DCC layout Wiring 2

WIRE… Why you must use the right wire for the job, wiring for best performance on the DCC layout and what do in various special trackwork circumstances

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On this page we will cover many things relating to layout wiring. Some may raise questions as to why and we have tried to add explanations where possible, however there may be things that still aren’t clear, as there is a limit to adding detail… too much and we create confusion!

In those cases, don’t sweat the theory or listen to the myriad bits of contrary advice on the web - for best results “Just do it”!

No, we don’t know it all and we are constantly learning, however if we write about it we know about it, so be confident. We have based 100% of our advice on real world experience in creating & troubleshooting layouts of many sizes in many paces across the world. The advice we give is what WE actually do ourselves and it never fails to work.

If you cannot answer your questions by reviewing these pages, please do feel free to email us directly and we will do our very best to give you some “One on One” assistance with your wiring problems or questions

Just click here with your mouse to bring up an email window.

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The importance of using the correct wire size and

Wiring for BEST POSSIBLE layout performance

For more than a decade, we have been providing advice and help to modellers all over the world. Often, when we discuss the issue of the importance of wiring a layout and in particular wire size on a forum or email group, others will chime in with “I do it differently” or “I use finer wire than that and is working fine” or similar contrary comment or advice.

We simply smile and let it go, all the while re-stating and reinforcing our recommendations to the person we are helping and doing our best to keep the modeller thinking and working in the right direction.

This is important to us: We know that some will be lucky, and get away with cutting corners - however for everyone who compromises and has no problems, there is at least one who will suffer the pain of poor running, random runaway trains, systems not working properly and decoders failing for no apparent reason…. NONE of those problems ever happen when the job is done right, and its not really any harder or much more expensive to be sure it’ll work the way you want so why compromise?

We fix many layouts for modellers, and the difference that small improvements make is astounding - almost all are actually quite easily fixed by changing power supply, re-arranging or improving wiring or correcting simple wiring errors that have made big problems for the layout owner. We would like you to get it right and enjoy the hobby!

Lets kick off with how you can know the wire size is correct. The chart below will help you buy the correct wires, but a few specific examples may give you a better “feel” for what is right and wrong.

Please note that if I refer to equivalents in house wiring, then they will apply to 230 / 240 volt systems such as in Europe or Australia, not the 110 volt US and Japanese systems.

The DCC POWER BUS:

This is the main pair of wires that runs under the layout taking the power from the DCC command station to all the track of the layout. Knowing how hard it is for most to resist a bit of free cable or compromise so there’s more cash left to buy loco’s, We’ve given you options—the correct size to use, and a smaller one that’ll probably be OK as long as you take care with the quality of soldered joints and connections etc...

Wire Size for the Bus

It should be relatively BIG wire. The larger the layout is, the thicker this wire should be. Here is a guideline: Please, DO compromise as little as possible!

Small: Up to 8 x 4 (2400x1200) , or shelf type layouts. 16 Gauge

If you must compromise, then no less than…. 18 gauge

Medium: Say up to full room layouts 12 x 20 (3600 x 6000). 14 Gauge

If you must compromise, then no less than…. 16 gauge

Large: Over 12 x 20, club layouts, very long layouts. 12 gauge

If you must compromise, then no less than…. 14 gauge

Calculating wire size:

AWG is now only commonly used in car audio references outside of the USA, and not all manufacturers use the simple copper area or diameter, so you may find that your supplier sells wire that is specified differently than the numbers shown below, for example, in UK, EU and Australia, it may be specified by both diameter of each strand of copper and qty of strands.

To work out the area, The formula is pi (∏) x the radius squared, and Pi (∏) is 3.14159265. work out one strand, then simply multiply by how many strands there are….

Example: A common wire size used by modellers is 32 x 0.2mm cable.

Therefore if the diameter of each strand is 0.2mm, its radius is 0.1mm, then…. The area of the strand is 0.1 x 0.1 x 3.14159265, or 0.0314159265 square mm

If there are 32 strands, the total cross sectional area of copper in that cable is close to 1mm, making it approximately 17 Gauge, good enough for a small layout, but a bit too light for anything much larger!

It is of course easier to give you a direct reference:

The following is taken from the MAPLIN UK Website, and shows their red/black twin cable selection. The part numbers are from Maplin in UK, but for all other European or Australian readers, the dimensions and specifications are just as relevant.

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Part Power Stranding copper area dim over

Number rating per wire of each wire insulation

XS70M 6A 2x20/0.19 0.57 5.6x2.8

XS71N 10A 2x28/0.19 0.79 6.0x3.0

XS72P 15A 2x42/0.19 1.19 6.6x3.3

XS73Q 20A 2x75/0.19 2.12 7.6x3.8

XS74R 25A 2x50/0.3 3.53 8.4x4.2

XS75S 35A 2x80/0.3 5.65 10.0x5.0

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So… using the above, we can see that we should perhaps use:

XS 70M for droppers (the smaller wires from bus to track).

