Guys
We're getting way over the top here. NCM railroad stated he had a 15ft bus run. Assuming a 5 amp power supply, 16 gauge is plenty for for his bus line. However, 14 and 12 ga is often cheaper and more available due to its use in house wiring.
Unless the wire has to flex, I personally prefer solid wire as easier to use. But that's me. Either will work just fine.
Shielding is unnecessary, as is wire twisting for the power bus - especially on a 15ft run. Both are nice to do, but given that the rails run precisely parallel for greater distances, taking precautions for the signal in the wiring is unnecessary.
What is important is that the feeder to bus wire connections be made as reliable as possible, and that feeder connections be spaced close enough to overcome electrical connectivity issues with rail joiners (and impedance issues with rail if you are using code 40 rail or smaller).
Overkill is very easy to achieve in this area. I know of a guy who has been successfully running a 5x10 ft layout with just 2 wires from his DCC unit to the track for over a year now. How long will that success last? Probably as long as he doesn't disturb his track. Suitcase connectors have been known to sometimes fail after several years in a hostile automotive or RV environment. How long will it take them to fail on a model railroad without the vibration and wetting that the automotive and RV installs get? And just how long does your layout stay intact before you move or change the layout anyway?
What I'm saying is that feeders every 3-12 ft depending on how good you feel about your rail joiners is good enough. Soldered feeder to bus joints, suitcase connectors, crimped spade lugs and terminal screws, and wire nuts are all reliable enough (if done correctly) for the expected lifetime of the wiring of a not-large fixed layout.
Modular layouts where modules are taken to shows have to take the wiring reliability a notch or two higher. Now, the subtle differences in reliability when being handled frequently and subjected to vibration during transport become important.
Bottom line for NCMrail: If you are one of those guys who likes his wiring neatly bundled, all tagged and color coded, then you will probably feel better about leaning towards more feeders and heavier gauge wire. On the other hand, if you can live with spaghetti wiring under the layout, and use whatever color and type wire is cheap and available, then spacing feeders further apart and using suitcase connectors is probably for you. Both systems will work just fine, becasue the difference in reliability comes down far more to care during installation rather than the reliability of a given technique.
my thoughts, your choices