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What Is a Sandwich Bus Duct?

By DHYN Engineering Team  ·  July 19, 2026

Quick answer: A sandwich bus duct is a compact type of busway where the copper or aluminum busbars sit flat against each other, separated only by thin insulation, inside a metal enclosure. The tight "sandwich" layout makes the duct smaller, cooler running and better protected than older air-insulated designs. It is the most common bus duct in modern buildings.

Open up a catalog from any busway manufacturer and the first product you will see is usually a sandwich bus duct. There is a reason it took over the market. Here is what it actually is, and when you want it.

What "sandwich" means

In older air-insulated ducts, the busbars hang apart with air gaps between phases. In a sandwich design, the phase bars are pressed together in one flat stack, each wrapped in epoxy film or similar insulation, and the whole stack slides into a steel or aluminum housing. There is almost no empty space inside. That is the entire trick, and nearly every advantage follows from it.

Why the industry moved to it

Size, mostly. Packing the bars flat makes the duct far smaller than an air-insulated equivalent, so it fits into tight shafts and ceilings. The tight stack also conducts heat out to the housing efficiently, which lets the duct run cooler at the same current. And with no air gaps, there is less room for dust and moisture to creep in, so a sandwich duct easily reaches IP54, and IP65 with the right joints.

There is an electrical bonus too. The flat, closely coupled bars have low reactance, which means lower voltage drop on long runs and better behavior under short-circuit forces.

Typical ratings

Most sandwich bus ducts on the market run from about 250A up to 6300A, at 400V or 690V, built to IEC 61439-6. Copper is the usual conductor; aluminum saves money on the larger sizes if you can accept a slightly bigger duct.

Where you would use it

Vertical risers in high-rise buildings, horizontal distribution across factory floors, feeds between a transformer and a main switchboard. Indoors, in dry conditions, it is the default answer. Outdoors, or anywhere wet, dusty or corrosive, look at a resin cast busway instead, because sandwich construction is not meant to be submerged or blasted with weather.

What to check before buying

Ask for the insulation class and temperature rise test results, the IP rating including at the joints, and the short-circuit withstand rating. On paper every supplier looks the same. The type test reports are where the differences show up.