전압 강하와 허용 전류: 코드 최소 전선보다 굵게 해야 할 때
NEC 310.16, NEC 210.19, NEC 215.2, IEC 60364 기준으로 열적 허용 전류와 장거리 전압 강하를 구분합니다.
허용 전류는 전선 과열을 보는 기준이고, 전압 강하는 부하단 전압 품질을 보는 기준입니다. 긴 회로는 두 검토가 모두 필요합니다.
This localized field guide uses the same engineering baseline as the English source: ampacity is checked with NEC 310.16 and related adjustment rules, while voltage drop is checked against practical branch and feeder targets. For public references, review the National Electrical Code, electrical wiring, and the International Electrotechnical Commission.
Ampacity tells you whether the conductor survives thermally. It does not prove that a 120 V load 140 ft away receives healthy voltage.
— Hommer Zhao, Technical Director
전압 강하와 허용 전류: 코드 최소 전선보다 굵게 해야 할 때
먼저 NEC 310.16, 단자 온도, 주변 온도, 도체 수 보정으로 열적으로 적합한지 확인합니다. 20 A 회로에 12 AWG 구리가 가능해도 거리가 길면 부족할 수 있습니다.
그 다음 실제 편도 거리, 부하 전류, 도체 재질로 전압 강하를 계산합니다. 120 V, 16 A, 120 ft에서 12 AWG 구리는 약 5.1%이므로 10 AWG가 더 적절합니다.
| Circuit | Minimum | Distance/load | Better move |
|---|---|---|---|
| 120 V branch | 12 AWG Cu / 20 A | 16 A at 120 ft | 10 AWG for about 3% |
| 240 V EVSE | 8 AWG Cu / 40 A | 32 A at 170 ft | Check feeder budget |
| Detached feeder | 6 AWG Cu / 60 A | 48 A at 180 ft | Use 4 AWG Cu or compare Al |
| 230 V motor | 10 AWG Cu / 30 A | 24 A at 210 ft | Check 8 AWG and startup |
| 277 V lighting | 12 AWG Cu / 20 A | 12 A at 220 ft | Grouping may control |
When a 20 A branch circuit passes 100 ft at 120 V, calculate it instead of relying on habit. The performance answer is often 10 AWG even when 12 AWG is the familiar minimum.
— Hommer Zhao, Technical Director
NEC / IEC checkpoints
NEC 310.16, NEC 310.15, NEC 210.19(A)(1), NEC 215.2(A)(1), NEC 430, and IEC 60364-5-52 should be reviewed together. Ampacity is the safety floor; voltage drop is the performance check.
A practical job record should show volts, amps, one-way distance, conductor material, selected conductor size, and calculated voltage drop. Those six values explain why a design uses a larger conductor than the breaker-size minimum.
In review, four numbers expose weak designs quickly: volts, amps, one-way feet, and conductor material. If they are visible, bad voltage-drop assumptions cannot hide.
— Hommer Zhao, Technical Director
FAQ
Can ampacity pass while voltage drop fails?
Yes. A 12 AWG copper conductor can be legal on a 20 A circuit, while 120 V at 16 A over 120 ft can exceed a 3% branch-circuit target.
Do I need a larger breaker when I upsize wire?
No. A 20 A circuit can use 10 AWG copper for voltage drop and still remain protected by a 20 A breaker.
Which NEC checks come first?
Start with NEC 310.16 and NEC 310.15 for ampacity, then check NEC 210.19(A)(1) and NEC 215.2(A)(1) voltage-drop guidance.
When does voltage drop usually control?
It often controls 120 V branches above 75 to 100 ft, long 240 V equipment circuits, and feeders that must preserve a 5% total budget.
How does IEC practice compare?
IEC 60364-5-52 also coordinates installation method, grouping, ambient correction, conductor size, and allowable voltage drop.
What numbers should I document?
Record voltage, load current, one-way distance, conductor material, conductor size, and calculated drop percentage.
자재 발주 전에 최소 전선과 한 단계 큰 전선을 계산기로 비교하세요.
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