Multiwire Branch Circuits Practice Questions

A. Two or more ungrounded conductors with a voltage between them, plus a grounded conductor with equal voltage between it and each of them, connected to the neutral of the system

Every clause of that sentence does work. The voltage between the ungrounded conductors is what forces them onto different phases or different legs, and the equal voltage to the grounded conductor is what makes that conductor a neutral instead of just another circuit wire. A definition item on this topic is asking you to notice both clauses rather than one.

NEC 2026 Article 100

B. So that the shared neutral carries the difference between the load currents rather than their sum

It is not a preference and it is not about tidy panels. On the same leg the return currents add, and the neutral is asked to carry roughly double what it was chosen for with nothing in the circuit that would notice. The definition builds the phase relationship in by requiring a voltage between the ungrounded conductors.

NEC 2026 Article 100

C. 7 amperes

With the two ungrounded conductors 180 degrees apart, the neutral returns only the difference between the two load currents, which is 16 minus 9, or 7 amperes. That is the whole point of the arrangement, since three conductors do the work of four. The 25 ampere answer is what the same two loads would put on the neutral if they were landed on the same leg.

NEC 2026 Article 100

B. The neutral carries 25 amperes and nothing in the circuit trips

The return currents now add instead of subtracting, so a conductor chosen to return one circuit carries the sum of two. No overcurrent device sees anything unusual, because each ungrounded conductor is still within its own rating. That silence is what makes this a routine inspection finding and an easy item to write, so a stem that describes where the breakers sit is asking about it.

NEC 2026 Article 100

A. About 14 amperes

Two equal currents 120 degrees apart do not cancel, and their sum comes out about the same size as either one. Balance empties the neutral only where the ungrounded conductors sit 180 degrees apart. In a raceway this circuit looks identical to a single-phase three-wire circuit, which is why the code counts its neutral as current-carrying and the other one as not.

NEC 2026 310.15(E)(1)

B. A means that will simultaneously disconnect all the ungrounded conductors

Open one ungrounded conductor and the other is still driving current through the shared neutral, so somebody who switched off the breaker they were told to switch off is holding a live conductor. The requirement attaches at the origin and it is written as disconnection rather than as tripping. That distinction is what most items on this section are built from.

NEC 2026 210.4

C. No, it attaches at the point of origin and what sits downstream has no bearing on it

The single utilization equipment condition is real, but it belongs to a different subsection, the one that limits a multiwire branch circuit to line-to-neutral loads. Borrowing that condition and bolting it onto the disconnect rule is the invented answer these items are built around. An item will quote one of the two subsections and offer the other as a choice.

NEC 2026 210.4

D. Where the circuit serves only dc line-to-neutral loads or single-phase alternating current line-to-neutral loads

That permission sits in the overcurrent article rather than in the branch-circuit article, which is why people go looking for it in the wrong place. It is conditioned on the kind of load and on the system, not on the occupancy or the panel. The handle ties themselves have to be identified for the purpose.

NEC 2026 240.15(B)

A. Simultaneous disconnection means throwing the handle takes both poles out together, while common trip means a fault on one pole opens the other as well

The branch-circuit article asks for disconnection, and a handle tie across two single-pole breakers provides it under the conditions the overcurrent article states. A two-pole common trip breaker also provides it and is what most panels get, but common trip is more than the section asked for. Reading the requirement as tripping rather than as disconnection produces the wrong answer on the handle tie item.

NEC 2026 210.4, 240.15(B)

B. Only line-to-neutral loads, unless one of the two exceptions applies

The general rule is a flat restriction, and both ways out of it are written as exceptions rather than as conditions inside the rule. One covers a circuit supplying only one utilization equipment, and the other covers a circuit whose ungrounded conductors are all opened simultaneously by the overcurrent device. Read the exceptions in your own book, because the second one is easy to confuse with the disconnect requirement two subsections earlier.

NEC 2026 210.4

C. The circuit supplies only one utilization equipment, or all its ungrounded conductors are opened simultaneously by the overcurrent device

There are exactly two exceptions and they are alternatives rather than a pair of conditions to be met together. The second one asks for opening by the overcurrent device, which is a stronger statement than the handle tie permission elsewhere. An answer built out of the disconnect rule is offered on purpose, because the two subsections sit a few lines apart.

NEC 2026 210.4

D. By wire markers, cable ties or similar means, in at least one location within the enclosure

Article 210 states the grouping requirement and then points at the grounded conductor article for how it is done, so the mechanics are written one article over. Grouping exists for the next person opening that panel, since the hazard behind every rule on this topic is somebody working on a circuit they believe is dead. Two multiwire circuits arriving in one raceway both get tied.

NEC 2026 210.4, 200.6(B)

A. The branch-circuit or feeder conductors enter from a cable or a raceway unique to that circuit, which makes the grouping obvious

The exception is narrower than most people remember it, and it turns on a cable or raceway that belongs to that circuit alone. A tidy panel is not the test and neither is the breaker arrangement. Two multiwire circuits sharing one raceway fail the condition and get tied.

