Railwayscenics beginners guide on DCC bus wiring

We have been asked several questions about DCC bus wiring many times, so have produced this page which we hope will answer many questions that you may have about installing a basic DCC bus to your model railway layout. The information here was that gained when I started with a DCC layout. There was so much information out on the internet, that I got confused. There appeared to be no right or wrong way to wire a DCC layout, so I did what I thought was right and it worked. This page describes what I did, and the thoughts and reasoning behind it without going into too much depth.

Caution

Please note that model railway electrics and electronics can be very dangerous. This website cannot be held responsible for injury or damage however caused by the use of any information on this website. If you are unsure about anything please ask a qualified electrician for help. Always fully read any instruction manuals supplied with any equipment and make sure you fully understand what you have read. Before using any electrical system you may have designed or made, please have it properly PAT tested to ensure that it is safe to use.

What is the difference between DC and DCC?

With a DC analogue system you only apply power to the tracks when you want a locomotive to move.

With a DCC system, all of the track is live all of the time, so you can move more than one locomotive at a time on the same piece of track.

What materials and tools will I need to wire my layout?

When wiring up a typical layout you may need any of the following tools and materials, regardless of scale and gauge of your layout.

1. DCC power bus wire of 24/0.2 or 32/0.2 in black and red (or another colour of your choice) depending on your layout size or scale chosen
2. Dropper wire of 1/0.6 single strand or 16/0.2 stranded to match your colour choice of bus wire.
3. Suitcase style connectors to join stranded dropper wires to bus wires
4. Wire strippers suitable for your choice of wire
5. Soldering iron of your choice, but at least 25w power and preferable 40w
6. Suitable non acidic flux that does not require washing or cleaning
7. Solder that is suitable for the materials used
8. Small file or scratch brush to clean side or underside of track before soldering
9. Pair of pliers to close suitcase style connectors
10. Insulation tape or heat shrink to insulate soldered joints
11. Electrical test meter with continuity function
12. Drill and various sized drill bits to drill through baseboard materials.

Why do I need a power bus system under my DCC layout?

Relying on joints in the track to transfer power could cause problems which may include:

1. Poor performance on longer layouts.
2. Voltage drop leading to reduced loco performance.
3. Distorted signals leading to loss of loco control.
4. Limitation on the number of locos and accessories that can be powered.
5. Short circuits can lead to loco or decoder damage as the control station cannot detect these quickly and cut the power.

To ensure better running the costs involved in adding a DCC power bus is small, compared to the cost of not having one.

How do I choose the correct size cable and wire for my power bus?

"What size wire should I use?" is the most frequently asked question from our customers when it comes to first planning a DCC layout and its wiring. Given that most new DCC systems have a power rating between 3 and 6 amps, selecting a suitable gauge of wire for the power bus is very important to avoid voltage drop, degradation of the digital signal and to ensure the short circuit detection system will work at all times. A small to medium sized layout with a power bus length of up to 30ft (10m) can be wired with 24/0.2mm copper cable whilst anything larger than that should be wired with 32/0.2mm copper cable. It can be beneficial if you use a large wire on a small layout, and it will allow you to have added capacity in case of future expansion or mind changes. Think about the future of your layout and if you are planning to add things like sound and lights which you currently do not have, then build that into the layout by using a larger wire.

All wire used should have an insulated cover. This is mainly for safety reasons. Some exhibition managers will not allow layouts that have been wired using un-insulated wires, regardless of the voltage and currents that they carry.

What is voltage drop, and should I be worried about it?

You also have to consider voltage drop. If your layout is large, you need a larger multi core cable to prevent this voltage drop. For example, a loco can draw 0.5A or more, lighting can be 50mA per carriage and the DCC bus can also power point motors, street lights, etc so a larger layout can draw several amps, with more than one loco running at a time. A typical 7/0.2 wire is limited to 1.4A but more significantly has a typical resistance of around 0.1 ohms per meter. Wiring a typical layout can use 10-15m of wire easily, so the wire would have a resistance of 1.5 ohms. At 1.4A this gives a voltage drop of 2.1V which could be enough to distort the signal to the point where a decoder struggles to interpret it. At higher currents, the loss will be greater still.

What size wire should I use for my dropper wires?

To maintain a high level of reliability where sound and lighting is uninterrupted, it is good practice to connect every length of rail to the power bus. Do not solely rely on rail joiners to carry the digital signal and current no matter how good the connection may seem. Rail joiners can work loose and could be a source of "noise" in the digital signal. Now saying that there are layouts that work and have droppers soldered to the rail joiners, so once again there is no right or wrong way to do this.

