How To Use Wire Ampacity Calculator(Fundamentals)

How To Use Wire Ampacity Calculator(Fundamentals)


The ampacity is an important rating and selection parameter of a wire in pcb manufacturing. It is determined by the diameter, length, conductivity, temperature, and heat dissipation factor of the conducting cable on board. The ampacity calculations ensure safe operation of the cables. The ampacity is the maximum allowed continuous current in wire ampacity calculator 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 rechecks 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:

Wire Ampacity Calculator

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 evaluat in pcb assembly 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.


Wire Ampacity Calculator1.png

Fig 1: The structure of a conventional high voltage-current carrying cable



Wire Ampacity Calculator2.png

Fig 2: A comparison of two cables with compensated for heat dissipation and without heat dissipation


Flickr photos from the group WELLPCB Project