Track buses for Digital Command Control

Track buses for Digital Command Control: Layouts wired for direct-current block control have several isolated track sections called blocks. Each block has an electrical switch that controls which DC throttle powers that track section. In theory, a Digital Command Control (DCC) layout could be wired as one large block with two wires feeding the power directly from the DCC booster to the track. However, on any model railroad larger than a test track, more wiring is necessary if you want your trains to run well.

Since DCC can power more than one locomotive at a time, more current is required to pass through the rails than with DC control. Small DCC systems usually have boosters that can supply at least 3 amps. Bigger systems can supply 5 amps, and large scale model railroads use boosters that can supply 10 amps. In DCC the voltage remains constant, and each locomotive draws as much current as it needs at the time. A train going up a grade draws more current than a locomotive running by itself on level track. Locomotives in larger scales draw more current than those in smaller scales. The sum of all the current to each locomotive is what the DCC booster supplies. A medium sized HO layout with just a few trains with three locomotives each can approach the capacity of a single 5 amp booster.

So how do you get the current to the trains? Nickel silver rail is a poor conductor of electricity and acts like a resistor. The longer the rail, the more resistance there is. Following Ohm’s Law, the more current going though a resistor, the larger the voltage drop from the booster to the locomotive.

I measured a piece of code 83 nickel silver rail and found the voltage drop was .057Ω per foot. That doesn’t sound like much, but on anything but the smallest layout, it adds up quickly. At a full 5 amps, one piece of 3-foot flextrack fed at one end can cause a voltage drop of 1.7 volts at the other. How? Because the current must travel three feet down one rail, pass through the locomotive, and return three feet down the other rail. Imagine what it would be like on the other side of a large model railroad.

Feeding a layout with only two wires may not only affect locomotive performance, but it could be destructive. Since DCC boosters can supply more current than a typical DC throttle, they’re equipped with internal circuit breakers that trip and remove power from the rails when a short circuit occurs. If not wired properly, the voltage drop through the rails may be so great that the booster isn’t able to detect a short. The full current capacity of the booster passing through whatever is causing the short can be destructive enough to melt a brass truck frame.

Fortunately, copper wire has a much lower resistance than nickel silver rail. The solution to the voltage drop problem is to run a track bus of heavy gauge copper wire beneath the layout, roughly parallel to the track, and smaller gauge track feeders from the bus to the rails. The size of the wire required depends on the length of run and the capacity of your booster. There’s no set maximum voltage drop that’s acceptable, but it has to be low enough so that the booster circuit breaker trips when a short occurs.

Each DCC manufacturer has different recommendations for acceptable wire size and length of wire runs. Different boosters may detect short circuits at different levels of voltage drop. It’s important that the wiring is capable of handling the current output of the booster and that the booster circuit breaker will trip.

For boosters that supply 5 amps and less, 14AWG for runs of less than 30 feet is fine. If you have a longer run or larger booster, consider using 12AWG or larger. You can determine the worst-case voltage drop by multiplying the wire resistance by your booster’s full current capacity. There is a handy calculator for this online at www.cirris.com/testing/resistance/wire.html. Enter the length of the run and bus wire size to determine the wire resistance. Remember to double the value to account for the voltage to the locomotive and the return.

There is a debate about whether solid or stranded wire is better for track buses. Either one will work. Digital Command Control is an AC waveform, so skin effect (AC flowing only through the outer layer of a conductor) will cause a slightly larger voltage drop in solid wire, but not enough to make a difference.

It’s important to color code your bus wires to make sure that the same bus wire goes to the same rail throughout the layout. I use red for one rail and black for the other rail. From the track bus, smaller gauge track feeders carry the current to the rail. I use 22AWG on my HO layout. This would be impractical for an N scale model railroad. For short runs between the bus and the rail, the gauge of the track feeder isn’t as important as the track bus gauge. Good electrical and mechanical contact to the rail is what’s important.

Nickel silver rail is a poor conductor, but rail joiners are worse. Over time they will loosen and the rail will oxidize. It’s a good idea to use feeders for every piece of rail and solder rail joints on short
sections of rail.

Digital Command Control signals are square waves, and consequently generate a lot of noise at all sorts of frequencies. This can cause interference with a number of electronic devices, including your DCC throttles through the DCC cab bus. Though not a hard and fast rule, the track bus should be routed a foot or so away from the cab bus.

Another way to cut down on interference is to twist the bus a few times per every foot or so. Don’t twist the bus so much that it is difficult to attach the feeders. Twisting the bus also has the effect of increasing the DCC signal fidelity. At the end of long buses, the square wave of the DCC signal can become distorted and cause excessive voltage that could potentially damage a decoder. NCE Corp. suggests placing a snubber at the end of a long bus. Digitrax doesn’t recommend using a snubber on its system.

As you wire your layout, frequently test your work by shorting the rails together at different locations. Your DCC booster should shut down. If it doesn’t, check to make sure you have enough track feeders. Dividing your track bus (and the track above it) into electrically isolated sections that are powered by additional boosters or protected by DCC circuit breakers will make it easier to diagnose short circuits.