Wiring standards part 1: Power

I've built (or more accurately, partially built) 5 previous indoor layouts, 3 in HO and 2 in N scale. I learned a lot from each one, but they were all small, varying in size between 32 and 44 square feet. On this layout I've found that there are some things you won't learn until you build that big layout, and one of them is how to set up a well-organized wiring system. As I've reached the point in the staging reconstruction where wireing is added, now is a good time to start describing the system. This topic will take a few posts.

When I started this layout I took my first shot at a wiring system, and a lot of things did go well. I set standards for the type of wire used for each purpose, developed a system of a track power bus with terminals and feeder wires, labeled each wire per a naming system, and diagramed it all in Visio. I consider it a good first effort, but as I'm redoing staging this is a good time to revise the system with the lessons learned.

Some of the changes being made are due to things I learned in the process of doing, and some due to things I read in books or on-line forums that caught my attention now that I'm actually wiring a sizable layout.

Note that this is *my* system, not a general purpose one. There are all kinds of details that were chosen to match my layout's specifics, such as it being N scale or the use of Digitrax DCC. At a later date I'll probably post on how/why I made those decisions, but for now those are just the givens that influenced the wiring design.

Power Cabinet

It all starts here. You want to have a cabinet that contains your power supply, your DCC command station, booster(s) (if needed), and any electronic equipment that can be kept centralized, like the power district units, in one location. This is partly for ease in problem debugging. It also helps to have it all in one enclosed cabinet to keep out dust, etc. The other point is that you want to minimize the wire length of the power bus from your power station to the most distant track. (I'll explain why shortly.) So it 's a good idea to put the power cabinet in a central place to keep the longest power bus length to a minimum. On my layout I'll put the cabinet under the lower deck, on the west wall just north of the stair case.

(Aside: What do the terms DCC and DC mean? Think of them as digital (DCC) and analog (DC). DC is the way model trains are traditionally powered, that is by a transformer that sends Direct Current (DC) to the rails and the locomotive speed is determined by the amount of power received. DCC is Digital Command Control. For DCC the system provides AC (Alternating Current) to the rails in a constant high voltage, and uses the rails as a digital bus to send electronic signals. Each locomotive is equipped with a decoder to interpret the signals, and each locomotive is assigned an electronic address. In this way the decoder is told by the DCC command station how much power to route to the electric motor, or how to operate lights, or what sounds to emit, etc. DCC is more expensive when you consider the cost of just running trains, but it has enormous benefits in terms of simplifying wiring and operations and the use of features like sound and lights, as well as the obvious benefit of running multiple trains without any extra effort. The large majority of sizable layouts use DCC these days. I'll post more about DCC and DC sometime in the future.)

Power Districts

If you spend any time on model railroading forums that include DCC as a topic you'll hear old timers advise you to set up power districts. This is not a lot of work, and in a room sized layout it makes sense.

A power district is not the same as the "power block" concept from DC cab control, although in both cases what you are doing is taking a section of track and electrically isolating it from the rest of the layout. However, a power district covers a very large area -- on a DC layout you might have a power district comprised of many power blocks. The power district idea is this: it's common for an electrical short to occur on your railroad, most typically when a locomotive derails and a metal connection is made across both rails. When a short occurs your DCC command station will sense it and shut down power before anything burns out, and with Digitrax an audible series of beeps is emitted. At that point everthing stops until you find the source of the short. In a large layout such a search can take a long time, meanwhile everyone stops and waits.

The solution is to divide the layout into electrically isolated power districts, each with its own short-sensing circuit breaker. The size of the district is up to your personal tastes. For me I'll probably have 8 districts, 4 per deck, with the south staging room being one district on each deck.

(If you are running scales larger than N you may find, depending on locomotive density, that you need multiple power boosters. Each power district can receive power from only one booster, so you power district size may be constrained by your booster setup. This is, in practice, a non-factor in N since our locomotives don't require much power..)

You can set up your track at installation time with the insulators or rail gaps in place to provide for power districts, but not actually add the circuit breakers until later.

