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The two main bus wire are not separated. They are either twisted or kept close together. The comment about being 6 inches apart refers to the main bus verses the throttle bus. Those two bus's should not be close together.
Contributed by Mark Gurries"If you have two loosely routed bus wires that carry the booster current out to the track and back, there are electrical benefits of twisting these two wires together into what is called a twisted pair. The first benefit you get happens when you place the two loose wires that carry equal but opposite AC current side by side to each other. When we say equal but opposite, think in terms of an AC power extension cord where we have a hot and neutral wire. Power goes out and back on these two wires. This side by side pairing reduces the wire's inductance which is a property of wire that creates noise and voltage spikes when they interacting with this type of AC power and electrical layout hazards we have with DCC. However this inductance reduction can only happen when the wires are super close to each other as in NO air space or gap between them. Just pure wire insulation. This is the type of wire construction found in your everyday AC power cord. Inductance reduction is very sensitive to the spacing and the closer and tighter it is, the better the results. Romex cable has too much spacing to offer much benefit. Zip cord is better but still has extra spacing down the middle where one does the "Un Zipping". Two individual wires, preferably different colors, from spools will work best. See section on RC filters/Terminators for additional tips and techniques.
Maintaining this very tight mechanical relationship over the entire run length of the pair is the key to success. If you're starting with two loose wires, such as from a spool of wire, the best way to achieve that goal is to twist them together to form a "twisted pair". The twisting action mechanically keeps the two wires close to each other all the time and forms a single cable that you can route around the layout easily.
Twisting these same two wires together also provides a second electrical benefit. It will reduce the twisted pair's ability to both spread electrical noise to and pickup electrical noise from OTHER wires nearby. In a sense, twisting the two wires together forms a shielded cable so to speak. The tighter the twist (higher twist rate per foot), the better the shielding effect you get.
These are the two reasons why telephone and high speed communication (Ethernet) wires are built in the form of twisted pair. Given that DCC track wiring is both communication and power all on one wire cable, layout wiring can benefit from this type cable technology.
Let's be practical about this. It is understood that when you run a track bus in the form of a twisted pair, you must untwist portions of the wire to permit one to make connections. Small/short untwisted sections will not ruin the overall benefit. The goal is to keep the far majority of the wire run twisted.
What to do if you have excessive inductance on an existing layout:
You don't have much choice. You either twist the bus wires or limit the length of your bus feeders from the booster to 30' (10m) or less. I don't like this any better than you do I'm an existing layout owner, too!"
You are correct in that a detection area the main bus should not be twisted.
"Caution: If you use twisted pair wiring within a detection block, it may contribute to false occupancy events. It is best to keep the wiring untwisted and keep that wiring as short as possible at the same time. This is best achieved by placing the block detector near the block it is going to detect as apposed at some central wiring panel. Running twisting cable up to the block detector is perfectly fine. Twisting after the detector is not recommended. See above diagram. "
At 12:53 PM 2/23/2014, you wrote: