What Gage wire do I need for the wiring ? The ballast wiring looks pretty
heavy, but the other items looks like I could get away with something
less.


Anyone?

I have 18 for everything right now and it seems ok (that's what I had on hand). I will change the ballast to 16 or 14 for peace of mind.

I was planning on 16 gauge as well

I also posted the same Q in the home theater forum. Iron Geko posted this:

Thanks Iron !

Wire Current Carrying Capacity
As printed in the National Electric Code 310-17, to follow are the maximum current carrying capacities for THHN type wire. Note temperature rating of the insulation (this chart is 90 degree C insulation rating) and assumes free air application (no conduit) (from OGR thread):

AWG (Gauge) Amperage Rating
18 18
16 24
14 35
12 40
10 55

Related input from Lisa Marie Tahtaras: "When wire heats up, its resistance increases. While this has little affect on a 120V circuit, it makes a lot of difference on a 12V circuit." The recommendation here is use a heavier gauge wire and quality connectors.


Distance/Amperage Chart
AWG wire sizes vs. usable output in amps. Distance is maximum length of wire in one side of the circuit (from Trains.com).

Distance 1A 3A 5A 10A
8 feet 22 20 18 16
12 feet 20 18 16 14
20 feet 18 16 14 12
30 feet 16 14 12 10

*THHN is a stranded single-conductor wire. Solid wire can handle slightly higher amperage but is a royal pain to work with. Bigger wire is always better so if in doubt, use a smaller gauge number. (Yeah, they're backwards. Smaller gauge is bigger wire.)
Hope this helps!

This would seem to indicate that within our boxes 18 or 20 would probably be sufficient given that the AMPS and distances we deal with (<10A and < 8')

Dave