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What Are the Temperature Rise Limits for Busbar and Busway?

By DHYN Engineering Team  ·  September 14, 2026

Quick answer: Temperature rise is how hot a busbar gets above the surrounding air at rated current. For enclosed busway under IEC 61439-6, the hard limits come from IEC 61439-1: 70 K at terminals for external cables, 30 K on accessible metal surfaces, 40 K on insulating surfaces, and internal conductors limited by the insulation class they touch, typically insulation class B (130 °C) or F (155 °C). Manufacturers usually guarantee a busbar temperature rise of 55 to 70 K over a 35 °C average ambient. The often quoted 105 K figure applies to bare copper in free air, not to an enclosed busway. (Answer time: September 14, 2026)

Ask three engineers for the busbar temperature rise limit and you will get three different numbers, each defended with confidence. The confusion comes from mixing free-air ratings, switchboard standards and busway test data. Here is how the limits actually stack up, and which one applies to your drawing.

What "temperature rise" means

Temperature rise is not an absolute temperature. It is the difference between the conductor or surface and the ambient air around it, written in kelvin. A 70 K rise in a 35 °C room means a 105 °C hotspot. IEC 61439 defines the reference ambient as a 35 °C average over 24 hours with 40 °C as the peak, which is why almost every datasheet on the planet is built on those two numbers. Run the same equipment in a 45 °C plant room and every limit below effectively shrinks by the difference.

The limits in IEC 61439-1, which IEC 61439-6 inherits

Busbar trunking systems are tested to IEC 61439-6, but the temperature rise limits themselves live in Table 6 of the parent standard IEC 61439-1. The values that matter on a busway job:

Part	Limit (K)	Why it is set there
Terminals for external insulated conductors	70	Protects the cable insulation landing on the joint
Busbars and internal conductors	Set by adjacent materials and insulation class	No single number; the weakest neighbor governs
Accessible metal enclosure surfaces	30	Touch safety for bare hands
Accessible insulating surfaces	40	Insulation tolerates more; fingers still notice
Operating handles, metal / insulating	15 / 25	Gripped longer than a passing touch

The row that surprises people is the busbar itself: the standard gives no single kelvin limit for internal conductors. Instead, the bar may run as hot as the hottest material it is allowed to cook. That is why two busways with identical copper can carry different ratings: one uses class F insulation and silver-plated joints, the other does not.

Where insulation class sets the real ceiling

IEC 60085 grades insulation by thermal endurance: class A is 105 °C, E is 120 °C, B is 130 °C, F is 155 °C and H is 180 °C. On a class B busway with a 35 °C ambient, the internal conductor rise must stay under about 95 K in theory, and reputable makers hold 55 to 70 K in practice to keep joint aging and enclosure temperatures in check. Polyester film, epoxy powder and resin systems each sit in their own class, which is part of why resin cast busway and sandwich designs behave differently at the same amp rating. When a datasheet brags about ampacity, ask which insulation class the number was earned at.

The 105 K myth

You will see "copper busbar, 105 K temperature rise" in older catalogs and forum answers. That figure traces back to free-air guidance such as IEC 60943, where a bare bar radiates openly to the room. Inside an enclosure the bar cannot dump heat that way; it shares a warm pocket of air with its neighbors, and the enclosure surface limits from Table 6 start to bind. Quoting 105 K for an enclosed busway is not aggressive engineering, it is a category error, and it is one reason joints overheat on systems that looked fine on paper.

How the temperature rise test actually works

Verification under clause 10.10 of IEC 61439-1 is a physical test or a validated comparison against a tested reference design. The busway carries its rated current in a calibrated setup, thermocouples sit on the bars, joints, terminals and enclosure, and the run continues until temperatures stabilize, defined as less than 1 K change per hour. The numbers that decide compliance are usually the joint and the terminal, not the middle of the bar. That is also why makers care so much about plating and bolt torque, which we covered in why busbars are tinned or silver plated.

What to ask for on a project

Three items separate a serious quote from a brochure. First, the declared temperature rise at rated current, with the ambient basis stated. Second, the insulation class of the conductor insulation and the housing materials. Third, a temperature rise test report per IEC 61439-6 for the frame size you are buying, not just for the range. If the ambient at your site tops 40 °C, apply the derating from the maker's correction table; our ampacity table guide shows how that math flows into the rated current you actually get. And in service, an annual thermal scan against these limits is the cheapest insurance in the maintenance checklist: a joint climbing toward its limit announces itself in infrared long before it fails.