Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsEasy
A commercial building requires a 120/208V, 3-phase, 4-wire service. The calculated demand load for the service is 150A. What is the minimum size copper service entrance conductor required for this service, assuming THWN insulation?
- A3/0 AWG
- B4/0 AWG
- C2/0 AWG
- D1/0 AWG
Show answer & explanationAnswer & explanation
Correct answer: D. 1/0 AWG
For a 150A service, using the 75°C column (THWN insulation) in NEC Table 310.16, 1/0 AWG copper conductors are rated for 150A. This meets the minimum requirement.
Why the other options are wrong
- A. 3/0 AWG copper is rated for 200A at 75°C, which is larger than necessary.
- B. 4/0 AWG copper is rated for 230A at 75°C, which is larger than necessary.
- C. 2/0 AWG copper is rated for 175A at 75°C, which is larger than necessary.
Service Conductor Sizing
The process of selecting the correct wire gauge for service entrance conductors based on the calculated load, conductor material, and insulation temperature rating.
- Based on calculated demand load.
- Refer to NEC Table 310.16 for ampacity.
- Insulation type dictates temperature rating column.
Memory trick: Service Conductors: C-A-T (Calculated Load, Ampacity Table, Temperature Rating)