Wire Size and Feeder Spacing for DCC
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 fix 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.
Digital Command Control comes with its own set of rules for wiring. The first mistake is the application of Direct Current/Analog practices to Digital Command Control. Many analog wiring concepts are counterproductive to good performance with Digital Command Control.
Selecting an Appropriate Wire Gauge
Proper Wiring is Central to the Proper Operation of your Digital Command Control System.
No Digital Command Control 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.
- Power Bus wires must be the correct size (gauge)
- The amount of power available to locomotives will be reduced if inadequately gauged wires are used
- Trains will run slower in areas which are poorly wired
- Consists or multiple trains in the same Power District will run at reduced speeds if the wiring is inadequate
- Wires which are too small may be a fire hazard when used with high current boosters (8 or more amps)
- Poor wiring can result in runaway locomotives. loss of control or damage due to shorts not being detected
Basic Rule: The DCC Signal will seek out the path of least inductance.
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 energy loss equals the current squared, times the resistance. This is expressed as I2*R loss (I squared times R). When current flows through a resistor, a voltage drop is created, equal to the current multiplied by the resistance (=I*R). The result is a drop in voltage.
- Example: If a run of wire has 1 Ohm of resistance, and you pass 1 amp through it, you will see a loss of 1 volt (I*R=1amp*1ohm=1Volt). If you pass 5 amps through it, your loss is 5 volts (5*1=5). You will also convert 25 watts of energy to heat (52*1=25*1=25 Watt).
- If the resistance is doubled to 2 Ohms, that voltage loss increases to 10 volts, and the power loss increases to 50W (52*2=25*2=50 Watt).
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.
One reason 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. As the wire gauge number increases, the inductive properties of the wire increase.
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 interferes with the over-current protection circuit's operation. Which could result in damage to your locomotive, or its decoder, due to excessive current was flow creating excessive heat. Or a short occurs, and something melts because the power was not interrupted. This happens with inadequate wiring, 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 increase the power dissipation by more than twice.
- Good heavy wiring goes a long way to preventing problems. 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 for current limiting purposes. 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.
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 DCC waveform is a square wave. Square waves are, by their nature, loaded with harmonics. Harmonics waste energy by not contributing to the power available. 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.
The idea of the power bus being a transmission line will be put forward in any discussion on DCC Wiring. While technically correct, the application of transmission line theory does not apply, as it was developed to explain phenomena on long telegraph lines. The DCC power bus is measured in meters, not kilometers. The load is dynamic, both in impedance and position, thus altering the length of the transmission line, making it difficult to present a solution to every situation.
Recommended Wire Gauges by Scale
- Recommended Wire Gauges Shown in an Ampacity Table Are for Copper Wire Only, at 60 Hertz, AC, Sine wave.
- 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
- Recommended wire gauge is for a 5-amp current draw, using copper wire.
- Feeders should be kept as short as possible.
|Wire Gauge Guidelines for a DCC Power Bus at 5 Amps|
|American Wire Gauge (AWG)|
|Scale||Bus Length||Feeder Length|
|< 21 ft||21 − 40 ft||> 40 ft||< 5 ft||< 10 ft|
|< 6m||6 − 12m||> 12m||< 1.5M||< 3m|
|H0||1:87.1||14||12 − 14||18 − 22||18 − 20|
|N||1:160||16||14||12||18 − 22||18 − 20|
|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|
- 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. Always try to keep the feeders as short as possible.
- 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.
|Wire Size Guidelines for DCC Power Bus|
|Metric Equivalents, mm2|
|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|
|H0||1:87.1||2.5||4.0 - 2.5||1.0 - 0.5|
|00||1:76.2||1.0 - 0.5|
|N||1:160||1.5||2.5||4.0||1.0 -0.5||1.0 - 0.5|
|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|
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 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.
For a trouble-free railroad, it is recommended that you follow these guidelines for feeder wire spacing.
|G (1:20.3 – 1:29)||Every 12 − 20 feet (4m-6m)|
|Every 3 to 6 feet|
|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.
|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 reliable electrical connections.
|Every piece of track should have its own feeder. |
Never rely on rail joiners for reliable electrical connections.
- Main article: Wire Sizes and Spacing/Alternate Feeder Table
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 as Code 55, more feeders will be needed as the impedance of the rail will have increased. Do not rely on rail joiners to conduct current.
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
- The Track Bus carries power from the booster to the track using heavy gauge wires
- 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.
Wiring a Power Bus
The Power Bus carries power from the booster to multiple points along the track. The bus follows the track from below, and feeders are regular intervals connect the track to the power bus. This is done to minimize any voltage losses caused by the impedance of the rails. As demonstrated by the table below, Code 100 rail is equivalent to a 24AWG wire. By paralleling the track with a heavy gauge power bus, the effect of this higher resistance can be negated.
- A 24-gauge wire has 9 times the resistance of a 14-gauge wire. The difference between 24 and 12 gauge is almost 15 times.
For the greatest benefit of the power bus, the wires should be kept close together to minimize their impedance. This may be referred to as Closing the Loop, as it keeps the magnetic field around the wires as tight as possible, reducing their inductive properties. See below for more details.
