Since you are looking for the PDF, here is a high-level summary of the six-step process you will find inside the official document:
Certification bodies (such as UL, CSA, VDE, or TÜV) accept the TR 60890 method as a valid verification route for assemblies built according to IEC 61439. Auditors will expect to see the calculation sheets. iec tr 60890 pdf
A common frustration: Searching "IEC TR 60890 PDF free download" yields many sketchy websites offering outdated or counterfeit copies. Here is the legal, safe, and accurate path: Since you are looking for the PDF, here
The report provides formulas to calculate the effective cooling surface area ( A_b ) (in m²). For a freestanding enclosure, it considers all sides. For wall-mounted units, the area against the wall is neglected. Here is the legal, safe, and accurate path:
IEC TR 60890 provides conservative estimates (±10–15%) compared to actual tests. It is accepted by IEC 61439-2 for PTTA verification (Clause 10.10.1) but not for fully type-tested assemblies (TTA) unless combined with test extrapolation.