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Multiwire Branch Circuits

Multiwire Branch CircuitsIf you look up the definition of multiwire circuit in Article 100, you won’t find it under “Multiwire Branch Circuits.” You’ll find it under “Branch Circuits, Multiwire.” So, what is it?

A multiwire circuit consists of two or more ungrounded conductors (with a voltage between them) and a grounded conductor (e.g., neutral). The grounded conductor has an equal voltage between it and each ungrounded conductor, plus it’s connected to the neutral (or grounded conductor) of the system.

Basically, a multiwire is two circuits that share a common neutral. The NEC permits you to consider it to be two circuits [210.4(A)].

The advantage of this wiring scheme is you save some wire by not running a neutral. The downsides include:

  • An open neutral, as might occur if you remove a receptacle (e.g., for replacement) without opening both breakers on the multiwire circuit, which can result in unacceptably high voltage on the operating circuit.
  • With today’s nonlinear loads, you can easily overload the neutral. And because of electronic ballasts, this is true even of lighting circuits, where multiwire circuits formerly seemed advantageous.
  • Instead of two neutral wires for each circuit, you have one longer neutral wire serving both. That means increased voltage drop and reduced energy efficiency.

If you weigh the tiny bit of installation cost-savings against the increased risk of loss of operations, these circuits are not justified for industrial and commercial applications.

Source: Mark Lamendola | Mindconnection