Busbar Heating: Ampacity to IEC 60287 and Faults to IEC 60865-1

Two different questions a busbar has to answer

A busbar (or any heavy conductor) has to survive two very different thermal duties, and they're governed by different standards:

• Continuous ampacity — how much current it can carry indefinitely without the metal exceeding its allowable operating temperature. This is a steady-state heat-balance problem, and IEC 60287 supplies the AC-resistance piece of it.
• Short-circuit withstand — how hot the bar gets during a brief, enormous fault current before protection clears it. This is a fast transient where the bar has no time to shed heat, and IEC 60865-1 gives the adiabatic method for it.

Continuous rating: it's a heat balance, not a table

At steady state the bar sits at whatever temperature makes the heat it generates equal the heat it sheds. Heat generated is P = I²·R_ac (current is RMS, which is defined precisely so this gives the correct average heating). Heat shed is natural convection plus radiation from the bar's surface, in series with any coating or overmould's conduction resistance (t/(k·A)). Balance the two and you get the operating temperature; find the current that lands it exactly at the allowable limit and you have the ampacity.

The subtlety IEC 60287 handles is the AC resistance. At DC the whole cross-section carries current evenly, but at AC the current crowds toward the surface (skin effect) and toward or away from neighbouring conductors (proximity effect), raising the effective resistance above the DC value. IEC 60287-1-1 gives the empirical skin-effect factor kₛ that corrects DC resistance to the AC value actually used in I²R. (For a bar built from several sections of different width, the heat also conducts axially between them — a fin-type problem — but each section still balances I²R against its own surface losses.)

Short-circuit: the adiabatic assumption (IEC 60865-1)

A fault current can be tens of kA for a fraction of a second. Over that time the bar simply can't shed a meaningful fraction of the heat — so IEC 60865-1 makes the adiabatic assumption: all the I²R energy goes into raising the metal's temperature, none escapes. That turns a hard thermal problem into a clean closed form relating the final temperature to the current density and the fault duration, using material constants (for copper K ≈ 226 A·√s/mm² and β = 234.5 °C; for aluminium 148 and 228 °C). Because conduction is negligible over a fault, each section is checked on its own.