The power district management components will be resident in my power cabinet. Digitrax provides a PM42 for this purpose, but it does not get good reviews on the internet Digitrax forums. I have one PM42 and I have to say I understand the complaints -- it's requires a complex set up and tuning it can be a challenge. This component is part of general electronics, not specific to your DCC manufacturer, so I don't have to use the Digitrax product. I'll research this later and report the results here. A likely candidate is the Power Shield from Tony's Trains.

Power Bus

For each power district you'll have a power bus, consisting of two wires, using your largest wire gauge, that runs the length of the district under the layout.

I've chosen to use 14 gauge solid wire, red and black, for the bus. The wire size is influenced by two things: 1) the peak current (amperage) you expect to draw on the district at any one time, and 2) the length of the wire.

For (1) you can calculate the amperage by figuring how many locomotives you'll run and what their likely amp draw will be (some recommend using the stall current). This can be hard as so many variables are involved. However, one of the nice things about N scale is that locomotives, especially those manufactured this century, are amperage misers. We typically rate them at 1 amp maximum but rarely do they ever get close to half that (even most HO motors don't hit 1 amp at stall current). For my layout , despite a prevalence of track and locomotives, I am going to try using a 5 amp Digitrax command station for the whole thing and based on comparisons to similar layouts I don't think there will be a problem. If I do have a problem I'll have to get a separate Digitrax 5 amp booster for the upper deck. Because of the 5 amp limitation I used the value "5 amps" for my wire gauge calculations.

For (2), the issue is resistance. The longer the wire, the more the resistance. If the resistance gets you high you'll get voltage drop and the locomotives drawing power near the end of the bus will run slower than those close to the power source. However, you can counter this with a thicker wire gauge, since thicker wire has less resistance. (Note that wire resistance is not like the width of a water pipe. That is, if you have 20' of 2" pipe and 1' of 1" pipe, the maximum water flow is determined by the smaller pipe, and all that wider pipe makes no difference. But with wire resistance, as short length of small gauge wire, such as the feeder wire between the power bus and the rails, does not contribute appreciably to the overall resistance of the wire length.)

So, get your amperage, get the length if wire, then look up what size you'll need in a table to avoid the voltage drop problem. There is probably such a table on line somewhere, but I've found that no two sources of info on this are alike, due the the variables involved. For example, the table on page 104 of Andy Sperandeo's book Easy Railroad Model Wiring tells us that for 20' and 5 amps use 14 gauge and for 30' use 12 gauge (larger numbers mean a smaller wire gauge). However, another Model Railroader book, Mike Polsgrove's DCC Projects & Applications, says on page 13 that if you have a 14 gauge bus you won't need to consider 12 gauge unless your run is longer than 80'. Same publisher, different info. Part of the difference may be that Andy's book is about general electrical topics, with DCC covering only one chapter near the end, and Mike's is focused solely on DCC. As a test I ran trains on track that was about 75' from the power source, with 5 locomotives running at once, and there was no evidence of voltage drop. So since in practice my longest bus run will be about 40' 14 gauge should be fine.

I've been using 14 gauge solid (not stranded) wire, one red and one black.

I buy the 14 gauge wire (solid, not stranded) in red and black colors from the electrical section at Home Depot. You won't find this gauge at many hobby or electronic stores, and in any event it's cheaper at Home Depot because they sell it in large bulk to electricians (14 gauge is commonly used in houses for wiring circuts for lighting and common outlets).

When installing the power bus I now twist the red and black wire together. This is because if you have two parallel wires over a distance a nature impedance will form that can cause interferance with your DCC system. The twisting prevents that.

I use a cheap Dymo LetraTag label printer I got at Target to create labels for the power bus and wrap the labels around the twisted wire pair, usually adding a layer of scotch tape on top to assure that the label stays in place. For the buses the label is of the form: PD-1 (for Power District-1). I looked at various products designed for adding number tags to wires and found this to be a cheaper solution if you expect to create a lot of labels -- and they also look nice.

The next post(s) will cover my wiring standards for actually getting the power from the bus to the track. Later I'll discuss auto-reversing wiring standards.