Wire Sizes and Spacing

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Selecting an Appropriate Wire Gauge

Wiring is central to the proper operation of your Digital Command Control System.

But no topic seems to elicit more debate or more misunderstandings than wiring. This article will attempt to explain some of the rationale behind DCC Wiring Practices. Most of the recommendations are based around technical reasons and Best Practices which have been found to work reliably.

Can You Use Wire That is Too Heavy?

It might not appear to be so, but experience has shown that because you can run more trains with more locomotives Digital Command Control, you usually will.

This means your electrical loads will be higher for a given layout. In addition, a voltage drop of just 2 volts is a big issue with Digital Command Control, and you can't compensate by cranking the throttle open a little more. Keeping these factors in mind, it's clear you will need heavier wire. A small table top layout can reduce these sizes a little bit without problems, but larger home or club layouts should adhere to these suggestions - you'll appreciate it in the long run. It's cheaper to do it right the first time than it is to tear it out and do it over!

The important issue with wire gauge is resistance. Heavier wire (a smaller gauge number) has less resistance than light gauge wire (with a larger gauge number.)

The resistance causes energy loss when current flows through the wire. This is lost through heat. The result is a drop in voltage. This is expressed as I^2R loss (I Squared R). When current flows through a resistor, a voltage drop is created, equal to the current multiplied by the resistance. The energy loss equals the current squared, times the resistance.

Basically, if a loop of wire has one ohm of resistance, and you pass 1 amp through it, you will see a loss of one volt. If you pass 5 amps, your loss is 5 volts You will also convert 25 watts of energy to heat. If the resistance is doubled, that voltage loss increases to 10 volts, and the power loss increases to 50W.

For the most part, the resistance of copper wire is so small that it is usually expressed as ohms per 100, or 1000 feet. For a short run of a few feet, the resistance is negligible. But it becomes an issue for a long run of wire. Remember, an equal length of wire is needed to complete the circuit, doubling the resistance of the circuit. High resistance caused by inadequate wiring can also prevent the proper operation of the booster's over current protection circuit, resulting in damage to track, rolling stock, and even your booster.

Keep in mind that one of the reasons for using heavy wire is to minimize effects caused by the nature of the DCC signal. While the DC resistance is quite low, the AC Impedance is not the same, and may be higher. In fact, a heavy AWG 12 wire may be equivalent to a 20 AWG wire when used with the digital DCC signal. (Remember, electrical codes are written with 60 Hz (or 50 Hz) alternating current in mind. Also, thou shalt not exceed 80% of the rated current capacity of the circuit in actual use.) This will introduce new problems in terms of voltage drop.
'Note
Recommended Wire Gauges Shown in Table Are For Copper Wire Only.'



Wire Size Guidelines
Wire sizes are American Wire Gauge (AWG)
BUS LENGTH
FEEDERS
1-20 FT
21-40FT
40+ FT
Up to 5 FT
Up to 10 FT
SCALE
G (1:20.3 / 1:29)
Gauge 1 or I (1:32)
O (1:32)
S (1:64)
12
10
8
16
14
HO (1:87.1)
14
12 - 14
18-22
18-20
TT (1:120)
14
20
N (1:160)
Z (1:220
16
14
12
18-22
18-20
N-Trak
12
18-22
18-20
N-Trak is based on the N-Trak Wiring RP for DCC http://ntrak.org/ntrak_powerpole_rp.htm


An important concept that is often ignored is the way the circuit protectors work. They are not sensitive to the amount of current. They react to the rate of change in current. A sudden spike will trigger them.
Poor wiring will interfere with their operation. Which could result in damage to your locomotive, or its decoder, because too much current was flowing, resulting in excessive heat. Or a short occurs, and something melts because the power was not interrupted. This happens because the wiring isn't heavy enough, interfering with the rate of change the booster sees. Or the voltage drop demands more current to maintain the same amount of power. A change from 1A to 1.5A in draw will also increase the power dissipation by more than twice.
Good heavy wiring goes a long way to preventing problems like that. Using AWG 14 or heavier is not overkill for a bus, and track feeders of AWG 18 are not that heavy. Choosing wire because it is cheaper or easier to hide is just asking for problems.

