CSLB C-10 Electrical ContractorWiring Methods and MaterialsHard
An engineer specifies two 3/0 AWG THHN copper conductors per phase, installed in parallel in separate conduits, to serve a 400A load. Each 3/0 AWG THHN conductor has an adjusted allowable ampacity of 225A. What is the total allowable ampacity of the parallel conductor set per phase, per NEC 310.10(H)?
- A337.5 amps
- B225 amps
- C450 amps
- D400 amps
Show answer & explanationAnswer & explanation
Correct answer: C. 450 amps
When conductors are installed in parallel under NEC 310.10(H), they must be identical in length, material, size, insulation, and termination, and the total ampacity equals the sum of each individual conductor's ampacity. Two 225A conductors in parallel yield 225A + 225A = 450A, which is sufficient to serve the 400A load.
Why the other options are wrong
- A. This value does not correspond to any standard doubling calculation.
- B. This is the ampacity of only one conductor, not the parallel set.
- D. 400A is the load requirement, not the calculated parallel ampacity.
Parallel Conductor Ampacity
Conductors run in parallel per NEC 310.10(H) must match in size, length, and material, with total ampacity equal to the sum of each conductor's individual ampacity.
- Applies to conductors 1/0 AWG and larger
- All parallel sets must be identical in every conduit
- Total ampacity = sum of each conductor's ampacity
Memory trick: Twins in parallel add their strength together