A commercial building has a 208Y/120V, 3-phase, 4-wire service. The calculated load for the service is 150A. The service entrance conductors are copper, installed in RMC, and rated for 75°C. What is the minimum size of the grounded (neutral) conductor required if there are no nonlinear loads and the maximum unbalanced load is 100A?
- A2/0 AWG copper
- B1/0 AWG copper
- C1 AWG copper
- D2 AWG copper
Show answer & explanationAnswer & explanation
Correct answer: C. 1 AWG copper
According to NEC 220.61(A), the grounded conductor must be sized to carry the maximum unbalanced load. The maximum unbalanced load is given as 100A. Therefore, the grounded conductor must have an ampacity of at least 100A. From NEC Table 310.16 (75°C column for copper), 1 AWG copper has an ampacity of 130A, which is sufficient. The overall service calculated load of 150A would require 1/0 AWG for ungrounded conductors, but the neutral is based on unbalanced load.
Why the other options are wrong
- A. 2/0 AWG copper is significantly larger than required for a 100A unbalanced load.
- B. 1/0 AWG copper has an ampacity of 150A (75°C), which is larger than required for a 100A unbalanced load and would typically be used for the ungrounded conductors of a 150A service.
- D. 2 AWG copper has an ampacity of 115A (75°C), which is greater than 100A, but 1 AWG is a more common size for this range and the next standard size that provides ample margin.
Service Grounded Conductor Sizing
The service grounded (neutral) conductor must be sized to carry the maximum unbalanced load, as determined by NEC 220.61. It must also not be smaller than the grounding electrode conductor, per NEC 250.24(C)(1).
- Sized for maximum unbalanced load (NEC 220.61).
- Minimum size is not less than the GEC (NEC 250.24(C)(1)).
- Nonlinear loads require special consideration (NEC 220.61(C)).
- Does not typically need 125% factor for continuous loads unless specified.
Memory trick: Neutral's Balance: Unbalanced current, plus GEC minimum, sets the size.