多線分岐回路（MWBC）は、1本の中性線を複数の非接地導体で共有する便利な方法です。ただし、分相120/240Vでは2本のホットが反対側のレッグにあることが前提で、同じレッグに載ると中性線電流が加算されます。
設計では、ブレーカー、導体サイズ、端子温度、箱内導体数、保護装置、電圧降下を一緒に確認します。ここでは NEC 210.4、300.13(B)、310.16、250.122 を前提に、15A、20A、30A の具体例で説明します。
Code References
This article references NEC 210.4 for multiwire branch circuits, NEC 210.4(B) for simultaneous disconnection, NEC 300.13(B) for neutral continuity, NEC 310.16 for conductor ampacity, and IEC-style conductor context from International Electrotechnical Commission. For background, see National Electrical Code and Electrical wiring.
What a Multiwire Branch Circuit Actually Is
多線分岐回路（MWBC）は、1本の中性線を複数の非接地導体で共有する便利な方法です。ただし、分相120/240Vでは2本のホットが反対側のレッグにあることが前提で、同じレッグに載ると中性線電流が加算されます。
設計では、ブレーカー、導体サイズ、端子温度、箱内導体数、保護装置、電圧降下を一緒に確認します。ここでは NEC 210.4、300.13(B)、310.16、250.122 を前提に、15A、20A、30A の具体例で説明します。 16Aの負荷を2つ同じレッグに置くと、中性線に32Aが流れる可能性があります。これは太い電線でごまかす問題ではなく、相配置を直すべき施工ミスです。
I treat every MWBC as a small load-balance calculation. On a 120/240V system, 18A on one leg and 7A on the other should leave about 11A on the neutral; if the neutral is carrying 25A, the breakers are on the wrong leg or the circuit has been modified incorrectly. — Hommer Zhao, Technical Director
Sizing Workflow for MWBC Conductors
設計では、ブレーカー、導体サイズ、端子温度、箱内導体数、保護装置、電圧降下を一緒に確認します。ここでは NEC 210.4、300.13(B)、310.16、250.122 を前提に、15A、20A、30A の具体例で説明します。
- Use 14 AWG copper only for 15A circuits, and only where the wiring method and terminals are suitable.
- Use 12 AWG copper for common 20A kitchen, laundry, receptacle, and workshop MWBC layouts.
- Use 10 AWG copper for 30A shared-neutral equipment circuits only when every device and termination is rated for that use.
- Size the neutral at least as large as the ungrounded conductors unless a specific engineered rule allows otherwise.
- Size the equipment grounding conductor from the overcurrent device under NEC 250.122, not from neutral current.
多線分岐回路（MWBC）は、1本の中性線を複数の非接地導体で共有する便利な方法です。ただし、分相120/240Vでは2本のホットが反対側のレッグにあることが前提で、同じレッグに載ると中性線電流が加算されます。 NEC 300.13(B)の目的は中性線の連続性です。コンセントを外しても、下流側のもう一方の回路の中性線を切ってはいけません。
Comparison Table: Common MWBC Layouts
設計では、ブレーカー、導体サイズ、端子温度、箱内導体数、保護装置、電圧降下を一緒に確認します。ここでは NEC 210.4、300.13(B)、310.16、250.122 を前提に、15A、20A、30A の具体例で説明します。
| Circuit Type | Typical Conductors | Neutral Current Check | Disconnect Requirement | Best Use |
|---|---|---|---|---|
| 15A, 120/240V split-phase MWBC | 14/3 copper with ground | Imbalance only, max 15A design | 2-pole breaker or listed handle tie | Lighting plus light receptacles where allowed |
| 20A, 120/240V split-phase MWBC | 12/3 copper with ground | Imbalance only, max 20A design | Common trip or tied handles per 210.4(B) | Kitchen countertop, laundry, garage, workshop receptacles |
| 30A, 120/240V equipment MWBC | 10/3 copper with ground | Only for 120V imbalance loads | 2-pole common disconnect | Listed equipment with mixed 120V and 240V loads |
| Three-phase 120/208V MWBC | Phase conductors plus shared neutral | Vector sum; harmonics may matter | 3-pole or listed simultaneous disconnect | Commercial receptacle banks and lighting rows |
