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Idaho Low Voltage Practice: Overcurrent protection and conductor sizing

Idaho Low Voltage Practice · Work through your exam, one topic at a time.

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Overcurrent protection and conductor sizing

Scope

Overcurrent and conductor-sizing questions test Article 240, Article 310, and the small-conductor rules. You must select an overcurrent device that protects the conductor after ampacity adjustment and correction, while respecting terminal ratings.

Core rules

  • Standard OCPD sizes are in 240.6: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A, and so on.
  • Table 310.16 gives allowable ampacities for copper and aluminum conductors at 60, 75, and 90 degrees C.
  • Adjust ampacity for ambient temperature (Table 310.15(B)(1)) and for bundled current-carrying conductors (310.15(C)(1)).
  • Small conductors have hard limits in 240.4(D): 14 AWG = 15 A, 12 AWG = 20 A, 10 AWG = 30 A, unless a specific exception applies.
  • Continuous loads are calculated at 125% for branch-circuit and feeder sizing (210.20, 215.2).

Worked example

A 12 AWG THHN copper conductor has a 90 degrees C ampacity of 30 A. It is installed with four current-carrying conductors in a raceway (80% adjustment) in a 40 degrees C ambient (0.91 correction). Adjusted ampacity = 30 x 0.80 x 0.91 = 21.8 A. The termination is rated 75 degrees C, so the 75 degrees C column gives 25 A, but 240.4(D) still limits 12 AWG to 20 A. The maximum OCPD is 20 A.

Traps

Using the 90 degrees C column for terminations; forgetting the 125% continuous load factor; applying adjustment twice; using the next-size-up rule without checking 240.4; and assuming a 30 A breaker is fine on 12 AWG.