CSLB C-10 Electrical ContractorPlanning and EstimatingEasy
An electrician is reviewing blueprints for a new commercial building. The general notes specify that all conductors for branch circuits are to be rated at 75°C. For a 120V, 20A general-purpose branch circuit, what is the minimum allowable ampacity of the conductors before applying any adjustment or correction factors, assuming copper conductors?
- A25 A
- B15 A
- C20 A
- D30 A
Show answer & explanationAnswer & explanation
Correct answer: C. 20 A
For a 20A branch circuit, the conductors must have an ampacity of at least 20A. NEC 240.4(D) specifies that the overcurrent protection for 14 AWG, 12 AWG, and 10 AWG copper conductors shall not exceed 15A, 20A, and 30A respectively. Thus, a 20A circuit requires conductors rated for at least 20A, typically 12 AWG copper.
Why the other options are wrong
- A. Incorrect. This is an overestimation.
- B. Incorrect. This would be for a 15A circuit.
- D. Incorrect. This is for a 30A circuit.
Conductor Ampacity for Branch Circuits
The maximum current a conductor can carry continuously under specified conditions without exceeding its temperature rating, determined by NEC Table 310.16 and restricted by NEC 240.4(D) for small conductors.
- NEC 240.4(D) limits OCPD for small conductors.
- Conductor ampacity must be equal to or greater than the branch circuit OCPD.
- Temperature rating of insulation affects ampacity (e.g., 60°C, 75°C, 90°C).
Memory trick: Ampacity for branch circuits: 'RATED' for the 'BREAKER', 'TEMP'erature matters.