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What is a Wire Amp Rating?

Posted by Rick Regole on June 29, 2020 | Updated on March 7, 2024

wire-ampacity-1Electrical circuits are capable of producing immense energy, but despite their powerful output, circuits are also quite delicate.

Ampacity is a rating system used in electrical work to define the maximum current of energy a conductor can carry without exceeding its temperature rating. The current is measured in “amperes” or “amps.” If an electrical circuit does not have the proper wire amp rating, the wire can overheat, causing irreversible damage to the conductors and wire insulation. In severe cases, overheating can even lead to fires, explosions, and serious injury.

Wire amp rating requirements are established by the National Electrical Code (NEC) and defined in their published codebook, which can be viewed online. The NEC sets the standard for electrical safety in residential, commercial, and industrial applications. Every three years, the organization releases updates to its codebook, and 2020 happens to be a benchmark year, including important changes to the standard wire ampacity guidelines. Fortunately, one of the goals of the NEC 2020 codebook is to clarify ampacity tables, which have been notoriously difficult to understand.

What determines a wire amp rating?

The ampacity of a circuit depends on the number and type of electrical devices connected to the circuit. To calculate the load requirement (the amount of electrical output needed to run all electrical devices connected to the circuit) for a circuit:

  • Add up the wattage (a metric used to define the operating power of a device) for all electrical devices that will consume power from the circuit.
  • Divide the total wattage by the voltage (the pressure from a power source that pushes the electrical current through a conducting loop) of the system.

Total Watts / Voltage = Amps

What factors influence a wire amp rating?

Ambient Temperature: The temperature of the environment in which the wire is used will affect the allowable temperature rise of the wire itself. For example, a wire rated for 90°C that operates in a 75°C environment only has an allowable temperature increase of 15°C.

Duty Cycle: In some applications, the level of required current can vary depending on the type of load. For example, an engine requires a larger current at start-up before leveling out.

System Components: If multiple wiring systems are installed within close proximity, they may not be able to disperse internally-generated heat (as designed). This can cause a rise in ambient temperature and the internal temperature of wires.

As you make decisions about electrical connections, carefully consider every detail, especially the ampacity. Circuits can quickly become hazardous if the proper components are not selected for the assembly. For this reason, we highly recommend consulting with an electrical connector and cable assembly expert before risking the health and safety of your product or the technicians who will work with your product.

Learn about the most important updates regarding wire amp rating in the 2020 NEC codebook.


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