Wire for Model Railroads
Model Railroaders are a thrifty bunch, and will often recycle wire from previous layouts, junked hardware, excess wire from other projects, and other sources. This can lead to a variety of different kinds of wires being used. Wires will typically be either copper, tinned copper (typical of some data cables and “hook up wire”) or in some cases aluminum although that’s less likely. Wire is typically stranded and insulated with inexpensive but bulky PVC insulation. Solid wire may be used for DCC bus wiring (although it’s hard to work with; I prefer stranded myself). And wire with specialty insulation, such as “magnet wire” with thin enamel insulation, may also be used.
Note: in almost all cases electrical wire will be made of soft annealed copper, however aluminum is sometimes used to reduce cost. Aluminum has a higher resistance, and thus a larger gauge wire will be required to carry the same current as copper.
If I were starting from scratch, knowing what I know now, I’d use tinned marine primary wire for DCC bus and control panel wiring, and tinned hook-up wire for track feeders, electronics wiring, and DCC decoders. I expect to use magnet wire for some specialty lighting and decoder applications, but not otherwise.
I cover the application of wire to model railroads on my DCC Layout Power page, but for additional information including some real-world resistance measurements of wire, see Allan Gartner’s Wiring for DCC site, in particular the Track Wiring page. Note that his resistance numbers are typically for a pair of wires of a given length, while mine are for a single wire, so mine need to be multiplied by two, or his divided by two, for comparison.
Wire Gauge and Size
The size of wire is measured in a number of different ways. In the U.S., the American Wire Gauge (AWG) system is still used. Other similar systems in the rest of the world (like British Standard Gauge) have largely been replaced with metric systems. In “gauge” systems, a number refers to a typical cross-section in an indirect manner (it can be calculated, but it’s usually easier to look it up in a table). In metric systems what is usually given is the number of strands (which can be 1 for solid wire) and the diameter of the strands, which can be used to directly calculate a cross-section. For example, 14 AWG wire has a cross-section of 1.94mm2 when made from 19 strands (typical) and 2.08 mm2 when solid. Although the solid wire can carry slightly more current because of the larger cross-sectional area, for all intents and purposes the two are identical. Identical metric wire would be specified as 19x0.36mm or 1x1.65mm wire. Metric 40x0.25mm also has a 2.0mm2 cross-section, and thus is roughly equivalent.
The following table lists two typical strandings plus sold wire, with the diameter (in millimeters) of each strand. If you have a digitial micrometer, you can measure an individual strand and count the number to determine what gauge the wire is. The cross-section in square millimeters is also listed. This is for for solid single-strand wire, but stranded wire will have a similar aggregate cross-section.
Some typical wire types
Wire used for a model railroad likely falls into one of the following categories:
Wire intended for in-conduit commercial AC use (identified on packaging as type THHN, TFFN or TFN) can be either stranded or solid, and typically uses bulky high-voltage PVC insulation with a slick nylon coating, making it much larger than other forms of wire. The advantage is that it can be relatively inexpensive (not that copper is ever cheap) and when used for bus wiring, bulk is rarely a problem. THHN is available with insulation in standard colors based on the electric code in effect in your area.
This wire can also be extracted from two-conductor “romex” used inside house walls, although in that case wire insulation colors will typically be limited to black and white.
Bus wires are the main application for this, particularly for long distances or when large-amperage command stations are used. The stiffness of the solid form of this type of wire makes it a poor fit for wiring inside control panels, and even stranded can be a bit stiff for such use due to the thick insulation. This wire is typically sold in 500’ spools, but some stores will sell it by the foot (at a higher price, of course).
I’ve read some misinformation online about thicker wire somehow being unsuitable for DCC frequencies. While there’s some minor skin effect reduction in effective capacity for larger wire, this never overrides the gain in capacity from having larger wire in the first place. And it’s generally a trivial reduction. If you need heavy wire to push high-current track bus lines over a longer distance, there’s no problem (other than how hard it is to work with) in using 12-gauge (2mm dia.) or even 10-gauge (2.5mm dia.) wire. That’s probably mainly an issue for garden railroads though.
Zip-cord (aka lamp cord, aka speaker cord), two stranded wires sharing a single jacket, although made for household AC use, is closer in structure to Primary Wire. Zip-cord is typically good-quality annealed copper wire. It comes in a variety of gauges, although typical lamp cord is 16ga or 18ga. Insulation is typically bulky PVC and not color-coded although often available in black, white or brown insulation. Speaker wire will have one of the two wires marked so that positive and negative aren’t reversed, but standard lamp cord may not.
