Question 1

What is wire ampacity?

Accepted Answer

Ampacity is the maximum continuous current-carrying capacity of an electrical conductor under specific conditions, as defined by NEC Article 100. It is determined by the conductor size (AWG or kcmil), material (copper or aluminum), insulation temperature rating (60°C, 75°C, or 90°C), ambient temperature, and number of conductors bundled together. NEC Table 310.16 provides base ampacity values for conductors in conduit, cable, or earth.

Question 2

How do I derate wire ampacity for high temperature?

Accepted Answer

Apply the temperature correction factor from NEC Table 310.15(B)(1). The table is based on a 30°C (86°F) ambient. If the actual ambient is higher, multiply the base ampacity by the correction factor for your temperature and wire rating. For example, a 90°C-rated conductor at 50°C ambient has a correction factor of 0.82, reducing its ampacity to 82% of the table value. If the ambient exceeds the conductor's temperature rating, it cannot be used at all.

Question 3

How does conduit fill affect ampacity?

Accepted Answer

When more than 3 current-carrying conductors are bundled in a conduit or cable, heat cannot dissipate as effectively. NEC Table 310.15(C)(1) requires derating: 80% for 4–6 conductors, 70% for 7–9, 50% for 10–20, 45% for 21–30, 40% for 31–40, and 35% for 41 or more conductors. Both the temperature correction factor and the conduit fill factor are applied simultaneously by multiplication.

Question 4

What wire size do I need for a 20-amp circuit?

Accepted Answer

For a standard 20-amp residential branch circuit (120/240V, 60°C or 75°C terminal rating), NEC Table 310.16 requires a minimum of #12 AWG copper. However, #12 AWG Cu has a 75°C ampacity of 25 A — NEC 240.4(D) limits it to a 20 A OCPD. If you need to derate for ambient temperature or conduit fill, you may need to upsize to #10 AWG or larger. Always verify that terminal ratings match the conductor temperature rating.

Question 5

What is the difference between 60°C and 75°C wire ampacity?

Accepted Answer

The temperature rating refers to the maximum operating temperature of the conductor's insulation, not the ambient temperature. Higher-rated insulation allows more current to flow before the conductor reaches its thermal limit. For example, #4 AWG copper: 60°C = 70 A, 75°C = 85 A, 90°C = 95 A. Most modern equipment terminals are rated for 75°C, making the 75°C column the most commonly used. The 90°C column may only be used when both the conductor insulation and equipment terminals are rated at 90°C.