed and tap-off flexibility typically win, and the enclosed busbars remove the derating that bundled cables suffer in a tray.
Properly specified and installed, a busway delivers decades of maintenance-light service and a clear upgrade path as your plant grows. Start from the sustained load, add headroom for harmonics and expansion, then match the short-circuit rating and IP class to the site—and the busway will outlast most of the equipment it feeds.
They are the same product. “Bus duct” is the older term; “busway” is the modern name for the enclosed busbar system. Both describe prefabricated conductors housed in a grounded metal enclosure.
Start from your continuous load current, add about 25 % spare for growth, then de-rate for ambient temperature above 35 °C and for enclosed routes. Specify the next standard rating above that figure and match it to your protective device.
Choose air-insulated when you need easy inspection, vertical risers, or the highest short-circuit ratings. Choose sandwich (compact) busway when wall and ceiling space is limited and ratings are in the mid range.
Use IP54 for typical indoor plant rooms. Specify IP65 or IP66 for outdoor runs, washdown areas, or dusty environments. A higher IP adds sealing cost but protects the joints.
Yes, where space and load justify it. Busway is faster to install and easier to re-tap, but at currents below about 250 A cable tray is usually more economical. Compare total installed cost, not just the material price.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.