Wire Size and Spacing for DCC

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Summary: Proper wiring results in less problems later. It is one of the most important aspects of Digital Command Control.

Wiring is one part of layout construction which is easier to do correctly the first time than to change or upgrade it later.

Some of the information presented may seem overly complex or unnecessary. The wiring practices described are based on Best Practices which have been demonstrated to work, both in a home layout and large modular layouts.

Wiring Issues

Digital Command Control comes with its own set of rules for wiring. The first mistake is the application of Direct Current/Analog practices to DCC wiring. Many analog wiring concepts are counterproductive to good DCC performance.

Selecting an Appropriate Wire Gauge


Good or Cheap. Pick One.
Proper Wiring is Central to the Proper Operation of your Digital Command Control System.

Wire gauge.

No DCC topic seems to elicit more debate or 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.

  1. Power Bus wires must be the correct size (gauge).
  2. The amount of power available to locomotives will be reduced if inadequately gauged wires are used.
  3. Trains will run slower in areas which are poorly wired
  4. Consists or multiple trains in the same Power District will run at reduced speeds if the wiring is inadequate
  5. Wires which are too small can be a fire hazard when used with high current boosters (8 or more amps)
  6. Poor wiring can result in runaway locomotives or damage due to shorts not being detected

Can You Use Wire Which Is Too Heavy?

Experience has shown that since you can run more trains with more locomotives using Digital Command Control, you will.

This means your electrical loads will be higher for a given layout. In addition, a voltage drop of only 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 is clear you will need a heavier gauge of wire. A small table top layout can reduce these sizes slightly 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 I2R 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 expressed as ohms per 100, or 1000 feet. For a short run of a few feet, the resistance is negligible. It becomes an issue with a long run of wire. Remember, an equal length of wire is needed to complete the circuit, doubling the resistance of the circuit. High impedance caused by inadequate wiring can prevent 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 gauge wire is to minimize the effects caused by the nature of the Digital Command Control signal. While the DC resistance (R) is quite low, the AC Impedance (Z) is not the same, and may be higher. In fact, a heavy AWG 12 wire may be electrically equivalent to a 20 AWG wire when used with a Digital Command Control 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.

Proper Operation of Overcurrent Protection Devices

The short circuit protection features of your booster are designed to protect the booster, not your expensive locomotive, from damage.

An important concept often ignored is the way circuit protectors work with many DCC boosters. They are not sensitive to the amount of current. They react to the rate of change in current. A sudden spike will trigger them. This allows them to cut off current flow much sooner than a device that relies on a set value. Which means power will be cut off long before significant damage occurs from excessive current flow which would not trip a device acting on a set value. Some boosters can provide an almost unbelievable amount of current for a very brief period of time, upwards of 60A!
Poor wiring will interfere with the over-current protection circuit's 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 adequate, 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 power 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.
Automotive tail lamps are not recommended as current limiting devices. This idea belongs in the analog era, not the DCC era.
Always test your wiring using the quarter test. The booster should cut out immediately. If not, there is a problem.

Harmonics

This is an important issue. Harmonics are multiples of the fundamental frequency. They cause distortion of the waveform. Harmonics Do Not contribute to the available power. This is mentioned in the NMRA S-9 Electrical Standard years before the NMRA Digital Command Control Standard was written, warning of possible damage to your motor by harmonic currents.

Why is this important with Digital Command Control?

The nature of the DCC waveform. A DCC signal consists of a square wave. Square waves are, by their nature, loaded with harmonics. Harmonics waste energy by not contributing to the power available to you. In extreme cases, half the power available will be wasted in the form of harmonic currents. With poor wiring, these harmonics can alter the shape of the waveform, distorting it to the point the decoder may not be able to read it. If Alternate Power Sources are enabled, the decoder could interpret the DCC waveform as being direct current, and a runaway locomotive occurs.

In DCC you probably will never see that happen, yet energy will still be lost in this manner. That is why the wiring is much heavier than that used for analog operations. Analog power packs will also waste energy when using pulse power modes, the difference being they supply one low current load, whereas your DCC booster and wiring supply multiple loads demanding a lot more current.

Recommended Wire Gauges by Scale


Note
Recommended Wire Gauges Shown in an Ampacity Table Are For Copper Wire Only, at 60 Hertz, AC, Sine wave. Keep that in mind when making comparisons.
This chart is based on a 5A maximum current on the bus. For an 8A booster, move to the next larger gauge (smaller number).
These guidelines are based on Technical Requirements and Best Practices.

