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Thursday, October 10, 2019
Question:
Per 2017 NEC, is shared neutral conductor permitted in multiwire branch circuits to serve line-to-neutral loads for lighting and/or receptacles installed indoor? Section 200.4(B) states that "unless specifically permitted elsewhere in this Code". Specific requirements in sections 215.4 and 225.7 permit multiple circuits to have a common or shared neutral conductor.
Jesus Santos
A
Answer:
Hey Jesus thanks for your question. Yes, and they are not limited to indoors. Multiwire branch circuits are allowed as stated in 210.4 under the conditions provided and can be considered as multiple circuits - even though they are not multiple circuits. The definition of Branch Circuit, Multiwire in Article 100 indicates it is "A" branch circuit and the ungrounded conductors have a voltage between them. As you mention the neutral conductor is "shared" between multiple ungrounded conductors from different phases (or lines) of a circuit. A multiwire circuit supplied from a three-phase system (A, B, C and Neutral conductor) has essentially the equivalent load capacity as three separate 2-wire branch circuits, but it is considered one circuit.
As you also mention, neutral conductors are not allowed for more than one branch circuit or one multiwire branch circuit or one set of ungrounded feeder conductors as stated in 200.4(A), unless specifically permitted elsewhere in this Code. For lighting equipment installed outdoors, a neutral conductor can be used for multiple circuits as stated in 225.7(B). In this arrangement there could be 2 A phase, 2 B phase, and 2 C phase 20 ampere ungrounded conductors with 1 (single) 40 ampere neutral conductor - used as a common neutral conductor. There are limits of how many sets of feeder conductors as stated in 215.4. Although the word "shared" is not used in the definition of Branch Circuit, Multiwire, in 210.4, 215.4, or 225.7 it is a good way of differentiating between common neutral conduct and those that are for a single branch circuit (2 wire or multiwire).