NEC 2026 200.6(B)

B. Branch-circuit conductors that pass through a box or conduit body without a loop of the kind the box fill rules describe, and without a splice or a termination

A conductor that only passes through has no connection anybody could work on in that enclosure, so there is nothing for the grouping to warn about. The exception borrows the loop description from the box fill rules, which is where a pass-through conductor is defined for counting purposes. A conductor that is spliced or terminated in the box is outside the exception.

NEC 2026 200.6(B), 314.16(B)(1)

C. From the equipment containing the overcurrent protective device, except as an underground provision permits otherwise

The general rule keeps the whole circuit tied to one piece of equipment, which is what makes a single simultaneous disconnect at the origin meaningful. The subsection carries a pointer to an underground wiring provision that permits an exception. A multiwire circuit assembled out of two different panels defeats every other rule on this page.

NEC 2026 210.4

D. About 192 volts

With the neutral gone the two loads become one series pair across 240 volts, so the current is 240 divided by 120 ohms, or 2 amperes, and the 96 ohm load sits at 192 volts while the 24 ohm load gets 48. The small load takes nearly the whole supply and burns out while the large one does nothing. Nothing trips, because 2 amperes is nowhere near either breaker rating, which is why the code refuses to let neutral continuity depend on a device connection.

NEC 2026 300.15(B)

A. It shall not depend on device connections such as lampholders and receptacles, where removing the device would interrupt it

Pulling one receptacle out of a two-gang box should not put 200 volts on a lamp at the other end of the circuit, and that is exactly what a neutral run through the device terminals would do. The fix is a pigtail so the neutral stays made up whatever happens to the device. This is an overvoltage failure rather than an overcurrent one, and no protective device on the circuit will catch it.

NEC 2026 300.15(B)

D. No, unless the code specifically permits it elsewhere

A neutral conductor is not to be used for more than one branch circuit, more than one multiwire branch circuit, or more than one set of ungrounded feeder conductors unless the code says otherwise. That is what makes a shared neutral a defined arrangement rather than a shortcut. The section that says it also points at a separate common neutral provision for feeders.

NEC 2026 200.6(A), 215.14

C. It is counted as a current-carrying conductor

That neutral carries about full load current when the two phases are matched, so it adds heat to the raceway and goes into the count on the system alone. The load type does not enter this case, because the geometry already settled it. The nonlinear trigger belongs to a four-wire wye circuit and reaches a different arrangement.

NEC 2026 310.15(E)(1)

D. Four

Each circuit has two ungrounded conductors, which is four, and each neutral carries only the unbalanced current from the other conductors of its own circuit, so neither neutral is counted. That leaves four current-carrying conductors, which is already past three and puts the raceway in the first adjustment band. Counting the neutrals as well gives six, and a raceway with six conductors sits in the same band, so this error can hide.

NEC 2026 310.15(E)(2), 310.15(C)(1)

A. Every conductor in the raceway counts for fill, including both neutrals and any equipment grounding conductors

Fill is about physical space, so a conductor takes up room whatever it is doing electrically. The derating count and the fill count are two different numbers taken off the same drawing, and a well-built item hands you one raceway and asks for both. A candidate carrying only one number in their head answers half of it correctly.

NEC 2026 Chapter 9 Table 1

B. It shall not be part of a multiwire branch circuit

A shared neutral means one fault or one loose connection can take out both circuits at once, which is not acceptable where the lights are part of the life safety system. The rule is a flat prohibition with no conditions attached. It is a one line section and a clean item to write.

NEC 2026 517.15

C. The branch circuit serving emergency lighting and power circuits

The emergency systems article and the health care article carry the same prohibition for the same reason, and each names its own class of circuit. Knowing that the pair exists is worth more than knowing either one alone, because an item can be aimed at either article. Neither rule turns on the occupancy of the room the circuit lands in.

NEC 2026 700.19

D. The article now carries a requirement for premises supplied from one nominal voltage system, as well as the existing rule for premises supplied from more than one

The single system case is new and it sends you to the general conductor identification rules, while the more than one system case keeps its own requirement to identify by phase or line and by nominal voltage system. An item that puts two systems in one raceway can be aimed at either article, depending on whether the stem asks about the neutral or about the phase conductors. Deciding which conductor the question is about is the whole item.

NEC 2026 210.5(C)

A. Each grounded conductor shall be identified by nominal voltage system

One system's grounded conductor may keep the ordinary white or gray finish, and the others take a different outer covering or another permitted means that distinguishes them. This is a separate requirement from the grouping rule, even though both sit in the same article and both come up in a crowded panel. An item about two systems in one enclosure is asking which of the two you noticed.

NEC 2026 200.6, 200.7