Wire Size Table, American Wire Gauge

Wire Gauge Guidelines for a DCC Power Bus
American Wire Gauge (AWG)
https://dccwiki.com/Wire_Sizes_and_Spacing
Scale Bus Length at 5A Feeder Wire Length
< 21 ft 21 − 40 > 40 ft < 5 ft < 10 ft
< 6m 6 − 12m > 12m < 1.5M < 3m
G 1:20.3 12 10 8 16 14
1:29
0 1:48
S 1:64
H0 1:87.1 14 12 − 14 18 − 22 18 − 20
00 1:76.2
TT 1:120 14 20
N 1:160 16 14 12 18 − 22 18 − 20
Z 1:220
N−Trak 16 14 12 18 − 22 18 − 20
N−Trak is based on the N−Trak Wiring RP for DCC http://ntrak.org/ntrak_powerpole_rp.htm
Quick Link to this page: https://2tra.in/o4WZH WireSizeQR.png
Main article: Wire Sizes and Spacing/Wire Resistance Table
Feeder Length is not the distance between feeders. It is the recommended maximum length between the track and the DCC Power Bus.
According the Big Book of DCC by Digitrax, runs up to 100 feet are possible with 12AWG wire, and 50 feet with 14AWG. This is for one leg, in theory it is possible to have a 100 foot run (14AWG) with the booster in the middle (50 feet on each side). Don't skimp on wiring, plan for future power needs.

Metric Equivilents

Wire Size Guidelines for DCC Power Bus
Metric Equivalents, mm2
https://dccwiki.com/Wire_Sizes_and_Spacing
Quick Link: https://2tra.in/o4WZH
Scale Bus Length Feeder Length
< 21 ft 21 − 40 > 40 ft < 5 ft < 10 ft
< 6m 6 − 12m > 12m < 1.5M < 3m
G 1:20.3 4.0 6.0 10 1.5 2.5
1:29
0 1:48
S 1:64
H0 1:87.1 4.0 4.0 - 2.5 1.0 - 0.5
00 1:76.2 1.0 - 0.5
TT 1:120 2.5 0.5
N 1:160 1.5 2.5 4.0 1.0 -0.5 1.0 - 0.5
Z 1:220
N−Trak 1.0 - 0.5 1.0 - 0.5
N−Trak is based on the N−Trak Wiring RP for DCC http://ntrak.org/ntrak_powerpole_rp.htm
Quick Link: https://2tra.in/o4WZH WireSizeQR.png

Metric Equivalents to American Wire Gauge

The link below has a table comparing AWG to Metric wire sizes to aid in converting between the two measurements.

Main article: Wire Sizes and Spacing/Metric to AWG Table

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 is recommended that you follow these guidelines for feeder wire spacing.

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)
00 (1:76)
Every 3 to 6 feet
TT
(1:120)
Depending on size of the layout:
Mainline ≤ 250 feet: every 4ft, 250 − 450ft of mainline: every 3 ft, > 450 ft: Every piece of track.
Short sections (≤ 5") should connect to a bigger section.
N
(1:160)
Every piece of track should have its own feeder.
Track pieces longer than 18" should have a feeder near each end.
Never rely on rail joiners for electrical connections
Z
(1:220)
Every piece of track should have its own feeder.
Never rely on rail joiners for electrical connections.
Main article: Wire Sizes and Spacing/Alternate Feeder Table

Feeder Tips

Distance between Feeders

This is directly related to the rail used. With Code 100 to Code 83 (HO Scale) rail, every three to six feet (1 – 2 metres) is recommended. With smaller rail, such at Code 55, more feeders will be needed as the impedance of the rail will have increased.

Length of Feeders

Feeders should be kept as short as practically possible. There will be exceptions, but as a rule, no longer than needed. Options also include routing the power bus for easier and shorter feeder installation, such as routing it diagonally across a yard. Or branching off from the power bus to feed a number of sidings.

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 differences in wire gauge. IDCs are made for joining two wires, and they are available for different gauges, as well as various combinations of wire gauges .

IDCs are also known as ScotchLoks (manufactured/invented by 3M) or sometimes called suitcase connectors. The 3M devices are superior in design and construction than the generic knock-offs.
3M makes a T-Tap, an IDC you crimp onto the wire, which has a tab which will accept an appropriately sized crimp-on female connector. These can be useful to make connections for feeders as well, with the advantage of being able to be disconnected.

Power (Track) Bus Wiring


  1. The Track Bus carries power from the booster to the track using heavy gauge wires
  2. Feeders (sometimes called droppers) connect the rails to the track bus, using lighter gauge wires

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

You can use either solid or stranded wire. Stranded wire is more flexible and will handle repeated bending. Repeated flexing of a solid wire will anneal the copper and making it brittle, which can then break. Stranded wire is also more tolerant of nicks caused during stripping. A nick in a solid conductor creates a weak point, where the wire may break.

Twisting the Bus Wires

There have been ongoing debates raging about the necessity of twisting the bus wires together since the introduction of DCC. One group says yes, the other believes it is not necessary.

Twisting your bus wires together serves two purposes:

  1. It looks neater
  2. Reduces the impedance by minimizing the inductive properties of the wires

The same can be achieved by tying the wires together with cable ties. This method actually makes it easier to separate the wires to add feeders.

If you are planning to add detection, do not twist the wires for a foot or two prior to and after the current transformer. This reduces parasitic currents due to capacitance between the wires. For maximum sensitivity keep both wires close to the CT to concentrate the flux.

