Managing voltage drop on long feeders — busduct vs cable
On long horizontal runs and rising mains, voltage drop quietly limits how far you can push the load. Here's how busduct compares to cable and where each makes sense.
Al Sanaya Engineering
·6 min read
IEC and most local codes ask designers to limit steady-state voltage drop to roughly 4% from source to load. On a long horizontal feeder or a tall rising main, that constraint usually drives the conductor selection more than the continuous current rating does.
Why busduct beats cable on long runs
A sandwich-type busduct has very low loop impedance because the phase conductors are tightly coupled and the housing returns flux. Compared to a parallel cable run of the same current rating, the per-metre voltage drop is typically 30–50% lower at unity power factor and the difference grows at lagging power factor.
That means you can either cover more distance for the same drop, or use a smaller conductor for the same distance — and reclaim shaft or tray space.
Where cable still wins
- Short, direct runs — cable beats busduct on installed cost
- Routes with many bends or branches — cable handles geometry better
- Highly congested ceiling spaces with no straight run available
- Single-load circuits (motors, individual panels) below the busduct break-even rating
How we approach the trade-off
On every project we run the voltage-drop calculation both ways at the design stage. The cross-over point depends on length, current, power factor and copper price — but for vertical risers above 30 m or horizontal feeders above 50 m at typical commercial loads, busduct almost always comes out ahead on total installed cost once labour, fittings and shaft space are included.
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