| Same-leg shared neutral mistake | Any size becomes unsafe if mislanded | Currents add: 16A + 16A = 32A | Must be corrected, not relabeled | Never acceptable as an MWBC |
多線分岐回路（MWBC）は、1本の中性線を複数の非接地導体で共有する便利な方法です。ただし、分相120/240Vでは2本のホットが反対側のレッグにあることが前提で、同じレッグに載ると中性線電流が加算されます。
Worked Examples With Specific Numbers
Example 1: Two 20A Kitchen Small-Appliance Circuits
設計では、ブレーカー、導体サイズ、端子温度、箱内導体数、保護装置、電圧降下を一緒に確認します。ここでは NEC 210.4、300.13(B)、310.16、250.122 を前提に、15A、20A、30A の具体例で説明します。 たとえば1,500Wは120Vで約12.5A、900Wは約7.5Aです。反対レッグなら中性線電流は約5Aです。
Example 2: Same Loads Placed on the Same Panel Leg
多線分岐回路（MWBC）は、1本の中性線を複数の非接地導体で共有する便利な方法です。ただし、分相120/240Vでは2本のホットが反対側のレッグにあることが前提で、同じレッグに載ると中性線電流が加算されます。 同じレッグに置くと電流は相殺されず加算されます。16Aと16Aなら中性線は32Aになります。
Example 3: 100-Foot Workshop Receptacle Run
設計では、ブレーカー、導体サイズ、端子温度、箱内導体数、保護装置、電圧降下を一緒に確認します。ここでは NEC 210.4、300.13(B)、310.16、250.122 を前提に、15A、20A、30A の具体例で説明します。 100 ft、約30 mの20A作業台回路では、12 AWGで3%電圧降下に近づくことがあり、10 AWGとの比較が実務的です。
Example 4: 120/208V Commercial Receptacle Row
多線分岐回路（MWBC）は、1本の中性線を複数の非接地導体で共有する便利な方法です。ただし、分相120/240Vでは2本のホットが反対側のレッグにあることが前提で、同じレッグに載ると中性線電流が加算されます。 120/208V三相では、ベクトル和だけでなく、LEDドライバ、UPS、PC負荷による高調波も確認します。
The breaker tie is not a cosmetic accessory. NEC 210.4(B) exists because a person servicing one receptacle must not open one hot conductor while the shared neutral and the other hot conductor remain energized in a confusing way. — Hommer Zhao, Technical Director
Shared Neutral Rules That Affect Wire Size
設計では、ブレーカー、導体サイズ、端子温度、箱内導体数、保護装置、電圧降下を一緒に確認します。ここでは NEC 210.4、300.13(B)、310.16、250.122 を前提に、15A、20A、30A の具体例で説明します。
多線分岐回路（MWBC）は、1本の中性線を複数の非接地導体で共有する便利な方法です。ただし、分相120/240Vでは2本のホットが反対側のレッグにあることが前提で、同じレッグに載ると中性線電流が加算されます。
Common Pitfall
設計では、ブレーカー、導体サイズ、端子温度、箱内導体数、保護装置、電圧降下を一緒に確認します。ここでは NEC 210.4、300.13(B)、310.16、250.122 を前提に、15A、20A、30A の具体例で説明します。 2本の12/2ケーブルの中性線が同じ箱にあるだけで、既設回路をMWBCに変えてはいけません。
GFCI, AFCI, and Receptacle Device Issues
多線分岐回路（MWBC）は、1本の中性線を複数の非接地導体で共有する便利な方法です。ただし、分相120/240Vでは2本のホットが反対側のレッグにあることが前提で、同じレッグに載ると中性線電流が加算されます。
設計では、ブレーカー、導体サイズ、端子温度、箱内導体数、保護装置、電圧降下を一緒に確認します。ここでは NEC 210.4、300.13(B)、310.16、250.122 を前提に、15A、20A、30A の具体例で説明します。
Voltage Drop and Calculator Use
設計では、ブレーカー、導体サイズ、端子温度、箱内導体数、保護装置、電圧降下を一緒に確認します。ここでは NEC 210.4、300.13(B)、310.16、250.122 を前提に、15A、20A、30A の具体例で説明します。 120Vの3%は3.6Vだけです。長いキッチン島や作業場の回路では実計算が必要です。
多線分岐回路（MWBC）は、1本の中性線を複数の非接地導体で共有する便利な方法です。ただし、分相120/240Vでは2本のホットが反対側のレッグにあることが前提で、同じレッグに載ると中性線電流が加算されます。 計算機には実負荷電流、片道距離、導体材料、電圧を入力し、必要に応じて12 AWGと10 AWGを比較します。
For long 20A MWBC runs, I often calculate twice: once for the loaded 120V path and once for the worst neutral imbalance. If 12 AWG is close to 3% drop at 100 feet, 10 AWG can be a better design even though the breaker remains 20A. — Hommer Zhao, Technical Director
Mistakes to Avoid Before Inspection
- Putting both ungrounded conductors on the same phase or leg, which can overload the neutral.
- Using two independent single-pole breakers without the simultaneous disconnect required by NEC 210.4(B).
