Railwayscenics guide on DCC bus wiring

DCC Bus wiring
We have been asked the question about DCC bus cable wiring many times. We have produced this page which we hope will answer many question you may have about installing a DCC bus to your model railway layout.

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. The general consensus is that a DCC bus should not be a continuous circle, it should have a break in it. 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.

Now if you start to consider voltage drop in your wires, its best to have the controller connection in the middle on 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 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 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 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.

Choosing the correct bus wire size
Given that most new DCC systems have a power rating of 3 to 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.

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.

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 are a source of ’noise’ in the digital signal.

Not all cable that supplies the track with power has to be of a large gauge. Whilst a minimum of 24/0.2 for 4.5 Amp N and OO gauge layouts is considered to be good practice, 32/0.2 for 6 Amps O gauge layouts, simple single strand 1/0.6 or multi strand 7/0.2 wire may be used to make the final link between the power bus and the running rails. Our 16/0.2 wire has an outside measurement of 1.6mm and is also suitable. This is the wire that we supply in all of our DCC starter wiring kits. This works on the assumption that even a 3ft long piece of rail may only 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. The above wire will carry up to 1.4 to 1.8 Amps easily. All dropper wires should be kept as short as possible. These figure may vary depending on things like whether the loco is fitted with lights and sound.

Connecting dropper wires to the bus wires
We sell 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 connectors in two sizes. The red connectors are suitable for all of our wire sizes. These mechanical connectors make the job of wiring DCC bus systems easier, but should not be used with a solid core wire.

Voltage drop
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.

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.

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 twist per foot (or 12 twists per meter).  If you twist all the wire 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 a dropper wires. Then attach the dropper wires. Then continue twisting until you get to the next dropper wires.

Lets 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/short untwisted sections will not ruin the overall benefit. The goal is to keep the far majority of the wire run twisted.

Why do I need a power bus system under my layout?
Relying on joints in the track to transfer power can cause many problems including:
Poor performance on longer layouts.
Voltage drop leading to reduced loco performance.
Distorted signals leading to loss of loco control.
Limitation on the number of locos and accessories that can be powered.
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.

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 of time.

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 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 which 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.

Keep your wiring neat and organized
Many people just run the wires under the base board and then think that out of sight is out of mind. Over time your layout wiring can become quite extensive even on small layouts. 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 signal and many more thing, it will become complicated. Where possible come up with a colour coded wiring plan, and keep to it. Try to remember as you go along to keep your wiring neat. Bundle wires that are going to the same areas together with plastic ties. 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.

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 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. The termination kits we sell, include two components, a resistor and a capacitor. Neither of these items are polarity sensitive. 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 allow you to join the power feed from the controller to the bus wires.

Micro DCC non solder layout wiring starter kit
Model Number: 218-009
Micro DCC bus kit
Click image above to taken to product page
DCC power bus wiring starter kit for small layouts
Model Number: 218-008
Small DCC bus kit
Click image above to taken to product page
DCC power bus wiring starter kit for medium layouts
Model Number: 218-005
Medium DCC bus kit
Click image above to taken to product page
DCC power bus wiring starter kit for large layouts
Model Number: 218-004
Large DCC bus kit
Click image above to taken to product page

We have a copy of a booklet produced by Nigel Burkin that explains the basics of DCC wiring. This can be downloaded by clicking here

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