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. It is not recommended when installation dimming systems for the following reasons:
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.