# How To Use Wire Ampacity Calculator(Fundamentals)

Published on June 29, 2021 , Last modified on July 26, 2024
by Hommer Zhao

The ampacity is a vital 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 the safe operation of the wires. The ampacity is the maximum allowed continuous current in the wire ampacity calculator that a cable can handle without compromising safety standards at 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 the wire-rated temperature, the cable may burn or catch fire, which breaches the safety byelaws. Therefore, the line 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, the different applicable input variables can be calculated by the other functional input variables. Therefore, designing a new system is an excellent practice to self-calculate and recheck environmental and material parameters. The resistance parameter of the cable also varies due to ambient temperature variations and material properties.

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## Wire Ampacity Calculator–Content

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. This is because the standard voltage systems operate relatively at higher currents. If the wire size or diameter is too small, it creates more power losses around it that cause a temperature rise; conversely, a higher diameter wire causes an increase in cost.

The ampacity calculation, according to the Neher-Mcgrath, is:

• Wire Ampacity Calculator
• Where I = Ampacity in amperes
• 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 a significant value 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.

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

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

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Hommer Zhao
Hi, I'm Hommer Zhao, the founder and Chief Editor at WellPCB. With extensive experience in the PCB industry, I oversee all content to ensure it meets the highest standards of accuracy and insight. We proudly serve over 4,000 customers globally. For inquiries or more information, don't hesitate to reach out. Your satisfaction is my top priority!