Wiring a DCC bus

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.

Choosing the correct bus wire size
Given that most new 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 around 20 to 25 ft. 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 hard to attach feeders. Worse, if your railway is already built, twisting your bus wires together is not really an option. While the more twists the better, more twists will make it much harder to attach feeders. Therefore, it is recommended that you apply only about 1 twist per foot (or 3 twists per meter).  If you twist all the wire before you attach the feeders, you may find it challenging to untwist the wire at the points you intend to attach the feeders. Try twisting the wire as you install the feeders. That is, twist the wire up to the point you intend to attach a feeder. Then attach the feeder. Then continue twisting until you get to the next feeder.

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.

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. Simply solder both together, then connect the free lead of the capacitor to the left bus wire, the free lead of the resistor to the right bus wire, or vice versa.

 

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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