전압 강하 결과는 첫 단계일 뿐입니다. 20A 분기회로가 12 AWG에서 8 AWG로 바뀌면 더 큰 도체도 단자, 전선관, 박스, 굽힘 공간, 보호 조건에 맞아야 합니다.

요약

전압 강하는 부하전류로, 허용전류는 별도로 확인합니다.
장비에 연결하기 전 NEC 110.14(C)를 확인합니다.
전선 크기를 바꾸면 박스 필과 전선관 필을 다시 계산합니다.
IEC 작업은 전압 강하와 설치 방법, 그룹 계수를 함께 봅니다.

Definitions that matter before you buy wire

전선 증설

전선 증설은 최소 허용전류보다 큰 도체를 선택해 전압 강하를 줄이는 것입니다.

단자 온도 정격

단자 온도 정격은 장비 러그에 적용할 허용전류 온도 열과 절연 온도를 정합니다.

박스 필

박스 필은 NEC 314.16에 따라 도체, 장치, 클램프, 접지선 공간을 확인하는 것입니다.

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

자재 목록을 승인하기 전에 아래 순서로 확인하면 전압 강하 해결이 단자나 충전 문제로 이어지는 것을 막을 수 있습니다.

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 작업장 분기회로에서 16A, 12 AWG 구리는 약 7.6%가 될 수 있습니다. 8 AWG는 약 3%에 가깝지만 더 큰 박스나 피그테일 접속이 필요할 수 있습니다.

"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

시공 전에 증설 도체 확인

먼저 전압 강하를 계산하고 전선 크기, 전선관 필, 감용 계산기로 제안 도체를 확인합니다.

전압 강하 계산기 열기 Wire Size Calculator Conduit Fill Calculator

전압 강하 후 전선 증설 체크리스트: 단자, 박스, 전선관

요약