Actually, you can use ANY of the diameters indicated below for droppers if they are short - ie if they are kept shorter than 300mm, then a smaller wire will be fine. When it comes to droppers, perhaps the best practical guideline is “use the largest wire you can efficiently and reliably solder tidily to the rails”!

XS72P for a small layouts track bus: (Its slightly smaller than the chart indicates, but that will be OK).

XS73Q for a medium layouts track bus: (Almost perfect in size!)

XS74R for a large layouts track bus: (Again, right on the money)

XS75S for a HUGE layout: (My own layout covers about 2000 square feet, and uses this size of cable for main bus. In fact I choose to run two “Power busses” - one for track, another for accessories etc, so that accessories don’t corrupt the main track bus signal!

Incidentally…. Cable can be much cheaper by the reel - For example, Maplins bulk discount is almost 50% if you buy a full 100 metre reel of one wire!

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NO… Its NOT about the power handling of the wires!

I can hear some of you thinking - he’s insane… why do we need such big wire, we are only carrying up to 5 the amps maximum available from my system?

Well, it is all about overcoming two things that WILL happen to your layouts power no matter what you do - voltage drop over distance and inductance created by the non-symmetrical DCC signal under power, which adds hugely to the overall potential for voltage drop on your layout. The added area of copper means that this will be at its lowest possible, and your layout will be “well fed” with power no matter how intensely you operate it!

Type of wire for bus and droppers:

Stranded or solid are both OK. The important thing is the cross-sectional area is correct. You will find however that stripping the bus to add droppers is easier with stranded wire, and unless you have “free supply” of an appropriate solid, I’d strongly recommend that you buy stranded.

Consistent colour coding is important:

The convention is red and black for track wiring, with red to one rail, black to the other. What colours you chose is up to you of curse, but try to be consistent with this, as maintaining polarity between under board wires and track is important!

How should you run the power bus?

The Power Bus runs from the track outputs of the DCC system and usually follows the general position of the track. You will only need one power bus for multiple tracks as long as they are reasonably close together. Rather than having the bus sake back and forth, make separate bus runs for branch lines, major yards etc and join them close to the DCC system. This is called a “Star” circuit.

There are three things to pay attention to here…

however most of you can probably relax, as all three can effectively and safely be ignored for smaller layouts - the wire runs are too short to create the sorts of problems that are indicated to a degree that will affect running. DO pay attention to them though if you have a layout that could be classes as medium or larger.

(1) Make it a T or star shaped bus, not a ring: This is good practice for all layouts, and there are good reasons for this on medium to large layouts but they are very technical—if you really need more info feel free to ask by email.

Place your DCC system so its track outputs are about the middle of the layout, and take the bus right and left from there. If there are several spurs to other track areas, fan them out from a central start point (the DCC system or booster) if possible.

(2) If you use twin wire (figure 8 cable) your layout is medium or large: Then TWIST the cable approximately 4 turns per foot as you run it. This twisting helps to counteract the creation of inductance in the wire and will significantly reduce voltage drop under load, as well as helping to keep the DCC signal clean.

No need to worry about this if your layout is small.

(3) If you prefer, you can run two separate wires rather than twin. If you do this, they should be more than 100mm apart wherever possible. This will also prevent the two wires interacting and creating and creating an inductance which will cause voltage drop and signal degradation. (In many ways, this “two wire” arrangement will be easier to manage and install than a twisted twin cable)

Taking care and getting this right is worth the effort. It will guarantee that your layouts wiring is off to a good start, and reduce the potential for later problem. There is one more thing that I strongly recommend is added to EVERY power bus.

Add a bus terminator/filter to each bus end point:

This is a simple device that will act as a filter to improve the quality of the DCC waveform, act as a suppressor for voltage spikes that are generated by every single short circuit on the layout, and because of that, adding these low cost devices will extend decoder life and improve overall layout reliability.

Bus terminators/filters are available ready to go in packs of two from DCC concepts (Click here to see or buy) however for those who like to do it themselves, here is how to make and install them if you want to have a go.

You will need for each terminator/filter you want to make:

One 0.1 microfarad ceramic capacitor (also called “monolithic” capacitors)

One 150 ohm resistor - (1 watt absolute minimum)

Neither is polarity sensitive. Simply connect them together, then connect the free lead of the capacitor to the left bus wire, the free lead of the resistor to the right or vice versa....

That’s it - a filter/terminator made and installed with 3 solder joints!

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Adding droppers to the Power Bus:

Droppers are the thinner wires that are connected to the power bus one end, and to the track at the other.

