An old rule-of-thumb (used in Sweden) is that one should reduce RMS current to around 70% of the standard (single cable) value. There are much more detailed tables available, but I wouldn't use them unless there are long cables, which justifies the extra mental effort.

Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.

We need the current carrying capacity [ampacity] of a circuit in order to fit a protection device. If we take the NEC 2014 recommendations Art. 310.15 Ampacities for Conductors Rated 0–2000 Volts. (2) Selection of Ampacity. Where more than one ampacity applies for a given circuit length, the LOWEST value shall be used.
Following all other standard, since the protection device ratings is connected with maximum current carrying capacity[ampacity] in the actual conditions then it is clear one has to comply with the NEC recommendations in any way.So the lowest ampacity has to be in the indoor position and this has to be taken into consideration.

You should consider the worst situation

If Derrated ampacity on conduit > Derrated ampacity on ladder, use first value, if not, use the other

Agree with JuanBC - you have to take the worst case, meaning the lowest current rating. I think you probably know that, but aren't happy about it.