Wire—we cannot do without it.
Every cable has one thing in common—wire—the conductive metal strands that carry the signals and power between connectors. One would think that wire is but a simple part of a successful cable design. However, it would be a mistake to simplify the importance of correct wire design.
Wire is sized from the smallest single strand of #40 to a half-inch flexible power cable for welding with 6,000 strands of #40 copper. The higher the number of strands, the higher the bend ability; a single 14-gauge solid wire is very hard to bend, whereas 266 strands of #38 wire equals the same electrical resistance as a 14-gauge wire, but it is easily bent with the right protective coating. Current carrying capability of any wire with power must be calculated and tested. Coatings are selected for their toughness, bend ability, and environmental survival, e.g., PVC, PTFE, PE (Polyethylene) ECTFE, PVDF, Teflon, and Nylon. Rubber is a popular coating which will allow easier bending with a high strand count but is sensitive to salt or UV environments.
Proper stress relief design is a major concern in a successful design of a cable. Even the softest multistrand wire will work harden over time, becoming brittle and broken if bent enough times. Cable assembly companies have testing machines that will bend a wire or stress relief through a tight radius until it creates an opening. Physical testing of the design is a priority with any high reliability cable assembly. Stress diffusers inside a wire, such as strong multi-fibers of poly prevent, pull stress from creating an opening within the strands of copper.
Voltage dielectric ratings of the coating are easily tested in the lab. However, real life throws items like solvent-based oils or salt on the wires, lowering the cable coating’s overall voltage rating with time. Many designs are not critical for voltage breakdown ratings. Any critical, medical, or automobile use will require certification of dielectric ratings in the given environment. Many types of wire coatings are not suitable for UV, strong chemical, or abrasive situation use.
Shielding and ground wires are used on many wire types. A thin wrap of aluminum is used in multi-wire bundles within one cable. A tinned or coated bare wire will be placed adjacent to the foil and used as a ground wire. On flat cable assemblies, conductive silver or PSA foil can be used as a shield, and twisted pairs of fine multi-strand wire are used for high-speed internet signals, matching the requirements for cat 5 through 8.
Stranding in bundles is achieved by braiding the conductor around each wire so that the conductor stretches uniformly when the cable bends. This type of cable is more durable because of its tension proof core. The more individual wire strands in a wire bundle, the more flexible, kink-resistant, break-resistant, and stronger the wire becomes. For applications that need even more flexibility, such as welding cables, a 2/0 wire has 5,292 strands of #36 gauge wire. Heavier braided cables are used for electrical connections that need a high degree of flexibility.
Within the wire industries, different types of non-standard cable are sometimes called for on the drawings. Examples are Litz wire with alternating directions of the twist of wires to eliminate resistance; and thermocouple wire can have different combinations of wire such as iron, chromel, and alumel to remove the minute voltage of connecting dissimilar metals. Flat cable assemblies in applications characterized by repeated motion should allow for tight bend radius, high life cycles under constant flexing, package size, and environmental conditions. The bend radius of flat cable depends on the gauge of the wire and type of conductors used in the cable. In general, the finer the gauge of the conductors, the smaller the allowable bend radius.
Solid wire is made up of one piece of copper wire protected with a coating. It is used when less flexible wire is needed, such as building infrastructures, vehicle controls, and outdoor applications. Magnet wire, also called enameled wire, is a solid wire which is insulated with a very thin coating to allow it to be very tightly coiled. These tight coils are used for applications such as inductors, transformers, motors, electromagnets, and speakers.
Stranded wire consists of multiple small wires bundled or wrapped together. Stranded wire is more flexible and has a higher resistance to metal fatigue than solid wire. It is used for circuit board connections because a solid wire would create too much stress on the solder joint. Stranded wire is also used for line cords where flexibility is required.
Tinsel wire is far more resistant to metal fatigue than stranded wire. It is used for applications that require high mechanical flexibility but low current-carrying capacities, such as cords for telephones, headphones, and small electrical appliances. It is made up of several strands of thin metal foil wrapped around a flexible nylon core. Multiple tinsel wires are usually jacketed with an insulating layer to form one conductor and are available as either a round profile or a flat cable.
Braided wire is composed of multiple small strands of wire that are braided together. Braided wires are better conductors than solid wires. They don’t easily break when flexed. Braided wires are often used as an electromagnetic shield in noise-reduction cables.
Coax wire has a single strand, solid inner copper wire, featuring insulation with a low Dk, low Df material such as Teflon, polytetrafluoroethylene (PTFE), polyethylene foam, and solid polyethylene, with a PVC outer coating. Additionally, fiber optic cables, although not a copper conductor, utilize a small strand of a special glass which transmits signals by light.
Among the common types of copper wire are: copper alloy wire, copper clad aluminum wire, copper nickel, and nickel-plated copper wire. Copper alloy wire is available in standard sizes, tensile strength and working temperature. The alloy options include Zirconium, Beryllium, Brass, Bronze, Titanium, and other metals. The choice of alloy typically alters the strength, solder ability, and durability.
Copper clad aluminum wire comes in different conductor diameters, insulation, and jacket thicknesses. They feature higher working temperatures, fire, oil, and ozone resistance, which give it conductivity with reduced weight. It is less expensive than pure copper wire while it has higher strength and greater electrical conductivity than pure aluminum wire. Copper nickel and nickel-plated copper wire are used for aerospace, defense, petrochemical, nuclear, and medical industries.
Whatever your design, choose your wires carefully.
Cooler Cao is an engineering manager at Cabletree Industrial Company. Cables, cable assemblies and wire harness are, more than ever, an integral part of today's electronics. With the development of automotive and medical diagnostics, electric automobiles, solar and wind power has come the design and develop of newer and more sophisticated connection devices.