e57 I will take a stab at your questions.
Keep in mind these are just my opinions and I have nothing to back it up.
So why is it you can have up to 30 conductors in any cross-section of gutter/wireway before the need for derating? (under 20% fill of course)
My guess is that the conductors will not be in a tight bundle like in a raceway or cable.
Why is it derating not applicable under 24"?
What about all those conductors on both sides of every panel?
What about junction boxes?
I believe the answer to those 3 question is heat conductivity.
Copper along with being a very good electrical conductor is an excellent conductor of heat.
With unbundled conductors on each side of the nipple, Jbox or panel acting as heat sinks the up to 24" long section of conductors that are tightly packed can not get much warmer than the conductors on each side.
Do switch legs and travelers count?
IMO they do although I doubt it matters much as generally derating does not get in the way for 15, 20 and 30 amp circuits until you exceed 9 conductors.
Ever wonder why 3 or more conductor cable is triplexed? (Twisted clock-wise) PS - you can get 12-8 w/ Gnd MC! And all are twisted the same way.
There may be other reasons but that 12-8 MC (which along with 10-8 MC we use often) would be much stiffer to bend without the twisting.
If all the conductors were simple parallel in the cable when you tried to bend the cable it would be difficult as the conductors on the outside of the bend need to be longer than the conductors on the inside of the bend.
By putting the twists in the cable it makes the bend much easer.
Try bending eight solid 12 AWGs taped together in a parallel bundle.
Then try the same thing with a twist.
Even quicker experiment, try to bend a 12/2 NM cable on its side. In other words with one conductor on the outside radius and one conductor on the inside radius.
Unless the conductors slide by each other or stretch it is imposable.
Bob
[This message has been edited by iwire (edited 03-18-2005).]