Re: Busbar Ampacity
Both Charlie and Jim are right. If you have seen a formula, it is most likely a "rule of thumb."
The standard for bus is neither UL nor NEMA, but ANSI/IEEE C37.23. The "formula" for calculating the ampacity is right up there with Neher-McGrath calcs. I don't have a ready copy, but as I recall, there are approximately 7-10 variables, including the "shape" of the bar itself.
In general, the ultimate rating is determined by a temperature rise test against a specific standard rating. In other words, if a continuous 1200A through a particular configuration, at specified ambient temperature, doesn't result in a rise above 90C, then it is a 1200A bus.