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:
- Since SCR dimmers produce non-sinusoidal wave form on their outputs, the neutral current can equal 1.7 times the single dimmer load current. This means that for three 20 amp dimmers, the neutral could be 34 amps. This requires running a larger AWG wire for neutral than the load.
- Not all manufacturers have the same phasing on consecutive dimmers. This makes it difficult to assign dimmer numbers to locations.
- Because the voltage drop on the neutral wire is dependent on the combination of three dimmers, the electronics cannot compensate for this varying voltage. This will cause some interaction between dimmers on the same neutral wire.
For use with relay systems it is important the there is a single disconnect for each line, so a multipole breaker would be required to have a common neutral.
The use of a shared neutral where the circuits come from individual breakers is not allowed by the NEC. If they use a multipole breaker, a single neutral is allowed. The idea is that if any of the circuits needs to be serviced, all of the circuits that share the neutral must also be de energized together
Common Neutral NEC Information
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.
Note: If you or your customers require more information on this issue, Please feel free to contact Project Management, or Application Engineering for more information.