Title say's it all.
None really. Just keeping them close together helps.
https://dccwiki.com/Wire_Sizes_and_Spacing#Twisting:_Is_It_Necessary.3F
The main purpose is to be neat, and reduce the inductance/impedance as much as possible. The difference between twisting and closely spaced wires isn't much, as shown in the above link.
Some say it's to improve the signal quality, to which I say nuts, at the DCC frequencies anyway. Plus you have it connected every 3-6 feet to a set of parallel conductors that by design are an EXACT distance apart and perfectly parallel - the track.
There are at least as many people running layouts using 2 conductor #12 landscaping wire for the bus as you individual conductors and either twist or don't twist them together.
Twisting can add inductance, which with an AC signal causes a slight power flow - so twisting wires beyond a block detector will likely result in false occupied readings even when no trains are present.
I think it's far more important to keep the track bus away from any AC power, or any low poower control signals under your layout, and make sure to have any crossings between types of wiring systems accur as close to 90 degrees as possible to avoid the stronger signal fron inducing current in the weaker one. In a grid type benchwoork - DCC in one bay and the signal bus in the next bay would usually be fine. If they need to cross over one another, have one make a sharp 90 degree bend, cross over the other line perpendicular to it, and then make a sharp 90 degree bend back if it needs to continue in parallel but on the other side.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Our club used an NCE five amp Power Pro. We ran #14 flexible speaker wire about forty feet and had no issues. No terminator either. That wire was tough to seperate, strip and attach feeders to while laying in a creeper with a raised back we had made.
We use to use Harbor Freight multimeters to view the DCC, about 13.6 VAC and one DCC EE online said the meter will not show spiking, so I used a Scope to view the DCC. No spikes.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
Twisting or keeping wires close together will not increase inductance, it will decrease it, and the burden seen by the booster. Inductors resist change in current flows.
Twisting the wires together increases the parasitic capacitance between them, leading to some current leakage, which is why you should not twist bus wires before and after a current transformer used for sensing occupancy. The capacitance could cause enough leakage current to result in false triggers.
True. Compared to a fixed 2 conductor wire, like landscape cable - not so much, but compared to two wires just kind of strung along in the same vicinity - a lot. I jsut had it backwards.