Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsHard
A convenience store is being wired with a 208Y/120V, 3-phase, 4-wire service. The calculated load for the service is 300A. The service entrance conductors are 350 kcmil copper with THWN insulation. What is the minimum size copper service grounded conductor (neutral) required?
- A3/0 AWG copper
- B4/0 AWG copper
- C1/0 AWG copper
- D2/0 AWG copper
Show answer & explanationAnswer & explanation
Correct answer: A. 3/0 AWG copper
First, the service grounded conductor must be sized to carry the maximum unbalanced load (NEC 220.61). Second, it must not be smaller than specified in NEC 250.24(D)(1) and Table 250.102(C)(1). For 350 kcmil copper service conductors, Table 250.102(C)(1) requires a minimum of 3/0 AWG copper grounded conductor.
Why the other options are wrong
- B. 4/0 AWG copper is larger than required.
- C. 1/0 AWG copper is too small for 350 kcmil service conductors.
- D. 2/0 AWG copper is too small for 350 kcmil service conductors.
Service Grounded Conductor Sizing (250.24 & 220.61)
The sizing of the neutral conductor at the service, which must carry the maximum unbalanced current (NEC 220.61) and also meet the minimum size requirements based on the ungrounded service conductors as per NEC 250.24(D)(1) and Table 250.102(C)(1).
- Sized for maximum unbalanced load.
- Minimum size from Table 250.102(C)(1).
- Must be the larger of the two requirements.
Memory trick: Neutral needs 'Balance Calc' (220.61) and 'Table Minimum' (250.102) – pick the biggest!