"What size wire should I use?" is always one of the first questions asked when someone starts to build a model railroad. Of course, the answer to that question depends on many things. Some of the variables are obvious, others not so much.
Different wiring projects call for different standards. Here are some guidelines for the most common wiring jobs you'll find. Note that when working with specific products, manufacturers will often recommend a wire size and type. If they do, follow their recommendations.
First, a few general notes about choosing wire. Wire gauges are coded by their size. The smaller the number, the larger the wire. The wire can also be purchased as solid or stranded. A solid and stranded wire of the same gauge will be the same total diameter. But a 12 gauge solid wire is one strand, while a 12 gauge stranded wire may be made up of more than a dozen much smaller individual strands spun together.
In most cases, stranded wire is preferred because the multiple threads provide more conductivity and flexibility. However, there are times when the solid wire has advantages, particularly when soldering in small locations. Some DCC systems also recommend solid wire for certain applications.
The insulation on the outside of the wire also varies greatly. In most cases, because we are dealing with relatively low voltage and amperage and in normally stable temperature and humidity climates, our wiring does not require any extra insulation. If you're experiencing problems with your wiring due to humidity, for example, you are going to face even greater issues with the trains and tracks themselves. Outdoor railroads are, of course, an exception! Heavier insulation not only adds to the cost of the wire but also makes it harder to bend.
The multi-conductor wire is also available. This is different from the stranded wire in that the different individual wires, each with their own color-coded insulation, is placed inside a secondary insulation wrap. Three conductor wire is common for household use. Multi-conductor wires with many strands of finer wire are more common for telecommunications and electronics. These can all have uses for your layout.
Although it will not have any impact on the performance of the wire within, choosing multiple colors of wire, and standardizing on a specific color for each function, will go a long way in making your wiring easier to install and easier to detect flaws later on. There are a few standards when it comes to color coding. DCC decoders are one good example where the wire colors are critical. Although not necessarily a mandated standard, white, black and or red are used for track power on most layouts - if for no other reason than these colors are easiest to find.
Track Bus and Feeders
As your layout grows beyond the basic oval of track that comes with most starter sets, a good power distribution network is essential to getting consistent performance from your train all the way around. These bus wires are often more important than the size of the power supply itself in getting good results.
You can also minimize voltage drop at the rail joints by soldering the rails together.
For most layouts, and most scales, Number 14 stranded wire will work best for your bus. If you have a very long run, you may want to consider Number 12 wire. Smaller layouts (a typical 4 foot by 8 foot being a good example) can usually get by with Number 16 since the length of the run from the power supply to the track is never more than a few feet.
While it is tempting to go with the biggest wire possible to guarantee excess capacity, there are drawbacks to putting in more than you need. Larger wire is usually more expensive and more challenging to work with. In addition to the wire itself, connectors like crimp-on terminals and terminal blocks must also be purchased in larger (and more expensive) sizes. Of course, if you happen to have enough Number 12 wire left over from a home renovation project to install all the buses on your small layout, it won't do any harm.
You don't want to try to solder even Number 16 gauge wire right to your rails, however. Smaller feeder wires are used to bridge the small gap from the bus to the track. Number 22 solid wire works best on most scales. Solid wire is preferable as it is much easier to solder to the rails.
Most feeders will be only a few inches long so the smaller diameter wire will not be a problem.
It is best to use at least two different colors for your track wiring (one for each rail.) White and black are common selections. Lionel uses red and black (red always for the center rail, black for the two outer rails) as a standard in its instructions and most 3-Rail O Gauge layouts are probably wired this way.
If you are wiring with a common rail for your blocks, then keeping the common rail the same color all the time while changing the color for the other rail in each block is also an option. Whatever colors and pattern you choose, just make sure to keep a good notebook handy to reference later.
Lighting and Accessories
Adding lights to buildings, streets, and other scenic accessories usually do not require anywhere near the amperage demand of the trains themselves. Number 26 to 20 wire (depending on the length of your bus) will be sufficient for most applications. If you are using only LEDs for lighting, you could get away with even less.
Just like with your track bus and feeders, running an accessory bus of slightly heavier wire to connect to each light or accessory with a smaller feeder is a good plan.
It is also a good idea to run your accessories off of a separate power supply and wire grid from the trains themselves. This conserves the trains' power supply for their needs and makes troubleshooting much easier.
Switch Machines and Motors
Regardless of your scale, all model railroad switch machines fall into one of two categories, a slow-motion electric motor or a "twin coil" relay. And again regardless of scale or manufacturer, switch machines of these two types will have similar needs and performance.
Twin coil machines have a higher current draw when in motion but are also much faster-acting than the motorized versions. In either case, however, voltage and amperage draw are low and peak for only brief periods.
Because of this, like with lighting and accessories, your power supply and wiring for switch machines do not need to be as robust. Again, an independent power supply and distribution bus is a good idea for your switches.
Number 24 to Number 20 wire will all work with most layouts. Again, if you have a very long run between switches, going a little larger is less likely to cause problems with voltage loss.
Control Panels and Electronic Projects
The back side of a control panel can turn into a "rat's nest" of wiring very quickly! With multiple switches, lights, power supplies and more all concentrated in a small space, color coding and neatness pay off.
The preferred choice for wiring control panels is telephone or telecommunications wire. This wire is very fine and can be found in many color combinations. Because the runs are short and power demands so low, this fine wire poses no safety threat.
The same is true for other electronic projects around your layout and inside buildings, train cars, and even locomotives. The smallest wire you can work with will be more than sufficient.