Multiwire branch circuits, the simultaneous disconnect at 210.4(B), and the condition candidates invent

What the exam asks about it

One question in our five hundred names a multiwire branch circuit in the stem, and it's worth reading in full because every version of this item takes its shape. Two 120 volt loads, opposite ungrounded conductors, a shared neutral, and a question about what Article 210 requires where the circuit originates. The answer is a means to disconnect all of the ungrounded conductors at the same time. All three wrong options bolt a condition onto that requirement.

The topic sits across several areas at once, which is why a single count understates it. A conductor count item asks how many current-carrying conductors are in the raceway. A grounding item asks why a shared neutral matters. A theory item asks what happens to two loads left in series across 240 volts. All three are multiwire items wearing other labels, and a candidate who has memorized only the disconnect rule answers one of the three.

The definition clause that decides it

Article 100 describes a multiwire branch circuit as two or more ungrounded conductors that have 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 in that sentence carries weight. The voltage between the ungrounded conductors is what puts them on different phases, or on different legs of a single-phase supply. The equal voltage to the grounded conductor is what earns that conductor the name neutral.

Two consequences fall out of the definition and both get tested. A two-wire 240 volt circuit has no neutral, so it sits outside the description entirely, while a three-wire 120/240 volt circuit is multiwire. And 210.4(C) then limits a multiwire circuit to line-to-neutral loads, with an exception for a circuit supplying a single piece of utilization equipment and another where all the ungrounded conductors open simultaneously at the overcurrent device.

The worked wiring and the long-hand arithmetic live in the multiwire branch circuits question set. This page is about what the Texas papers do with it.

The disconnect at the origin

210.4(B) calls for a means that simultaneously disconnects all the ungrounded conductors of a multiwire branch circuit, at the point where the circuit originates.

The reason is the shared neutral seen from the other direction. Open one ungrounded conductor and the other is still driving current through the neutral, so somebody who switched off the breaker they were told to switch off is holding a conductor that's live through a load, with nothing about the installation telling them so.

Read the requirement as disconnection, because that word is where items get built. A handle tie across two single-pole breakers satisfies it for a circuit serving only line-to-neutral loads, and the permission for that sits at 240.15(B)(1) in the overcurrent article. A two-pole common-trip breaker satisfies it as well and is what most panels get. Common trip goes further than 210.4(B) asked for: throw the handle and both poles leave together, while clearing a fault on one pole leaves the other free to stay closed.

The condition candidates invent

The condition candidates invent is that the disconnect rule bites only where the circuit feeds a single piece of utilization equipment. The requirement attaches where the circuit originates, and the equipment at the far end has no part in deciding whether it applies.

That invented condition is a real rule wearing the wrong hat. 210.4(C) does limit what a multiwire branch circuit may supply, and one of its exceptions is a circuit supplying a single piece of utilization equipment. The exception belongs to the supply rule. An item will quote one of those two subsections and offer the other as an answer, and the two sit a few lines apart in the book, which is exactly what makes the swap work.

A second invented condition turns the rule on receptacles, and a third turns it on the voltage of the individual loads. Both do the same job: a plausible qualifier attached to a requirement that carries none. Where an option adds a condition to a disconnect rule, treat the addition as the thing being tested.

The open neutral, worked through

This is the part that makes everything else on the page obvious, and it's the part almost nobody can explain on demand.

Lose the neutral somewhere between the panel and the loads and the two 120 volt loads stop being two circuits. They become one series pair across the full 240 volts, and the supply divides between them in proportion to their resistances.

The figures here are invented for the example and none of them comes out of a code table. Say a lamp load measures 240 ohms, which would draw half an ampere at 120 volts, and a heater measures 12 ohms, which would draw ten. In series across 240 volts the pair draws about 0.95 amperes, because the lamp holds nearly all of the resistance and therefore sets the current. Multiply back out and the lamp sits at about 229 volts while the heater has about 11. The small load takes nearly the whole supply and burns out, while the large load does nothing at all.

Nothing in that sequence trips anything. Under an ampere is a small fraction of what the heater was drawing before the neutral opened, and it's roughly double what the lamp was drawing, which is the detail that kills the lamp. An open neutral is an overvoltage event, and an overcurrent device is the wrong instrument for it. Our page on series and parallel circuits works the same voltage division with no code attached.

Both legs on one busbar

The quieter failure is both ungrounded conductors landed on the same busbar leg.

Two loads of 9 amperes each, on legs 180 degrees apart, leave the neutral carrying the difference between them, which here is nothing at all. Put them on the same leg and the neutral carries the sum, 18 amperes, on a conductor chosen to return one circuit.

That is the entire reason the ungrounded conductors of a multiwire circuit come from different phases. It has nothing to do with a tidy panel schedule and it's a long way from being a preference. The failure is quiet, which is what makes it dangerous and what makes it examinable, since nothing trips and the neutral runs hot inside a wall for as long as both circuits are loaded.