While this could be used for bus wires, at least for shorter ones, it’s a fairly poor choice due to the bulky insulation and typical wire gauge. It’s also not really suitable for insulation-displacement taps, or for stripping mid-wire, to connect feeders. Finally, placing the two wires really close to each other without twisting them around each other can actually cause some signal degradation for DCC, which makes zip-cord even less desirable.
It can still be used for smaller layouts where you aren’t approaching the limits of length or power handling.
Primary wire (also known as Marine Wire) is low-voltage wire in larger gauges, typically of stranded copper. Insulation is typically PVC. Because of this, and because the insulation doesn’t need to be as thick as in wire for commercial AC use, it is more flexible than THHN. This may be available in relatively short lengths (I get it in 25’ packs from the local home supply store; it’s also available in 100’ spools). This is suitable for DCC bus wiring and wire inside larger control panels. Primary wire is available with insulation in a variety of colors.
This wire may be found in automotive-supply and marine-supply stores, among other sources. The marine form is often tinned to resist corrosion, which likely has some long-term benefit in model railroad applications.
Hook-Up wire is a specialty wire available in a wide range of sizes. This is typically stranded wire of either copper or tinned copper, using PVC or more specialized insulation. Tinned wire is coated with tin (or in some cases silver or gold, but those materials provide little real benefit for the cost) and has a slightly higher initial resistance, but will increase in resistance less over time, as tin does not oxidize the way copper does (an oxide coating increases wire resistance). Tinned wire can be distinguished from aluminum wire by scraping a bare piece of wire with a razor blade, which will reveal the reddish color of the copper if present. This is typically sold in 100’ spools, although it is sometimes available in shorter sizes. Hook-up wire is available with insulation in a variety of colors.
Wire recycled from computer cables is likely to be similar if not identical to hook-up wire.
Some (rather expensive) forms of hook-up wire use specialty insulations that are thinner than typical PVC insulation, which may be useful for decoder installations, but these really don’t reduce the size a lot (maybe by about 10%). If the size of the wire is an overriding concern, consider using magnet wire instead.
Hook-Up wire is available in a range of gauges suitable for track feeders, control-panel wiring, and decoder wiring.
Note: Digitrax sells 30ga “decoder wire” which appears to be tinned 7-strand (0.1mm diameter strands) hook-up wire with a cross-sectional area of about 0.057mm2.
Magnet wire, also called winding wire or enameled copper wire, is a solid wire, typically of annealed copper. Insulation is a thin layer of any of a number of compounds, which typically requires scraping with a knife blade or burning in a flame to remove (wire-strippers won’t work because it’s too thin and tightly bound to the wire). Some versions can be directly soldered, because the hot solder melts off the insulation, but that’s not true of all.
One big advantage of magnet wire is that the thin insulation makes the wire easily concealed when adding lighting to structures. While aluminum is sometimes used, the larger size required for a given current negates one of the largest advantages of magnet wire: the small diameter. In 30ga, typical magnet wire has an outside diameter (including insulation) of just 0.29mm, while ordinary stranded hook-up wire is 0.81mm in diameter, nearly three times the diameter.
The downsides to magnet wire are that it isn’t color-coded, and it’s not normally sold in smaller lengths. The smallest I could find online for 30ga wire was a 1/2 lb spool (it’s sold by weight), which contains about 1,600 feet (500m) of wire, for US$60. Priced per foot, that’s not a bad price, but how much of it will you ever use? It’s also solid wire, so using it where it could flex (e.g., connecting a decoder to truck-mounted pickups) might be prone to failure over time, but in most decoder applications flexing probably wouldn’t be an issue.
Note: I found 100’ spools of magnet wire at a local electronics hobby store, so you may be able to find it in reasonable lengths if you look.
Why Stranded Wire?
Stranded wire carries the same current as solid wire of the same cross-section (or gauge), so what’s the difference? Stranded wire is much more flexible. In larger wire (like a DCC bus) this makes solid wire harder to work with. Solid wire also doesn’t work well with insulation displacement connectors (suitcase connectors) or crimp-on terminals (although you can apparently buy versions of the latter designed for such use). If wire is going to be subject to flexing, solid wire is also more likely to break over time, although in most model railroad applications this isn’t likely to be a problem. The exception would be a modular layout with a DCC bus that’s repeatedly connected to other modules and disconnected; those linkages get a lot of flexing.
