I haven’t written about multiwire branch circuits in quite some time, but this issue keeps coming up when I provide training on electrical safety, so I believe it’s time to revisit it. In the training class I teach on NFPA 70E, Standard for Electrical Safety in the Workplace, I provide this real-life example.

An electrical contractor was changing ballasts in the classrooms of an older school building. Since classes were still in session, some of the classrooms were in use while he was working in other, unoccupied classrooms. He had been instructed to turn off only the lighting circuit he was working on and leave the others in operation. He turned off a circuit breaker in the panelboard, locked it off and then checked the wall switch in the classroom with a voltage tester to ensure the lighting circuit was truly off.

Upon removing the lens in the luminaire and accessing the ballast, he was shocked, fell off the ladder and broke his leg. What should he have done to be safer and avoid this incident?

What to do differently

First, he should have checked in the panelboard to determine if the circuits he would be working on were individual branch circuits with a single hot conductor and a dedicated neutral conductor or multiwire branch circuits. A multiwire branch circuit is defined in Article 100 as “a branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a neutral conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral conductor of the system.”

A multiwire branch circuit can consist of either a two-pole, single-phase circuit or a three-pole circuit breaker for a three-phase circuit, with the circuits in each case sharing a neutral.

Multiwire branch circuits are specifically covered in 210.4(A) through (D) in the National Electrical Code with general requirements located in (A) stating the following: “Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire circuit shall be permitted to be considered as multiple circuits. Except as permitted in 300.3(B)(4), all conductors of a multiwire branch circuit shall originate from the equipment containing the branch-circuit overcurrent protective device or protective devices.”

A multiwire branch circuit can consist of either a two-pole, single-phase circuit or a three-pole circuit breaker for a three-phase circuit, with the circuits in each case sharing a neutral. The single-phase, three-wire circuit could consist of two single-pole circuit breakers with an identified handle tie between the two breakers or a two-pole internal trip circuit breaker. 

A three-phase circuit could use three single-pole circuit breakers with an identified handle tie or a three-pole internal trip breaker. A two-pole circuit breaker or two single-pole circuit breakers with an identified handle tie could supply a duplex receptacle, with the breakable tabs on the hot side removed and a single neutral on the neutral side of the receptacle.

Code changes to note

Prior to the 2008 edition, the NEC required a two-pole internal trip or two single-pole circuit breakers with identified handle ties for multiwire branch circuits supplying duplex receptacles. However, in the 2008 NEC, Section 210.8 was reworded to require simultaneous disconnection of all ungrounded conductors in any multiwire branch circuit. 

This simultaneous disconnection reduces the risk of electric shock for a person working on equipment supplied by a multiwire branch circuit. Knowing of this Code change could have kept the electrical contractor in our example above from getting injured.

There is another major issue involving multiwire branch circuits, as covered in 210.4(D), involving grouping as follows: “The ungrounded and grounded circuit conductors of each multiwire branch circuit shall be grouped in accordance with 200.4(B).” 

When accessing 200.4(B), the text states that “where more than one neutral conductor is associated with different circuits in an enclosure, the grounded circuit conductors must be identified or grouped to correspond with the particular ungrounded circuit conductors within that multiwire branch circuit.” 

For example, in the panelboard, a wireway or a box, the two or three hot conductors of a multiwire branch circuit and its corresponding neutral must be grouped by cable ties, marking or similar means from other multiwire branch circuits or single circuits. This ensures that the neutral conductor of each multiwire branch circuit will be de-energized when that circuit breaker has been turned off. If someone were to tie all of the neutrals together in a wireway, a person could encounter an energized neutral after turning off a particular multiwire branch circuit, with potentially grave consequences.

Simply knowing the history of the NEC and the reason and dates for certain changes can affect how electricians work on electrical installations—and their safety.