DCC Bus Wiring on Model Railroads:
To Twist or Not?

As we prepare to begin construction of a new layout for our new Nottawasaga Model Railway in our newly renovated premises at Bygone Days in Collingwood, Ontario, our members are in the midst of discussions about everything from the the trackplan to scenery construction. A major topic concerns DCC bus wiring. Should the wires be twisted or not? There is considerable debate about this topic throughout the world of model railroading. It seems to be more of a concern with DCC than with DC layout wiring. Following is a transcript of emails among club members about DCC bus wiring that you may find both interesting and useful when building your own model railroad. Club member, John Houghton, took the lead on doing the research about DCC bus wiring.

[John H]:

I did the research and the consensus is to twist the DCC Bus wires. A loose twist will work fine. The reason is to reduce interference both in transmission and reception on the bus.

If you want the full details, read the link below that summarizes all the points, and then some.

http://www.dccwiki.com/Wire_sizes_and_spacing#To_Twist.2C_or_Not_to_Twist 

Some interesting points from my research: In the above link it is recommended that if current detection systems are being used for detecting trains then do not twist the DCC power bus lines. Run two parallel lines separated by 2” and not more than 6” apart. Do not twist the feeders either when using current detection. I found no reference on the NCE site or in the command station manuals about how to run the bus. Their site did reference a link that does recommend twisting the bus.

https://sites.google.com/site/markgurries/home/dcc-general-best-practices/wiring-planing/twisted-pair-wiring

The Digitrax site does not reference the issue either. However, in some of their product manuals, they show bus wires as parallel lines. I think more for clarity rather than an indication of how to. I found one photograph on the Digitrax site that showed the wiring installation of a small module, with the main bus wires parallel and separated by about 2"-3”. The Digitrax Loconet bus should not be twisted. Some have tried to use ethernet cable for Loconet and the twisted pairs inside cause major issues with Loconet. The use of 6 wire telephone cable is all that can be used on Loconet. The Loconet bus should be 3”-4” away from any DCC buss lines.<

The Model Railroad Hobbyist Forum site and articles in their digital magazine recommend twisted pair bus lines.

Of more importance is the selection of the proper size of bus wire and and feeder wires to the track. For HO gage and the size of our layout, the main bus should be AWG12 and feeders AWG 18-22. Insulated, stranded wire is recommended primarily for easy in stringing and routing the wire. The first link above has a chart on recommended wire sizes for distance of run. Termination of the bus wires is not recommended so runs are left open at the end and insulated for safety.

Additional findings, especially for our layout, is the use of power districts and additional boosters. Spreads the current loads around the layout, reduces the hazards of high current boosters and melted equipment in case of a short. Allows more engines to be used on the layout without over taxing the system. Circuit breakers to protect for shorts and power boosters for load distribution to be considered.

Additional points: Stringing Twisted pair is a lot easier and takes up less space. If multiple bus lines are running parallel to each other, separate the twisted pairs by 2”. Analog bus lines, as in 5VDC and 12VDC need not be twisted, However, I twist mine to reduce the potential of DCC interference being received on these bus lines and less need to filter out any potential unwanted signal before powering digital equipment like Arduinos, electronic controls, etc.

[Bill H]: I never twisted my bus wires but I did try to keep them apart, usually about a foot or so. However wires to different districts often ran close to each other. I only separated the + from the -. I never had any problems but maybe I was just lucky. I did have some screwy things happen with some of my signals that might have been a result of DCC interference. Never did find the cause. I used current detection. I haven't wired much of the new layout yet. Busy building bridges!

[Bill P]: Martin and Doug. Did we not use Ethernet wire for some of our loco net wiring?

[Martin]: We did, and we still have some lengths of co-axial made up, but we don't use it on the exhibition layout any more. Perhaps we should throw it out.

I think we have to give serious consideration to John's comments. His layout runs flawlessly and it seems that twisting the bus wires is not uncommon.

The italicized first two lines in the DCCWiki article perhaps say it all.

"This topic is fiercely debated . . . endlessly and without any real solid conclusions . . . " It seems to me that if twisting the bus wires is such a no-no then it wouldn't be debated and if it were then there would be definite conclusions."The same section goes on to say, in point 2, "There are two reasons for twisting the pair of wires together . . . This become really important with long runs."

I think we can safely say we'll have some long runs on the new layout. In the second paragraph under "Mutual Inductance" it states the wire only need be twisted 3 to 5 turns per metre. This is about once every 8 to 12 inches. Under "Twisting: Is it Necessary?" it says in the first sentence that you should twist if your bus is going to be 30 feet or more. Lastly, under "Other Opinions" it states that slightly twisting the wires together will virtually eliminate radio interference. Makes me wonder whether we shouldn't revisit the wiring on the exhibition layout. Perhaps this is one of the causes of our intermittent wonky running at shows. My vote is to lightly twist.

[Ken D]: Does twisting mean using alternating holes every 16" (cross members) or is the twist actually wrapping the wires around each other (touching)?

[John H]: Twisting means twisting the wire pair around each other first. When stringing the wire, the twisted pair will then go through one hole in the framework. The number of twists per foot, meter, need not be many as indicated in the links I provided. The point is, even with a minimal number of twists, the advantage to reduced interference is increased over non twisted wire. Too tight a twist and it is hard to add and solder the feeder wires.

With 12AWG it is a matter of fixing one end of the wires together, in a vice for instance, and simply hand twisting the wires around each other. If there is a hand drill with a large enough chuck to grab the other end of the wire pair, twisting is a little simpler. Then string the bus through the layout.

 I use 12AWG coloured wire on my bus with Black being the (-) and the coloured being the (+). So power districts are: red/black, white/black, blue/black, green/black.  Yes, the DCC signal has a polarity so hooking up a power district backwards will cause a short. 


More DCC bus wiring resources

If you're ready to learn more about dcc bus wiring and try some circuit wiring for yourself, you might like to check out this site. Part 1 of the program is a free, downloadable tutorial so you can try before you buy. 

Click here for Twisted Pair Electronics.

Should you like to contribute to our conversation, go to my Contact Me page and send me your thoughts so I can add them here or join the conversation by posting your comments below.

Go to model railroad wiring for some helpful tips.

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