In non-dimmed applications, branch circuits are often installed with three (or two) circuits sharing a common neutral conductor, where each of the circuits are on a different phase. This is known as a "multi-wire" 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, or three conductors (two hot, one neutral) in the case of a two-circuit multi-wire arrangement
Three Phase Multi-wire Branch Circuit
Two Phase Multi-wire Branch Circuit
The US National Electrical Code (NEC) section 210.4 requires that if you use a multi-wire branch circuit, there must be a single point of disconnection:
[NEC 2020] 210.4(B) Disconnecting Means. Each multi-wire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.
This is a provision made for safety. With a shared neutral, it would not be possible to safely service equipment on one branch while the other two are energized. In practical terms, this requires a multi-pole circuit breaker or multiple single-pole breakers connected by Listed handle ties to feed the multi-wire circuit. This forces disconnection of power to all three branch circuits when any one is turned off, to ensure the safety of anyone working on the circuit.
Dimming Systems - General
ETC does not recommend the use of multi-wire branch circuits with phase-control dimmers (either forward or reverse-phase), for the following reasons:
Since dimmers draw a non-sinusoidal current, the neutral current in a multiphase system can theoretically equal 1.7 times the single dimmer load current, depending on dimmer settings. This means that for three 20A dimmers, the neutral current could be 34A. This requires running a larger wire for neutral than the three hot conductors
Because the voltage drop on the neutral wire is dependent on the combination of levels of multiple dimmers, the electronics cannot compensate for this varying voltage. This will cause some interaction between dimmers on the same neutral wire. This could show up as performance issues, such as one dimmer flickering when another is at a specific level.
Dimming Systems – Sensor and Unison
Because the circuit breakers in ETC’s Sensor and Unison racks have no means of tying breaker handles on different phases together, you cannot use Sensor or Unison racks to feed a multi-wire branch circuit of loads and be compliant with the NEC. For Sensor and Unison dimming systems, all loads must have a separate neutral.
Dimming Systems – Distributed (Foundry, Echo dimmers)
Because the current rating of distributed architectural dimmers is typically lower than a theatrical dimmer, the risk of overcurrent in the neutral conductor (point 1 above) is lessened.
However, it is still likely that item (2) could cause problems, especially in the event of a mixture of load types and levels.
Therefore, ETC also strongly recommends not using multi-wire branch circuits to feed distributed dimmers.
If it is unavoidable, for example in a retrofit installation, care should be taken to:
Minimize the different load types on branch circuits fed by multi-wire branch circuits
Take time to try a variety of dimmer levels during commissioning, to look for any problems or areas where circuit intensities interact and cause problems such as flickering.
In switching-only systems, the phase current cannot exceed the neutral current, so it is acceptable to use a multi-wire branch circuit to feed a system of distributed relays.
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.