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Isolated Phase vs Segregated vs Non-Segregated Bus Duct: What Is the Difference?

By DHYN Engineering Team  ·  September 21, 2026

Quick answer: These are the three constructions of metal-enclosed bus defined by IEEE C37.23, used for medium voltage and very high current feeders. Non-segregated bus duct puts all three phases in one shared enclosure with air between them; it is the cheapest and the most common for switchgear and transformer connections up to about 38 kV. Segregated bus duct adds metal barriers between phases inside the common housing, so a fault on one phase cannot easily become a phase-to-phase fault. Isolated phase bus duct gives every phase its own separate enclosure and is reserved for generator main leads at power plants, carrying 10 to 30 kA or more. For ordinary building distribution below 1000 V, none of these apply; that world runs on IEC 61439-6 busbar trunking. (Answer time: September 21, 2026)

Buyers outside North America meet these terms late, usually on a power plant or large industrial tender, and the vocabulary is confusing because "bus duct" here means something quite different from the sandwich busway in a shopping mall. Same words, different product family, different standard. Here is the clean separation.

The family: metal-enclosed bus under IEEE C37.23

Low voltage busway is governed by IEC 61439-6 or UL 857 and tops out at 1000 V. Metal-enclosed bus picks up above that: 4.16 kV, 13.8 kV, 24 kV and beyond, at currents from 1200 A to well past 30,000 A. IEEE C37.23 defines the constructions and the test regime. All three types use bare or insulated conductors on support insulators inside a grounded aluminum enclosure, usually outdoor-rated, connecting generators, transformers and medium voltage switchgear where cables would need impossible bending radii and huge parallel runs.

Non-segregated phase bus duct

All three phase conductors share one enclosure, separated only by air and their support insulators. It is compact, simple and the least expensive of the three, which is why it carries most of the medium voltage market: transformer secondary to switchgear lineup, tie connections between boards, feeds to large motors or rectifiers. The trade-off is inside the name. With phases sharing a housing, a foreign object, heavy condensation or an insulator failure can escalate into a phase-to-phase fault. On a well-designed, well-maintained indoor installation that risk is acceptable, and decades of service history back it up.

Segregated phase bus duct

Same shared outer housing, but continuous metal barriers divide the interior so each phase sits in its own compartment. The barriers block the classic escalation path from single-phase fault to multi-phase fault, and they reduce the chance of a tool or a loose part bridging phases during maintenance. The cost sits between the other two. You see segregated construction specified on critical industrial feeders, utility auxiliary systems and anywhere the owner has been burned once by a phase-to-phase event and refuses to have another.

Isolated phase bus duct

Each phase conductor gets its own fully separate tubular enclosure, with air space between the three housings. A phase-to-phase fault becomes essentially impossible, and the enclosure design manages the large induced currents that come with generator-level loading: the housings are electrically bonded and shorted at the ends so circulating currents cancel the external magnetic field. This is the bus you see leaving a generator at 15 to 27 kV and 10 to 40 kA, walking across the plant to the step-up transformer. It is engineered equipment, often force-cooled at the top ratings, and nothing about it is a catalog item.

Choosing between them

	Non-segregated	Segregated	Isolated phase
Enclosure	One housing, all phases	One housing, phase barriers	One housing per phase
Phase-to-phase fault risk	Possible	Largely blocked	Essentially eliminated
Typical duty	MV switchgear and transformer links	Critical industrial and utility feeders	Generator main leads, power plants
Relative cost	Lowest	Middle	Highest, fully engineered

The decision usually writes itself. Standard 13.8 kV plant distribution is non-segregated until the owner's reliability policy says otherwise. Anything feeding a unit where an outage costs real money per hour tends to segregated. Generators get isolated phase because there is no serious alternative at those currents. Whichever type lands on your drawing, ask for the short-circuit rating with the same rigor you would on a low voltage feeder; our article on bus duct short-circuit ratings explains Icw and Ipk in plain terms, and what is a bus duct maps the low voltage side of the family if that is what your project actually needs.