Wires and Cables: An Overview
Types of Wires and Cables Used in the Electrical Industry
The electrical industry uses a large variety of wires and cables primarily for carrying current from one location to another. The size of these wires and cables depends on the amount of current to be transferred, with larger diameters able to carry higher values of current. The specification of an electrical wire defines its current capacity and its insulation type. The ASTM standard B 258. defines the wire dimensions used in the electrical industry.
Overview of a Wire
A wire is a single strand of a malleable metal drawn into a long cylindrical form. Usually, the industry uses wires made from metals that are very good conductors—ones with low resistivity—such as steel, aluminum, copper, silver, and gold. A layer of insulating material coats a wire and prevents it from leaking electric current, except from where a connection is necessary. Beyond a certain diameter, single strand wires are difficult to handle because they are no longer as flexible.
Cross section of an unspecified TECK cable
Advantages of Stranded Wire over Solid Wire
Manufacturers use multiple strands of wires to make cables with larger diameters. A cable will have two or more wires wrapped together in one insulating jacket. Cables meant for high frequencies such as for RF, suffer from higher resistance because of skin effect. This forces a larger part of the high-frequency current to flow through the outer periphery of the wire’s cross-section as compared to that flowing through its central part. Rather than using a single core solid wire, using multiple insulated wires stranded together helps to overcome skin effect by offering a larger surface area for current flow.
Larger diameter cables carrying high currents use multiple cores to retain flexibility. Cables with multiple cores offer higher resistance to metal fatigue. Cables are more suitable when connecting between circuits where the rigidity of a solid wire would produce high amounts of stress because of movement during assembly or service. For instance, AC line cords for electric appliances, welding electrode cables, control cables between machine parts, trailing machine cables, and numerous others use flexible cables.
Defining Wires and Cables with AWG
The North American industry follows the American Wire Gauge (AWG), a system of standardized wire gauges, for defining the diameters of nonferrous, solid, round, electrically conducting wires.
Similar to other wire gauging systems such as the Standard Wire Gauge (SWG), AWG too follows increasing gauge numbers for denoting decreasing wire diameters. This has evolved from the requirement of larger number of drawing processes necessary to produce wires of thinner diameter.
Although the industry uses AWG tables primarily for single, round, solid conductors, it is useful for determining the AWG size of stranded wires as well, by using the cross-sectional area of an equivalent solid conductor. As strands of wires will have small gaps in between, the stranded wire has a slightly larger diameter overall than that of a solid wire of the same AWG.
Purpose of Electrical Ground Cable
The industry uses electrical ground cables to connect the neutral part of an electrical system to one or more earth electrodes—most electrical systems require system grounding or earthing for implementing safety requirements. The purpose of earthing is to bypass leakage currents from the neutral parts of the system to the earth, thereby limiting their voltage to that of the earth during lightning events, and during accidental contacts with higher voltage lines. The ground cable along with system grounding also helps to stabilize the voltages during normal operation.
While portable electrical devices using metal cases use an interconnecting plug that has a pin to connect to the system earth, permanently installed electrical equipment usually have a permanently connected grounding cable. The local or national wiring regulation determines the size of the power ground cable. In large structures, the ground cable is usually bonded to available pipework or structural steel.
Difference between Grounding and Bonding
While grounding is the process of connecting a ground cable to link the neutral parts of an electrical system to the earth electrode for implementing safety, bonding is the act of forming an electrical joint between two conductors. The joint may be between two wires, two equipment, or a wire and an earthed pipe. Usually, bonding connects all metal parts not carrying current during normal operation, and brings them to the same electric potential.
TECK and ACIC Cables
TECK cables are typically made of bare copper conductors with cross-linked polyethylene insulation. All TECK cables have a built-in bare ground wire. These cables can operate within a temperature range of 90°C in wet or dry conditions, with an emergency temperature range of 130°C.
TECK style cables come with an inner and outer PVC jacket that provides additional protection from external physical and chemical damage over the typical aluminum interlocked armored products. The PVC jackets significantly reduce the amount of corrosive acidic gases that evolve under fire conditions. The additional safety jackets make the TECK cables more expensive than standard armored products.
Cross section of an ACIC cable
Typically, Armored Control and Instrumentation Cables (ACIC) are multi-conductor cables containing stranded bare copper conductors, XLPE insulation, PVC inner and outer jackets, a bare copper ground wire, and interlocked aluminum armor. In addition to resistance against heat, corrosion, and moisture, the ACIC cables also offer impact and crush resistance. They offer high tensile strength, high dielectric strength, and low dielectric loss.