National Electrical Code Ampacity Tables

What is Ampacity?

When we measure ampacity, also known as ampere capacity, we’re referring to the highest electrical current a wire can safely carry without overheating or exceeding insulation or jacketing protection. A wire’s ampacity rating is dictated by the National Electric Code (NEC).

Ampacity depends on several factors, including the metal used to make the wire, its insulation, and the ambient temperature. Using the ampacity rating, we can figure out how well a given wire dissipates heat as current passes through it.

It’s important to remember that as more current moves through a conductor, more heat is generated. Ensure conductors have enough space around them to properly dissipate heat. Wires that surpass their ampacity ratings can potentially generate excessive heat, leading to insulation failure and fires.

Copper Conductors:

NEC Table 310.16 – Ampacities of Insulated Conductors with Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried)

These ampacities are valid for an ambient temperature of 30°C (86°F). Higher ambient temperatures require derated ampacity. Additionally, if more than three conductors are bundled together, the ampacity must be derated to allow for heat dissipation.

Aluminum and Copper Clad Steel (CCS) Conductors:

NEC Table 310.16 – Ampacities of Insulated Conductors with Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried)

These ampacities are valid for an ambient temperature of 30°C (86°F). Higher ambient temperatures require derated ampacity. Additionally, if more than three conductors are bundled together, the ampacity must be derated to allow for heat dissipation.