This information primarily applies in the United States. Shared neutral circuits are extremely uncommon in European systems.
In non-dimmed applications, branch circuits are often installed with three circuits sharing a common neutral conductor, where each of the three circuits is on a different phase.
This is known as a "multiwire" branch circuit arrangement. It is used to save costs because only four total conductors (three hot, one neutral) are used to feed three branch circuits.
This type of multi-wire branch circuit arrangement is not recommended for use with phase-control dimmers on new installations because it can cause voltage-drop interaction between the three branch circuits, as well as neutral conductor overloading.
Previously, it could sometimes be acceptable for the existing shared neutral circuit to be connected to a dimming system, provided that the three phase conductors remained on separate phases and that the current on the three branches was low enough that the neutral was rated for over 130% of any individual phase current.
However, in a new development, the 2008 NEC section (210.4), adds a new condition requiring multiwire branch circuits to be fed from a multi-pole breaker, or three single-pole breakers with a handle-tie identified for the purpose. This essentially eliminates new installations or retrofits using common neutrals on branch circuits fed from dimmer racks, because the handles of single-pole breakers in the dimmers cannot be mechanically tied together.
The exact wording of the 2008 NEC is:
2008-210.4(B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.
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