The Basics of DCC Power
Bus and Dropper Wiring
By Nigel Burkin
The basics of DCC Power Bus and Dropper Wiring
2
Copyright Nigel Burkin © 2011.
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The basics of DCC Power Bus and Dropper Wiring
Contents:
Introduction........................................................................................................................... 4
Materials and tools................................................................................................................ 5
Choosing wire .......................................................................................................................5
Important considerations....................................................................................................... 7
Connect each length of rail to the power bus for reliable power supply and
don’t rely on rail joiners for electrical conductivity.....................................................7
Remember the difference in electrical resistance between nickel silver
rails and copper wire.................................................................................................7
When planning layout wiring, clearly define which rail is the ‘left rail’
and which is to be the ‘right rail’................................................................................7
Choose suitable wire to meet likely current demand................................................8
Allow for future expansion........................................................................................ 8
Single core bell wire can be used for droppers but kept as short as possible..........9
Avoid making loops in the power bus wiring. Radial or linear wiring is
considered best practice...........................................................................................9
Multi strand or single strand cable can be used for the main power bus..................9
'Scotchlok' type connectors save time but increase costs........................................9
Using Scotchlok-type insulation displacement connectors:...................................................10
Making a start on wiring.........................................................................................................11
To conclude............................................................................................................................12
3
The Basics of DCC Power Bus and Dropper Wiring
4
Introduction:
Users of Digital Command Control (DCC) systems very soon take one of its principle benefits as a given: fully
functioning lighting effects and sound. The key feature of DCC layouts is that the track is always powered or
live, something which can take a little getting used to when it flies in the face of how conventional model
railway control works. On conventional (analogue) layouts, trains are driven by changing the power in the track
using a simple controller whilst sections are directly controlled to isolate locomotive using switches on a control
panel. If you do not want a locomotive to move, it's isolated from the rest of the layout in a dead section until
switched back on. This has obvious implications for operational features such as lighting effects. No power
means no lights. The same goes for a train in motion which will have lights illuminated until power is turned
down to nothing to slow the train down to a stop. Unless fancy electronics are included in the model, the lights
go out as the train comes to a stand. Not very realistic.
DCC overcomes all of these operational shortcomings by placing the controller (decoder) in each locomotive,
EMU and DMU. With power constantly live in the track, there is always power for sound and lights, amongst
other things. The decoder is the controller, providing power to motor and on-board systems as and when it is
called upon though its unique address and digital signals from the command station. Such functionality
requires a different approach to layout wiring and this article explains how to wire up a layout with a basic DCC
power bus so that all of your track is constantly powered enabling decoders to do their job. It looks at the
simplest wiring needed to get a layout ready for DCC operation, from base station to the furthest piece of track
so trains can be run, track and equipment tested and bedded down. The materials and type of wiring is also
described, together with materials and options for refining the wiring for particular circumstances.
Good layout wiring is essential for the reliable operation of DCC systems and decoders rely on a constant supply of
current to keep sound decoders and lighting systems constantly working even when locomotives are stationary. Many
ready to run models come complete with sophisticated lighting circuits including cab and interior lighting together
with sockets to accept plug and play decoders.
The basics of DCC Power Bus and Dropper Wiring
5
Materials and tools:
When wiring up a typical layout I use the following tools and materials, regardless of whether it consists of a
UK or US outline theme; or whether its N gauge, HO gauge, OO gauge or O gauge.
 DCC bus wire of 24/0.2 gauge in black and red.
 Consider the use of 32/0.2 gauge if wiring runs are likely to exceed 25 feet or you are faced with the
higher current demands of an O gauge layout (or both!).
 DCC bus to track dropper wire of 1/0.7 gauge bell wire (in black and red) which is single strand
making soldering to the running rails much easier to do.
 Carr’s ‘Speedy’ solder, a non-corrosive general purpose solder can be used for electrical wiring.
 Wire strippers – don’t use your teeth!
 Carr’s Orange Label flux for those areas where a little more help with soldering is needed. Orange
Label flux is non corrosive.
 Mid range soldering iron: a 25 Watt iron is more than adequate.
 Modelling knife, sharp blades and a steady hand.
 'Scotchlok' type suitcase insulation displacement connectors, otherwise known as blade tap
connectors.
 Pliers strong enough for use on 'Scotchlok' connectors or suitable crimps if there are a lot of
connectors to install.
 Suitable connectors and cable capable of carry 5 Amps (8 Amps for O gauge) if the layout design is
intended to be portable.
 Insulation tape.
 A power drill for drilling holes through the baseboard top.
 2mm drill bit.
