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
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.
Bare Copper (ABC)
Tinned Copper (ATC)
Plated Copper (SPC)
Nickel Plated Copper (NPC)
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.