Wiring

Track Bus

Bus wires carry the power from the booster to the track, but don't directly connect to the track. Feeder wires handle that task.

Below is some information to get the most of your system but using the correct wire types, gauge, and installation methods.

You can use either solid or stranded wire. Stranded wire is more flexible and will better handle repeated bending. (Repeated flexing will anneal the copper and make it brittle, which can break).

To Twist, or Not to Twist

This topic is fiercely debated on internet forums and DCC related mailing lists, endlessly and without any real solid conclusions. Seems everyone has an opinion, just like they do with wire sizes…

To Twist or Not to Twist, That is the Question…

There is always a large debate on the twisting of track bus wire. Keep in mind that DCC track buses are an Unbalanced Pair: One wire (A) is held to ground while the other (B) is energized, then they flip when A carries a signal and B is held to ground. This is happening constantly.

There are two reasons for twisting the pair of wires together:

  1. Reduce interference, both caused and received by the DCC system. Your track bus can be thought of as a large antenna.
  2. Reduce mutual inductance between the two wires forming the bus. This becomes really important with long runs.

Interference

If your DCC system is causing electrical noise, this is one way to reduce it. Since many people don’t listen to AM Radio anymore, you might be radiating a large amount of RF energy without realizing it. Since TV is now in the digital domain, you won’t be yelled at for introducing noise into the picture either.

Interference could be interpreted by loco decoders and could cause havoc on the system. This interference can come from outside the layout, or be caused by the bus wires themselves or nearby signal buses. It also can reduce any interference the track bus may cause by inducing a signal in a low power signalling bus, such as your throttle network or occupancy detection system. If you do twist the wires and have a detection system, the section of bus being monitored should not be twisted, and the wires spaced apart, to reduce capacitance between them. Otherwise a small leakage current can cause issues by raising the noise floor..

Mutual Inductance

When a pair of wires makes a long run parallel to each other, the one wire induces currents in the other. This was discovered in the early days of telephony. In fact, Alexander Graham Bell discovered that by twisting the two wires together, interference and inductance were reduced, meaning the signal was stronger and clearer at the other end of the line.

One way to reduce the inductance is to space the wires apart. The further apart, the better, but, just to save you some time and space, more than 15cm or 6” is overkill. The other way is to twist the wires, with about three to five twists per metre. Doing so alters the phase relationships between the two wires, reducing any induced currents in the wire.

The negative side of the coin is that attaching feeder wires to the bus wire could get complicated if you are not consistent with your colour coding, however, the wires are not twisted a great deal so it shouldn't be too difficult.

Is It Necessary?

Insert Heated Debate Here

You should consider twisting your bus wires if they are going to be 30 feet or more. Twisting is more important on outdoor railroads where runs can sometimes be a 100 feet or more. If you compare two long and widely spaced wires (inches apart) with two long but closely spaced wires (1mm apart), the former have a lot less inductance which is better. The amount of inductance you have in your wire directly relates to the degree of DCC waveform distortion and other problems such as large voltage spikes.

Large voltage spikes are created during intermittent short circuits caused by derailments or other electrical track issues. Inductors store current in a magnetic field, and when the current increases or decreases, it resists that change by inducing a voltage in the wire. That voltage, according to Lenz’s Law, with be positive when the current increases, or negative when it decreases.