Choosing wire:
Choosing the correct grade or gauge of wire is important to avoid voltage drop. The main bus wires must be of
a fairly hefty gauge to avoid voltage drop over longer runs as well as being able to carry the current demand of
a number of operating locomotives, up to the maximum output of the DCC system. Dropper wires, the wire that
links the power bus to the track is not so critical because it will only used in short lengths and will only carry
power intermittently in most cases, as a locomotive crosses onto the length of track it supplies.
When planning your shopping list, do not be tempted to use poor grade wire or rely on the track to carry
The basics of DCC Power Bus and Dropper Wiring
6
current to all parts of the layout. Avoid smaller gauges of wire thinking that your current demands are not high.
What happens should you upgrade a simple starter system to a fully fledged 5 Amp advanced system? Do not
build in obsolescence into your layout which will cost you more time, frustration and money in the future.
Choose wire and connectors carefully. It may have to carry up to 5 Amps of current and voltage drop must be avoided.
Using a minimum of 24/0.2 wire for the power bus will ensure reliable operation on small to medium-sized layouts.
Wire strippers are a 'must' and suitcase style insulation displacement connectors can speed up the job of installing
dropper wires.
The basics of DCC Power Bus and Dropper Wiring
7
Important considerations:
Connect each length of rail to the power bus for reliable power supply and don’t rely on rail joiners for
electrical conductivity.
The primary feature of DCC that brings so many of the benefits of the operating system is having a constant
supply of current to the track. This enables the operation of digital sound systems and running lights together
with independent control of the locomotive regardless of whether the train is in motion or not. 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 rely on rail joiners to carry current no matter how good the connection may seem
to be when the layout is built. Rail joiners can and frequently do work loose and are a source of ’noise’ in the
digital signal. As dirt and other grit works into the joints, the signal may become further impaired.
Dropper wires connect every section of rail to the power bus (A) on the author's layouts ensuring reliable power
supply. Note the soldered joints (C) connecting the dropper wires (B) to the power bus. The models shown are Dapol
N gauge Class 153s.
Remember the difference in electrical resistance between nickel silver rail and copper wire.
There is a temptation to think that the running rails can carry a digital signal and power supply over long
distances to remote areas of the layout. It is equally tempting to provide just one supply to sidings and fiddle
yard roads in the belief that complete wiring is not necessary. Poor power supply will corrupt the DCC signal
and make the all-important short circuit detection system unreliable. Every part of the layout must be in contact
with the power bus. The cable must be able to carry the maximum current load of the digital system to be used
on the layout. In the case of most high end systems, 5 Amps is common and 1 to 2 Amps typical for starter
systems. Remember, when tempted into saving a few pounds in the purchase of wire, large gauge copper wire
has much lower resistance to electrical current than steel or nickel silver rail and the incremental increase in
cost is still a tiny fraction of the overall cost of building the layout.
When planning layout wiring, clearly define which rail is the ‘left rail’ and which is to be the ‘right rail’.
Allocate a cable colour to each, label them and apply the protocol consistently throughout the layout wiring
project. Take care to identify the rails with circular layouts and be sure that the outer rail and inner rails are also
correctly related to ‘left’ and ‘right’ rails.
The basics of DCC Power Bus and Dropper Wiring
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The reason for this practice becomes obvious when buying and using add-on components which need to be
wired correctly with the left and right rail wiring clearly defined so they remain in sync with the command
station. Such components include reversing modules, block occupancy detection for asymmetrical DCC
operation and power district boosters. Out-of-sync wiring will only result in frustrating short circuits when a train
enters a reverse loop or an adjacent power district. Short circuits could damage expensive equipment and
decoders too.
Choose suitable wire to meet likely current demand:
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 important to avoid voltage drop, degradation of the digital signal and to ensure the short
circuit detection system will work at all times and quickly! A medium size layout with power bus runs 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.
Allow for future expansion.
Plan for expansion! Whilst a basic entry level system of 1 to 2 Amps may prove adequate for layout operations
today, upgrading to a high end system to enjoy more of the features of DCC may put your power bus wiring
under stress if cable of less than 24/0.2 has been used. Smaller cable gauge of 16/0.2 or less is fine for low
current applications but will offer both considerably more resistance to higher current loads and also to the
effective operation of short circuit detectors in the base station. Short circuit protection does much to protect
expensive equipment on the layout and decoders from serious damage!
Dropper wires at the surface of the layout, one for each section of rail, ready for trimming and soldering to the rails.
Droppers can be composed of 1/0.7 single core bell wire or even 7/0.2 equipment wire. The stiffness of the former
makes it easier to solder to rails and to shape so it can be kept out of the way when doing other wiring jobs. A
Graham Farish N gauge Class 08 is shown in this picture.
The basics of DCC Power Bus and Dropper Wiring
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Single core bell wire can be used for droppers but kept as short as possible.
Not all of the cable that supplies the track with power has to be of an large (and relatively expensive) gauge.
