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How Much Clearance Does a Busway Need in an Electrical Shaft?

By DHYN Engineering Team  ·  September 23, 2026

Quick answer: A busway shaft needs three clearances, not one. Allow roughly 100 to 150 mm of free space around the housing for installation and heat dissipation, 350 to 600 mm in front of the tap-off outlets so plug-in boxes can be inserted, opened and operated, and clear access at every joint for torque checks. If the tap-off box counts as equipment that needs servicing, North American jobs also apply the NEC 110.26 working space rules, typically 914 mm (3 ft) deep at 600 V. (Answer time: September 23, 2026)

Shaft space is the most expensive real estate in a high-rise, so every project tries to shrink it. That is fine until the busway arrives and the tap-off handle cannot clear the wall. We have seen risers that fit perfectly on the section drawing and failed on site because nobody separated the three different kinds of space a bus duct actually needs.

1. Clearance around the housing

The duct itself needs room to go in and room to breathe. Sections come in 2 to 3 m lengths and have to be maneuvered upright inside the shaft, joined, and then torqued. Most manufacturers ask for about 100 to 150 mm of free space around the housing as a practical minimum. Less than that and the installer's hands, the torque wrench and the joint covers simply do not fit. This gap also lets the housing shed heat. A sandwich bus duct relies on its enclosure as a heatsink, and a duct pressed tight against concrete runs hotter than its test report promised. Our temperature rise article explains why that margin matters.

2. Working space in front of tap-off outlets

This is the one that gets forgotten. A plug-in tap-off box projects outward from the duct face, often 300 to 500 mm once its cover is open, and the electrician has to stand in front of it to insert it, close the breaker and read the meter. Plan for 350 to 600 mm of clear space on the outlet side as a floor, and more where boxes are large. On projects built to US practice, NEC 110.26 treats that tap-off as electrical equipment requiring examination and servicing: the working space table calls for about 914 mm (3 ft) of depth at up to 600 V under normal conditions. Local codes differ, but the physics of a person standing in front of a live box does not.

3. Access at joints

Every joint is a future inspection point. Commissioning requires torque checks and insulation tests, and annual maintenance includes a thermal scan of each joint. If a joint lands inside a slab penetration or hard against the shaft wall, nobody will ever check it, and joints are exactly where bus ducts fail first. The rule of thumb we give designers: keep joints at least 300 mm away from slab edges and walls, and stagger them so no joint sits inside the fire barrier zone. The fire stopping detail itself is covered in our fire wall penetration guide.

Do not forget the vertical dimension

Clearance is not only horizontal. Leave room at the top of the riser for the end feed unit and for thermal growth, because a long riser expands upward and needs somewhere to go. At the bottom, keep the first section clear of any zone where water collects; shafts leak from above and puddle at the base. And remember that spring hangers at each floor need working room too. The support side of the layout is covered in busway hangers and supports, and the full riser picture in what is a rising main busbar trunking system.

The mistakes that shrink shafts after the drawing is done

Three patterns repeat. The shaft is sized around the duct dimensions alone, and the tap-off side is left with 100 mm because the section view looked spacious. Civil tolerances eat the margin: a wall cast 50 mm out of true is common, and it comes straight out of your clearance. And late value engineering swaps a wider shaft for a narrower one without telling the electrical designer. Our advice is boring but works: mark the tap-off operating space on the architectural drawings, not only on the electrical ones, and get the shaft dimensions confirmed before the busway goes into production, not after.