For more on Twisting and Impedance please read:

Main article: Wire Sizes and Spacing/Twisting the Bus

Inductance and Impedance Reduction

Managing the Impedance of your track bus is important.

Three Steps to Managing Inductance:

  1. Use heavy gauge wires for your track bus
  2. Avoid long runs
  3. Keep the pair close together.

It is important to understand that the reactive components of your wiring's impedance have an effect on phase relationships. Since a DCC signal is a pulse made from a fundamental frequency and its multiples (harmonics), as those relationships are altered by the reactive properties of the wiring distortion of the DCC signal results.

Impedance

In DCC, the impedance of the wire is important. The Reactive Component can be much larger than the resistive properties of the wire, controlling the Reactive Inductive component is important for good electrical performance of your wiring.

Impedance (Z) is calucated using the following formula: Z = √(R2 + X2)

Where X is the reactive component, calculated as (XL – XC)
  • XL is Reactive Inductance
  • XC is Reactive Capacitance
  • Frequency determines the Reactive component.

Inductance

The two properties of wire which impact inductance are the gauge and length. Longer wires have increased inductance. Increasing the gauge (reducing the diameter) of that wire will increase the inductance too. To counteract the impedance of a long run, a heavy wire should be used. An effective method to decrease the inductance/impedance is to keep the wires close. The closer the better. The most effective solution 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. Tying the bus wires together is also a very effective approach.

Attaching feeder wires to twisted bus wires gets complicated if you are not consistent with your colour coding, however, if the wires are not twisted a great deal so it shouldn't be too difficult. Tying the two wires together with cable ties is just as effective, plus it is easier to connect feeders.

Capacitance

Twisting the bus wires will create a capacitor, with a value of 1 to 2 pF per inch. Doesn't sound like much, but that can be as much as 480pF over 20 feet. An excessive amount of twists in the bus should be avoided, a loose twist is enough. This creates a leakage path for the DCC signal, causing issues with current sensing block detection systems. If a block detection system is planned, keep this in mind while the track bus is being installed. Wires feeding a detection block should not be twisted together for at least a foot before and after the current sensor.

Capacitance is determined by the parameters of the wire: Larger gauge (lower numbered) wires have more surface area, plus the thickness of the insulation which forms the dielectric (insulating) portion of a capacitor. The resulting capacitance is created by the area of the plates (the wire's surface) and the distance (thickness of the dielectric) between them.

Power/Track Bus Impedance

Impedance Measurements on a loop made of two 12AWG wires, ~36 feet each for a total loop of ~72'. Average impedance of each wire is 0.8Ω, L = 12.6μH, resistance 0.09Ω. Measurements made at 10kHz.

Bus Wires Z (Impedance, Ohms) Inductance (μH) at 10kHz
https://dccwiki.com/Wire_Sizes_and_Spacing
Parallel, >1 foot spacing 1.38 22
Parallel, Tight 0.57 9
Loose Twist 0.50 8
12AWG has a DC resistance of 1.6Ω per 1000 feet. This loop would be 72/1000 × 1.6 = 0.12Ω. The measured impedance was 1.38Ω, which, at 5A, would result in a voltage drop of 600mV. By reducing the impedance to 0.6Ω, VDrop becomes 300mV.

Rail Resistance, Nickel Silver

The following table gives the impedance of various codes of rail. The impedance was found with 1A (at 60Hz) current flowing through the sample, using a comparator feeding a detector set at 50μV. By injecting a negative impedance, the impedance of the rail is found when the measuring system is brought into balance.

Code of Rail Impedance per metre, mΩ Equivalent Wire Gauge Strands/Gauge
https://dccwiki.com/Rail_Size
100 76 24 7/32
83 108 26 19/38
70 206 28 19/40
  1. The wire used for an equivalent is stranded. Since the measurements were made at 60Hz, impedance better reflects the results.
  2. Rail resistance measurement uncertainty is >50ppm. Actual resistance will vary by manufacturer due to alloy and profile.

Some multifunction decoders have increased sensitivity to waveform distortion because:

  • The manufacturer has designed them to reject out-of-specification waveforms.
  • The default setting is to continue as it was in the absence of a valid waveform (many decoders simply shut down after a packet timeout period). This can make some brands of decoders less sensitive to dirty track resets, at the increased risk of loss of control when waveforms are badly distorted.
  • Failure to remove the EU-mandated interference suppression inductors from the loco motherboard is another cause of loss-of-control at speed.

To isolate this issue:

  1. remove the vehicle from the track,
  2. place in a cradle or on a short section of isolated flex-track (restrain the vehicle if needed),
  3. power supplied with jumpers directly from the command station's track output.
    1. disconnect the command station from the layout's power bus first!

This process will isolate whether the problem is layout or vehicle wiring.

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. Also see the section on Inductance above.

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. If the output of the booster is a high impedance, this usually minimizes the issue. A low impedance output driving the track bus with moving higher impedance loads on it will create a transmission line, and ringing is quite possible.

As a rule, they should only be installed if needed, and after proper investigation such as measuring the waveform with an oscilloscope.

See Also

  • Wiring - Primary wiring article.