電圧降下の結果は第一段階です。20A回路が12 AWGから8 AWGになる場合でも、太い導体は端子、管路、ボックス、曲げスペース、保護条件に適合しなければなりません。

要約

電圧降下は負荷電流で計算し、許容電流は別に確認します。
機器へ接続する前に NEC 110.14(C) を確認します。
サイズ変更後はボックス容量と管路充填を再計算します。
IECでは敷設方法、グループ係数、電圧降下を一緒に確認します。

Definitions that matter before you buy wire

電線サイズアップ

電線サイズアップとは、最小許容電流より大きい導体を選び、電圧降下を減らすことです。

端子温度定格

端子温度定格は、機器ラグで使える許容電流欄と絶縁温度を決めます。

ボックス容量

ボックス容量は NEC 314.16 による導体、器具、クランプ、接地線の容積確認です。

For NEC jobs, the critical references are 110.14(C) for terminal temperature, 210.19(A)(1) and 215.2(A)(1) informational notes for voltage-drop design targets, 300.17 and Chapter 9 for raceway fill, 310.15 for adjustment and correction, 312.6 for wire-bending space, and 314.16 for box volume. For IEC work, coordinate the result with IEC 60364 and IEC 60364-5-52 cable selection. Background references for the National Electrical Code and the International Electrotechnical Commission help when the calculation note goes to an owner or inspector.

"The calculator can tell you 8 AWG solves a 3% target, but the field question is whether the device terminal, box volume, and bending space also accept 8 AWG." — Hommer Zhao, electrical calculator editor

Five-step field checklist

材料承認前に以下の順序で確認し、電圧降下を解決して端子や充填の問題を作らないようにします。

1. Keep ampacity and voltage drop separate

Example: a 48A EVSE on a 60A breaker uses 48A in the voltage-drop calculator, while the branch-circuit rating still follows the 125% continuous-load rule in NEC 210.20(A) and 625.41.

2. Confirm the terminal range and temperature column

A breaker or receptacle may list 14-10 AWG only. If the voltage-drop result needs 8 AWG, use a listed splice or terminal kit; do not trim strands.

3. Recalculate box fill and bending room

Changing three 12 AWG conductors to three 8 AWG conductors can move a device box from acceptable to undersized under NEC 314.16. Larger feeders may also trigger NEC 312.6 bending-space checks.

4. Recheck raceway fill and derating

A conduit that was fine for 6 AWG THHN may exceed the fill limit with 4 AWG, and more than three current-carrying conductors can require NEC 310.15 adjustment.

5. Document the final reason for the larger conductor

The note should say "upsized for voltage drop, overcurrent device unchanged" when that is true. This avoids confusion during inspection and troubleshooting.

Worked comparison table

Circuit	Voltage-drop result	Post-calculation check	Practical decision
20A workshop receptacle	120V, 16A, 185 ft: 12 AWG near 7.6%; 8 AWG near 3.1%	NEC 314.16 box fill; device terminal may not accept 8 AWG	Use 8 AWG run with 12 AWG pigtail only if splice and box volume are listed and adequate.
48A EV charger	240V, 150 ft: 6 AWG about 2.95%; 4 AWG about 1.85%	NEC 110.14(C), 625.41, lug range, conduit fill	If 6 AWG meets the target, avoid unnecessary 4 AWG unless owner requires tighter drop.
60A detached garage feeder	120/240V, 140 ft: 6 AWG Cu may exceed 3%; 4 AWG Cu near 2%	Feeder neutral load, grounding conductor, panel lug range	Upsize ungrounded and neutral as needed; keep EGC sizing tied to NEC 250.122 unless upsizing rules apply.
480V 40 hp motor feeder	52A running, 260 ft: 6 AWG around 2-3% depending conductor data	Motor-starting voltage sag, NEC 430 sizing, terminal temperature	Check running drop and starting drop separately; do not size only from breaker rating.
400V IEC machine	32A, 70 m: 10 mm2 about 2.2%; 16 mm2 about 1.4%	IEC 60364-5-52 installation method, grouping, ambient	If grouped with six loaded circuits, thermal derating may control over voltage drop.

Real field note

185 ftの作業場回路では、16Aで12 AWG銅線が約7.6%になることがあります。8 AWGなら約3%に近づきますが、より大きい箱やピグテールが必要になる場合があります。

"When a long branch circuit fails at 7.6%, I do not approve the larger conductor until the box-fill math is on the same note. Voltage quality and physical installation have to pass together." — Hommer Zhao, electrical calculator editor

FAQ

Do I need to upsize the breaker when I upsize wire for voltage drop?

Usually no. If the load and overcurrent device are already correct, the larger conductor can be used only to reduce voltage drop. Example: a 20A receptacle circuit may still use a 20A breaker with 8 AWG conductors, subject to terminals and splices.

Can I put 8 AWG wire directly on a 20A receptacle?

Only if the device is listed for that conductor size, which many common receptacles are not. A listed splice to a smaller pigtail is often needed, and the box must pass NEC 314.16.

Should equipment grounding conductors be upsized too?

For NEC work, equipment grounding conductor sizing starts with 250.122. If ungrounded conductors are increased for voltage drop, 250.122(B) can require proportional EGC upsizing.

Does conduit fill change when only one conductor is upsized?

Yes. Raceway fill is based on the actual conductor areas in the raceway. A change from 6 AWG to 4 AWG can affect Chapter 9 fill even when the circuit ampacity did not change.

How should IEC users treat this checklist?

Use the same workflow: calculate voltage drop, then verify current-carrying capacity, installation method, ambient temperature, grouping, and termination limits under IEC 60364-5-52 and local rules.

What should I write in the permit note?

Write the actual current, one-way length, conductor material and size, voltage drop percent, and the phrase "conductor upsized for voltage drop; OCPD unchanged" when applicable.

"The clean note is short: 48A actual load, 150 ft one-way, 6 AWG copper, 2.95% drop, 60A circuit. That tells the installer and reviewer what changed and what did not." — Hommer Zhao, electrical calculator editor

施工前にサイズアップ導体を確認

まず電圧降下を計算し、導体サイズ、管路充填、ディレーティングも確認します。

電圧降下計算機を開く Wire Size Calculator Conduit Fill Calculator

電圧降下後の電線サイズアップ点検表：端子・ボックス・管路

要約