Shared-Neutral Circuit With Breakers on the Same Leg
At a small shop, a 120/240 V single-phase panel feeds a 20 A multiwire branch circuit run in 12 AWG copper. Circuit A supplies 16 A of resistance heating; circuit B supplies 12 A of lighting. The two ungrounded conductors share one neutral. A replacement breaker was installed in the wrong panel slot, so both breakers are on the same bus leg. The neutral is noticeably warm, and a receptacle voltage drops from 121 V to 116 V when both loads run. The owner proposes replacing the 12 AWG neutral with 10 AWG and leaving the breakers where they are. A helper says the neutral current is only the difference between the two loads and cannot be the problem.
1. Calculate the neutral current with the breakers on opposite legs and with both breakers on the same leg, assuming unity power factor. Show the reasoning.
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For a 120/240 V single-phase, three-wire circuit, the two ungrounded conductors are 180° apart when connected to opposite bus legs. With unity power factor, the neutral current is the phasor difference of the two line currents:
- Normal, opposite legs: In = |16 A − 12 A| = 4 A.
- Faulted, same leg: the two currents are in phase, so the neutral carries their sum: In = 16 A + 12 A = 28 A.
The helper is applying the opposite-leg rule to a same-leg installation. Once both breakers are on the same leg, the shared neutral is no longer a balanced-circuit neutral; it is carrying the total return current of both loads. A 12 AWG conductor on a 20 A circuit is not suitable for a continuous 28 A return current, which explains the warm neutral. The 5 V drop (121 V to 116 V) also indicates excessive impedance somewhere in the circuit, possibly including a poor neutral connection, and should be investigated rather than masked by upsizing one conductor.
2. Does replacing the 12 AWG neutral with 10 AWG correct the installation? What must be changed?
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No. Upsizing the neutral may increase its current-carrying capacity, but it does not correct the mispositioned breaker or remove the 28 A same-leg return current from the shared neutral. It also does not address the voltage drop or possible poor termination. The circuit would remain a code and fire hazard because the neutral is still overloaded and the multiwire branch circuit is not arranged as intended.
The necessary corrections are:
- Move one breaker to the opposite bus leg so the two ungrounded conductors are 180° apart.
- Install a listed handle tie or common-trip arrangement for the two breakers, as required for a multiwire branch circuit.
- Group the conductors and identify the shared neutral so the circuit can be maintained safely.
- Verify the neutral is continuous and not dependent on a device removal; check terminations and torque.
- Re-test voltage drop and neutral current under both loads.
Simply installing a larger neutral could leave a 28 A current on a conductor that is still part of a miswired circuit and could conceal a bad connection.
3. What code and failure risks should be discussed with the owner and helper?
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The main code issue is that a multiwire branch circuit must have the ungrounded conductors arranged so the neutral carries only the imbalance under normal operation. Breakers on the same leg defeat that arrangement. The installation also needs the required simultaneous disconnect means and grouping/identification so a worker does not open the neutral while the other circuit remains energized.
Failure risks include:
- Neutral overheating and insulation damage from the 28 A return current.
- Poor voltage regulation and possible equipment malfunction from the 5 V drop.
- Shock hazard if the shared neutral is opened while one circuit remains energized.
- Nuisance tripping or missed overcurrent protection because the neutral is not protected by either 20 A breaker.
- Hidden damage at terminations from heat cycling.
The owner should understand that the 10 AWG neutral proposal treats a symptom, not the miswiring. The helper should be corrected: the difference rule applies only when the two hots are on opposite legs.