CSLB C-10 Electrical ContractorEquipment and InstallationHard
An electrician is installing two 3-phase, 480-volt feeders in the same conduit. Each feeder consists of three 4 AWG THHN copper conductors. What is the maximum number of current-carrying conductors for the purpose of adjusting ampacity according to NEC Table 310.15(C)(1)?
- A4
- B6
- C3
- D7
Show answer & explanationAnswer & explanation
Correct answer: B. 6
Each 3-phase feeder has three current-carrying conductors (L1, L2, L3). Since there are two such feeders in the same conduit, the total number of current-carrying conductors is 3 + 3 = 6. The neutral conductor in a 3-phase, 4-wire system with balanced loads is not counted as a current-carrying conductor for ampacity adjustment, but the question specifies 3-phase feeders, implying 3 current-carrying conductors per feeder.
Why the other options are wrong
- A. This is incorrect; it might apply to some single-phase multiwire circuits but not two 3-phase feeders.
- C. This would be correct for a single 3-phase feeder.
- D. This would be for two 3-phase feeders plus a neutral or ground, but the question specifies 'current-carrying conductors' for ampacity adjustment.
Ampacity Adjustment for Multiple Conductors
When more than three current-carrying conductors are installed in a raceway or cable, the ampacity of each conductor must be reduced by an adjustment factor from NEC Table 310.15(C)(1).
- Only current-carrying conductors are counted.
- Neutral conductors in a balanced 3-phase, 4-wire system are generally not counted.
- Equipment grounding conductors are never counted.
Memory trick: Count Conductors: 'L'ive 'P'hases 'O'nly (unless neutral is unbalanced).