There are many types of wire available on the market. Thin wire, thick wire, flexible, stranded or solid. Copper. Aluminum. The construction of the wire makes a difference by giving the finished product certain properties.
Which is best for DCC Wiring? As with all things in Model Railroading, there are a lot of myths surrounding wire too.
What is THHN Wire?
The two types of wire you will see may have the following marked on the insulation:
- THHN or
THHN means Thermoplastic High Heat resistant Nylon-coated
THWN means Thermoplastic Heat and Water resistant Nylon-coated
Neither one identifies a specific type or the properties of the wire. There will be other markings, or a product number, which actually identifies the wire type (solid or stranded) and gauge.
These two types of insulation are very commonly found in residential wiring. So any building supply store will have wire like this. Other types of insulation include vinyl (can be soft or hard).
If you are recycling wire, or using some old wire, examine the insulation to ensure it is intact. Some plastics will lose their elasticity over time, and the insulation can crack and/or flake off, leaving exposed conductors. Wire like that should be sold to a metal recycler instead of being used.
Types of Copper Wire
|Gauge||Area, CM||Resistance 1000m 25C||Mass, 1000m|
|Minimize Proximity Effect||N||Y|
As the chart shows, stranded has a lot of desirable properties. In addition, you can use crimped on connections or IDCs (such as Scotchloks) with stranded. Solid conductors don't lend themselves well to those techniques.
There is a lot of debate about the merits of stranded versus solid wire.
Solid wire is stiffer and harder to handle as a result. Stranded wire is flexible, making routing a lot easier. It is also easier to terminate with crimp on terminations, and is also better when using Insulation Displacement Connectors (IDC), better known as ScotchLoks. Solid wire can be easily connected using Marrettes (or wire nuts).
In terms of electrical performance, solid and stranded wire is the same at low frequencies. The skin effect is minimal at the frequencies used for DCC operations. Skin effect only becomes an issue at high frequencies, in the MegaHertz region. In fact, high power transmitters don't use wire, they use hollow tubes, or steel wire clad with a layer of copper to move the energy, and waveguides are used for moving the RF energy to the antenna.
At frequencies in the audio range, at 20 kHz the signal will use 75% of a 12 AWG wire's 93 thou cross section. In fact, solid wire is superior in performance compared to stranded. (Remember that next time the salesman tries to sell you expensive super fine stranded speaker wire…)
Since the DCC signal does not use the entire cross section of the wire, the AC resistance is higher than the DC resistance. This is why heavier wire is used, among other reasons.
Aluminum is not easily available, and it not recommended for use in electrical systems. For ease of installation and long term reliability, copper is the appropriate choice. Copper can also be soldered, which is possible with aluminum as well, but the process is a lot more complex than soldering materials like copper and copper alloys (including nickel silver.)
Copper has all the advantages. It is the benchmark for conductivity, and with modern metallurgy, copper can often exceed the 100% benchmark.
- Tensile Strength
- Creep resistance
- Corrosion Resistance
- Thermal Conductivity
- Ease of Use
These properties make copper the ideal conductor, and prevent a lot of issues that will occur with other metals, such as connections loosening because of temperature and creep caused by it.
If some kind soul offers you aluminum wire, graciously accept it.
Then take it to a recycler, and use the money he gave you to buy copper.
At one time aluminum was considered a precious metal, more valuable than gold or silver. As the price declined, it became popular for wiring, and has been used for that purpose since the 19th century.
Compared to copper, aluminum has 1.6 times the resistance, and 1.26 times the diameter of an equivalent copper conductor.
Aluminum wire is not readily available anymore, because of various issues surrounding its usage. Aluminum requires special techniques, materials and devices to use it, and for the average person, this will be nothing but problematic. Therefore, for ease of installation and long term reliablity, don't bother with aluminum. Stick with copper, as it is easy to work with, and reliable.
The only advantage aluminum has ever had over copper is that the equivalent conductor is lighter and cheaper. Copper's advantages more than outweigh the cost difference between the two. In North America residential wiring is 100% copper because the benefits outweigh any cost savings.
CCS is Copper Clad Steel. This wire is not suited for DCC applications.
CCAW or CCA
Copper Clad Aluminum Wire (or just Copper Clad Aluminum) is another type of wire which isn't suited for DCC applications. It can be found in places where soldering and light weight are important, like voice coils or building wiring. It is also used in unshielded twisted pair network cables, but can come at the expense of speed and reliablity.
The resistance of a conductor at DC (0 Hz) depends on its cross sectional area. A conductor with a larger area has a lower resistance. The resistance also depends on frequency because the effective cross sectional area changes with frequency. For alternating currents (AC), the skin effect causes the resistance to increase with increasing frequency.
For low frequencies, the effect is negligible. For AC at frequencies high enough that the skin depth is small compared to the conductor size, the skin effect causes most of the current to flow near the conductor's surface. At high enough frequencies, the interior of a large conductor does not carry much current.
- At 60 Hz, the skin depth of a copper wire is about 1/3 of an inch (8.5 mm).
- At 60,000 Hz (60 kHz), the skin depth of copper wire is about 0.01 inches or 10 thou (0.25 mm / 250 micrometers).
- At 6,000,000 Hz (6 MHz)  the skin depth of copper wire is about 0.001 inches or 1 thou (25 µm).
Round conductors larger than a few skin depths don't conduct much current near their axis, so the central material isn't used effectively.
When larger area conductors are needed, measures are taken to reduce skin effect. One method is the use of a hollow pipe with the conducting wall thickness approximately that of the skin-depth at intended frequency.
In copper, the skin depth can be seen to fall according to the square root of frequency:
|Frequency||Skin depth (μm)|
The cross section of a 12 AWG wire is 93 thou or 2.1mm. AWG 10 is 2.6mm, AWG 18 is 1.02mm.
The above metric values are not cross sections but OD. In the metric world wire and cable are identified by cross section in mm2. The values as shown in the AWG table the page links to shows the correct values.
American Wire Gauge with Metric Equivalents
|AWG||Diameter (Inches)||millimetres||millimetres squared||Nearest Size (mm)|
Taken from the article on Wikipedia
- Wiring - Primary wiring article.
- Wire sizes and spacing - Help determine wire size and feeder spacing