Because they extend from under the layout to the track above the baseboard this CAN be a chore, but you can also make it much easier by planning head and pre-attaching droppers to rails as you lay them… and to make is pleasant and quick, using a couple of specialised tools we’ll tell you about soon will save a massive amount of time and frustration.

As to the physical bit, perhaps you can save yourself a lot of frustration by (my preference) attaching the wires to the rails before you lay the track, or making the layout in sections that can be wired on edge so you can stand up while wiring… For those about to dive under an already fixed lot of layout baseboards... we have a couple of sanity saving ideas to tell you about later on, so take heart!

Droppers are usually between 1/2mm and 1mm in copper cross sectional area—that’s somewhere between 18 and 20 gauge, however if they are kept short, then anywhere down to 24 gauge will be OK.

As a rule of thumb that will serve you well, use the largest dropper wire that you can reliably and tidily solder to the rails.

We strongly suggest that droppers are created with stranded wire—small solid wire is prone to fracture and failure.

Here’s a tidy, easily managed suggestion for connection of Droppers to the power bus. It uses tag strips which we sell in packs of 20, and he tools to make it an easy task are the excellent DCCconcepts Power Bus strippers and our exclusive fine wire strippers, plus our track cutters for snipping the tag strip loops.

Step one: Decide where the tag strips will be mounted & do it.

They should be placed close to the bus and not too far from wherever droppers will be needed, but easy for you to get to for soldering!

Step two: Group together the droppers close to the tag strip and cut to length, strip and tin them… making sure there is a little free wire for easy maneuvering of their stripped ends into the eyelet holes on the tag strip terminals. Insert the dropper wires into the “eyelet holes” of the tag strip and solder in place.

You will find this precision stripper is lower in cost than you might imagine, and you will wonder how you did without it for both layout wiring and decoder installation once you own one!

(You will find that depending on your wire choice, the small eyelet holes in the tag strips are big enough for say three ~ four dropper wires in each hole and happily, being pre-tinned eyelets, soldering to them is dead easy!)

Step three: Ascertain exactly where the bus will need to be stripped to meet up with the tag strips, then strip that section of the bus.

This will take less than a second for a perfect job every time with our Power Bus Strippers.

Step four: Cut one side of the outer ring terminals of the tag strip and bend them so you can easily insert the stripped but unbroken track bus into the loop.

The example above shows what we mean - note that we’ve already soldered the dropper to this tag strip too!

Step five: Now insert the bus wire into the cut eyelet loop and close the loop with fingers or pliers.

The image below shows what we mean! Droppers in small eyelet holes, bus wires in the larger tag strip type hole.

The centre terminal has a purpose too by the way—it’s the perfect place for the turnout frog wire connections when you use correctly wired live frog turnouts.

Double check that you have the correct wire in each loop and them solder in place.

Easy wasn’t it—tidy too, with every bit of bare wire safely held away form all other wires, and in a clear access position for easy maintenance later!

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Adding a Programming track:

The programming track is a separate section of track where programming (setting the locomotive address and/or other internal parameters) takes place.

A programming track is needed to complete the necessary start up programming for most decoder-equipped locos. It is sensible to do all “fist programming” on the programming track, as it is current limited and if you have made an installation error, the lower power of the programming track will not allow decoder damage to occur.

(For example, if you have mis-wired the decoder, it will just show an “error” message… if the same error is placed on the main track, it will “let the smoke out” of the decoder and it will be damaged!)

The programming track can not supply enough power to run a locomotive.

Because “service mode” or programming track mode broadcasts programming data to all loco’s on the track, the programming track must of course be electrically separate from the main trackage.

If your system has a separate pair of outputs for a programming track, proceed as below - It can if you wish be a totally separate length of track, however many modellers find the end of a convenient spur or siding will work fine.

Creating a programming track is simple – Just cut rail gaps or install insulated joiners in both rails directly across the track from each other. Allow enough length for your longest locomotive, and connect either directly from the DCC system.

This diagramme shows how you can make one siding serve two purposes by connecting it via a double-pole double-throw switch that is used to either isolate this segment from the rest of your layout or let it work as a standard siding when programming isn’t wanted.

If your system supports “Service Mode” or “Programming Track Mode” but does NOT have provision for a separated programming track (examples, Roco, Digitrax Zephyr, Bachmann Dynamis)

….then you can use the method below which will ADD this useful ability to your layout without the need to replace your whole system - All you need is the very clever NCE Auto SW which is a low cost but effective device that senses the software of your system wants to programme and automatically switches off the main line so you will ONLY programme the loco on the programming track!

Note that when using the programming track, the remainder of the layout is of course stopped.

Once the primary programming such as setting the address is completed, if your controller allows “programming on the main”, then all other decoder tuning and setting up” can be done on the main line, even when the loco is already moving if needed!

