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Before you connect premises wiring to a supply system, know what kind of system you have and what kind you’re connecting to. The nominal voltage is one obvious consideration. But another critical consideration is the grounded conductor (which is often the neutral).
For any grounded conductor of the interior system (premises wiring), the supply system must contain a corresponding grounded conductor [200.3]. If this isn’t the case, you’ll have to change the interior wiring before connecting to the supply. Obviously, the best time to sort this out is when the wiring still exists only inside a CAD drawing.
This requirement does have an exception. You don’t need a grounded conductor if you’re connecting a listed utility-interactive inverter and that inverter is identified for use in distributed resource generation systems (e.g., photovoltaics).
Remember, this is referring to grounded conductors not grounding conductors. A grounded conductor is any conductor that’s intentionally grounded [Article 100].
The neutral and the grounded conductor are typically, but not necessarily, the same thing. A neutral is a conductor that’s connected to the neutral point of a system. Unlike the ground conductor, it’s expected to carry current under normal conditions.
When your premises wiring has a neutral conductor, you generally need to stick to one neutral per circuit rather than share a neutral [200.4]. Even where the NEC permits this sharing, good engineering practices may rule it out. An example would be an application containing high harmonic loads; don’t use shared neutrals.
Source: Mark Lamendola | Mindconnection