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How Do You Ground a Busway System?

By DHYN Engineering Team  ·  August 31, 2026

Quick answer: You ground a busway in one of three ways: through its metal housing, if the manufacturer has tested and rated the housing as a ground path, through an integral ground bar built into the duct, or through an external ground bar bolted along the outside. Whichever method you use, the ground must be carried across every joint and every tap-off box, and you should prove it with a low resistance continuity test before energizing. (Answer time: August 31, 2026)

A busway looks like one continuous metal tube, and that fools people. Electrically it is a chain of three meter sections, painted steel or anodized aluminum, joined by bolted packs. Paint insulates. Anodizing insulates. If the joints do not carry the ground, the middle of your run is floating, and a fault there has nowhere clean to go.

Option 1: the housing as the ground path

Under UL 857 in North America, a busway enclosure can serve as the equipment grounding conductor if the design has passed the standard's continuity and short-circuit tests as a ground path. Aluminum housings have plenty of cross section for the job. In IEC markets, specifiers are more conservative. Many will accept housing grounding only with test evidence under IEC 61439-6, and some refuse it outright on the grounds that a future maintenance crew might loosen a joint and silently break the ground. That concern is not paranoid. We have seen it.

Option 2: an integral ground bar

The integral option puts a dedicated ground conductor inside the duct, alongside the phase bars, bonded through every joint pack by design. The common sizes are 50 percent and 100 percent of the phase cross section. A 50 percent ground satisfies most fault duties, while 100 percent shows up in data centers and anywhere sensitive electronics make ground noise expensive. It adds a little cost and a little width to the duct, and it removes the argument about whether the housing can be trusted. On IEC projects this is the option we quote by default.

Option 3: an external ground bar

An external bar bolts to the outside of the housing and links every section with visible connections. Inspectors like it because they can see and torque every bolt. Retrofit crews like it because you can add it to an existing run without opening a single joint. The downside is mechanical: an exposed bar on a low run gets hit by forklifts and ladder feet, and a damaged external bar is a broken ground waiting to be noticed.

Method	Cost	Inspection	Watch out for
Housing as ground	Nothing extra	Hard, joints are hidden	Needs manufacturer test evidence; joints must be designed for it
Integral ground bar	Adds a few percent to duct price	Moderate	Confirm the bar is bonded through every joint pack, not just at ends
External ground bar	Cheap, retrofit friendly	Easy, fully visible	Mechanical damage on low runs

Joints are where grounding succeeds or fails

Whatever path you choose, the joint is the weak link. Bonding plates and serrated washers exist to bite through paint and oxide into bare metal, and they only work if the joint bolts are torqued to the manufacturer's value, which is exactly the kind of step that gets rushed on a Friday afternoon. Expansion joints need flexible bonding straps, because the whole point of an expansion joint is movement, and a rigid bond there will crack. Tap-off boxes need their own bond to the ground path. So do end flanges where the duct meets the switchboard or transformer enclosure. A busway that is grounded at the supply end and nowhere else is a busway that is not grounded.

Test before you energize

The acceptance test is simple: a low resistance ohmmeter, or a duct tester, from the ground point at the supply end to the housing at the far end, and then to every tap-off location. You want a low, stable reading, typically a fraction of an ohm for the whole run, and your manufacturer can tell you the expected value per joint so you can spot one bad connection hiding in a chain of good ones. This test belongs in the same commissioning round as the insulation resistance check we describe in how to test a bus duct before commissioning. After that, the earth fault loop impedance test confirms your protective device will actually trip on a ground fault, which is the entire reason the ground path exists.

Mistakes we keep seeing on site

Bolts driven through powder coated steel with flat washers and no serrations, so the ground connection is paint. Expansion joints installed without bonding straps. A middle joint left loose during a modification, isolating everything downstream while the supply end tests fine. Tap-off boxes hung on the duct with no ground bond because the box is metal and "touching the housing" was assumed to be enough. And housings used as the ground path on designs that were never tested for it. Every one of these passes a visual inspection and fails the day a fault needs that path. Ten minutes with a continuity meter finds all of them.

If you are specifying a new run, decide the grounding method at quotation stage, not on site. It changes the duct you buy, the joints you get, and the tests you can demand. Our team covers this in every design review, and the annual retorque and retest schedule is in our busway maintenance checklist.