Derating Electrical Conductors: A Guide to NEC Ampacity Adjustments

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In the National Electrical Code (NEC), ampacity adjustments are made to the rated ampacity of a conductor to account for various factors such as ambient temperature, the number of current-carrying conductors in a raceway or cable, conductor bundling, and conductor insulation types.

These adjustments ensure that the conductors are safely carrying their intended loads without overheating. Here are some common ampacity adjustments as per NEC guidelines:

Ampacity Adjustment Factors:

  1. Ambient Temperature Adjustment (Tambient):
    • Conductors are assumed to be installed in an environment at a temperature of 30°C (86°F) unless otherwise specified.
    • If the actual ambient temperature is different, correction factors from NEC Table 310.15(B)(2)(a) are applied.
    • For example, if the actual ambient temperature is 40°C (104°F), the correction factor would be 0.91.
  2. Conductor Insulation Temperature Rating (Tc):
    • Conductors have different insulation temperature ratings (for example, 60°C, 75°C, 90°C).
    • Ampacity tables in the NEC (such as Table 310.16) provide ampacity ratings for different insulation types.
    • The ampacity of the conductor cannot exceed the temperature rating of its insulation.
  3. Adjustment for Bundled Conductors:
    • When multiple conductors are bundled together in a raceway or cable, they generate more heat due to reduced airflow.
    • Adjustment factors are provided in NEC Table 310.15(B)(3)(a) based on the number of current-carrying conductors in the bundle.
    • For example, with 4 to 6 current-carrying conductors, the adjustment factor is 0.80.
  4. Adjustment for Ambient Temperature (30°C/86°F) and More Than 3 Current-Carrying Conductors:
    • When both of these conditions are met, additional adjustment factors are applied.
    • Adjustment factors are provided in NEC Table 310.15(B)(3)(c).
    • For example, with 4 to 6 current-carrying conductors and an ambient temperature over 30°C (86°F), the adjustment factor is 0.70.
  5. Adjustment for Raceway Fill:
    • When conductors are placed in a raceway or conduit, the number of conductors affects the allowable ampacity.

Calculation Example:

Let’s say we have a scenario with the following data:

  • Conductor: 12 AWG THHN copper
  • Ambient Temperature: 40°C (104°F)
  • Three current-carrying conductors in a raceway
  • Insulation Type: THHN (90°C)
  1. Ambient Temperature Adjustment (Tambient):
    • From NEC Table 310.15(B)(2)(a), for 40°C, the correction factor is 0.91.
  2. Adjustment for Three Current-Carrying Conductors:
    • From NEC Table 310.15(B)(3)(a), for three current-carrying conductors, the adjustment factor is 0.80.
  3. Original Ampacity from Table 310.16:
    • Table 310.16 shows the ampacity of 12 AWG THHN copper as 30 amperes at 90°C.
  4. Calculate Adjusted Ampacity:
    • Adjusted Ampacity = Original Ampacity * Tambient * Adjustment Factor for Three Conductors
    • Adjusted Ampacity = 30 A * 0.91 * 0.80 = 21.84 amperes

Result:

  • The adjusted ampacity for the 12 AWG THHN copper conductor with three current-carrying conductors at an ambient temperature of 40°C (104°F) is approximately 21.84 amperes.

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