Whilst a minimum of 24/0.2 for 5 Amp N and OO gauge layouts is considered to be good practice together with
32/0.2 for 8 Amps O gauge layouts; simple single strand 1/0.7 bell wire may be used to make the final link
between the power bus and the running rails. This works on the premise that even a 3ft long piece of rail may
only have up to 3 locomotives working on it at a time and modern OO gauge locomotive motors are unlikely to
draw more that 0.5 Amps each under full load, unless it’s a Heljan Class 47! Bell wire will carry up to 3 Amps
for short periods of time; the time it takes a locomotive to pass over that length of track and only if the
connection (or dropper) is kept as short as possible: no more than 4 to 6 inches.
Avoid making loops in the power bus wiring. Radial or linear wiring is considered best practice.
If creating a ‘ring main’ of the power bus was your plan, may I persuade you not to go down that route. Most, if
not all, DCC equipment manufactures would suggest a linear power bus, kept as short as possible, is best
practice, even for an oval or continuous run layout scheme. Some modellers will wire the layout in a star or
radial pattern with each leg of the star kept as short as possible. This is a consideration for circular track plans,
so install double rail breaks to coincide with the break in the power bus at the far end of a circular layout.
Remember, the longer the power bus, the greater the chance that electrical resistance (even in copper wire)
will result in voltage drop and digital signal degradation making short bus runs in a radial pattern desirable. For
example, if a power bus wire is likely to be longer than 25 feet, upgrade from 24/0.2 gauge to 32/0.2 gauge to
prevent voltage drop at the furthest ends of the layout or wire the layout with two shorter runs of wire by
placing the DCC base station at a central point of the layout rather than the end.
In the United States, modellers use a simple test to see if power supply is adequate to trigger the short circuit
detection mechanism in the base station, and that test is called the 'dime' test. With the digital system fully
powered up, a coin is placed on the track to see how quickly the command station shuts down. More than a
microsecond of delay and you have a problem! Solve it by installing more dropper wires or by beefing up the
power bus.
Multi strand or single strand cable can be used for the main power bus.
Choosing suitable wire for a power bus is a matter of personal judgement. I personally buy the highest grade I
can to ensure adequate power supply, room for system expansion, durability and to avoid signal degradation.
Power bus wires can be either single or multi strand; it matters not as long as the wire gauge will comfortably
carry the peak current load. Multi-strand wire works better with 'Scotchlok' type connectors whilst single strand
wire is rigid enough to be shaped to match the track above at retain its shape. One downside of single strand
copper wire is that it can be harder to feed along a layout and to bend in tight corners.
One location on one of my layouts was wired using self adhesive copper tape. The same principles of current
load apply when selecting and using copper tape for part or all of the power bus. I chose to use it in an area
where ordinary cable runs could droop and become snagged by storage boxes under the baseboards. It is
also used at one location on my fixed home layout where there is an operator access point (people underpass)
and the last thing I want to do is strangle my operators with 24/0.2 cable!
'Scotchlok' type connectors save time but increase costs.
Mechanical connectors can save a great deal of time when it comes to connecting or splicing dropper wires to
the power bus. Most connectors, such as the Scotchlok type, are insulation displacement blade taps which
have a blade that connects both the main cable and the dropper wire without having to manually strip
insulation from the former or make a break in it at any point. The main power bus wires are simply run the
length of the layout in whatever form that suits the layout in whatever form that suits the layout design, linear
or radial, without a break in them because joins could become a weak point in the wiring. Work out which
length of running rail are to be connected to the power bus (ideally all of them!) and drill holes through the
base board top and track bed to accommodate the dropper wires. Each dropper should be colour coded to
The basics of DCC Power Bus and Dropper Wiring
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match the power bus wires for identification. Position along side the power bus, keeping them as short as
possible and fold the Scotchlok connector over the power bus wire and thread the dropper wire into it before
crimping tight with pliers or a crimping tool. Test the connection before moving on to the next one. The
connectors can take up more space and cost a little extra over the hard wire soldered splice where a length of
insulation is stripped from the bus wire and 15-20mm of dropper wire wound around at least 4 times before
flooding with solder. Scotchlok connectors cost a little more, however save a great deal of time compared to
hard wiring methods.
Using Scotchlok-type insulation displacement connectors:
Slip the connector over the power bus and introduce the end of the dropper to the narrow slot in the side of the
connector. Do not strip insulation from either wire.
Using crimps or a special tool (not shown) press the splicing blade down so it cuts into the two wires. This makes the
connection. Once done, fold over the insulating lid.
The basics of DCC Power Bus and Dropper Wiring
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Wire up the power bus in as simple a way as possible to get the layout into operation. Introduce
further wiring to assist with fault finding and power sub-division at a later date.
