Home  ›  Blog  ›  Busway Expansion Fittings

When Does a Busway Need an Expansion Fitting?

By DHYN Engineering Team  ·  September 21, 2026

Quick answer: A busway needs an expansion fitting in two situations. First, on long straight runs where thermal growth outgrows what the joints can absorb; most manufacturers set that threshold somewhere between 40 and 60 m. Second, wherever the run crosses a building expansion or seismic joint. Copper grows about 16.5 µm per meter per kelvin and aluminum about 23, so a 60 m run cycling 40 K moves 40 to 55 mm. Give that movement nowhere to go and it turns into bowed sections and loose, hot joints. (Answer time: September 21, 2026)

Thermal expansion is boring physics right up until it opens a joint. We get called to sites where a bus duct has visibly snaked sideways between hangers, or where joint bolts that were torqued at commissioning now turn by hand. Nine times out of ten the run was a long one, installed straight and rigid, with no provision for movement. An expansion fitting is a cheap part that prevents exactly this.

What an expansion fitting actually is

It is a short busway section, usually 1 to 1.5 m, built with sliding or telescoping conductors. The bars on one side overlap the bars on the other through flexible braided links or spring-loaded sliding contacts, so the housing can lengthen and shorten by typically plus or minus 25 to 50 mm while carrying full rated current. Electrically it is just another part of the run. Mechanically it is the one place the duct is allowed to move. Everything else about it, the rating, the insulation, the IP grade, matches the rest of the line.

Trigger 1: long straight runs

The math is simple and worth doing once for your project. Linear growth equals length times temperature swing times the expansion coefficient:

Conductor	Coefficient (µm/m·K)	30 m run, 40 K swing	60 m run, 40 K swing
Copper	~16.5	~20 mm	~40 mm
Aluminum	~23	~28 mm	~55 mm

Forty kelvin is not exotic. It is the difference between a cold warehouse at commissioning and a summer afternoon at full load. Outdoor runs see more; our outdoor installation article covers fittings every 40 to 60 m for exactly that reason. The published rule from most catalogs: fit an expansion unit once a straight run passes roughly 40 to 60 m, and follow the specific manufacturer's number because joint designs differ. When in doubt, add the fitting. It costs far less than the rework.

Trigger 2: building expansion and seismic joints

The second trigger has nothing to do with the busway heating up. Large buildings are split into structural blocks that move independently, and the joints between them can shift tens of millimeters vertically and laterally during settlement, wind sway or an earthquake. A rigid bus duct crossing that line takes the movement as bending stress, and the duct always loses. Wherever the route crosses a structural expansion joint, the drawing should show a flexible or expansion section at that exact point. In seismic zones this is not optional detail; it is part of the bracing design, the same way the hanger layout is. Our piece on busway hangers and supports explains the support side of that picture.

What about vertical risers?

Tall risers grow too, often more than horizontal runs because they are long and fully loaded. The difference is that a riser absorbs movement through its supports rather than a dedicated fitting. Spring hangers at each floor carry the weight while letting the duct extend, and the stack grows upward a few millimeters per floor. Clamping a riser rigidly top and bottom is the vertical version of forgetting the expansion fitting, and it fails the same way: stressed joints that run hot on the first thermal scan. More on this in vertical vs horizontal bus duct runs.

The mistakes we keep seeing

Three come back constantly. First, long feeders in logistics halls and factories installed with no expansion unit because "the catalog run was only 80 m and it looked fine." It looks fine for about a year. Second, a rigid standard section bolted across a building joint because nobody checked the structural drawings; the civil engineer's joint and the electrical route are on different sheets, and nobody overlaid them. Third, using an expansion fitting as a correction tool for misalignment. It is designed for axial movement, not for taking out a 30 mm installation offset, and forcing it sideways destroys the sliding contacts. If a run is already in and running hot at the joints, the fix starts with a thermal scan and torque check, then a conversation about where movement should be allowed. Our joint overheating guide covers the diagnostic side.