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Beginners guide to DCC wiring

There are myths and questions about DCC bus, hopefully we can answer some of the confusion. There is a lot of information out on the internet. There is more than one way to wire a DCC layout as you will see as we go over the detail. We hope will be to answer many questions that you may have about installing a basic DCC bus to your model railway layout – it is very worthwhile.

A word of Warning…!

Electricity and electronics if handled wrongly can be very dangerous. This applies to Model Railways as well and this website and cannot be held responsible for injury or damage however caused from 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. If in doubt ask…!

DC and DCC, what is the difference?

With a DC analogue system, you only apply power to the tracks when you want a single locomotive on a single track or loop to move, as you increase the voltage via the speed control knob the speed of the locomotive increases and as you reduce the voltage the speed reduces. A very simple system, but if you wish to run several locomotives at one time then you need more controllers, block switching or track isolating sections and so forth, this can lead to very complicated wiring and track sectioning.

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. You can use only one speed control to drive more than one train at a time. It works by sending signals down the track to the ‘Decoder’ inside the locomotive to adjust speed and other functions. You can of course have extra handsets or ‘Cab’s’ if you wish to have more than one operator.

What is so important about a power bus system under my DCC layout?

Just relaying on track and track joiners for a layout can create a number of issues particular on the extremities from the controller:

  1. Poor performance on longer layouts due to voltage drop at the layout extremities which will lead to a reduced locomotive performance
  2. Messages sent to the Locomotive Decoder can become distorted leading to the loss of control of the Locomotive leading to the need to use the emergency stop button
  3. Will quickly reach the maximum number of Locomotives and accessories that can be operated at the same time.

The costs are small compared to not having a bus circuit and having the above issues.

How do I wire a DCC Bus and what tools will I need?

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 7/0.2 or 16/0.2 stranded to match your colour choice of bus wire.
  3. DCC Track Connection Boards, enough for every Track Dropper
  4. 00 Gauge Power Track Joiners
  5. Wire strippers
  6. Screw driver
  7. Electrical test meter with continuity function
  8. Drill and various sized drill bits for various drilling duties

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. If you have a continuous ring then you will increase the Amp’s the Bus is able to carry, if you use 24/02 cable, this is capable of carrying 4.5A, therefore if in a ring it will double to 9Amps. (32/02 is rated at 6Amps, therefore a ring will be capable of 12Amps) 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 filter/snubbers 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 equal distance to the controller feeds.

Now, think about voltage drop, if you put the controller at one end of an end-to-end type DCC Bus, then you have one long line that will increase the voltage drop. It would be best to connect the controller a central as possible on the DCC Bus, this will significantly reduce voltage drop and each leg of the end-to-end is about half the cable length. This is considered as ‘Y’ (star) type of a Bus circuit. If you have a continuous ring DCC Bus, then you can connect the controller at any pint, the voltage drop will remain the same at all parts of the ring.

If you are running two three of four main tracks, there is nothing 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 PCB Controller connection board that allows for multiple DDC Buses.

Cable size and how to calculate for my power bus?

The following gives you’re the general rule of thumb for cable sizing, also need to bear in mind that each locomotive will draw somewhere between 0.25A-0.5A when sizing up the Command Station:

  • A DCC system with a rated output of 3A, and a board size of 1200 x 2400 (8’x4’) will have a bus length of a minimum of 10M (30’) can be wired with 24/0.2mm and the Droppers in 16/0.2mm.
  • A DCC system with a rated output of 6A, and a board size of 1200 x 2400 (8’x4’) will have a bus length of up to 10M (30’) can be wired with 32/0.2mm and the Droppers in 16/0.2mm.
  • cable whilst anything larger than that should be wired with 32/0.2mm cable. It is of benefit if you use a large wire on a small layout, future planning to is important and a larger cable size will help to ‘future proof’ your layout. Think for the future and plan for “what if” you may add extra accessories and trains.

Should I be worried about voltage drop?

Yes, you also have to consider voltage drop. If your layout is large, you need a larger multi core cable to prevent this drop in voltage. The Amps for a layout can build quickly, 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 is the best size cable for my track power droppers?