Why Copper Wire?
Most wire is copper, some is aluminum. Copper costs more, so why use it? Copper conducts more current with less voltage loss than aluminum of the same gauge. You also need to be careful when connecting aluminum wire: if connected to copper wire or brass terminals, the joint will corrode. Terminals need to be tin-plated to avoid this. You can certainly use aluminum wire if you exercise care, but I’d avoid it.
If you use aluminum wire, the general rule of thumb is to step up two wire gauges to get the same ampacity. Thus, if you need to use a 14 gauge copper wire, you should 12 gauge in aluminum.
Why Tinned Wire?
Why is tinned wire important, and just what does “tinned” mean, anyway? In soldering, to “tin” a wire is to flow molten solder onto it, which makes later soldering it to some other object much easier as the solder already has a good bond to oxide-free copper. Solder itself is made of tin mixed with other elements (usually 60% tin and 40% lead plus a few trace elements, but there are now lead-free solders that contain other materials). The purpose of those other elements is to make it easier to apply the solder; it’s the tin that’s important. And “tinned” wire is wire that has been coated over its entire length with tin, through either electoplating or dipping in molten tin. So this saves you a little work, but is that it?
Note: you can buy “tinned” wire that is coated with silver, or gold, but these are for exotic uses and have a matching cost. Any normal “tinned” wire will be tin-coated.
The benefit of tin, in either form, is that it doesn’t corrode the way copper does (this is also why terminal strips are typically tin-plated). Bare copper wire exposed to air with any moisture will soon form a less-conductive outer layer, and for AC applications (like DCC) this can have a significant effect. It’s also a problem with DC, but typically only with very excessive corrosion, such as is found on copper wire used in salt-water boats. And even if the wire is insulated, moisture will work its way under the insulation over time, and gradually coat the whole surface this way.
For wire in a damp basement (and most basements are damp at least some of the time), and which is likely to remain in a model railroad layout for decades, it is going to be subject to corrosion. Unless you live on the coast, the effect is likely to be marginal and I can’t quantify it in either case. But it would seem to me that using “tinned” wire for model railroad wiring is a good long-term investment, though not a requirement.
The most common “decoder wire” seems to be 30ga. While many people (including me) have used 30ga wire for decoders with no problems, it is somewhat on the borderline. It can carry 120mA using the most conservative numbers (and 400 or more by some tables), and that’s sufficient for normal conditions in an N-scale train, and it may well be safe even under stalled conditions given the safety margins factored in. There are likely plenty of models and usages which could get away with even thinner wire. There are also situations where it would be inadequate.
You can get “hook up wire” in a variety of gauges from electronics suppliers. This is typically sold in these gauges in 100’ (30m) spools, and for stranded copper runs about US$18 per spool with PVC insulation. However, to minimize size thinner, but more expensive, insulations are available at a bit over $30 per spool. And, of course, you could use magnet wire for the smallest outer diameter.
For my EMUs, if I want to be absolutely safe, I should use 26ga (0.4mm) wire for the pick-ups and motor, and 31 ga (0.23mm) for the lighting. That’s extremely conservative however, and 28ga is likely fine, while 30ga is probably adequate. Note that with bulb lighting or larger (or older) motors that draw more current, large wires will be required.
I’m considering ordering 100’ spools of 28ga wire with Polyphenylene Ether (PPE) insulation, with the following Digikey part numbers for my installs:
6710 RD005-ND, Red
6710 BK005-ND, Black
6710 OR005-ND, Orange
6710 SL005-ND, Gray
I could, however, end up going with 30ga versions of this or using magnet wire in either gauge, as it’s even smaller. I need to do more work on my ampacity tables before I convince myself which is best.
If I do use this, it is tinned-copper stranded wire, which I like for this application. There’s a cost to this, and I have to get it in sufficient bulk that I’ll probably never have to buy any more (I think I can do about 200 installs with a 100’ spool of each color). I’m also not likely to use the same wire for the lights, and may stick with the Digitrax 30ga wire for those, or even use something smaller.
One of the nice things about this wire, from a purely non-technical standpoint, is that it uses a more environmentally-friendly type of insulation rather than typical PVC. It contains no halogens or hazardous substances that would prevent recycling. See the manufacturer’s page for further information.