- Breaking the shared neutral through a receptacle instead of pigtailing it for continuity under NEC 300.13(B).
- Assuming a single-pole GFCI or AFCI breaker will work with a shared neutral without a listed wiring method.
- Forgetting box-fill volume when 12/3 cable, pigtails, device yokes, clamps, and equipment grounds meet in a small box.
- Ignoring voltage drop because the ampacity chart says the wire is large enough for the breaker.
For breaker and conductor pairing, compare this guide with the breaker size and wire size chart and the residential wiring guide then verify conductor ampacity with the ampacity calculator.
FAQ
What wire size is used for a 20A multiwire branch circuit?
A typical 20A copper MWBC uses 12 AWG ungrounded conductors and a 12 AWG shared neutral. NEC 240.4(D) commonly limits 12 AWG copper branch circuits to 20A, and NEC 210.4 controls the shared-neutral arrangement.
Can two 15A circuits share one neutral?
Yes, two 15A circuits can share one neutral when they are arranged as a legal MWBC. A typical copper cable is 14/3 with ground, the two hot conductors must be on opposite legs, and simultaneous disconnection is required under NEC 210.4(B).
How much current flows on the neutral of an MWBC?
On a 120/240V split-phase MWBC with opposite legs, the neutral carries the imbalance. If one leg carries 14A and the other carries 9A, the neutral carries about 5A. If the breakers are on the same leg, currents add instead.
Do multiwire branch circuits need a common-trip breaker?
NEC 210.4(B) requires simultaneous disconnection of all ungrounded conductors at the origin. A common-trip 2-pole breaker is common, but a listed handle tie may be allowed for some branch-circuit layouts depending on the application and local code.
Can an MWBC use GFCI or AFCI protection?
Yes, but the device must be matched to the shared-neutral circuit. A 2-pole GFCI/AFCI breaker or other listed method normally monitors both ungrounded conductors and the neutral together; two unrelated single-pole devices usually create nuisance trips or improper sensing.
Should I upsize wire for a long MWBC run?
Often yes. A 20A MWBC may be code-sized with 12 AWG copper, but a 100-foot 120V run near 16A can approach the common 3% voltage-drop design target. Checking 10 AWG in the calculator is a practical comparison.
Bottom Line
多線分岐回路（MWBC）は、1本の中性線を複数の非接地導体で共有する便利な方法です。ただし、分相120/240Vでは2本のホットが反対側のレッグにあることが前提で、同じレッグに載ると中性線電流が加算されます。
設計では、ブレーカー、導体サイズ、端子温度、箱内導体数、保護装置、電圧降下を一緒に確認します。ここでは NEC 210.4、300.13(B)、310.16、250.122 を前提に、15A、20A、30A の具体例で説明します。
Need help checking an MWBC design?
設計では、ブレーカー、導体サイズ、端子温度、箱内導体数、保護装置、電圧降下を一緒に確認します。ここでは NEC 210.4、300.13(B)、310.16、250.122 を前提に、15A、20A、30A の具体例で説明します。
Contact Wire Gauge Calculator多線分岐回路の電線サイズと NEC 実務ガイド: Field Verification Table
Before you close out 多線分岐回路の電線サイズと nec 実務ガイド, it helps to cross-check the same five items that inspectors and experienced installers review in the field: load basis, breaker protection, voltage drop, derating, and grounding or enclosure space. The underlying logic is consistent across the National Electrical Code and the International Electrotechnical Commission: use the actual load, verify the conductor against installation conditions, and only then lock in protection and layout details.
| Design Check | What to Verify | Practical Number | Typical Code Reference | Best Tool or Follow-Up |
|---|---|---|---|---|
| Load Basis | Start from nameplate load, calculated load, or connected VA before picking a conductor. | Continuous loads are usually checked at 125%. | NEC 210.19(A)(1) and 215.2(A)(1) | Use the main wire gauge calculator for the first pass. |
| Breaker Match | Protect the conductor ampacity instead of assuming the breaker sets wire size by itself. | 16A continuous becomes a 20A conductor check. | NEC 240.4 and 240.6(A) | Compare against the breaker sizing guide before trim-out. |
| Voltage Drop | Long runs often require larger wire even when ampacity already passes. | Design target is about 3% branch and 5% feeder plus branch. | NEC informational notes to 210.19 and 215.2 | Run a second check in the voltage drop calculator. |
| Derating | Account for ambient temperature, rooftop heat, and more than three current-carrying conductors. | 90 C insulation may still terminate on a 75 C or 60 C limit. | NEC 310.15 and Table 310.16 | Confirm with the ampacity calculator before ordering wire. |
| Grounding and Fill | Check equipment grounds, conduit fill, and box space as separate calculations. | A 60A feeder often uses a 10 AWG copper EGC under NEC 250.122. | NEC 250.122, 314.16, and Chapter 9 | Cross-check the ground wire and conduit fill guides before inspection. |
“If a circuit will run for 3 hours or more, I treat the 125% continuous-load check as non-negotiable. A 16A design current turning into a 20A conductor decision is exactly the kind of detail that prevents nuisance heat and callbacks.”
