A commercial building requires a 400A, 480Y/277V, 3-phase, 4-wire service. The service entrance conductors are 500 kcmil copper. What is the minimum size grounded (neutral) conductor required for this service, assuming the maximum unbalanced load is 250A and there are no nonlinear loads?
- A4/0 AWG copper
- B300 kcmil copper
- C250 kcmil copper
- D3/0 AWG copper
Show answer & explanationAnswer & explanation
Correct answer: C. 250 kcmil copper
According to NEC 220.61(A), the grounded conductor must be sized for the maximum unbalanced load. The maximum unbalanced load is 250A. Per NEC Table 310.16, 250 kcmil copper conductor is rated for 255A at 75°C, making it the minimum size. NEC 250.24(C)(1) requires the grounded conductor not to be smaller than the minimum size specified in Table 250.102(C)(1), which for a 400A service is 2/0 AWG copper. However, the calculated unbalanced load takes precedence if larger. 250 kcmil is larger than 2/0 AWG and satisfies the 250A load.
Why the other options are wrong
- A. 4/0 AWG copper is rated for 230A at 75°C, which is insufficient for 250A.
- B. 300 kcmil copper is rated for 285A at 75°C, which is larger than required as a minimum for 250A.
- D. 3/0 AWG copper is rated for 200A at 75°C, which is insufficient for 250A.
Service Grounded Conductor Sizing
The process of determining the minimum size of the grounded (neutral) conductor for a service, based on the maximum unbalanced load and minimum requirements.
- Sized for maximum unbalanced load (NEC 220.61).
- Must not be smaller than required by NEC Table 250.102(C)(1).
- Nonlinear loads require special consideration (NEC 220.61(C)).
Memory trick: Balance the load, then check the table; neutral size is critical, no fable.