This section addresses a multiwire branch circuit which is defined in Article 100. This is a branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.
Figure 210.4-1 depicts a typical multiwire branch circuit found in many residential and similar applications. In this case, a two-pole circuit breaker, or two single pole circuit breakers with a handle tie, powers two conductors in a single branch circuit that share a single neutral.
Figure 210.4-1: This demonstrates a circuit breaker with a common handle tie to disconnect both circuits that share a single grounded conductor.
This section is broken into 4 first level subdivisions including the following:
(A) General
The general requirements for a multiwire branch circuit provides the permission for a multiwire branch-circuit as well as permission for it to be considered as multiple circuits. This section requires that all conductors of a multiwire branch circuit originate from the equipment containing the branch-circuit overcurrent protective device(s). the only exception to this is for those that are permitted by 300.3(B)(4).
Section 300.3(B) is titled “Conductors of the Same Circuit.” 300.3(B) requires that all conductors of the same circuit, which in this case includes the grounded conductor and equipment grounding conductor for this multiwire branch circuit, be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord. That is unless it is permitted in 300.3(B)(1) through (B)(4). 300.3(B)(4), which is referenced as an exception of sorts in 210.4, addresses “Column-Width Panelboard Enclosures” and identifies that the grounded conductor (neutral conductor), is permitted to originate in the pull box where an auxiliary gutter runs between a column-width panelboard and a pull box. (Ref. Figure 210.4-2)
(Courtesy of Ryan Jackson)
Informational Note No. 1 reminds the user of the Code and the installer of a multi-wire branch circuit that a 3-phase, 4-wire, wye-connected power system that is used to power nonlinear loads may be such that the phase currents do not add to zero and will have currents returning through the neutral. This would require the designer to consider the size of the neutral conductor to ensure proper ampacity.
Informational Note No. 2 is simply a reference to Section 300.13(B) to ensure continuity of the grounded conductors on multiwire circuits.
(B) Disconnecting Means
Here are the requirements that when followed ensure both ungrounded conductors are opened when the handle of the disconnecting means, the circuit breaker as shown in Figure 210.4-1. Note that the overcurrent protective device when tripped does not have to open all of the poles of the circuit breaker.
(C) Line-to-Neutral Loads
This section ensures that loads are not connected between the hot conductors of the multiwire branch circuit. Loads are to only be connected between line and neutral.
There are two exceptions to this rule. The first addresses when a multiwire branch circuit supplies only one utilization equipment and the second is when a multiwire branch circuit supplies line to line loads when all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch circuit overcurrent device.
(D) Grouping
This section points the user of NFPA 70 back to 200.4(B) which focuses on neutral conductors and specifically when you have more than one neutral conductor associated with different circuits. This requires grouping those conductors. From a safety perspective, the shared neutral installations present unique challenges for the electrical worker. Grouping conductors together help ensure awareness of how those conductors are related and which disconnect(s) must be opened to establish an electrically safe working condition.
NFPA 70-2023 Changes
New language was added to (A) to reference 300.3(B)(4) as an exception to this rule. This is to address those specific times in these smaller panelboard installations when the grounded conductor may not originate in the same enclosure as the circuit breaker. Both exceptions of 210.4(C) were reworded for clarity but not changed in regard to their intent. The informational note was changed to a complete sentence to comply with the 2020 National Electrical Code Style manual section 3.1.4.1 and the word “where” was replaced to “if” in compliance with the same style manual section 3.3.4.