“Once branch-circuit voltage drop gets close to 3%, I stop debating and price the next conductor size. Moving from 12 AWG to 10 AWG on a 120V run is usually cheaper than troubleshooting low-voltage performance later.”
“The breaker, phase conductor, and equipment ground are related, but they are not the same calculation. I may upsize a 60A feeder to 4 AWG copper for distance and still keep the grounding conductor at 10 AWG copper because NEC 250.122 keys it to the overcurrent device.”
How to Use This With the Calculator
The calculator gives you a fast starting point, but serious installations still need one more pass for voltage drop, conductor temperature rating, and code-specific exceptions. That last review is where most inspection problems get removed before material is pulled.
多線分岐回路の電線サイズと NEC 実務ガイド: Practical Number Checks
The easiest way to keep 多線分岐回路の電線サイズと nec 実務ガイド practical is to sanity-check a few common field numbers before you order wire or close walls. On a 120V branch circuit carrying a 16A continuous load, the 125% rule pushes the conductor check to 20A. That is why 12 AWG copper becomes the real starting point instead of 14 AWG, even before you think about distance. If that same run stretches to 110 feet one way, voltage drop often pushes the design to 10 AWG while the breaker stays at 20A because the load has not changed.
The same logic shows up in larger work. A 7.5 HP, 460V three-phase motor with a full-load current around 11A does not mean you can stop at an 11A wire decision. Motor circuits, feeder calculations, and equipment grounding all apply their own code logic, and the conductor selected from ampacity tables still has to survive ambient temperature, rooftop heat, or bundling. That is why experienced electricians compare the load calculation against conductor ampacity, then against raceway or box space, and only then against the final breaker or fuse size.
Residential work needs the same discipline. A box-fill calculation that lands at 24.75 cubic inches on a 12 AWG two-gang box, or a detached garage feeder that picks up 3.6V of drop on a 120V leg, is already telling you the installation is too close to the edge. Use the long-distance wire guide when length is the problem, and cross-check enclosure constraints with the box fill guide or the conduit fill guide. Those second-pass checks are where most field rework gets avoided.
多線分岐回路の電線サイズと NEC 実務ガイド: Frequently Asked Questions
How do I know when 多線分岐回路の電線サイズと nec 実務ガイド needs a larger conductor than a simple chart shows?
If the run is long, the load is continuous for 3 hours or more, or the conductors are bundled in hot ambient conditions, the simple chart is only the starting point. A 20A circuit may still need 10 AWG instead of 12 AWG once the 125% rule or a 3% voltage-drop target is applied.
Does the 125% continuous-load rule matter for 多線分岐回路の電線サイズと nec 実務ガイド?
Yes, whenever the load is expected to run at maximum current for 3 hours or more. Under NEC 210.19(A)(1) and 215.2(A)(1), a 24A continuous load is treated as 30A for conductor sizing, which is why field calculations often move up one breaker and wire size from the first rough estimate.
What voltage-drop target is practical when planning 多線分岐回路の電線サイズと nec 実務ガイド?
The common design target is about 3% on a branch circuit and 5% total for feeder plus branch circuit. That is not a mandatory blanket rule in every NEC application, but it is the benchmark many electricians use to decide when a 100-foot to 200-foot run should be upsized.
Can I upsize wire without increasing breaker size for 多線分岐回路の電線サイズと nec 実務ガイド?
Yes. Upsizing for voltage drop or future durability does not automatically require a larger breaker. A common example is a 20A circuit that moves from 12 AWG to 10 AWG copper on a long run while the breaker remains 20A because the load and overcurrent protection have not changed.
Which code checks should I finish before calling 多線分岐回路の電線サイズと nec 実務ガイド complete?
At minimum, verify conductor ampacity in NEC Table 310.16, breaker protection in NEC 240.4 and 240.6, voltage drop design assumptions, grounding in NEC 250.122, and enclosure or raceway space in NEC 314.16 or Chapter 9. For international work, align the same review with IEC-style conductor and protection practices.
Next Steps
If you want to validate this topic against real project numbers, start with the wire gauge calculator, then cross-check longer runs in the voltage drop calculator, and verify conductor adjustments with the ampacity calculator. If you want us to add another worked example or application note, contact us here.