Ambient Temperature Correction Factor

I would be inclined to use an ambient temperature of 30 degrees for an outdoor  installation. That in my view represents the highest temperature that is regularly achieved. I do not feel that the presumed ambient temperature has to reflect the highest temperature ever achieved, but the highest likely/regularly achieved temperature.

For a large or complex installation, it would in some cases be reasonable to apply engineering judgement and consider not just the load in amps, but also the nature of the load. If the load is primarily street lighting or decorative lighting, then a lower ambient could reasonably be presumed on the grounds that operation during the heat of the day in very unlikely.

On the other hand, if the load is primarily air conditioning or refrigeration, then I might be more cautious and allow for a 40 degree ambient on the grounds that the load is likely to be greatest during an exceptional heatwave.

For a large or long hour load. consider the cost of losses in a marginally sized cable, this may suggest significant upsizing.

For a domestic outbuilding, loads might include space heating and a clothes dryer, significant use of which is improbable in a heatwave.

Finally, it might not matter much ! on a small and simple job, going up cable size from say 4mm to 6mm is affordable so as to remove any doubt or concern. On a large job, it is likely that cables will sized according to voltage drop rather than current rating.