I plan use DCC on my next layout, it'll be my first time with it. I picked up a book on wiring for DCC, and it notes that the wires for the main power bus should be parallel in order to avoid creating interference.
The illustration in the book-- as well as several pictures I've seen posted online-- looks like drawing A, below left.
My probably silly question is: is there a reason to do it like that, instead of like B, which is how I'd do it for a DC layout?
I think you may have misunderstood. The arrangement at B is for long runs where the signals can get corrupted with parallel runs. So, for bus runs nearing 30', say, it wouldn't hurt, according to my own understanding, to do some wraps of the one wire around the other. For runs between 0-20 feet or so, parallel wires are highly unlikely to interfere with the DCC signal.
Others will offer their understanding too, but this is mine.
I only kept my bus wires apart to make it easier to connect the feeders. It's pretty obvious when one bus lead is under the right hand rail and one is under the left which feeder connects to which bus wire. Cross-capacitance and crosstalk are probbaly not an issue with short runs on small layouts, but over a longer distance it might come into play. Larger scale, but pay attention to how the wires are arranged on high tension towers - it's NOT random and NOT for convenience. Of course, with maybe 15 volts amx there's not going to be any arcing with DCC track bus wires - at least, I'd HOPE not. But arcing isn't the only factor, if the wires are improperly oriented in a power line it will cause a greater transmission loss. This can happen in lower voltage situations as well but again, if unless your layout fills a barn and you run more then 50' bus lengths (which you shouldn't have even if your layout DOES fill a barn), there should be no problems.
--Randy
Modeling the Reading Railroad in the 1950's
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I've heard pros and cons on twisting the bus wires. Believe it may be warranted on long runs. Say over 50 feet. I still look at the track and wonder how to twist the rails.
We use zip wire on our NTRAK layouts.Can't get any closer together or parallel than that. Even in Louisville this summer with over 500 modules filling 50,000 square feet, our longest power district was 80 feet. With the booster placed at the center of a district, that's still only 40 feet to the end of the bus.
What should be observed is to try and keep the command bus a foot or so from the tatrack power bus. If it has to cross, do so as close to 90degrees as possible.
Martin Myers
Same with any other cables runnign around the layout, particularly ones carrying detection information. They should be kept away from runnign parallel with DCC track power, and cross at 90 degrees to avoid induced currents and false signals.
And true, no matte rhow you twist the bus wires, or keep ot as far apart as practical, you can;t change the fact that it's all wired to a parallel bus with a small fixed distance between the conductors - also known as the 'track'.
I placed the buss wires for each of my blocks about 1.5 inches apart and run them below the layout with feeders coming up through layout to the tracks and haven't had any interference problems.
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I normally bundle wires into cables as much as I can, all the separate wires wrapped up together and fastened to a single "line" for neatness' sake.
If I understand the above responses correctly, there's no need to keep the two wires of the main power bus separate, I can bundle them as I normally would, but I should keep data-carrying wires away from them. Correct?
For the most part, that would be accurate. I wouldn't bundle AC power wires with the track power either - such as a power feed to structure lamps. And if you are using Digitrax Transponding, or Railcom, I wouldn't bundle different detection sections together - when the decoder is transmitting back it is now a signal carrying wire - and a relatively weak one at that.