Not all copper wire is created equal, but knowing when to use which type of wire and where can potentially save you a ton of time, money, and headaches.
For the same reason you wouldn’t use a Honda Fit to tow a camper, you likely wouldn’t use THHN wire in a direct burial situation – it simply isn’t a good wire for the job. The same can be said for deciding when to use a solid copper wire versus a stranded version.
At first glance, it might seem like every type of copper cabling does the same job. However, the distinction between stranded and solid wire becomes more important when considering the application.
To make solid copper wire, rods are drawn down using a series of smaller and smaller dies to reach the gauge needed for the application it’s being used for.
As the copper is stretched out, it gets thinner and more brittle. To fix the brittleness and prepare the cable for use, it goes through a process called annealing. During annealing, the wire is heated, making it stronger and more ductile.
Stranded or bunched cable is multiple individual strands of smaller gauge wire twisted together to create a larger gauge product.
To make stranded copper, individual wires are drawn and annealed. Once they’re ready, they’re then run through another machine that twists them around a center wire.
Depending on the gauge needed, these strands can be composed of seven or more 30 or 34 AWG (American Wire Gauge) strands. As more strands are added, the resulting wire gains more flexibility.
It also matters how the wires are twisted. For example:
To answer this question, the first thing that needs to be done is to assess the situation.
Is the project going to be one where flexibility is crucial? Is there a threat of corrosion? How long is the run? All of these play a critical role in determining which cable is needed.
Solid copper is stronger than stranded copper. This is because the conductor is thicker and is one solid piece of metal, as opposed to a bunch of smaller cables twisted together.
Solid copper is cheaper for manufacturers to produce because the process is faster and includes fewer steps – it only needs to be drawn, annealed, and insulated (if necessary).
Stranded copper follows the same process, but the wires have to be twisted together to form the finished product. This adds more time and costs to the manufacturing process.
Solid wire is strong and rigid. Those qualities are great for applications that require strength, but not so much for pliability. Stranded wire is much more flexible, making it easier to route and work within tight spaces.
Despite the final gauge being the same, a solid wire will have less surface area than a stranded or bundled one, giving it a higher load capacity and less impedance. Solid wire also has a lower voltage drop over long runs but heats up faster than stranded cable.
Stranded has a higher surface area because it’s made up of several different wires rather than only one conductor. There is a larger voltage drop over long runs, but stranded wire doesn’t heat up as quickly because of air gaps that form as the wire is twisted together, allowing heat to dissipate along the length of it.
A single, solid wire will withstand corrosion better than a stranded one because there are fewer spots for corrosion to occur.
Remember those air gaps mentioned a few sections ago? Those same air gaps that make for a flexible, easy to bend wire also provide many more opportunities for corrosion to eat away at the individual strands. Conductors can also be tinned to impede corrosion.
Solid cable is strong, making it an excellent fit for situations where ruggedness is key. It works well in outdoor situations with approved sunlight-resistant insulation and is good for direct burial applications, including as a tracer wire.
Stranded copper is easier for installers to handle in tight areas and inside buildings because it is more flexible and faster to manually manipulate.
Cutting corners or using the wrong wire may be cheaper or faster at first, but those early mistakes have the potential to become major problems as the years wear on.
When choosing which type of electrical wiring or cabling to go with, consider the environment, need, and the overall project itself. Those clues can go a long way toward determining the right wire type for the job.