CSLB C-10 Electrical ContractorWiring Methods and MaterialsMedium
A continuous 30A load is supplied by copper THWN conductors terminating on breakers rated for 75°C. Using the 125% continuous load factor and NEC Table 310.16 (75°C column), what is the minimum conductor size required?
- A8 AWG
- B4 AWG
- C6 AWG
- D10 AWG
Show answer & explanationAnswer & explanation
Correct answer: A. 8 AWG
Required ampacity = 30A × 1.25 = 37.5A. At the 75°C column, 10 AWG copper is rated for only 35A (too low), while 8 AWG copper is rated for 50A, which satisfies the requirement. Therefore 8 AWG is the minimum size.
Why the other options are wrong
- B. 4 AWG greatly exceeds the requirement and is not the minimum size.
- C. 6 AWG is larger than necessary and not the minimum size.
- D. 10 AWG at 75°C is rated 35A, below the 37.5A requirement.
Continuous Load Conductor Sizing
Conductors supplying continuous loads (3 hours or more) must be sized at 125% of the load current using the appropriate temperature column of Table 310.16.
- 125% factor applies to continuous loads per NEC 210.19(A).
- Use the ampacity column matching the equipment's terminal temperature rating.
- Round up to the next standard conductor size that meets or exceeds the requirement.
Memory trick: 'Continuous loads need a quarter more' — multiply by 1.25.