Journeyman Electrician Exam (NEC)Wiring Methods and MaterialsHard
A client requests a new 120/240-volt, 100-amp single-phase service entrance for a small workshop. The service conductors will be installed in a wet location. What is the minimum size XHHW-2 aluminum service-entrance conductor required for this service?
- A1 AWG
- B1/0 AWG
- C2/0 AWG
- D2 AWG
Show answer & explanationAnswer & explanation
Correct answer: B. 1/0 AWG
For a 100-amp 120/240-volt single-phase service to a dwelling unit (or similar small service like a workshop), NEC Table 310.12, Note 3 (or 310.15(B)(7) for dwelling services) allows for a reduction in conductor size. For 100 amps, 1/0 AWG aluminum (XHHW-2 has a 90°C rating) is the minimum size permitted. The table explicitly lists 1/0 AWG aluminum for 100A services.
Why the other options are wrong
- A. 1 AWG aluminum is rated for 115 amps (75°C) or 130 amps (90°C), larger than needed but 1/0 AWG is the specific answer for 100A service.
- C. 2/0 AWG aluminum is rated for 130 amps (75°C) or 150 amps (90°C), which is larger than required.
- D. 2 AWG aluminum is typically rated for 90 amps (75°C column) or 100 amps (90°C column), but not for 100A service as per the specific service conductor rules.
Service Conductor Sizing (Dwelling/Small Services)
Special rules in the NEC (e.g., 310.12, 310.15(B)(7)) allow for smaller service-entrance conductors for dwelling units and small services compared to general ampacity tables, based on the service rating.
- Specific tables (e.g., 310.12, 310.15(B)(7)) apply to service conductors.
- Allows for smaller conductors than general ampacity tables for dwelling units.
- Considers the diversified nature of residential loads.
Memory trick: Service conductors are special, check the 'dwelling discount' for the size.