Busbars inside a metal enclosure carry thousands of amps in a neat, compact and easily extended route.
A bus duct is a prefabricated metal enclosure containing copper or aluminium busbars that carries high current from transformers to switchboards and loads. It often replaces several parallel runs of heavy cable.
What Is a Bus Duct?
A bus duct, also called busway, is a factory built system of insulated busbars inside a grounded sheet metal housing. It carries power between transformers, switchboards and distribution points, working on the same principles covered in our busbar sizing guide.
Current Midwest describes busway as a sheet metal duct containing copper or aluminium busbars, with ratings from about 100 A up to 6500 A. It was first introduced in 1932 for the automotive industry.
Sections bolt together like building blocks, with elbows, tees and flanged ends. Tap off units plug into outlets along the length to feed machines or distribution boards.
Where a transformer feeds a main switchboard through a short, high current link, busway is often neater than many parallel cables pulled through a crowded cable tray.
5 Smart Reasons to Choose Busway
Reason 3 is why busway is loved in factories and data centres. When a production line changes, a plug in unit can be moved instead of pulling new cable.
Reason 5 matters at high fault levels near transformers. Check the rated short time current against results from short circuit current calculation.
Main Types of Busway
No outlets, used to move bulk power from point to point.
Outlets along the length accept tap off units.
Bars pressed together with thin insulation for compact size.
Bars separated by air and supports.
At medium voltage, non segregated, segregated and isolated phase bus ducts are used, especially between generators and transformers in power plants.
Data centres use busway overhead to feed rack power units, as discussed in data center power distribution.
Bus Duct vs Cable Comparison
| Feature | Bus Duct | Parallel Cables |
|---|---|---|
| Space needed | Small | Large trays or trenches |
| Installation time | Short | Long for heavy runs |
| Future changes | Easy with tap offs | Needs new cables |
| Voltage drop | Low | Higher due to spacing |
| Initial cost | Higher for short runs | Lower for short runs |
| Fire load | Low | Higher insulation volume |
For long feeders above roughly 1000 A, busway often wins on total installed cost once labour, trays and space are included. For small currents, cable is usually cheaper.
Always compare the full picture, including cable derating for grouping, which can force much larger cables than expected.
Bus Duct Voltage Drop Formula
R, X = resistance and reactance per metre per phase
I = load current, L = length
Worked example, 1600 A copper busway:
R = 0.03 mΩ per m, X = 0.02 mΩ per m (example values)
L = 50 m, power factor 0.85, sin φ = 0.527
Vd = 1.732 × 1600 × 50 × (0.0255 + 0.0105) mΩ
Vd ≈ 5.0 V, about 1.2 percent of 415 V
Always take R and X from the maker catalogue for the exact rating and material. Aluminium busway has higher resistance than copper for the same current rating.
The result should meet the limits used in voltage drop calculation, typically a few percent for feeders.
Installation and Maintenance Tips
Keep bus duct routes away from corrosive vapours and water unless the enclosure rating suits the area. Current Midwest notes busway cannot be installed where corrosive vapours are present.
Choose the enclosure IP rating to suit the site, as explained in IP rating. Outdoor sections need weatherproof covers and drainage.
Torque every joint to the maker value and recheck during shutdowns. Thermal imaging of joints under load is the best way to catch loose connections early.
Where Busway Is Used
Busway often feeds a motor control center or several distribution boards from one main incomer.
Fire barriers are needed where the duct passes through walls and floors. Use the maker approved fire stop kits to maintain the building fire rating.
Busway Voltage Drop Calculator
Use catalogue R and X at operating temperature for accurate results. The calculator treats a balanced three phase load.
- Compact and tidy high current routes.
- Quick installation and easy extension.
- Low voltage drop and reactance.
- Low fire load compared with cables.
- Higher cost for short, low current runs.
- Needs accurate route planning.
- Joints require periodic inspection.
- Not suitable for corrosive areas without protection.
Busway Selection and Application Guide
Busway Explained Video
Bus Duct FAQ
Related Articles
- Busbar Sizing Calculation Guide
- Voltage Drop Calculation
- Motor Control Center Explained
- Cable Tray Fill Calculation
- Tier IV Data Center Power Architecture
External References
- Bus Duct Basics, Current Midwest
- SENTRON Busway Guide, Siemens
- Busway Products, Eaton
- Busway, Wikipedia
What We Learn Today
- Busway carries high current in a compact, prefabricated metal enclosure.
- Plug in tap offs make it easy to add or move loads.
- Low reactance gives low voltage drop, but joints need regular checks.