The ampacity is an important rating and selection parameter of a wire. It is determined by the diameter, length, conductivity, temperature and heat dissipation factor of the conducting cable. The ampacity calculations ensure safe operation of the cables. The ampacity is the maximum allowed continuous current that a cable can handle through it without compromising safety standards at the ambient temperatures. As the current moves through a conductor it causes an IR voltage drop and creates heat according to I2R loss. If the heat in the ambient temperature exceeds wire rated temperature the cable may burn or catch fire that breaches the safety byelaws. The cable is recommended to operate below this temperature range or otherwise heat must be dissipated in the environment. There are many tables available for wire and their allowed ampacity range. However, ampacity can be calculated by the different applicable input variables and to design a new system it is a good practice to self-calculate and recheck the environmental and material parameters. The resistance parameter of the cable also varies due to ambient temperature variations and material properties.
The dc resistance of a conductor can be calculated using the following formula:
Where R = Resistance of the conductor
ρ = conductivity of conductor, measured in ohms (Ω)
l = length of the conductor
A = effective cross sectional area of the conductor
The voltage drop in low voltage systems becomes more significant due to IR drop. The low voltage systems relatively operate at higher currents. If the wire size or diameter of the wire is too small it creates more power losses around it that causes rise in temperature, on the other hand, a higher diameter wire causes increase in cost.
The ampacity calculation according to the Neher-Mcgrath is:
Where I = Ampacity in kAmperes
Tc = conductor temperature
Ta = Ambient temperature
delTD = conductor temperature rise
Rdc = conductor DC resistance
Yc = conductor loss increment
Rca = conductor thermal resistance
The voltage drop in a conductor with current flow becomes significant and can be estimated by ohm’s law:
I = current in amperes, measured in Amperes
V = voltage drop across the conductor ends
R = resistance of the conductor
Longer cables have more dc-resistance and high voltage drops across them so the higher heat losses.
Fig 1: The structure of a conventional high voltage-current carrying cable
Fig 2: A comparison of two cables with compensated for heat dissipation and without heat dissipation