- Is sharing neutrals an acceptable practice? Is it code compliant?
Sharing neutrals is perfectly legal in most situations. The NEC term for several ungrounded conductors sharing a neutral is a "multiwire branch circuit." The definition is in Article 100. The key is, a multiwire branch circuit has a voltage between all the conductors of the circuit - two conductors on the "A" phase is not a multiwire branch circuit.
So, code compliance aside, is it a good idea? Many electricians do not approve of them, as disconnecting a neutral when one or more of the ungrounded conductors are live results in damage to equipment, and a shock hazard to the person working on the circuit.
The portion of the neutral that is connected to energized loads is carrying current, and can be lethal.
That said, 70% of members polled (
here) use multiwire branch circuits (MWBCs), and proudly so. They are more efficient in terms of voltage drop, and in material used. Simply put, using one neutral for three line-to-neutral circuits in a commercial setting saves 66% of conductor over using dedicated grounded conductors for each circuit.
As for the potential for shock hazard, electricians need to be aware of the wiring method and disconnect and lock out all ungrounded conductors supplying a MWBC - just as they would if it were a simple two-wire circuit.
In the 2008 cycle, it appears that all handles supplying MWBCs will have to be tied together to prevent this hazard. Opinions abound about their use, but one thing is for certain; MWBCs continue to be used, and everyone should be aware of their benefits and drawbacks, for the safety of both the electrician and the equipment of the user.
Be sure to check out some applicable sections: 210.4, 501.40, 502.40, 505.21.
Related links:
Inspecting MWBC's in the panel, Aug 2007. Started by Dan Smith, Discusses the new requirements on MWBC's coming up in the 2008, and a discussion about the pros and cons of MWBCs ensued.