Wiring with isolatable areas, power districts, extra boosters, added handsets etc:

All these subjects are covered by this drawing, repeated here wit some modifications from the NCE instruction manual from my own DCC system. Remember it is an example of how parts should interconnect on a larger system and it is NOT an indication of what may be needed for the quite modest layout it illustrates! While it shows NCE gear, the same principals apply to ALL brands. I hope it is self explanatory.

If you’d like to see it more clearly, download it & print or view at full size.

Isolatable areas:

No, you don’t really need them for running trains, but it is really sensible to break up the layout into sub-sections so that you can trouble-shoot problems that may happen in the future. These should be logically thought through - ie station area, yard, engine facility, various sections of main line etc.

**At its simplest, each section becomes a sub bus which can be shut down with a DPDT switch that breaks both power bus lines.

**Next level is to use a circuit breaker for each section. The image below shows these, the 3 section EB-3 and one section EB-1 are both automatic circuit breakers from NCE, and we also sell the excellent DCC Specialties OG-CB.

Power or Sub districts:

**For really large “sub districts” of the layout - or super heavily loaded areas, you might need more power than the system itself can supply, so you would then add power boosters… the booster would then in turn feed circuit breakers connected to local areas within the “sub” or “power District” that it feeds.

Boosters can be seen here:

To be honest its really rare for all but the large layout to ever need a second booster for track power (especially if accessory bus is separate and on its own booster) so don’t worry about needing lots of boosters - you probably won’t!

The Throttle or data bus:

This is the wire that additional walk around throttle will connect to—indicated to the right of the PH Pro control box in the image above. They form a series connection with the “UTP Panels”, which are most commonly linked with 6 wire cables to the system control box (using 6p6c Telco type connectors).

Setting up a control bus like this is very useful for mid to larger layouts where it will be helpful to follow the trains, or to move to a new control position to carry out operations.

Wiring for Accessories:

Plan ahead here: if you are going to have lot of accessories that will need connecting to the power bus, then it is well worth considering installing a totally separate power bus just for the accessories.

This will prevent feedback and emf from accessory items creating interference on the track bus, and make sure both the accessories and the trains always perform at their best. I very, very strongly recommend that you do this if you intend to add computer control later.

The accessory bus should ideally be of the same weight wire as the track bus, and should be installed with the same level of care. Initially or for a small layout, it can share the same booster as the track, however as the load grows, a separate booster for accessories will be a much better answer.

Using the correct wire size is very, very important for some accessories:

Its important to match the wire to the accessory too. The wire weight needed will depend on the current draw of the accessory, from heavy for solenoids, to light for low current things like LED lamps in signals and detectors etc…

Think about and plan colour codes for your wiring - and stick to them!

Strike a consistent set of insulation colour standards for each wire weight - it will make trouble shooting easier later on.

There are only so many wire colours available , so if you redefine use differently for each wire weight, re-using the same colour for a different purpose, that’s OK as you can still tell them apart by their size.

For complex wiring, do also consider labeling wires with tags made from white shrink tube and written on, tags made from masking tape etc….

Which wire for which accessory? CHECK each accessory, and as a rule of thumb stick to these sizes:

** use 2mm or larger for solenoids and electromagnets

** Use 1~1.5mm for medium power devices

** Use 0.5~1mm or similar for low power items and data communication.

If the run is long, especially for heavy current devices, go even heavier than the above rule of thumb sizes on the wire gauge if you can - its best to be safe as voltage drop gets much worse with length!

Special Wire: If the accessory instructions say “use twisted pair cable” or have special cable specs, USE IT - these things aren’t suggested for nothing, and failing to do so, especially if wires are long, will bring disappointment.

Solenoids: These are often very high current devices and will always work best with heavy wire, as they peak at up to 5 amps (depending on brand) at the moment of throw, and heavy wire will prevent voltage drop at this critical moment. We recommend no smaller than 1.5mm and preferably 2mm wire for solenoid wiring.

Lighting, switches, LED etc that use low current: Where multiple wires are needed between accessories and their switches colour coding is very important, as is tidiness. We find “alarm wire” to be excellent in many cases - it is low cost, 6 core wire with adequate power handling for most light duty applications, and with all 6 wires encased in one cable, its tidy and easy to keep organized well for later tracing and troubleshooting. Additionally, its usually clean white covering can be written on to identify each wire (use a fine tipped permanent marker pen).

Detectors: Most are happy with any good quality fine wire such as the alarm wire we mention above or any reasonable quality fine twin wire, but some digital devices may need specific cables to be used.

Read the manufacturers instructions for sophisticated devices carefully, as if it needs special wire and you use just anything, intermittent results are very likely!

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Well, that’s it for this page… for now anyway. On the next page, we cover many special wiring challenges that face the modeller, including reversing loops, wiring live frog turnouts and special turnout types and several other things that may be of help to you. We hope its all helping you!

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do you still have questions? No problem – just click here to email us and ask, we’ll be happy to help!

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