CSLB C-10 Electrical ContractorPlanning and EstimatingMedium
A client is upgrading the service for a single-family dwelling. The existing service is 100A, and the proposed upgrade is to 200A. The main service panel is located 60 feet from the utility transformer and meter. What is the minimum size of service entrance conductors (copper, THWN-2) required for this 200A service, assuming the conductors are installed in conduit?
- A3/0 AWG
- B2 AWG
- C2/0 AWG
- D1/0 AWG
Show answer & explanationAnswer & explanation
Correct answer: C. 2/0 AWG
For a 200A dwelling service, NEC Table 310.12 (formerly Table 310.15(B)(7)) allows for a reduction in conductor size. For copper conductors, a 2/0 AWG THWN-2 conductor is permitted for a 200A service.
Why the other options are wrong
- A. 3/0 AWG copper is oversized for a 200A dwelling service, leading to unnecessary cost.
- B. 2 AWG copper is rated for 115A (75°C) or 130A (90°C), insufficient for 200A service.
- D. 1/0 AWG copper is rated for 150A (75°C) or 170A (90°C), also insufficient for 200A service.
Dwelling Service Conductor Sizing
Special provisions in the NEC allow for smaller service entrance conductors for dwelling units than would typically be required for a general 200A load, due to inherent diversity of loads.
- NEC 310.12 (formerly 310.15(B)(7)) provides specific tables for dwelling service conductors.
- This table applies to 120/240V, 3-wire, single-phase dwelling services.
- It accounts for the diversity of loads in a typical home, reducing the required conductor size.
Memory trick: Dwelling service: 'Special' rules 'Reduce' conductor 'Size'.