It is important to maintain good supply to the track via the droppers. Ideally a connection to each piece of running track, with the thought that there could be 2 Locomotives in a length of track. A Lcoc with full load is anywhere between 05 and 0.75A each. 16/02 Wire has a load carrying of 3A, I would make this the rule of thumb for a 00-gauge track dropper. Bigger gauge may need bigger droppers like 24/02 or 32/02.

Ideally you need to keep the rack droppers as short as possible for 2 reasons, one is tidiness, but more important to reduce voltage drop as much as possible. We sell Track Power joiners with 200mm of wire attached in 7/02 and 16/02, these would have to be a tight track connection and provides an easy method of connecting track power, but if they are loose, they can induce ‘noise’ on the track power. You can also solder the cable direct to the track, although a fiddly jab to do…

How do I connect dropper wires to the bus wires?

There are a few options for connecting the Rail Track droppers to the Bus circuit, one such option is to solder join and cover with a heat shrink sleave. This is a tedious method and if you want to change anything you have to unsolder or cut out the joint. Another option is we sell a Track Dropper Connection board to join the droppers to the bus. This is a non-solder system using screw type connectors, but you can undo and reuse in the event of alterations.

What are power districts and do I need them?

Power districts are a way of creating sections on the layout to help power distribution. Each section of track is electrically separated form the next section with its own Booster Power supply, allowing for better power and DCC management. If you plan to run a large number of trains, it is essential to make sure all parts of the layout can supply all the power efficiently and safely. Power Districts can help with that. This will divide the total track power into smaller manageable sections.

So how many power Districts do I need? Well, this will depend on how big the layout is, how many trains and how many accessories to I plan to run. There are many ways you can split a layout into Power Districts, like up Main, Down Main, Hidden sidings and so on. It is possible to have a number of power districts using only one power supply. Generally, every power district should have its own power booster and circuit protection.

Why twist the two bus wires together?

If you have long bus runs, you need to twist your bus wires. This will quite simply reduce interference.

Twisting your bus wires together is easy using a cordless drill, measure the length you require for the 2 colours of cable, tie one end off onto a static object (I use a door handle or something similar) put the other end of the wire into the chuck of the drill, pull out till taught, then spin the drill till the desired twist is achieved, it is recommended that you apply only about 4 twist per foot (or 12 twists per meter). Continue onto the next section up to the next dropper wires.

Small or short untwisted sections are acceptable and will not affect the overall benefit. The goal is to keep the far majority of the wire run twisted.

DCC bus termination filter/snubber

We would recommend on a long DCC bus run fitting one of our DCC End of line Termination filter/Snubber boards. The DCC Termination Filter boards is a relatively simple cheap device that will filter out noise and voltage spikes created by motor 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.

Tidy wiring is safe wiring

Tidy wiring and cable routing under the baseboard is essential for a safe and reliable railway. Just because it is not visible means it can be any old how, this can lead to short circuits, and make fault finding very difficult. Start tidy and keep it tidy, make up a colour scheme and stick to it very religiously. Your layout may start small and simple but more and more working effect items are coming on the market, this will lead untimely to more wiring. Think about building lighting, street lighting, Electric point motors and so on. Use some form of cable management like clips and zip ties to hold your wiring in place. Label cabling up and make a record so they are easy to identify at a later date when you have forgotten what all the cables do…

Testing

Testing a DCC Bus on a layout is very simple using a Multimeter using the continuity setting to make sure you get continuity from the Bus to the Track, also to make sure you don’t get any short circuits. Install a section, test it, move on to the next. This way you know from the work go that all should be ok. Once you have completed the installation of the Bus circuit, one more final test to complete is to make sure the short circuit function works by connecting your controller, make sure no trains are on the track, apply power and put a metal object across the rails. The controller should shut down, then you know all is well.

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.

Conclusion

Don’t be fooled into thinking that 7/0.2 for Droppers and 16/02 Bus is ok for all layout sizes, this possibly maybe ok for a very small layout like a small N-gauge. Go for the 24/0.2 for the Bus as the smallest option, 32/0.2 if you can.

Please do not use 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.

Model Rial Electrics DCC Wiring Kits

We have a range of DCC start kits, a few items have been mentioned, but click here to see the kits that are available. All items are available separately as well, please browse the E-Shop

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