It's also a routine inspection finding and an easy one to write an item about. Where a stem describes which busbar the breakers sit on, that's what it's asking about. On the job the correction is cheap while the inspector is still standing there and expensive once the drywall is up.

Counting the shared neutral

Where the number of current-carrying conductors in a raceway or cable exceeds three, NEC 310.15(C)(1) reduces the ampacity of each one by an adjustment factor. Whether the shared neutral is one of those conductors depends on the system it came from.

A single-phase three-wire multiwire circuit and a three-wire circuit taken from a four-wire wye look identical in a raceway and count differently. One has a neutral that goes quiet when the loads match. The other has a neutral that carries a full share when the loads match, because the two ungrounded conductors sit 120 degrees apart. That difference decides the adjustment band, and it's where most conductor-count items are aimed.

Conduit fill takes no notice of any of it. Every conductor in the raceway occupies room whatever it's doing electrically, so the fill count and the derating count are two separate numbers off one drawing, and questions pair them for exactly that reason. Our page on rounding to the next standard size covers what happens to each of those counts once the division is done.

What to do about it before test day

Article 210 also requires the ungrounded and grounded conductors of each multiwire circuit to be grouped, and it does that by pointing at the grouping and identification rules for grounded conductors in Article 200. Cable ties or wire markers, in at least one location inside the enclosure. The exception reaches conductors entering from a cable or raceway unique to that circuit, which makes the grouping obvious with no ties at all. Two multiwire circuits arriving in one raceway get tied.

Three things to do before you sit down.

1. Tab 210.4 in your own copy and read (A) through (D) in one sitting, so the disconnect at (B) and the supply limit at (C) are separated in your memory by the layout of the page itself.
2. Work the open neutral once with your own numbers until the voltage division is something you can derive at the desk under time pressure.
3. Read the grouping exception in Article 200, where it's actually written, since that's the half candidates quote from Article 210 and get wrong.

Texas adopted the 2026 edition at 16 TAC 73.100, effective 1 September 2026, and several articles were reorganized for it. The exam is open book with your own copy, so where a memorized section number and the page in front of you disagree, the page wins. Cities amend on top of the state adoption under Tex. Occ. Code 1305.201(c), which changes what an inspector holds you to on the job while leaving the exam sitting on the state-adopted edition.

Questions people ask

What is a multiwire branch circuit?

Article 100 describes it as a branch circuit of two or more ungrounded conductors that have a voltage between them, plus a neutral conductor that has equal voltage between it and each of those ungrounded conductors and is connected to the neutral of the system. The practical effect is that three conductors do the work of four, because two circuits share one return path and that path carries less than the sum of what it returns. The equal voltage clause is what makes the shared conductor a neutral, and every clause in the description does work.

Does a multiwire branch circuit need a handle tie?

It needs a means that simultaneously disconnects all the ungrounded conductors at the point where the circuit originates, under NEC 210.4(B). A handle tie is one way of providing it, and NEC 240.15(B)(1) permits individual single-pole breakers with identified handle ties as the protection for each ungrounded conductor of a multiwire circuit serving only line-to-neutral loads. A two-pole common-trip breaker satisfies the requirement as well. Read 210.4(B) as a requirement for disconnection, because that reading is what exam items are built on.

Why is a shared neutral dangerous?

Because of what happens when it opens. Lose the neutral between the panel and the loads and the two 120 volt loads become one series pair across 240 volts, with the supply dividing between them in proportion to their resistances, so the small load takes most of the voltage and burns out while the large one does nothing. Nothing trips, since that's an overvoltage event. The quieter hazard is landing both ungrounded conductors on the same busbar leg, which has the neutral carrying the sum of the two load currents.

Is a 240 volt circuit a multiwire branch circuit?

A two-wire 240 volt circuit sits outside the Article 100 description, because that description needs a grounded conductor with equal voltage between it and each ungrounded conductor. With two ungrounded conductors and no neutral there's nothing to satisfy that clause. A three-wire 120/240 volt circuit is multiwire. NEC 210.4(C) then has a multiwire branch circuit supply only line-to-neutral loads, with one exception for a circuit supplying a single piece of utilization equipment and another where all the ungrounded conductors open simultaneously at the overcurrent device.

Does the shared neutral count as a current-carrying conductor?

It depends on the system the circuit came from, which is why the item is worth slowing down for. NEC 310.15(C)(1) applies an adjustment factor where the number of current-carrying conductors in a raceway or cable exceeds three. A single-phase three-wire multiwire circuit and a three-wire circuit taken from a four-wire wye look identical in a raceway, and one has a neutral that goes quiet when the loads match while the other has a neutral carrying a full share. Conduit fill counts every conductor either way.

Which code edition does the Texas exam use?

The edition Texas has adopted, which is the 2026 National Electrical Code from 1 September 2026 under 16 TAC 73.100. The exam is open book and you bring your own current copy. Several articles were reorganized for that edition, so a section number carried over from an older book can land you in the wrong place while the clock runs. A job you're working on is a separate question, since the authority having jurisdiction usually applies the edition in force when the permit was issued, plus that city's own amendments.