I suspect that table is simply based on normal UK weather conditions and the traditional lack of air conditioning - almost anywhere in the UK will experience air temperatures up to 25ºC at some point in the year - often rather higher (hence the usual default of 30ºC). Nothing stopping you extrapolating if you need to though.

   - Andy.

rotten said:

Hi all, I'm doing a design of a circuit where the specified ambient temperature of the building is 20C.

Before deviating from the standard approach in BS 7671, it might be worth thinking a little further. For example:

• Who specified this temperature and how is it to be maintained?
• If the ambient temperature is to be maintained by HVAC or another system, are circuits interlocked to curtail load when the cooling fails?
• Whilst the ambient temperature in rooms might be maintained at this level, is that ambient temperature maintained in all 'services' areas such as risers, above ceilings, etc.?

In addition, moving down from even 30 to 20 possibly won't have much impact on current-carrying capacity ... but if you end up selecting a lower csa cable, that would have an adverse effect on voltage drop (although volt-drop resistance-per-metre is also lower for a lower temperature). Certainly for standardized 'general purpose' circuits like the ones shown in the OSG, when RCDs are used for ADS, volt drop is actually the deciding factor ... in which case, since volt-drop is dependent on temperature too, you might get a few more metres from the same CSA, so for example if the volt-drop is assumed at 70 deg C for ambient of 30, you can adjust to conductor temp of 60 deg C for an ambient of 20.