The primary reason a neutral wire is often missing in older switch boxes is due to a wiring configuration known as a switch loop. In this historical setup, the power source, or line, first runs directly to the light fixture box, and only two wires are extended down to the switch. The neutral wire in a residential electrical circuit serves as the return path for the electrical current, completing the circuit back to the electrical panel and the utility transformer. It is insulated and carries current under normal conditions back to the transformer. Second question: the code now (I believe beginning in 2011) requires the neutral at most switch locations but there are also many exceptions, e. closets, non-habitable rooms, switches installed with conduit to allow for. Circuits use hot, neutral, and ground wires. Clearly, neutrals are vital components of every home.
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