DCC Complete, Track Wiring

Track Wiring

Main Power Bus

Power to run our model locomotives has to be put on the rails or track from the Command Station or Booster. We use wires to do this, as the interface or connection between the Booster and track. Since the DCC system allows for controlling multiple locomotives individually on the same piece of track, we do not need to separate our wiring into "control blocks" as we have done when using DC control. (Don't confuse the "control block" wiring that previously used toggle switches, with DCC block type wiring or Power Districts, which will be explained as we go along.) This makes the task of wiring our model railroad layouts simpler. However, because the DCC system is capable of providing more current to the track than our old DC system, the size of the wires used in various places is different.

The key to insuring that the wiring on any layout is correct for a DCC system is to perform and pass, a shorting test, also called the "Quarter test". All DCC Command Station Boosters have built-in current protection. If the current protection device, usually an electronic circuit breaker built into the unit, does not trip and turn off the track power, the wiring is not sufficient in conductor size. The test then, is to place a quarter on the track at various places around the layout and see if the circuit breaker trips and shuts down the track power. So what happens if the wiring is too small? If a short occurs on the layout, the small wire size will not allow enough current to flow from the Booster to the short, and the circuit breaker will not trip and will not stop the current flow. If the current flow does not stop, it is possible to melt or damage some parts of whatever is causing the short or, if left on a long time, burn the insulation off of the wires, which in turn may cause a fire. For that reason, it is important that the wiring be of the right size (or larger) and be done correctly.

Different size layouts have different wiring needs. However, all layouts require a Main Bus of some type. This bus is actually the main trunk line and is of heavier wire that runs under the layout and starts at the Command Station Booster and usually follows the track around the layout. If you have a small layout that has a circle of track, do not connect the bus together at the ends to make a loop. Also, do not connect the track together electrically to make a loop. Insulate the ends of both rails with an insulated rail joiner. The gaps both in the track and bus wiring should be at the farthest distance away from the Booster. The reason we do this is, if the bus and track were in a loop, the DCC signal could be corrupted because the signal coming from both directions at the same time could cancel parts of the signal out, and control of our locomotives may become intermittent.

So the main question now becomes: "What size wire do I need for My power bus?"

Power Bus Wire Size

Small layouts can use smaller wire. That doesn't mean telephone wire, which is 24 gauge, or AWG 24 (American Wire Gauge). That means smaller than AWG 12. (Typical modern house wiring is three conductors of solid AWG 12 in a cable called Romex.)

A small layout, which is 8 feet by 12 feet or smaller can use 16 gauge wire. Layouts larger than this should use AWG 12. A layout that will fit in a single car garage can use AWG 12. A layout that fills a two-car garage can also use AWG 12, but should be broken up into separate Power Districts, each with their own Booster.

You may well ask at this point, Why such large wire? Two main reasons. The larger the wire, the less resistance. And, the larger the layout, the more locomotives we put on it, thus the more current needed. (This is also the reason some layouts need separate power districts with their individual Boosters.) Since current travels down the wire, the longer the wire, the more resistance. So you see, if the wire size is large, the resistance will be less.

Others have gone into great detail and technical explanations as to why the bus wires should be a certain size, so I am not going to repeat them here. After all, this is supposed to be a hobby. The rule of thumb is: If the layout does not pass the quarter test, and/or the locomotives slow down when farthest away from the Command Station or Booster, the wire size is too small, or you don't have enough feeders from the Bus to the Track. (Smaller wire from the track to the Power Bus.) This now brings up another point. You have the bus wires in place but they are too small.

How do you fix it, short of tearing out the wires and replacing them? By running a parallel set of wires of the same gauge or larger. When you add parallel wires, you effectively increase the current capacity and reduce the resistance. You are now using two wires as one. When you run the second set of wires, you do need to connect them to the old wires (observing the polarity or phase so they will not be shorted) at about every three to six feet. There is a non technical rule of "Three" that can be used when running parallel wires. If both wires are the same size, say AWG 16, subtract 3. You get 13. Thus the two parallel AWG 16 wires are equal to a single AWG 13 wire.

Another factor to consider is, should the bus be solid wire or stranded wire. Again the rule of thumb is, if the wire will be flexed, bent, or under vibration, use stranded wire. If the wire will be installed and not be continuously flexed, it can be solid. Wiring in buildings is solid, wiring in automobiles is stranded.

Because our Bus wires are usually AWG 12, in most cases they will not fit into the track connections on most Boosters. If this is the case with the system you have, you will need to connect a pair of smaller shorter wires between the main Bus and the Booster. These wires should be as large as possible and still fit into the Booster connection. AWG 16 will usually work. These wires should be six inches or less in length, and should be soldered to the Bus wires, or connected to the Bus through a terminal block.

Power Districts

The purpose of a power district is to increase the power on the layout tracks of a large layout. One Command Station Booster is usually not enough to power a large layout as found in a two-car garage or larger.

A Power District is an independent section of layout bus wiring which is connected to or powered by a Command Station or Booster. As an independently powered section, if a short occurs in one power district, only that section of layout is affected. The remaining section will still keep running.