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What Is Wire Ampacity and Why Its Calculation Is Critical

Posted by Rick Regole on August 29, 2016 | Updated on April 3, 2024

Ampacity and why it needs to be right

What is Wire Ampacity?

Wire ampacity is the maximum electrical current (Amperes or “Amps”) that safely exist in a given size of conductor. Wires are made of 2 basic components: a copper conductor and the wire insulation that surrounds it. The temperature of the conductor will rise as the current level increases. The wire ampacity calculation is critical in that it will determine the wire size and temperature rating of the wire insulation required for the application. Choose a conductor that is too small for the given electrical load and you could end up with an overheating condition that could lead to wire insulation damage, shortened lifespan and ultimately melting of the wire insulation causing an electrical fire. Electrical fires consume tens of thousands of structures each year that result in billions of dollars in damages, thousands of civilian injuries of which some are fatal. So using wires that have the right ampacity is critical to safety.

Note: Wire insulation materials also vary for specific applications. Besides the temperature rating, other wire insulation attributes include:

  • Voltage rating
  • Abrasion resistance
  • Non-reactivity
  • Flexibility
  • Flammability
  • Density
  • Agency approvals
  • Low smoke (for automotive / in-cabin applications)

Calculating Wire Ampacity

Begin calculating wire ampacity with the National Electrical Code (NEC), which provides tables (Article 310) that list the ampacity for any given conductor size relative to the temperature rating of the wire insulation. The tables show the ampacities as wires in free air and up to 3 wires in a raceway (this includes conduit, cable jackets, etc). If more than 3 wires carry current in a raceway, then you will have to de-rate the ampacity of the wires based on the factors provided by Article 310.

The ampacity of the wire is determined by the amount of current in the wire at the point in which the conductor’s temperature rises 30°C.

The ampacity rating of higher temperature rated wires is greater than lower temperature rated wires for any given size of conductor. The conductor size selected needs to match the electrical load. The load is what will determine the level of current in the system. It is not un-common for devices to be marked with load size (like Wattage or Amps and Volts). Ampacity can be calculated by dividing the Wattage by the rated voltage. The quotient is the expected Amperage of the circuit.

In many instances, a circuit’s unique application correction factors (as described above) will warrant the need for ampacity adjustments. There are four conditions that will determine whether a correction factor is required:

  1. Ambient temperature – The temperature rating of a wire must include the ambient temperature in the application. If the temperature rating of a wire is 90°C and it needs to be placed in a 75°C ambient condition, then the allowable temperature rise of the wire is 15°C (90°C minus 75°C). In this example, the Ampacity of the conductor size specified will need to be de-rated by 50% (based on being limited to a 15°C rise instead of a 30°C rise).
  2. Duty cycle – many times the level of current in applications will vary over time depending on the type of loads. Electrical motors, for example, draw a large amount of current at start up for a short period of time and then the current level reduces as the motor reaches the steady state rpm. In these cases, the wire size is selected such that the temperature rise of the wire does not exceed 30°C.
  3. Overall system requirements – Consider the load and maximum temperature rating of the devices and equipment that make up the electrical system. Sometimes they can be the limiting factor and not the wire. In other cases, the equipment can generate its own heat which will require a higher temperature rating for the wire.
  4. Effect of adjacent load carrying conductors / rate of heat dissipation – More than three current carrying conductors (ground wires are not considered current carrying for these calculations) in a cable or conduit or enclosure affects the ampacity of the wiring as described above. In some cases, multiple wiring systems that generate heat are placed together in a common enclosure that impedes the ability of the wiring to dissipate its own internally generated heat which causes additional temperature rise. This condition also requires that each circuit be evaluated to confirm that the maximum 30°C temperature rise is not exceeded.


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