Not all cable that supplies the track with power has to be of a large gauge. Whilst a minimum of 24/0.2 (4.5 amp) and 32/0.2 (6 amp) wires are good for the power bus, it is possible to use smaller wires for the dropper wires. Whilst single strand 1/0.6 (1.8 amp) or multi strand 7/0.2 (1.4 amp) can be used, we consider them to be too small, so recommend that 16/0.2 (3 amp) wires should be used to make the final link between the power bus and the running rails. This is the wire that we supply in all of our DCC starter wiring kits and on our pre-wired rail joiners. This works on the assumption that even a 3ft long piece of rail may have up to 2 locomotives working on it, and modern OO gauge locomotive motors are unlikely to draw more than 0.5 to 0.75 amps each under full load. All dropper wires should be kept as short as possible which will reduce any voltage drop and help keep things tidy under the baseboard. These figures may vary depending on things like whether the loco is fitted with lights and sound.

How do I connect dropper wires to the bus wires

We sell a wide range of connectors to join the droppers to the bus, but you should really solder the connection, and then cover the joint with a heat shrink material or insulation tape. We sell scotchlok style insulation displacement connectors in two sizes. The red connectors are suitable for all of our wire sizes. These insulation displacement connectors make the job of wiring DCC bus systems easier, but should not be used with a solid core wire.

Should a DCC bus be a continuous ring?

If you read online about wiring a DCC bus, there appears to be no right and wrong way of doing it. Some use continuous runs of bus wires whilst others say not to do it and always have a break somewhere. I decided to build my power bus with a break, as that is what I thought was best at the time as there was no direct proof that one way was better than the other. If you could not leave a gap in the loop, you could not build end-to-end layouts. This is why we supply two termination filters in our kit, one for each end. Remember that the bus wiring not only supplies power to your locos it also carries the digital signal needed to operate the decoders inside the locos.

Adding a break or gap in your DCC bus wiring also means that you should also do the same in your track. What this means is that a continuous circle of track must have plastic or insulated joiners in both rails at some point in the circle to break the loop. We recommend that this is put in a similar place to the break in the bus wires, which should be equidistant to the controller feeds.

Now if you start to consider voltage drop in your wires, its best to have the controller connection in the middle of this broken ring. I always suggest that people consider their power bus as a capital T or Y. The base of the letter is where the power inputs from the controller and is joined to the bus centrally to either end using the supplied large choc block connector in our termination filter kit. This will help with voltage drop as the length of any bus wire is about half the length of the track run.

If you are running two three or four main tracks, nothing is saying that you cannot run the same number of buses under the layout. This will enable you to keep the dropper wires as short as possible and will also permit the introduction of different power districts at a later date. They can for the time being all be wired into the larger choc block connector supplied in your kit. They can all use the same colour wire if you wish, but using different colours will aid any fault finding at a later date.

What are power districts and do I need them?

Power districts allow you to have more control over the layout and the locomotives on it. It also helps to spread the power used across the whole layout and can be most useful on larger layouts by using more than separated one power bus. This does make the wiring a bit more complicated but can give better results.

A power district is a section of track that is powered separately from other sections of track on a model railway layout. Power districts also help with DCC power management. If you are planning to run lots of trains at the same time, you will need to make sure your DCC system can supply all your power needs efficiently and safely. Adding power districts to your layout can help with that. By dividing your layout into districts, you divide the total track power available into smaller more manageable pieces.

How many power districts you have usually depends on the size of your completed layout. Generally, you could split a layout up into up line, down line, and hidden sidings. That would give three power districts. You could also separate any large siding areas on the layout. For larger layouts, some power districts may have additional power boosters if they are a long way from the main power supply. Even on a small layout, though, you might want (or need) more than one power district. Separating a layout into power districts means that if one section becomes inoperable for some reason (like a short circuit, for instance) the rest of the layout can still be used whilst tracing the faults in the others. It is possible to have several power districts using only one power supply. Generally, every power district should have its own power booster and circuit protection.

Should I twist the two bus wires together?

If you have long bus runs, you should really twist your bus wires. This will also greatly reduce interference.

Twisting your bus wires together is easy. Once twisted, however, it is harder to attach dropper wires. Worse, if your railway is already built, twisting your bus wires together is not really an option. Therefore, it is recommended that you apply only about 4 twists per foot (or 12 twists per meter). If you twist all the wires before you attach the dropper wires, you may find it challenging to untwist the wire at the points you intend to attach the dropper wires. Try twisting the wire as you install the dropper wires. That is, twist the wire up to the point you intend to attach dropper wires. Then attach the dropper wires. Then continue twisting until you get to the next dropper wires.

