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Busway Thermal Derating: The Four Factors That Shrink Your Ampacity

By DHYN Engineering Team  ·  September 30, 2026

Quick answer: The ampacity on a busway datasheet assumes a 35°C average ambient, indoor installation, horizontal mounting at low altitude. Four factors shrink that number in the real world: higher ambient temperature, altitude above 2000 m, vertical mounting (around 5%), and harmonic-rich loads. Multiply the factors together, and if the result falls below your load current, step up to the next size.

Nobody gets in trouble for the duct that is one size too big. Plenty of projects get in trouble for the duct that was sized off the headline rating and installed on a 45°C roof. Here is how the derating factors actually work.

What the nameplate assumes

Under IEC 61439-6 the rated current is verified by temperature-rise test at a reference ambient: 35°C average over 24 hours, with peaks up to 40°C. Indoors, in still air, mounted as tested. Every real site that differs from those conditions eats into the margin, and the four corrections below are the ones that matter. Our temperature rise limits article explains the test itself.

Ambient temperature

This is the big one. A busway sheds heat through its housing, and warmer air carries less heat away. Manufacturer derating curves typically knock off roughly 1 to 2% of ampacity per degree above the reference ambient, which means a duct on a Middle East roof at 50°C average can lose 20 to 30% of its headline rating. If the run is outdoors in a hot climate, read outdoor busway installation requirements alongside this, because sun shields and resin cast options change the picture.

Altitude

Thin air cools worse. IEC 61439 assumes installations up to 2000 m, and above that the reduced air density lowers both cooling and dielectric strength. Manufacturers publish altitude correction factors; as a working figure, expect to lose several percent of ampacity per 1000 m above 2000 m. Mexico City, Johannesburg, Bogotá, half of western China: all above the line. Check before you quote the standard rating.

Mounting orientation

A duct tested flat does not behave identically on edge or standing vertical. Vertical risers run a touch warmer because the chimney effect inside the housing concentrates heat at the top, and the common industry correction is about 5%. We covered the details in vertical vs horizontal bus duct runs. Flatwise versus edgewise horizontal mounting can carry a small correction too, so check the datasheet footnotes rather than assuming.

Harmonics

Nonlinear loads add harmonic currents that heat the bars without showing up in the fundamental current your clamp meter reads on each phase. NEMA BU 1 guidance and manufacturer data both point the same way: where THDi is high, derate the duct or oversize the neutral, often both. The neutral side of the problem is in bus duct neutral sizing.

Putting the factors together

Factors multiply, they do not add. Worked example: a 2000 A duct, 45°C ambient (factor 0.90 from the maker's curve), vertical riser (0.95), high harmonic load (0.95). That gives 2000 x 0.90 x 0.95 x 0.95, about 1625 A. If your design current is 1600 A, you are at the edge with zero growth margin, so step up to the 2500 A frame. The full sizing sequence, including short-circuit and voltage drop, is in our bus duct sizing calculation.

Mistakes that keep coming back

Using the bare busbar ampacity from a handbook instead of the enclosed duct rating, which we warned about in how to read a bus duct ampacity table. Adding the factors instead of multiplying them. And derating the duct but forgetting the tap-off boxes, which have their own temperature limits. When in doubt, send us the site conditions with the inquiry. Derating at the factory is a line on a datasheet; derating after installation is a replacement order.