What is Cable Ampacity?

What Is Cable Ampacity?

Understanding ampacity is a vital part of cable selection and installation. Ampacity relies on a complex calculation with many different factors, and getting it wrong can lead to significant safety concerns. It is vital to consider how this rating affects the need for adjustments to the cable’s operating environment.

We’ll go over cable and wire ampacity and how you can select the right products for your application.

What Is Wire Ampacity?

Wire ampacity refers to the highest amount of current a conductor can carry without compromising safety at ambient temperatures. This rating is determined by the National Electric Code (NEC), which lists these ratings according to real-world examples. Most cable manufacturers provide ampacity ratings for their products.

The term “ampacity” comes from “ampere capacity.” It depends on a wide range of factors, like the material the wire’s insulation is made of and the ambient temperature in which it is used. With this information, we can calculate how well heat is able to dissipate away from the cable. Heat is generated when a current moves through a wire. If the heat can’t dissipate and exceeds the cable’s temperature rating, the cable can fail and potentially cause a fire.

Several factors can affect a wire or cable’s ability to dissipate heat and, by extension, its ampacity, including:

  • Jacket and insulation materials.
  • Conductor size.
  • Device amperage.
  • Ambient temperature.
  • Grouping, bundling or enclosures.

If the ampacity is affected by these elements, corrections need to be made to prevent unsafe operating conditions. For example, if the ambient temperatures increase, the amperage running through the wire must be reduced to maintain a safe current.

Why Is Ampacity Important?

Ampacity is a critical consideration for safety and adherence to NEC. If it isn’t top of mind, there can be serious consequences for anyone nearby or working on the installation. Some problems associated with overlooking ampacity requirements include risks for:

  • Damage to the insulation: When heat exceeds the capacity of the insulation, it begins to damage it. Even if it doesn’t cause a fire, this damage can affect the performance of the wire, especially as it degrades over time.
  • Electrocution: Exceeding ampacity also creates the risk of arc flashes and electrocution to electrical workers involved.
  • Electrical fire: Excess heat can lead to fire hazards, which endanger property and anyone in the vicinity of the wiring.
  • Injury and death: Any of these issues could create a clear risk for passersby and electrical workers.

How to Calculate Wire Ampacity

Wire ampacity can be a complex calculation. Heat dissipation depends on a wide range of factors, notably ambient temperature and insulation material. This maximum temperature rating is primarily determined by insulation, and the temperature rating heavily influences the current-carrying capacity.

To calculate wire ampacity, you’ll want to start by referring to NEC Table 310.15(C)(1). This table tells us the ampacities of wire in free air, with up to three wires in a raceway — that includes in cable jackets and conduits. The conductor size needs to match the electrical load, which will determine the available current.

Devices are typically marked with the load size in either wattage, amps or volts. To calculate ampacity, you’ll need to divide the device’s wattage by the cable’s rated voltage. This number tells us the wire current capacities if the conductor’s temperature rises 30 degrees Celsius.

The calculation is incredibly simplified with the help of these ampacity charts. The actual equation looks like this:

  • I = { ∆Φ – Wd[½T1+n(T2+T3+T4)] / RT1+nR(1+λ1)T2+nR(1+λ12)(T3+T4) }1/2

The calculation for ampacity requires the following information:

  • I: The permissible current rating.
  • ∆Φ (K): The rise in temperature for the conductor.
  • n: The number of same-size conductors carrying the same load.
  • R (Ω/m): The alternating current resistance for each unit length at the max operating temperature.
  • Wd (W/m): The dielectric loss for each unit length for the conductor’s insulation.
  • T(K m/W): The thermal resistance for each unit length between a conductor and the sheath.
  • T2 (K m/W): The thermal resistance for each unit length of the bedding between the sheath and the armor in
  • T3 (K m/W): The thermal resistance for each unit length of the cable’s external sheath.
  • T4 (K m/W): The thermal resistance for each unit length between the cable’s surrounding medium and its surface.
  • λ1: The ratio of losses in the metal sheath to the total losses in all cable conductors.
  • λ2The ratio of losses in the cable’s armoring to the total losses in all conductors of the cable.

If there are any complicating factors, such as changes in temperature or an increase in the load, you will need to make ampacity adjustments, which we’ll review next.

Making Ampacity Adjustments

Certain conditions will call for adjustments to the ampacity rating. These include:

  • Ambient temperatures: Wire ratings are based on specific ambient temperatures — usually 30 degrees Celsius, or 86 degrees Fahrenheit. If your ambient temperature is different, you’ll need to adjust it according to the NEC’s tables.
  • Duty cycle: Different types of loads will affect the wire’s current. For example, electrical motors pull a lot of current when they first start up, but only for a short time. Then, the current reduces and stays steady. The wire needs to be capable of handling load spikes and changes without causing the wire’s temperature to rise by more than 30 degrees Celsius.
  • System requirements: The equipment involved in an electrical system has load and maximum temperature ratings. They could limit a wire’s ampacity or generate more heat during operation. This would, of course, increase the need for a higher temperature rating.
  • Nearby wiring systems: If your cable, conduit or enclosure has more than three current-carrying conductors, ampacity is likely to be affected. These bundles affect the wire’s ability to dissipate the heat it generates and, in turn, cause an increase in temperature. If you’re using more than three wires, you’ll need to evaluate each circuit to ensure you do not exceed 30 degrees Celsius.

Adjustment factors are provided in the NEC’s tables.

Master Ampacity With AerosUSA

At AerosUSA, wire and cable are our lifeblood. We offer a wide range of wire and cable protection products to help you find the ideal solution for even the most specific of installations. Our knowledgeable staff is well-versed in ampacity and can answer any questions you still have, such as how placing your wire in the conduit will affect its ampacity.

We’ve been in the industry for years and have built up a reputation as a dependable supplier. Browse our cable protection and strain relief products today, or reach out to an expert with any questions about the ampacity of your installation.


Reviewed for accuracy by: George Sims.
George Sims is an engineering and service-oriented leader in Cable Protection and Cable Management Products. Focus is on 100% commitment to customer satisfaction. AerosUSA is a small, agile, independent company whose focus is on our customers.