Cable Design

Conductor Material
The ability of a material to act as a conductor, semi-conductor or insulator is determined by that material's molecular structure.

Copper
Copper is by far the most versatile and the most widely used conductor material. It is also compatible with numerous coatings to enhance termination and retard corrosion. Annealed copper conductors provide better flex life than hard copper conductors.

Copper Clad Steel
Copper covered steel is utilized when greater strength than that of solid copper conductor is required and where some of the conductivity of solid copper can be sacrificed. Copper clad steel consists of a steel core with a concentric copper covering thoroughly bonded to it. The most widely used grades are:

• High Strength - 40% Conductivity
• High Strength - 30% Conductivity

The above conductivity is expressed in terms of conductivity of a solid copper wire of equal diameter.

Where greater flexibility is necessary, the annealed grade should be specified since it employs a soft steel core with the flexibility near that of copper but with twice the strength. High strength will be achieved by using the hard drawn form. In the applications of high frequency transmission, no loss of conductivity is evident from that of solid copper due to transmission along the copper surface (skin effect). However, at power frequencies, the conductivity is 30 or 40% that of copper wire.

High Strength Alloys
Greater breaking strength and flex life are achieved by alloying copper with cadmium chromium, cadmium, chromium and zirconium. With only a slight increase in resistivity compared with copper clad steel, these alloys allow size and weight reduction to be achieved in electronic and aerospace applications.

Cadmium Chromium copper provides the highest conductivity of the above four alloys and is suitable for high temperature application.

Ampacity
Ampacity (or current carrying capacity) is determined by a number of factors;

• The maximum continuous thermal performance of the covering insulation,
• By the heat generated in the cable (result of conductor and insulation loses)
• By the heat-dissipating properties of the cable and its environment.

Heat generated in a conductor varies as the square of the applied current. The factors influencing current carrying capacity are:

• Conductivity of Conductor Material
  The higher conductivity materials such as silver and copper possess higher current carrying capacity compared with alloys or aluminum hence generating less heat.
• Conductor Size
  Ampacity varies directly with conductor size and will increase as the diameter increases.
• Insulation Material
  The specific heat of the insulating material will determine its ability to conduct heat through the wall to the surrounding medium (air, water, etc.) In no case should the conductor temperature exceed the thermal rating of the insulation.
• Surrounding Temperature
  Ambient conditions such as a higher air temperature will reduce heat transfer away from the conductor.

Stranding
Stranded conductor constructions were developed as a means of overcoming the rigidity of solid wires. For any given wire size, the greater the number of strands with corresponding decrease in individual strand size, the more flexible and costly the conductor.

An increase in diameter must be associated with the use of stranded wires; resistance and weight are affected as well, depending on the number of strands and lay length used.

There are specific numbers of strands which lend themselves to round configurations, i.e., 7, 12, 19, 27 and 37. Normally beyond 37 strands, rope type constructions are utilized consisting of 7 or 19 strand groups

Strand Construction

Bunched
Conductor strands of any number twisted together in the same direction without regard to the geometric arrangement.

True Concentric
A central wire surrounded by layers of helically laid wires. Each layer has reversed lay direction and an increasing lay length in each succeeding layer. The inner layer will support the outer layers to prevent migration of strand that can occur in bunch constructions.

Unidirectional Concentric
A central wire surrounded by one or more layers of helically laid wires with same direction of lay and increasing lay length in each succeeding layer. It has an advantage of much greater flexibility and flex life than true concentric.

Unilay
A multi-layer of helically laid wires with the same direction and same lay length for each layer.

Equilay
Composed of multi-layers of helically laid wire, with the direction of lay reversed for succeeding layers. As the name designates, all layers have equal lay length.

Rope
Is cabled groups of any of the above stranded members. It is standard to use a number of groups that provide a round construction (7, 13, 19, 27). Rope lay is basically used for large gauge (No. 10 AWG and larger) constructions that consist of a central core stranded member surrounded by one or more layers of stranded members.

Bunch Stranding

True Concentric & Equilay Stranding

Unidirectional Concentric & Unilay Stranding

Rope Stranding

Conductor Coatings

Bare copper conductor will oxidize from exposure to the atmosphere forming copper oxide on the surface. Oxidation and other types of corrosion are accelerated by the presence of heat, moisture, and some insulating materials such as rubber. The oxide film is a poor conducting material and must be removed to assure a good, reliable terminal connection. To prevent corrosion and enhance terminating (soldering), bare copper is coated with a metal that is not susceptible to oxidation and corrosion. Contact resistance between conductors and terminals is reduced with coating materials like tin, silver and nickel.

Tin is the most frequently used coating; however, nickel and silver are used for specific applications.

Tin
The least expensive coating for ordinary usage is tin. It is a soldering aid and is specified when that type of terminating method is used.

Tinned Copper
Normally a film thickness of 20 micro-inches (.000020") is applied to each strand. The strands are twisted together to form the tinned copper conductor.

Heavy Tinned Copper
Carries a heavier tin thickness on the individual strand - 100 micro-inches on smaller than 30 AWG strands; 150 micro-inches on 30 AWG and larger.

Prefused Copper
Consists of twisted strands of heavy tinned copper fused with heat along the length.

Overcoated Copper
Consists of tinned strands of copper twisted together followed by a tin coating over the twisted conductor. The finished product is bonded along its entire length.

Topcoated Copper
Consists of bare copper strands twisted together, with the resulting conductor given a coating of tin. The finished product is bonded along its entire length.

Silver
Silver is primarily electroplated to copper and then drawn down to the proper conductor size with a resulting 40 micro-inch coating. Silver-coated conductors are reliable for continuous temperature application through 200°C. Although higher in cost than tinned copper, silver coated conductors have a lower resistance, than either tin or nickel coated conductors. At higher frequencies, the current density is at the
conductor surface (skin effect) thereby making this highly conductive coating material the most effective of all coatings.

Nickel
Nickel plating is considered suitable for continuous service up to 260°C. At these elevated temperatures, nickel does not tarnish as does silver.