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What Is a Busway Tap-Off Box and How Do You Choose One?

By DHYN Engineering Team  ·  September 23, 2026

Quick answer: A tap-off box, also called a plug-in unit or bus plug, is the device that draws power out of a plug-in busway. Its contacts stab into the busbars at an outlet point, and inside the box sits the protection for the downstream circuit: a fuse switch or a molded case circuit breaker, sometimes with metering. Plug-in types run roughly 16 to 630 A, with bolted units covering higher ratings. Choose one by the protection your load needs, whether you want energy metering, and always by the busway maker's own stab interface, because boxes and ducts are not interchangeable between brands. (Answer time: September 23, 2026)

The busway itself is the boring part of a distribution system. The tap-off box is where people actually interact with it, where circuits get added, and where most on-site mistakes happen. A quick tour of what is inside and how to pick the right one.

What the box actually does

Three jobs in one enclosure. It makes the electrical connection to the busbars through spring-loaded stabs or bolted jaws. It protects the downstream circuit, so a fault in one machine trips that box instead of the whole riser. And it gives the operator a local disconnect and, increasingly, metering for that load. Outlet openings on a plug-in busway sit at regular intervals, commonly every 0.5 to 1.5 m, so loads can be connected wherever the floor plan demands. The difference between this and a plain feeder duct is covered in feeder vs plug-in busway.

Fusible switch or circuit breaker

Two protection philosophies. Fusible tap-offs use a switch plus fuses: simple, very high interrupting capacity, cheap, and unforgiving, because a blown fuse means a spare, a ladder and downtime. Circuit breaker boxes use an MCCB: resettable, adjustable trip settings, easier to coordinate with downstream devices, and more expensive. North American practice leans heavily on fusible bus plugs; IEC markets lean toward breakers. Neither is wrong. Match what the maintenance team already stocks and knows.

Plug-in or bolted

Plug-in boxes clamp or stab onto the outlet and can often be inserted with the busway live, thanks to safety interlocks that keep the contacts covered until the box is fully seated and grounded. That hot-swappable feature is the main reason facilities buy plug-in busway in the first place. Bolted tap-off units connect at the joint or a dedicated point with hardware, need a shutdown to install, and make sense at higher ratings where plug-in stabs reach their limit. Above roughly 630 A, expect bolted.

Metering and monitoring

Modern tap-offs can carry current transformers and a small meter reporting amps, kWh and sometimes power quality per circuit. In data centers this is standard practice, feeding per-rack consumption into the DCIM. Our intelligent busway article covers the monitoring layer, and bus duct for data centers shows how metered tap-offs fit a real hall layout.

Mistakes that come back to bite

Four we see repeatedly. Mixing brands: the stab pitch, phase spacing and grounding geometry differ between manufacturers, and a box forced onto another maker's duct is a fire waiting for a thermal camera. Wrong phase orientation on three-phase boxes, which shows up as reversed rotation at the motor, so mark rotation before energizing. Exceeding the stab rating by upsizing the breaker inside an old box. And ordering boxes before the floor layout is frozen, then discovering the handle faces the wall or the cable entry lands on a beam. Decide ratings, orientation and cable exit direction on the drawing, in the factory it is free, on site it is a rework order.