derating factors for enclosed conductors

I am hoping someone will be able to give me the specific derating factor for the current carrying capacity of conductors enclosed in air. I also need the source of the info ie ref to a standard.
I have looked at a number of cable handbooks and standards and codes and have found heaps on enclosed conductors in thermal insulation and overall derating factors but have not been able to pin down this specific factor. eg of application: internal cabling run in wiring duct of a metalic switchboard.
I need this to extrapolate data from manufacturers handbook.
i hope someone can help me out.
Oskin

Rating of busbars

While there are is plenty of information out there on the sizing of busbars for electrical distribution, I am after some rather specific info regarding the rating of tinned or silver plated busbars.
I read somewhere once that silver plated busbars can be uprated by a factor due to the fact that the surface does not oxidise.
Does anyone know what this factor is?
Why?..Is it due to the skin effect with ac current or simply the impedence increases due to the oxidisation.
And I would like some kind of reference or standard that I can quote to back this up if this is the case.
Some flexible busbar manufacturers have much higher current ratings that you would expect for the csa of the copper, and I have seen references to that being due to this oxidisation effect.
And finally would tinning have the same effect if this is true?