Summary: 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.
Solid versus Stranded
There is endless debate about the validity of one's wire choices in solid or stranded wire. Both types have their benefits, as well as their negative aspects. For most layout wiring, stranded wire is the better choice for ease of handling, as it is a lot more flexible.
Solid wire is rugged and usually cheaper. It is preferred in applications where there is little vibration, twisting or bending. It is easily damaged during stripping if the conductor is nicked. Excessive bending or flexing will cause self-annealing at that point, with the probability of the wire breaking there in the future. Solid wire is ideal for outdoor use or applications where corrosion is a problem.
Stranded wire is much more flexible, making installation easier. It can better withstand the twisting and bending forces during installation, and vibration over time. It is less vulnerable to surface damage such as nicked wires or scratching during stripping. Solid wires can break with repeated flexing, especially if it was nicked during a stripping operation.
RJ plugs also terminate better on stranded wire. (Only use RJ connectors designed for the type (solid/stranded) of wire being used.) Many IDC terminations work better with stranded wire. Wires with a higher strand count have thinner conductors, but more of them, making the cable much more flexible than those with a lesser strand count.
Stranded wire should not be tinned prior to insertion into a terminal block using clamps, screws, or crimped on terminals. The ability of stranded wire to conform to the surface of the connection mechanism is an advantage. Crimp on terminals were designed to replace soldering by creating a gas tight connection when crimped, which is not possible if the wire is tinned. If solder can wick into a crimped connection, the crimp is of poor quality.
What is THHN Wire?
- THHN: Thermoplastic High Heat Resistant Nylon-coated
- THWN: Thermoplastic Heat and Water-resistant Nylon-coated
- TFFN/TFN: Thermoplastic Flexible Fixture Nylon, similar to THHN insulation
This designation indicates the type of insulation on the wire. 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 types of insulation are very commonly found in residential wiring. TFFN may be called fixture wire, it comes in a smaller range of gauges due to its applications. Any building supply store will have wire like this.
In no way do the materials used for insulation determine the properties of the conductor. Don't get fixated on a "type" based on the insulating material used. The gauge and metal used are the most important properties.
If you are recycling wire, or using some old wire, examine the insulation to ensure it is intact. Some plastics 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
Also see Wire Sizes and Spacing/Wire Resistance Table for more information.
|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.
AWG to Metric Equivalent Conversion Table
|AWG||Diameter (Inches)||millimetres||millimetres squared||Nearest Size (mm2)|
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, a 20 kHz 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 reliability, 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.
CCAW or CCA
Copper Clad Aluminum Wire (or just Copper Clad Aluminum) is another type of wire which is not suited for DCC applications. It can be found in places where light weight is important, like voice coils. It is also used in low-cost unshielded twisted pair network cables, but can come at the expense of speed and reliability. Low-cost Ethernet cables made from this material (marked as CCA or CCE) violate the standards for CAT5e and CAT6 cabling, and will reduce network speed.
Avoid this type of wire! It is hard to solder to, and one of its properties is higher DC resistance than pure copper. It is not as rugged as copper, which can lead to problems where the wires are pulled, twisted and bent during installation. The only advantage is cost, and that advantage is outweighed by the disadvantages.
CCS is Copper Clad Steel. This wire is not suited for DCC applications.
For the frequencies and currents present on the DCC Power Bus, the skin effect is negligible and can be ignored. This section is included for information purposes only.
What is the Skin Effect?
|Frequency||Skin depth (μm)|
The resistance of a conductor at DC currents (0 Hz) depends on its cross-sectional area. A conductor with a larger cross-sectional area has a lower resistance. The conductor's resistance depends on both current and frequency as the effective cross sectional area changes. The repulsion between electrons is called the skin effect, which reduces the effective cross section of the conductor. In DC it is caused by the amount of current, with alternating currents (AC), the skin effect increases with both current and frequency.
For low currents and frequencies, the effect is negligible. For AC 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. High power broadcast transmitters often employ this method. The reduced mass of a pipe saves on cost as well.
In copper, the skin depth can be seen to fall according to the square root of frequency, as shown in the table.
The cross section of a 12 AWG wire is 93 thou or 2.1mm. AWG 10 is 2.6mm, AWG 18 is 1.02mm.
When used with AC, a wire gains a new property: Inductance. Inductance fights any change in current, and is proportional to both the diameter and the length of the conductor. The resulting impedance is determined by the frequency of the signal on the wire. Managing inductance is important, hence the recommendation to keep bus wires close together to minimize this property.
- Wiring - Primary wiring article.
- Wire sizes and spacing - Help determine wire size and feeder spacing