Should a short occur, the current will suddenly increase and ‘’charge’’ the inductor. When the current is cut off by the overcurrent protection circuit, the magnetic field collapses and the inductor rapidly discharges into the circuit. This causes a voltage spike to appear. This is exactly what happens in your car’s ignition system: The ‘’coil’’ is in fact a transformer, and current flowing through the low voltage coil creates a magnetic field, which is storing energy. The ‘’points’’ or a switch then opens, the current stops, and the magnetic field collapses, which creates a huge voltage in the secondary coil. This voltage is thousands of times greater in magnitude than the 12V used in the primary circuit, which has enough potential to break across the gap in the spark plug. Televisions with a CRT used much the same trick to create the high voltages needed to energize the CRT.

Relays and solenoids do the same thing, which is why they have circuits added to prevent/reduce the damage the kick back can cause.

Other Opinions

"The track power bus wires should generally be parallel to each other. Slightly twisting the track power bus wires together will virtually eliminate radio interference, but this is not absolutely necessary. Avoid non-parallel wiring which might be tempting when running wires through and around various obstacles. This prevents unnecessary electronic emanations. The trains will not care, but reception of distant AM radio stations might experience some interference if track power bus wires are neither twisted nor parallel."


DIGITAL COMMAND CONTROL; Stan Ames, Rutger Friberg, and Ed Loizeaux, Alt om Hobby AB, 1998, page 38, paragraph 4.1.1

Digitrax doesn’t require it, and suggests that proper selection of wire gauge and feeder lengths kept to a minimum are essential to reducing resistance and power loss. 


Terminating Bus Wires

This is another DCC topic that gets a lot of ink (or maybe electrons moving) on a regular basis.

In general, bus wires should not need termination. However, you may find it beneficial on pre-installed long wire runs and/or in situations in which your experiencing control problems, such as decoders losing their programming or worse, a decoder blowing up. Refer to the your system manual to see what is recommended. The RC Network can absorb some of a voltage spike by giving it an alternate path, instead of your decoder’s front end.

For more information on Bus Snubbers or Terminators, see Bus Termination.

Some manufacturers recommend the installation of a bus terminator, others do not. Digitrax doesn’t recommend them, while some command stations may have the equivalent built in.

Feeder Wires

Track soldered at the joints, with feeder wires

Feeder wires are wires that connect the track to the bus. That is, every few feet, a set of wires run from the bus to the track. The goal is to make sure that there are no voltage drops and that the train has full power available to it. The benefits are that the train will not slow down. Also, this helps to ensure that the booster's short circuit protection will work.

Feeder Spacing

For a trouble-free railroad, it's recommended that you follow these guidelines for feeder wire spacing.

Feeder Spacing Guidelines
Scale Feeder Spacing
G (1:20.3-1:29) Every 12-20 feet (4m-6m)
I (1:32)
O (1:48)
S (1:64)
HO (1:87.1) Every 3 to 6 feet
TT (1:120) Depending on size of the layout:
up to 250ft of mainline: every 4ft
250ft-450ft of mainline:every 3 ft
more than 450ft of mainline: every piece of track, for short pieces (up to 5") keep on connected to a bigger piece
N (1:160) Every separate piece of track should have its own feeder.
Track pieces over 18" should have a feeder near each end.
Never rely on rail joiners for electrical connections!
Z (1:220) Every separate piece of track should have its own feeder.

Never rely on rail joiners for electrical connections!

Feeder Tips

Don't Place Feeders at the End of a Short Section

If you have a very short block or track section, and will only have one set of feeders, place it in the middle instead of at either end. Don't worry if you can't get it exactly in the middle. There is the ideal and then there is the practical: aim for the ideal, but keep the practical in sight.

Feeders can be installed in a variety of ways. Marrettes, splices or IDCs (Insulation Displacement Connectors). When choosing a mechanical method of joining the feeder to the track bus, make sure it can handle the difference in gauge. IDCs are made for joining two wires, and they are available for different gauges.

IDCs are also known as ScotchLoks (manufactured/invented by 3M) or sometimes called 'suitcase connectors'.

See Also

  • Wiring - Primary wiring article.
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