Texas Electrician ExamTexas journeyman & master

Checked against primary sources 2026-08-24

One neutral doing two jobs, and what happens when it lets go

Two circuits sharing one neutral is efficient wiring and a specific hazard. The exam tests both halves, and the hazard is the half most people cannot explain.

On this page
  1. What the definition is doing
  2. The arithmetic that decides which phase
  3. The disconnect at the origin
  4. The open neutral, worked through
  5. Grouping, and telling one conductor from another
  6. Counting the shared neutral
  7. Reading the question
  8. What this page cites

What the definition is doing

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 is load bearing. The voltage between the ungrounded conductors is what forces them onto different phases, or onto different legs of a single-phase supply. The equal voltage to the grounded conductor is what makes that conductor a neutral rather than just another circuit wire.

The practical point of the arrangement is that three conductors do the work of four. Two 120 volt circuits share one return path, and the return path carries less than the sum of what it returns. Done right it saves copper and panel space. Done wrong it produces the two failures below, and the exam is interested in both.

The arithmetic that decides which phase

Series and parallel circuits share the opposite quantity, and that is the whole trick In a series circuit there is one path, so the current is the same everywhere and the voltages across each element add up to the source. In a parallel circuit every branch sees the same voltage, and the branch currents add up to the total. Nearly every basic theory question is testing whether you know which quantity is shared. SERIES ~ one path same current everywhere the voltages add up PARALLEL ~ many paths same voltage everywhere the currents add up
Series shares current. Parallel shares voltage. Name which one you have before touching a formula. Electrical theory, not a code rule. Appears on the theory portion of most licensing exams.

Take two loads of 9 amperes each. On a single-phase three-wire supply, put them on ungrounded conductors 180 degrees apart and the neutral carries 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 sized to return one circuit.

That is the entire reason the ungrounded conductors of a multiwire branch circuit come from different phases. It is not about a tidy panel schedule and it is not a preference. It is a conductor being asked to carry roughly double what it was chosen for, with no device anywhere in the circuit that will notice.

The failure is quiet, which is what makes it dangerous and what makes it examinable. Nothing trips. The neutral just runs hot inside a wall for as long as both circuits are loaded, and it does that for years.

Two ungrounded conductors of one multiwire circuit landed on the same busbar leg is a routine inspection finding, and it is an easy item to write. If a stem describes where the breakers sit in a panel, that is what it is asking about.

The disconnect at the origin

Article 210 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 again, seen from the other direction. Open one ungrounded conductor and the other is still driving current through the neutral. Somebody who switched off the breaker they were told to switch off is now holding a conductor that is live through a load, and there is nothing about the installation that tells them so.

Read the requirement as disconnection and not as tripping, because that is the distinction items are built on. 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 in the overcurrent article rather than in this one. A two-pole common-trip breaker satisfies it as well and is what most panels get. Common trip is not what the branch circuit article asked for. Throw the handle and both poles leave together. Clear a fault on one pole and the other is free to stay closed.

The condition candidates invent is that the rule only bites where the circuit feeds a single piece of utilization equipment. It does not. The requirement attaches where the circuit originates, and what sits at the far end has no bearing on whether it applies.

That invented condition is a real rule wearing the wrong hat. Article 210 does limit what a multiwire branch circuit may supply, and one of the exceptions to that limit is a circuit supplying a single piece of utilization equipment. The exception belongs to the supply rule. It has no bearing on the disconnect, and an item will quote one of the two and offer the other as an answer.

The open neutral, worked through

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

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 rather than by anything either of them wants.

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. Say a heater measures 12 ohms, which would draw ten. Put them in series across 240 volts and the pair draws about 0.95 amperes, because the lamp is nearly all of the resistance and the lamp therefore sets the current.

Multiply that back out and the lamp is sitting at about 229 volts while the heater has about 11. The small load takes nearly the whole supply and burns out. The large load does nothing. What the customer reports is that half the house went dead and the other half destroyed itself, which sounds like two faults and is one.

Nothing in that sequence trips anything. Under an ampere is nowhere near the rating of either breaker, and it is a small fraction of what the heater was drawing before the neutral went. Note that it is roughly double what the lamp was drawing, which is the detail that kills the lamp. An open neutral is an overvoltage event rather than an overcurrent event, and an overcurrent device is the wrong instrument for it entirely.

That single fact justifies the rest of the topic. It is why you do not lift a neutral in an energized panel, why the ungrounded conductors open together, and why an item that asks what happens to the loads is asking about voltage division and not about tripping.

Grouping, and telling one conductor from another

Article 210 also calls for the ungrounded and grounded conductors of each multiwire branch circuit to be grouped, and it does that by pointing at the grouping and identification rules for grounded conductors in Article 200. Cable ties, wire markers or a similar means, in at least one location inside the enclosure.

The exception is narrower than most people remember it. It reaches conductors that enter from a cable or a raceway unique to that circuit, which is the thing that makes the grouping obvious without any ties. A tidy panel is not the test and neither is the way the breakers are arranged. Two multiwire circuits arriving in one raceway get tied.

Grouping exists for the next person in the panel. The hazard running through every rule on this page is somebody working on a circuit they believe is dead, and a bundle at the origin is the cheapest possible way of telling them which conductors belong together.

Two identification rules sit either side of that one and they are easy to swap. Article 200 governs grounded conductors, and where more than one system shares an enclosure the neutrals have to be distinguishable from each other. Article 210 governs ungrounded conductors, and in the 2026 edition it reaches a premises supplied from a single nominal voltage system as well as one supplied from more than one.

So an item that puts two systems in one raceway can be aimed at either article, and which one depends on whether the stem asks about the neutral or about the phase conductors. Deciding that first is the whole item.

Counting the shared neutral

Where the number of current-carrying conductors in a raceway or cable exceeds three, the ampacity of each conductor shall be reduced by the applicable adjustment factor (NEC 310.15(C)(1)). 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 of them has a neutral that goes quiet when the loads match. The other has a neutral that carries full current when the loads match.

That distinction has its own page here, because it decides the adjustment band and it is where most conductor-count items are aimed.

Conduit fill does not care about any of it. Every conductor in the raceway takes up room whatever it is doing electrically, so the fill count and the derating count are two different numbers off the same drawing. Questions pair them for exactly that reason.

Reading the question

One habit is worth more than the rest of this page. When a stem mentions a neutral shared by two circuits, decide first whether the question is about normal operation or about a fault. Everything about a multiwire circuit that surprises people happens in the second case, and the answers written for the first case are still sitting on the screen.

What this page cites

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Every rule here is cited to the section it comes from, and corrections are logged in public with the date. This is study material rather than electrical advice, and it does not replace your own code book or your inspector.