DCC Bus wiring
We have been asked the question about bus cable wiring many times. We
always advise customers to purchase the largest wire they can reasonably afford.
This is not advice which is business related, it is sound advice.
Relying on joints in the track to transfer power can cause many problems
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.
Choosing the correct bus wire size
Given that most high end systems have a power rating of 3 to 5 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.
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 of the 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 works on the premise that even a 3ft long piece
of rail may only have up to 3 locomotives working on it and modern OO gauge
locomotive motors are unlikely to draw more that 0.5Amps each under full load.
The above wire will carry up to 1.4 to 1.8 Amps easily. Dropper wires should be
kept as short as possible.
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 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..
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, 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.
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 both 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