The location of a booster should allow the power bus to fed from the center, with an equal length of bus on either side of the booster, to minimize the total bus length. Instead of a 50-foot run, splitting it into two runs of 25 feet is better from an electrical standpoint.
If the track plan forms a loop, there should be a double gap at the point farthest from the booster. The power bus should not form an electrical loop either.
Feeders are short lengths of light gauge wires used to connect the track at regular intervals to the power bus. They can be 18 to 22 gauge, as they will not be very long. Again, they should be kept close.
They will be soldered to the track, and can be connected to the power bus by soldering or mechanical means, such as insulation displacement connectors (IDC) such as the 3M ScotchLok® or their equivalent. Another type of IDC is the "T Tap", named for its appearance. The IDC is crimped onto the bus and feeder wire, or with a T Tap, the tap is crimped onto the bus. A feeder wire terminated with a crimped-on connector is mated with a connector in the tap to complete the connection.
IDCs such as ScotchLoks are available from a number of suppliers and in a number of wire gauge combinations. Always use the correct IDC for reliable performance
Power Bus Alignment
In many cases the power bus will follow the main line. For other applications, some variations may be required.
Multiple Track Mainlines
One power bus can supply a double or multiple track mainline, as well as adjacent sidings. There will be more feeders to install.
Sidings and Branch Lines
Sidings and branch lines which are not parallel to the main line and its power bus can be supplied by their own bus, or by using a sub bus tapped off from the power bus.
One way to easily wire a yard with multiple tracks is to zig-zag the power bus beneath the yard, forming a V or W shape, to minimize the length of the feeders.
Some modellers desire the ability to control the application of power to their locomotive storage and ready tracks. This is possible using a SPST or DPST toggle switch. What is important is to avoid attempting to save on wire by only running a one wire loop to the switch and back. This will introduce impedance into the circuit which can cause problems. To minimize the impedance, create a sub-bus off the power bus with a pair of heavy gauge wires, which has the switch inserted in one or both wires, then run that bus to the storage or ready tracks. Much like the power bus, this keeps the impedance low. Running a single wire from a remote location to the fascia and back to the location introduces unnecessary inductance into the loop.
If the ability to control power to these tracks is not an issue, the techniques used to wire branch lines, sidings and yards would apply.
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:
- It looks neater
- 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. While this is an advanced topic, it is important to understand the basic principles to avoid creating problems when wiring.
Three Steps to Managing Inductance:
- Use heavy gauge wire
- Avoid long runs
- 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. This results in some frequencies travelling faster than others, distorting the pulse shape.
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 calculated 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.
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.
The impedance of the wire is dominated by the inductance. When the inductance is lowered, so is the impedance. A 1m length of 10AWG has an inductance of 1.32µH, which at 8kHz results in a inductive reactance component of 66mΩ. The same length of wire has a resistive component of 3.3mΩ. The impedance (Z) computes to 66mΩ. This is why heavier wire is used than that of analog applications. A 14AWG wire has an impedance of 71mΩ per metre. If the wiring loop is 5m long (10m total) 10AWG is 660mΩ compared to the 710mΩ of 14AWG.
An effective method to decrease the inductance/impedance is keeping 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. This is sometimes referred to as Closing the Loop.
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, 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.
Mutual Inductance is the property where the flux around one wire induces a current in another. This is the principle by which transformers work. Mutual inductance can also be additive or subtractive in nature.
By keeping the two bus wires close together, the net effect of their mutual inductance is to reduce the total inductance. As k, the coefficient of coupling increases, the resulting inductance decreases, as per the formula:
- Total Inductance = L1 + L2 – 2M
- Where M = k × √(L1 × L2)
- k is a value from 0 to 0.9. Perfect coupling is considered to be 0.9.
For example, if each bus wire were to possess an inductance of 50µHenries:
- k= 0.8 for a tightly coupled pair, LTotal = 20µH
- k = 0.4, for a loose coupling such as wires several inches apart, LTotal = 60µH
- With no coupling, (k = 0 the total inductance of the loop would be 100µH.
- The k values are just for illustration of the effect of mutual inductance and coupling effects.
Ref: Lenk, Databook for Electronic Technicians and Engineers
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 number of twists in the bus should be avoided. A tight twist will create more capacitance than a loose twist.
Capacitance 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 or more 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|
|Parallel, >1 foot spacing||1.38||22|
As this table demonstrates, the Inductive Reactance (XL) is the dominant factor in the resultant impedance. By managing the inductance, the voltage drop can be kept to a minimum.
- 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. This does not include the burden imposed by the multifunction decoder(s) and the track.
- A calculated approximate value for a 72-foot straight 12AWG wire is: 30µH/foot (1.9Ω) plus 120mΩ, for a total impedance of 2Ω.
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|
- The wire used for an equivalent is stranded. Since the measurements were made at 60Hz, impedance better reflects the results.
- 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:
- Remove the vehicle from the track,
- Place in a cradle or on a short section of isolated flex-track (restrain the vehicle if needed),
- Power supplied with jumpers directly from the command station's track output.
- Disconnect the command station from the layout's power bus first!
This process will isolate whether the problem is layout or vehicle wiring.
- "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 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.
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. Share this page
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