The AOLP session covered some of the same topics, more about UL 1838 and how it applies to what we do.
The 12ga wire issue was an eye-opener for me as well. Here's how I now see it: (please fill me in if I'm wrong...I don't want to go to the next conference and see I've screwed it up for another year!)
You cannot use wire that is rated at less than the current rating of the circuit protection of the circuit. So, 25A breakers need to be at least 12ga.
The fixture wires are much smaller (18ga or 20ga sometimes), but should be minimized (trimmed off) to not be excessively long.
25' fixture leads are fine IF they are tied into a hub that is fused, with the fuse being rated at no more than the load rating of the wire. So, if using 16ga 25' leads, your hub fuse cannot be more than 12.5A (the amp rating of 16ga wire) correct?
This is the problem I have with certain manufacturers that do not provide secondary protection at the transformer. If using a 600w transformer and you only have a 5A fuse on the primary, would you not need to use wire rated at no less than that amount on the secondary, or 50A, which would mean you can use no less than 8ga wire? Maybe thats why I have seen so many of them burn up 12ga wires.
In order to be able to use 12 guage main runs, and 16ga branch runs, you'll need a 25A breaker, with a 12.5A fuse at the hub, which would in effect be tertiary circuit protection (primary being the panel breaker, secondary being the 25A low-voltage breaker).
Now, does the 80% rule not apply to this? If so, the breaker would have to be rated for 20A to use 12ga wire would it not? Therefore any load on the 25A breaker would need to be on 10ga or larger wire runs?