Step Two: Understanding Wire and Cable Construction Materials

1 - Primary conductor insulation can be silicone rubber, Teflon* or mica. 2 - Binder tape can be a mica tape composite when a heat or flame resistant barrier is required. 3 - Outer protective jacket can be fiberglass or aramid (Kevlar*). Metallic braid armor can be applied overall. 4 - Conductor can be tin- or nickel-coated copper or pure nickel. 5 - Protective layer over primary conductor insulation is usually fiberglass braid color-coded to specification. 6 - If required, a moisture and fluid barrier is usually Teflon.

First, look at the foundation of cable construction: the conductor. Then, consider which insulation, jacketing, and shielding materials lead to optimal wire or cable performance for your specific application. Conductor choice depends on: ampacity and voltage requirements, method of termination, and environmental conditions.

Conductors

There are three principle types of metallic wire permitted by the NEC - aluminum, copper and nickel. Variations of these three conductor types - copper-clad aluminum, nickel-coated copper, and silver-plated copper - may provide a specific benefit, such as enhanced resistance to corrosion or enhanced conductivity.

Nickel, for example, has roughly 60% the conductivity of copper, but has a melting point nearly double that of copper, making it ideal for high-temperature applications from 540 C to 1000 C. A nickel conductor will withstand more heat before it sustains any damage and will tolerate a higher ampacity than a copper or plated conductor of the same size. A larger diameter wire may be required, however, to accommodate the initial voltage demand in a particular circuit due to nickel's reduced conductivity. (Refer to the NEC for ampacity ratings and conductor sizing charts for each of these metals.)

Conductors may be constructed of either solid or stranded wire. Solid wire is easier to terminate, solder, or weld than stranded wire and, in corrosive environments, there is less surface area that can be affected by reagents. Stranded wire, which is more flexible and less likely to suffer mechanical fatigue than solid wire, is your best bet for applications subjected to vibrations or frequent movement. After deciding upon your conductor material, consider the inner and outer layers of insulation appropriate for your application.

Insulation Materials

Insulation, jacketing, and metallic shields serve a variety of electrical and physical functions and are an important part of wire and cable selection. Primary insulation must be a good electrical insulator to contain and channel the voltage, while the secondary insulation is used to protect the primary insulation and conductor against physical damage. The outer layers, braiding and jacketing, provide additional mechanical protection to the insulation with jacketing providing environmental protection as well. Finally, metallic shielding can be used to provide heavy-duty physical protection in harsh environments and to prevent electrically generated interference that may be caused by motor ignition and shut-off, fluorescent tubes in lighting ballasts, and relays.

Many insulation, jacketing, and metallic shield materials, which may be considered individually or in combination as a composite, are available for high-temperature wire and cable applications.

Choosing an appropriate insulating material is a process of weighing the balance of desirable characteristics against those that may be undesirable for a particular application. For example, mica is an excellent insulation material for 450 C temperature exposure, but not for a traveling cable which will be continually bent and flexed. Mica would probably fail very quickly in this application due to its embrittlement after exposure to heat. Another construction alternative would be to use layers of PTFE protected by layers of a fiberglass serve. This construction would allow the cable to remain flexible, but it would not have the heat resistance of mica, resulting in a relatively short life span.

The solution? Select a composite of mica with intermediate layers of fiberglass braid, mica, and more fiberglass braid. This choice provides the heat resistance of mica and extends the cable's service life by diminishing the effects of flexing upon the cable. But, when considering a composite cable, it is important to remember that it is ultimately only as strong as its weakest component, and the components need to be compatible.

When selecting high-temperature wire or cable, first consider all the basic requirements that you take into account when selecting normal wiring materials: current-carrying capacity, voltage rating, dielectric strength of insulation materials, and ambient temperature. Next, consider the insulation materials and how they are affected by the application environment. When selecting insulation materials for a high-temperature application, the first consideration is usually the relative heat resistance of the material. There are several temperature thresholds that serve to group materials (See ratings chart).