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UL 857 vs IEC 61439-6: What Is the Difference for Busway?

By DHYN Engineering Team  ·  September 28, 2026

Quick answer: UL 857 is the North American product standard for busways, used alongside NEMA BU 1 application guidance and NEC installation rules. IEC 61439-6 is the international equivalent used across Europe, the Middle East, Asia and most export markets. Both verify a busway through testing, but they differ in temperature-rise limits, short-circuit test methods and how enclosure protection is rated. Specify whichever standard your project's authority having jurisdiction recognizes.

If you buy or sell busway across borders, you will meet both standards on the same project eventually. Here is a practical comparison, without the standards-nerd fog.

Who governs what

In the US and Canada, busways fall under UL 857, with CSA C22.2 No. 27 as the Canadian twin. Listing by UL or another NRTL such as Intertek (ETL) is what inspectors look for. Almost everywhere else, the governing document is IEC 61439-6, the busbar trunking part of the IEC 61439 low-voltage assembly family, which replaced the old IEC 60439 series back in 2014. We have a separate article explaining IEC 61439-6 itself if you want the deeper dive.

Temperature rise

Both standards cap how hot the busway may run, but they measure it differently. IEC 61439-6 works through the general rules of IEC 61439-1, whose Table 6 limits terminals to a 70 K rise and lets the insulation class set the ceiling for the bars. UL 857 applies its own temperature-rise table with thermocouple measurements on a specified test setup. The numbers look similar on paper but are not directly convertible, because the mounting, ambient conditions and test currents differ. Treat each standard's rating as valid inside its own system.

Short-circuit ratings

Under IEC, a busway carries a rated short-time withstand current (Icw, typically one second) together with a peak withstand value. Under UL 857, the busway is marked with a short-circuit current rating in kA, proven by test at a stated voltage. When you compare datasheets across the two systems, make sure you are comparing the same kind of number. A "65 kA" UL rating and a "65 kA Icw" IEC rating are cousins, not twins.

Enclosure protection: IP versus NEMA

IEC busways are rated with IP codes per IEC 60529: IP54 for indoor distribution, IP65 or IP66 for wet or dusty duty, up to IP68 for resin cast products. North American practice uses NEMA enclosure types per NEMA 250: NEMA 1 and 12 indoors, 3R for outdoor rain resistance, 4 and 4X for hose-directed water and corrosion. There is no exact conversion, but the rough working equivalents look like this:

Application	IEC rating	Closest NEMA type
Indoor, clean electrical rooms	IP54	NEMA 12
Outdoor, rain exposure	IP55–IP66	NEMA 3R / 4
Washdown or corrosive areas	IP66 + coating	NEMA 4X
Buried or submerged duty	IP68 (resin cast)	No direct NEMA equivalent

If the IP system is new to you, our IP54 vs IP65 vs IP68 article walks through it.

Which one to specify

Follow the project, not your preference. North American jobs need listed product to UL 857, full stop. Export projects to the IEC world need type test reports against IEC 61439-6. And if you serve both markets, ask the manufacturer early whether they hold both certifications, because retrofitting a design from one standard into the other is expensive. DHYN builds to IEC 61439-6 with full type test documentation, and we are happy to discuss North American certification requirements on a project basis.