Cables are available with all the usual conductor materials, the most popular being aluminium, electrolytic annealed bare copper (ABC), annealed tinned copper (ATC), silver plated copper (SPC), nickel plated copper (NPC), pure nickel and special conductors for thermocouple extension and compensating cables.

Critical conductor selection variables are composition of conductor materials, conductor area and stranding. Expected service temperature should be considered first because conductor materials vary in heat resistance. Next to be evaluated should be the conductor’s capacity to carry current without surpassing the temperature rating of the conductor and the insulation. Conductor material and conductor area, are the key determinants of this capacity.

Finally the decision should be made on how much flexibility is required, for this determines whether a stranded conductor is required or not, as the cost increases. Stranded constructions are used in cables for improved conductor flexibility, flex life and ease in handling. Stranded conductors also offer greater resistance to vibration and bending during assembly and in service.

During conductor selection, copper should be evaluated first. It is a more efficient conductor than any other common conductor options, in terms of both conductivity & cost. But at elevated temperatures copper tends to oxidize unless coated with a more heat-resistant metal such as nickel, silver as indicated below.

Copper Conductor MAX. TEMP. RATING Annealed Bare Copper (ABC) 125°C Annealed Tinned Copper (ATC) 150°C Silver Plated Copper (SPC) 200°C Nickel Plated Copper (NPC) 250°C

Current Carrying Capacity (Ampacity): It is the current a conductor can carry before the temperature of the conductor and the insulation rises beyond a permissible limit. The following are the key factors in determining ampacity.

Conductor Size & Material: Conductivity of various conductor materials varies widely. These variances affect current carrying capacity. Also, as the conductor is reduced in diameter and mass, ampacity decreases.

Amperage: As applied current rises, greater conductor heat is generated and the temperature of the wire rises.

Ambient Temperature: As the ambient temperature (the temperature of the air surrounding the cable) rises, less current-generated heat is required to reach the temperature rating of the insulation. Thus ampacity is governed also by contribution of ambient heat.

Insulation Type: Heat dissipation through insulation varies with insulation type. The rate of dissipation affects total heat and, therefore, the ampacity. The dissipation problem becomes even more complicated when the wire or cable is enclosed in a tightly confined space.

For these reasons, assigning the current carrying capacity to a conductor is an exact process. Consequently design engineers responsible for making such decisions may evaluate wire constructions empirically, using guidelines established by various standards like IS 3961, IEE, etc.