From the DCC Wiki: http://www.dccwiki.com/Converting_to_DCC
This article describes the steps on converting your layout from Analog (Direct Current DC) to Digital Command Control DCC. Remember, converting from analog to DCC can be costly and time consuming - but it's very rewarding once you have done so. Also, the first step (upgrading your locomotives) should be done first and you can complete this task at your own pace. The track wiring needs to be done all at once since you cannot run Analog and Digital Command Control at the same time.
Whether you are converting a Direct Current layout, or building a new layout, wiring is one of the most important aspects of Digital Command Control.
First Things First
This article assumes that: You have read the DCC Tutorial and have the basic principals of DCC down; That you have selected a DCC system, you may or may not have purchased it yet; and you have read the Wiring article.
For many people, you can simply disconnect the analog power pack and simply hook up the two wires from the DCC system to where you used to hook up the two wires from the analog power pack. It should be noted, that this is the most basic, easiest, and quickest way to get your track running DCC. However, you should create separate power districts to handle shorts and reverse sections.
Simply put, you can convert quickly once you have a few engines and wires connected. You will want to go back through your system to make sure your booster detects shorts, and if you have any reversing loops, make sure those are handled properly.
If you're impatient and want to get your locomotives running on DCC before you've converted them all you can set up your layout for Mixed DCC and Analog Operation.
Unlike old fashioned direct current systems, the Digital Command Control booster can put out a lot more power, from five to eight or more amperes of current. Compared to the typical direct current (or Analog) setup using multiple power supplies with a maximum output of one or two amps, this is a lot more power. A short can do a lot of damage quickly.
When converting to DCC, the first and most important thing on the list is to verify that the wiring is capable of handling the increased power capacity you will have. Light gauge wire is not up to the task!
As a rule of thumb, the main source of power to the track, the TRACK or POWER BUS, should be at least 14AWG. Bigger is always better! For drops to the track, 18 to 22 AWG is sufficient. Short lengths of a lighter gauge wire will also be adequate if you need them to connect your track bus to the booster.
Generally , many older layouts were wired in stages using a variety of wires and techniques, with many splices and connections. This can make troubleshooting difficult, and if doing an upgrade, replacing the wiring is probably a better option.
Sloppy wiring and poor wiring practices can lead to new problems, like poor operations, burnt out decoders, short circuits causing damage and even runaway locomotives. So it is better to rewire the layout, document what you are doing, and use adequate wire for the task.
- You will see claims that 14AWG is overkill, but in reality, it is a "Best Practice". There are three things to remember: This is not Direct Current, it is not 60 Hertz, and the high frequency digital waveform doesn't behave like them.
For added insurance, test your track wiring using the Quarter Test. If it fails, find the problem and solve it.
DC and DCC?
Yes, and NO.
In the early days of DCC (twenty years ago), the ability to run both Digital and Analog simultaneously was considered important. Many would have forgone the improvements that DCC could offer if they suddenly couldn't use any Analog/DC equipment. Today, not really an issue, with low cost decoders available, and even locomotives with a decoder installed available at the hobby shop.
Why not? Because an oversight can do a lot of damage when suddenly the high current DCC signal is connected to the DC trackage by accident.
In the beginning there were devices available designed to cut out if the two worlds were to meet, but most have been discontinued due to lack of interest today.
While you can disconnect your analog throttle/power pack and use it as a power supply for your DCC System, as a rule it is only a temporary solution. The typical power supply used for an analog layout (or your power pack) will not have enough voltage for satisfactory operation, nor can it supply enough current to be of any real use.
Obtain a power supply with the voltage and current ratings appropriate for your system. Check your owners manual for recommended power supplies, or the minimum and maximum voltages, and whether you need a Direct Current (DC) or an Alternating Current (AC) input. In some cases, either will do. For current, you should have at the minimum the current rating your booster is capable of. Larger high current power supplies can be an unnecessary expense. A few amps extra isn't an issue.
Common Rail Wiring
This can be tricky with DCC. If the entire layout is wired as one single electrical circuit, or Power District, everything should be fine. Best to be avoided. Also becomes an issue with reversing loops or tracks that cross each other.
This is the DCC term for blocks used with Cab Control and other methods of running multiple trains. Their advantage comes from their creation using power management devices, which also have circuit breakers in them. This allows for electrically separate sections of track, and should a short occur, only that segment is affected. All the other trains in the other power districts keep running as if nothing has happened.
A reversing section is created with the track loops back onto itself, and what was the right rail becomes the left rail. These are dealt with using mechanical means, such as a switch, or automatically using an Auto Reverser (AR) device. In either case, gaps must be cut in the rails to eliminate shorts on both sides of the loop.
One of the more troubling issues. The Closure or Point Rails must not be electrically connected together. If so, a metal wheel can bridge between it and the Stock Rail, causing a short. As DCC systems are much more sensitive to shorts, this will cause problems.
One way to avoid this is to use a DCC Friendly Turnout, or modify those that can be modified to eliminate this issue.
Phasing is very important if you are going to use multiple boosters. Both should be connected to the track in the same manner. To verify, a simple DMM on the AC Volts Range will suffice: Place one probe on either side of the gap and read the voltage. It should be Zero. If you see voltage, the boosters are out of phase. When a locomotive bridges the gap between their districts, they will start to cycle endlessly if they have Auto Reverse capability. To prevent that, disable the auto reverse function. Then one will shut down. Correcting the phase relationship between the two will stop that from happening.
Also, connect a heavy gauge return wire between the boosters, as per the manufacturer's instructions, so that any return current should this happen can pass through this wire instead of your throttle network wiring.
If you have a functioning signaling system, some consideration is needed here. For DCC, a complete rethink and reconstruction may be needed, but if it uses optical methods independent of the track, there shouldn't be too many issues.
Accessories, like point motors for animation and switches can be hooked up to stationary decoders. This allows you to control your switches and animation through the DCC system.
One of the first steps you need to do is convert all the locomotives you wish to run on the layout to DCC by installing a decoder. Be sure that you select mobile decoders that are compatible with both analog (DC) systems as well as DCC. This feature is called Analog Conversion and allows you to pace yourself (and/or your wallet) in converting your fleet from analog to DCC. Once this is done, you're nearly half way there.
You don't have to convert your entire fleet, and probably won't. Just choose the ones you want to run, and you can expand the DCC Equipped fleet later. Some locomotives will be difficult to convert, those can be sent to the display track or sold off, much like a real railroad does.
Analog Conversion is useful if you install decoders but don't have a DCC system in place yet. Later, when you have completed the conversion, consider switching the decoders to NMRA Digital Only operation, to prevent runaways. Unless you like to "lease" extra power to other railroads...