Whilst it is tempting to introduce complexity to the basic power bus from the beginning, experienced DCC
users will install the minimum of power bus to get the layout running, with the minimum of whistles and bells.
Power bus wiring can be divided up for the introduction of sub-power districts and electrical blocks for easier
fault finding at a later date as the layout is developed. 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.
The layout can be run (in its un-scenicked state) to bed in the track, find faults, check turnouts and ensure
turnouts are correctly wired for crossing vee polarity switching. Primarily, such testing will ensure that good
power supply is available to all parts of the layout with minimal voltage drop. An exception to this ‘keep it
simple, stupid’ approach is when a reverse loop is included in the track plan. A module capable of
automatically changing the polarity of track current as a train enters and traverses the loop must be
incorporated from the start, together with the correct isolating rail joiners to make the reverse loop independent
of the rest of the layout. However, introducing power districts and sub-power districts could be built in at a later
date.
Making a start on wiring:
Each section of track and length of rail is carefully identified and a 2mm diameter hole drilled through the
baseboard to through to the underside to accept a dropper wire. This is a task that can take a little time and
the resulting saw dust will need vacuuming afterwards.
Place the digital system in a convenient location on the layout; ideally in a central position rather than one end
or the other so wiring runs can be kept to a minimum. Work out the best route for the main power bus wires
from the digital system to the extreme ends of the layout. It is quite acceptable to have several branches in the
main power bus and they can be tapped onto the main run of bus wire using Scotchlok insulation displacement
connectors designed for larger gauge wire (coloured blue in the Nairnshire Modelling Supplies range). The
route should take into account the greatest concentrations of dropper wires to keep them as short as possible.
With the power bus wires in place (check your equipment manual for any need to introduce a twist in the
power bus), install droppers one at a time from the spool, bending them at the baseboard surface to prevent
them dropping through to the floor. Cut the length from the spool or coil of wire when the desired length has
been obtained thus saving wire. Do not solder to the rails at this point. When using insulation displacement
connectors, do not strip the end of the dropper wire before feeding it from the top side of the layout to the
power bus. If using a soldered connection such as a spliced join to the bus, strip around 25mm from the end of
the dropper wire before inserting in the hole.
Once all the dropper wires are installed, look around and double check you have the right colours for the
correct rail (left or right rail) so to avoid any confusion when connecting everything up. The inexpensive way of
connecting up the droppers is to strip around 15 mm of insulation from the power bus wires and wrap the
stripped end of the dropper to form a direct splice that can be flooded with solder. You can attach a number of
droppers to one area of stripped power bus especially if room is too tight to accommodate more that an one or
two insulation displacement connectors.
When using insulation connectors, use the red ones for connection droppers as these are designed for small
gauge wire, including bell wire. They do work well with 24/0.2 bus wire too, although when splicing bus wire
together, use the blue ones.
With all the wires connected to the power bus, start the job of stripping a few millimetres of insulation from the
track end of the dropper wires and soldering them to the rails. There are many views on the nest way to do
this. I prefer to solder to the outside and sometimes trade appearance for reliability. For example, a slightly
larger than usual blob of solder can be trimmed and will definitely hold your dropper in place for ever! Note that
the use of single strand bell wire makes soldering droppers to the rails so much easier to do and the wire can
be bent to hold whatever position is desired whilst wiring and soldering work is undertaken.
The basics of DCC Power Bus and Dropper Wiring
12
Once all droppers are fitted and before adding ANY DCC devices or connecting up the DCC system, use a
conventional 12v DC power pack to test the layout by running a locomotive around it. Short circuits will be
easy to detect and to rectify. Assuming that you have wired up turnouts properly and made sure there's no
confusion between your left and right rail wiring colours, it should all work well. Look out for any metal tools
lying on the track if a short persists and wiring all appears to be fine. Sometimes, a small sliver of metal
shaving from track building can bridge an insulation gap and cause problems, so look for that sort of thing too.
To conclude:
The initial wiring of a new digital layout need not be any more complex than stringing two power bus wires
along the length of the layout, installing droppers to connect each running rail to it and plugging in the DCC
base station. Some turnouts such as Peco ‘Electrofrog’ turnouts and hand built ones with metal crossing vees
will need to be wired for polarity switching and I plan to write a separate article on how to do that in the near
future. Testing and bedding in the track before adding complexity to the layout is good practice. Remember,
the installation of a DCC system and associated wiring is not an end in its self but one of the processes
necessary to enjoy the many benefits DCC has to offer.
There's a lot of wiring work required to get a complex layout up and running. Unfortunately, layout wiring is one of
those jobs in which the time and effort is not immediately visible at the surface of the layout, unlike scenery or
structures. However, when the trains begin to run with sound and lights, the benefits of your hard work will be
realised!
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