IEC TR 60890

IEC TR 60890 is a technical report describing a method of verifying temperature rise in low-voltage assemblies by calculation instead of by test. It is the route most manufacturers reach for first because it needs no accredited laboratory. The constraint is that the method is bounded: IEC 61439-1 limits the calculation route to assemblies rated up to 1600 A, with components derated to no more than 80 percent of their free-air rating, and the calculation carries added safety margins to cover the approximation. Effective power losses are calculated from the rated current of the circuits, so diversity enters the calculation route only through the cap that total load current cannot exceed the assembly's rated current. Rated diversity factors belong to the test route.