Stranded wires is used in applications where the wire must move. For example, stranded wire is found in the doors of cars, the arm of robots, bilge pumps on marine vessel, and power cords for electric device. Stranded wire include many thin strand of copper twisted together, while solid wire include one solid rod of copper.
Solid wire is employed in applications where the wire will not move, such as inside an wall. However, applications that experience constant flexing of the wire require stranded wire; the individual strands of stranded wire allow for the wire to bend without crack. The more strands in stranded wire, the most flexible the stranded wire.
How to choose the right stranded wire
However, stranded wire with more strands cost more and requires more care during termination. Stranded wire with seven strand is flexible enough for most electrical connections. For applications that require tighter bend, such as cable carrier, a technician uses stranded wire with nineteen strands.
For robotics and drag chains, people use stranded wire with thirty-seven strands because thirty-seven strand stranded wire can survive million of movement cycles. The number of strands in stranded wire must be matched to the movement that the wire will perform. Ampacity is a measurement of the amount of current that a wire can carry before it overheat.
The ampacity ratings for stranded wire are determined for applications where the ambient air temperature are 30°C, and the wire is a single stranded wire in free space. If many wire are bundled together in a conduit, or if the temperature of the wire and its surroundings increase, the safe current capacity of stranded wire decrease. For example, stranded wire of a specific gauge can carry 20 amp at 30°C in free space.
However, that same stranded wire will have to be derated for a hot attic area with other electrical wire. These adjustments must be made to the ampacity ratings of stranded wire to ensure the safety of the wire and the individual that live in the area. Termination is the process of connect to a terminal.
Terminals are a common point of failure for stranded wire because the strands of the stranded wire will splay out when tighten in the terminal. One way of reducing the spread of the strands of stranded wire is to twist the strands in the direction of the clock before insertion into the terminal. However, the most reliable termination method for stranded wire is the use of a ferrule.
A ferrule is a metal tube that is crimped over the stranded wire to squeeze the stranded wire into a solid cylinder. The solid cylinder will not splay out when the device in which the stranded wire is install vibrates; additionally, the ferrule prevent the issue with stranded wire connection after several month of installation. The type of insulation for stranded wire is one of the factor that determines the temperature and chemical resistance of the stranded wire.
For example, stranded wire with insulation of PVC can be used in indoor application, but the PVC will soften to temperatures above 75°C. For applications with high heat, such as furnace, silicone insulation that remains flexible between subzero temperature and 200°C is used. For the most challenging environment, such as medical device or aerospace application, PTFE insulation that can withstand higher temperature and chemicals than silicone or PVC is used. The insulation for stranded wire can be chosen according to the expected temperature and chemical environment around the stranded wire; however, this factor is easily ignored in favor of lower cost for stranded wire.
If stranded wire is to be used in environments that is humid, wet, or exposed to the outside and marine environment, the stranded wire may be coated in tinned copper to provide corrosion protection. This tinned copper will protect the wire from oxidizing in these environment. Additionally, the strands of tinned copper will accept solder more readily than bare copper strand.
Bare copper wire is the default type of stranded wire for dry indoor electrical panel. The bare copper is less expensive than tinned copper, and the corrosion protection are not needed in dry indoor environment. To choose stranded wire for an application, the movement of the wire must be consider, as well as the temperature and moisture in the environment.
The strand count of stranded wire must be chosen according to the movement of the wire, the insulation type of stranded wire must be chosen according to the temperature and moisture exposure of the wire, and the gauge of stranded wire is chosen according to the amount of current that must travel through the wire. By matching each of these factor for stranded wire to the requirements of the application, stranded wire will perform correct for many years.
