Laying Out Your Power Bus
I get all kinds of questions from folks about wiring issues and one common one is how main power buses should be laid out under a layout. Most layouts I’ve crawled under follow a pretty simple pattern with the bus running directly below the track and straight out from the booster, with feeders attached to the tracks directly overhead. I have seen a few with a branching pattern where sub-buses run out to feed a yard or short branch. Occasionally some folks get real fancy and install wiring that could be confused as a model of the human circulatory system. Another option is a star-shaped pattern where several main buses fan out from one central location. This is the pattern some manufacturers suggest. For an image of that one just hold out your hand and imagine your palm is the booster and each finger is a power bus, all radiating out from a central position.
The star design works well for a centralized power setup, with all the boosters in one location, but for large layouts a distributed design may be better. With this design the boosters and/or power managers are distributed around the layout near the tracks they power. This approach reduces the lengths of power bus runs and saves on the cost and trouble of installing a lot of extra wire. In addition it reduces your potential problems with inductance related issues such as voltage drops, loss of control, and decoder killer voltage spikes.
But, what about loops–modular clubs use them, and so do individuals. Many small layouts are nothing but loops–sometimes folded over or with branches in or off them. Actually wiring a loop is very easy, if you cut it in two. You can cut gaps in both rails making it in effect one long straight piece of track that just happens to have a big curve in it, with a long power bus feeding it. Or you can use multiple gaps and matching power buses. By cutting it up you are creating blocks that can be better managed with power managers like the PSX power saver series from DCC Specialties.
These configurations should all work and should not be prone to inductance related issues as long as you twist your bus wires, or keep total length under 30′ and amperage under 5 amps. Things start to get dicey when you’re using individual wires that are not twisted or zipped together. Using loose, individual lengths of wire is a great way to create inductance related problems. Why is this a concern? If the two wires are placed tightly alongside one another then the electromagnetic signal tends to cancel out inductance.
Another issue is electronic noise cancellation, both coming from the bus wires and from other electronic sources in the room. When the two bus wires are tightly twisted it will tend to reduce the effects of this noise. This electronic noise not only affects the DCC signal in the power bus, it also can impact on signals being carried in adjacent wires such as your throttle bus, and that is also a good reason for keeping about 6″ between them.
For my power bus wires I have been using a 100′ spool of 14 ga zip cord designed for use as speaker wire. Larry Meir, who designs all those neat devices like the RRAmpmeter and the PSX power managers, has told me that he measured the DCC signal on a 100′ length of zip cord speaker wire and saw no effects of inductance. Larry feels the close proximity of the two wires in the zip cord acted the same as if they were twisted pairs and inductance was eliminated.
So why would zip cord be a better choice than twisted pairs? Well, it saves all the work involved in getting those long sections of individual wires twisted properly. When twisting the wires you need to make sure that they actually are twisting evenly, instead of one wire merely wrapping around the other. The idea is to have equal lengths of wire in as close a proximity as possible, and twisting will do that, but so will zip cord. Zip cord also saves some money since a twisted pair of wires is about 1/3 shorter than the original untwisted pairs. Finally I figure that a lot of folks will be more likely to use zip cord than having to twist their own wires, and if they do that’s a good thing in my book.