Let us be practical about this. It is understood that when you run a track bus in the form of a twisted pair, you must untwist portions of the wire to permit one to make connections. Small or short untwisted sections will not ruin the overall benefit. The goal is to keep the far majority of the wire run twisted.

DCC bus termination/filter

On long DCC bus runs, it could be worth adding a terminator kit to all open ends of your DCC wiring. The termination filter is a relatively simple cheap device that will filter out noise and voltage spikes created by a motors brushes, intermittent wheel to rail contact, and intermittent short circuits, and will improve the quality of the DCC waveform  by minimizing reflections caused by the open end of the bus. Adding these low cost devices may extend decoder life and improve overall layout reliability. The termination kits we sell, include two components, a resistor, and a capacitor. Neither of these items is polarity sensitive. A Capacitor across the two wires will cause a dead short to AC signals, such as the digital waveform used for DCC. The resistor is in the circuit to prevent it from being a dead short between the two bus lines.

We also include two small choc block connectors which are fitted to the ends of the DCC bus wires and allow fitment of the termination filters and a larger chock block connector that allows you to join the power feed from the controller to the bus wires.

Instructions on how to fit the termination kits that we supply and how to use the scotchlok style connectors can be found here.

Keep your wiring neat and organized

Many people just run the wires under the baseboard and then think that out of sight is out of mind. Over time your layout wiring can become quite extensive even on a small layout. When you start things may be small and simple, but over time when you add electric point motors, led lights for track detection, building lighting, electrically operated signals, and any other electrical accessory, it can become complicated and a tangled mess. Where possible come up with a colour coded wiring plan and keep to it. Try to keep your wiring neat and try to bundle wires that are going to the same areas together, using either plastic ties or clips. Secure the wiring to the underside of the layout to carry these wire bundles from one place to another. Label your wires so you know where they go or what they are for. Keep a written record of what you do to any wiring and why you added it. You will thank yourself many times over when you have an electrical fault in the future and all your wires are red.

Testing

It is all too easy to race along and wire the complete layout in one go. You should electrically test for continuity on each baseboard as you progress. Use a test meter to make sure you have good soldered joints to the track, and also good connections to the bus wires. Once you are happy, move on to the next section. It is also possible to connect up a controller to test that power can be applied, but do make sure everything is working fine before placing a loco on the rails. A short or bad connection will be easier to find now, rather than when the whole layout is wired.

The final test to be carried out before moving on is the coin test. This is where you place a coin across the live rails and see whether your command station shuts the power off. If it does it is all well and good. If it does not cut power you have to trace to find the reasons why. When carrying out this coin test make sure you have only the controller connected and no locos standing on any tracks as damage may be caused. DCC track signal is neither AC nor DC. DCC is digital data sent in the form of Pulse Width Modulation on the rails so only a purpose built DCC meter or an oscilloscope will give you an accurate reading. You can get an approximate voltage with a regular analogue or digital multimeter set to AC Volts.

K.I.S.S. (Keep It Simple Stupid)

You should firstly wire up the power bus in a simple way, to get the layout operational. If you wish to you can introduce further advanced wiring to assist with fault finding and power sub-division at a later date. This has the advantage of placing the layout into use much sooner and spreads the cost of ancillary power management equipment over a longer period.

This does mean that the layout can be run to test the track, layout, operational capabilities, find faults, check turnouts and ensure turnouts are correctly wired for crossing polarity switching and many other things. Primarily, such testing will ensure that a good power supply is available with minimal voltage drop.

Conclusion

There are a lot of websites that say that 7/0.2 wire for the droppers and 16/0.2 electrical cable for the ring main should be fine for use as DCC bus wiring. I personally disagree. Saying that this setup would work for a small layout that would run only a few locomotives at the same time. We sell more 32/0.2 red and black wire for DCC bus cables with 16/0.2 wire being used for dropper cable than any other wire size. That in it self must say something.

Avoid using any non insulated materials under the baseboard, such as copper tapes, bare wires and un insulated connections. They are generally just plain dangerous, and safety is important.

Our DCC starter wiring kits

We have briefly outlined the components that are available in the different starter wiring kits that are available on the website. Please click on any of the product names below to be taken to the products page to see full specifications and a list of the components included. We have also included the model numbers of all the parts used which will allow you to find additional items should you require them.

Other websites of interest

There is also a very good website by Mark Gurries with all sorts of DCC information which can be found here. I personally find this website less confusing that the one by Brian Lambert, but both contain a wealth of information that some may find helpful.