Power cable Constructions

Modern power cables come in a variety of sizes, materials and types, each uniquely suited to its purpose.In the industry, large single insulated conductors are sometimes referred to as power cables.The cable consists of three major parts: conductor, insulation layer, and protective sheath.The composition of individual cables varies by application.Structure and materials are determined by three main factors:

• Working voltage, which determines the insulation thickness;

• Current carrying capacity, which determines the cross-sectional dimensions of the conductor;

• Environmental conditions such as temperature, water, chemical or sunlight exposure, and mechanical shock determine the form and composition of the outer cable sheath.

Cables intended for direct burial or exposed installations may also include metal armor in the form of wire wrapped around the cable, or corrugated tape wrapped around the cable.The armor may be made of steel or aluminum and, while grounded, is not intended to carry current during normal operation.Power cables are sometimes installed in conduits, including electrical conduits and cable trays that may contain one or more conductors.When intended for use inside buildings, non-metallic sheathed building cables (NM-B) consist of two or more conductors (plus a grounding conductor) enclosed in a heat-resistant thermoplastic insulating sheath.It has advantages over armored construction cable because it is lighter and easier to handle, and its sheath is easier to work with. 

Power cables use stranded copper or aluminum conductors, although small power cables may use solid conductors up to a size 1/0.(For a detailed discussion of copper cables, see: Copper Wire and Cable.).Cables may include uninsulated conductors for circuit neutral or ground connections.A ground conductor connects the case/chassis of the device to ground to prevent electric shock.These uninsulated versions are known as bare or tinned bare conductors.The entire assembly can be round or flat. Non-conductive filler strands can be added to components to maintain their shape.The fill material can be made in a non-hygroscopic version if the application requires it.

Special purpose power cables for overhead applications are often bundled with high strength alloy, ACSR or aluminum welded miters.Such cables are called aerial cables or pre-assembled aerial cables (PAC). PACs can be ordered unjacketed, however, this has been less common in recent years due to the lower added cost of providing a polymer sheath.For vertical applications, cables may contain armored wires on top of a jacket, steel or Kevlar.Armored wire is periodically attached to support plates to help support the weight of the cable.Each floor of a building, tower or structure may include a support slab. This cable will be referred to as armored riser cable.For shorter vertical transitions (perhaps 30-150 feet), unarmored cable can be used in conjunction with basket (Kellum) clamps or even specially designed conduit plugs.

Material specifications for cable jackets often take into account resistance to water, oil, sunlight, underground conditions, chemical vapors, shock, fire or high temperatures. In nuclear industry applications, cables may have special requirements against ionizing radiation.Cable materials used in transportation applications may be specified to not produce significant amounts of smoke (low smoke zero halogen) when burned.Cables intended for direct burial must account for damage from backfilling or excavation.HDPE or polypropylene sheaths are typically used for this purpose. Cables for use in subways (underground vaults) may be prioritized for oil, fire or low smoke resistance.Today, very few cables still have an integral lead sheath. However, some utilities may still install paper-insulated lead-clad cables in distribution circuits.Transmission or submarine cables are more likely to use lead sheathing.However, lead content is declining and few manufacturers make such products today. When cables have to be laid in places exposed to mechanical damage (industrial locations), they can be protected with flexible steel tape or wire armor or covered with a waterproof sheath.Hybrid cables can include conductors for control signals or also optical fibers for data.