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How Do You Read a Bus Duct Ampacity Table?

By DHYN Engineering Team  ·  September 7, 2026

Quick answer: A bus duct ampacity table tells you the continuous current each busway size can carry without exceeding its tested temperature rise. To use one correctly you read four things, not one: the base rating at the reference conditions (usually 35 °C ambient, horizontal mounting), the correction factor for your actual ambient, the factor for vertical or edgewise mounting, and the neutral rating if your loads generate harmonics. And one warning before anything else: the bare copper busbar ampacity tables that dominate search results do not apply to enclosed busway. (Answer time: September 7, 2026)

We see the same mistake a few times a year in tender documents. An engineer Googles an ampacity table, finds a bare-busbar chart, sizes the copper from it, and then wonders why the enclosed bus duct built around that copper runs hot. This article is the table-reading guide we wish came stapled to every catalog.

Why bare busbar tables don't work for bus duct

A bare bar in free air radiates and convects its heat straight into the room. A conductor inside a busway housing is wrapped in insulation and steel or aluminum, so it gets rid of heat far less easily. The numbers differ more than people expect. A 1/4 x 6 inch copper bar, roughly 6 x 150 mm, carries about 1700 A in free air at a 65 K rise according to the classic bare-bar tables. Put two of those bars per phase inside a sandwich busway and the assembly is typically rated around 2500 A, not 3400 A. Same copper, same current density on paper, about 25 percent less usable current once it lives in a box. That gap is exactly why IEC 61439-6 requires every busway rating to come from a temperature rise test on the complete assembly, not from a calculation on the conductor alone.

What a real bus duct ampacity table looks like

Here is a simplified version of the table structure you will find in a busway catalog, for a copper sandwich busway at the IEC 61439-6 reference conditions of 35 °C average ambient over 24 hours, mounted horizontally, IP54:

Rated current (A)	Correction at 40 °C	Correction at 45 °C	Correction at 50 °C	Vertical mounting
800	0.95	0.90	0.85	0.95
1250	0.95	0.90	0.85	0.95
1600	0.95	0.90	0.85	0.95
2500	0.95	0.90	0.85	0.95
4000	0.95	0.90	0.85	0.95

The left column is the headline number everyone quotes. The columns to the right are where the rating survives contact with your actual site. Values differ by manufacturer, so treat these as typical and always pull the factors from the specific catalog you are buying.

The five factors that move the number

Ambient temperature. The reference is 35 °C average, 40 °C peak. A plant room at 45 °C knocks roughly 10 percent off. A rooftop run in direct sun can need a further step, which is why our outdoor installation guide pushes sun shields or a one-size derating.

Mounting orientation. A vertical riser stacks its own warm air, so most makers apply about a 0.95 factor to vertical runs. Horizontal edgewise mounting is worse than flat. Check which orientation the base rating assumes.

Altitude. Above about 2000 m the thinner air cools worse, and IEC 61439 practice is to apply a correction. Data halls in Mexico City and mining sites in the Andes both trip over this one.

Harmonics. Nonlinear loads push third harmonics into the neutral, and in a badly balanced office or data hall the neutral can carry as much current as a phase, sometimes more. If that is your load profile, specify a 100 percent or 200 percent neutral and treat the ampacity table as applying to all four conductors, not three.

Temperature rise class. Two busways can both claim 1600 A and not be the same product. One was tested at a 55 K rise, the other at 65 K. The 65 K unit uses less copper to hit the same nameplate number and runs hotter doing it. The rise class is printed in the test data, and it is worth asking for, because a 55 K design gives you margin and cooler joints for the life of the run.

A worked example: reading the table backwards

Say your design current is 1450 A after demand factors, the run sits in a 45 °C plant room, and a 40 meter section of it is a vertical riser. Work backwards through the corrections. Required base rating = 1450 divided by 0.90 for ambient, then divided by 0.95 for the vertical section: about 1696 A. The 1600 A size fails the check. The 2000 A size passes with margin. This is the same selection logic we walk through in the busway sizing guide, and once the rating is fixed you still owe the run a voltage drop calculation and a check against the available fault level, covered in our short-circuit rating explainer. Ampacity settles the thermal question; it does not settle those two.

Mistakes we keep seeing

Sizing from a bare-bar table, as covered above. Reading the headline rating and ignoring the correction columns, so a 2500 A duct installed in a 50 °C tunnel behaves like a 2100 A one. Forgetting that an IP68 cast resin section and an IP54 sandwich section of the same nominal rating shed heat differently. Treating the ampacity as the protection setting: the breaker protects the duct, and the duct rating is not a target to load up to. And finally, comparing two quotes on rated current alone without asking each maker for the temperature rise class behind it. If you are weighing conductor material as part of that comparison, the trade-offs are in our aluminum vs copper busway guide.