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Wiring a DCC power bus

10 August 2010

One of the most quoted myths of Digital Command Control (DCC) is that it is purely a two wire system where two wires connected to one location on the layout is all that is needed for full operation. In some instances, a small, simple layout can be supplied with all of the current it needs using just two cables, such as an Inglenook type where the lengths of running rail are short and turnouts wired to be live on both the clear and diverging routes.

However, most layouts are usually larger and more complex than Inglenooks and no two layouts are ever the same. Whilst simple DCC power bus wiring consists of two main bus wires, connections to the track via droppers is an essential part of the wiring of the power bus, which for many is more than the quoted two wires. Furthermore, there is another set of cables that must be installed on all but the tiniest layouts called a ‘controller bus’, which is necessary for connecting one or more controllers to the layout.

In the first of a short series on DCC layout wiring, this article takes a look at the power bus and associated dropper wires, the set of wires which supplies current from the DCC base station to the track – all of it. It looks at the simplest wiring needed to get a layout ready for DCC operation, so trains can be run, track and equipment tested and bedded down.

When wiring up a typical layout, regardless of whether it consists of a UK or US outline theme, I use the following tools and materials.

 1.   DCC bus wire of 24/0.2 gauge in black and red (32/0.2 gauge if wiring runs are likely to exceed 25 feet).
 2.   DCC bus to track dropper wire of 1/0.7 bell wire which is single strand making soldering to the running rails
       much easier to do.
 3.   Carr’s ‘Speedy’ solder, a non-corrosive general purpose solder for electrical wiring.
 4.   Wire strippers – don’t use your teeth!
 5.   Carr’s Orange flux which is non corrosive.
 6.   Mid range soldering iron: a 25 Watt iron is more than adequate.
 7.   Modelling knife, sharp blades and a steady hand.
 8.   Scotchlok suitcase insulation displacement connectors, otherwise known as blade tap connectors.
 9.   Pliers strong enough for use on Scotchlok connectors or suitable crimps if there are a lot of connectors to use.
10.  Suitable connectors capable of carry 5 Amps (8 Amps for O gauge) if the layout design is intended to be portable.

I present my top ten tips for basic DCC power bus wiring which have ensured reliable operation on my own layouts, both portable and fixed. They will get you up and running in no time!

1. 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 them 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 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.

2. Remember the difference in electrical resistance between nickel silver rails and copper wire.

There is a temptation to think that the running rails can carry a digital signal and power supply 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 which should be of suitable gauge multi or single strand copper cable with adequate insulation.

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 cable; large gauge copper wire has much lower resistance to electrical current than steel or nickel silver rail.

3. Clearly define a left and right running rail and allocate a wire colour to each.

The basics of power bus wiring on an N gauge layout with Dapol Class 153s in attendance. The power bus wires (A) are black and red on this layout, with the connecting droppers (B) clearly seen connecting the running rails to the power bus via a soldered splice (C).

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.

Many add-on components 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.

4. Choose a suitable gauge of power bus wire to suit the required 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.

5. Allow for future 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!

6. 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, 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 and modern OO gauge locomotive motors are unlikely to draw more that 0.5Amps 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.

7. 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 or coin test. With the digital system fully powered up, place a coin on the track and 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.

8. 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. Multistrand wire works better with Scotchlok 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 is hard to feed along a layout and harder 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!

9. Scotchlok 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 connect 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 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 hardwiring methods.

10. Wire up the power bus in as simple a way as possible to get the layout 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.

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. Test and bed in the track before adding complexity to the layout using circuit breakers, power districts and power management models which will be discussed in the next article in this series. Remember, the installation of a DCC system and associated wiring is not an end in its self but one of the processes necessary for that all-important authentic operation of the layout.

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