电压降计算器
电压降计算器用于确定电线由于导体电阻而产生的电压损失。输入电源电压、负载电流、导线规格（AWG）和单程布线距离，即时计算电压降（伏）、百分比降幅、负载端电压，以及是否符合 NEC 规范（支线回路 3%，综合最大 5%）。
Voltage Drop Calculator
Calculate voltage drop, voltage at load, and NEC compliance for electrical wiring.
Enter the one-way wire run distance. The formula accounts for the return conductor automatically.
Frequently Asked Questions
电压降计算器是如何工作的？
电压降 = 电流(A) × 导线电阻(Ω)。导线电阻 = 电阻率(ρ) × 导线长度 / 截面积。对于单相交流或直流电路，导线长度取两倍线路距离（往返）。本计算器内置铜线和铝线的电阻率，根据线径和长度自动计算。
电压降的允许范围是多少？
电气工程标准（如美国 NEC 规范）建议：从配电箱到任何负载的总电压降不超过 5%（其中主馈线不超过 3%，支路不超过 2%）。过大的电压降会导致设备效率低下、发热和寿命缩短，严重时可能损坏设备。
如何减少电压降？
减少电压降的方法：①使用更大截面积的导线（导线越粗，电阻越小）；②缩短导线长度（将配电箱移近负载）；③提高供电电压（如从 120V 升至 240V，电流减半，压降减少 75%）；④减少线路负载。
铜线和铝线的电压降有何不同？
铜的电阻率（1.72×10⁻⁸ Ω·m）低于铝（2.82×10⁻⁸ Ω·m），相同截面积铜线电阻约为铝线的 61%。因此，铝线产生的电压降约为铜线的 1.64 倍。使用铝线时需选择更大规格（通常大 1-2 号 AWG）来补偿更高的电阻率。
Voltage Drop Calculator: Complete Guide to NEC Compliance & Wire Sizing
The Voltage Drop Calculator determines how much voltage is lost in a wire run due to conductor resistance. It calculates voltage drop in volts, percentage drop, voltage available at the load, and whether the installation meets NEC guidelines. Use it to select the correct wire gauge for circuits in homes, commercial buildings, and industrial facilities.
Table of Contents
Voltage Drop Formula
The voltage drop formula used by this calculator is based on Ohm's Law and accounts for conductor resistance:
Single-Phase (2-wire):
Vdrop = 2 × I × R × (L / 1000)Three-Phase (3-wire):
Vdrop = √3 × I × R × (L / 1000)- I — load current in amperes (A)
- R — conductor resistance in ohms per 1,000 feet (Ω/1000ft)
- L — one-way wire run length in feet
- 2 — accounts for both outgoing and return conductors (single-phase)
- √3 ≈ 1.732 — three-phase power factor
Drop Percentage:
% Drop = (Vdrop / Vsource) × 100NEC Guidelines (3% / 5%)
The National Electrical Code (NEC) Article 210.19(A) Informational Note No. 4 recommends limiting voltage drop to maintain efficient and safe equipment operation:
| Circuit Type | NEC Recommended Max | Notes |
|---|---|---|
| Branch circuit | 3% | Outlet to equipment |
| Feeder circuit | 3% | Panel to subpanel or outlet |
| Combined (feeder + branch) | 5% | Total from service entrance to load |
Note: These are NEC recommendations, not mandatory code requirements. Some applications — such as motors, medical equipment, and sensitive electronics — may require tighter limits (1–2%). Always verify with local codes and equipment specifications.
Wire Gauge Resistance Table (Copper, 75°C)
Resistance values are in ohms per 1,000 feet (Ω/1000ft) for copper conductors at 75°C, per NEC Chapter 9 Table 9.
| AWG / kcmil | Resistance (Ω/1000ft) | Typical Ampacity (75°C) | Common Use |
|---|---|---|---|
| 14 AWG | 3.14 | 15 A | Lighting, small outlets |
| 12 AWG | 1.98 | 20 A | General outlets, kitchen |
| 10 AWG | 1.24 | 30 A | Dryers, A/C, EV chargers |
| 8 AWG | 0.778 | 50 A | Ranges, large A/C |
| 6 AWG | 0.491 | 65 A | Sub-panels, service |
| 4 AWG | 0.308 | 85 A | Large motors, service |
| 3 AWG | 0.245 | 100 A | Service entrance |
| 2 AWG | 0.194 | 115 A | Service entrance |
| 1 AWG | 0.154 | 130 A | Service entrance |
| 1/0 AWG | 0.122 | 150 A | Service feeder |
| 2/0 AWG | 0.0967 | 175 A | Service feeder |
| 3/0 AWG | 0.0766 | 200 A | Service feeder |
| 4/0 AWG | 0.0608 | 230 A | Large service |
Ampacity values per NEC Table 310.15(B)(16) for copper conductors with THHN/THWN-2 insulation in conduit at 75°C. Derate for bundling, ambient temperature, and conduit fill.
How to Use the Voltage Drop Calculator
- Select phase type — single-phase for most residential, three-phase for commercial/industrial.
- Enter source voltage — use presets (120V, 208V, 240V, 480V) or type your own value.
- Enter load current — the maximum continuous current draw of the load in amperes.
- Select wire gauge — choose the AWG size of the conductors being used.
- Enter one-way distance — the run length from the panel to the load in feet (the calculator doubles this for single-phase).
- Click Calculate — view voltage drop, voltage at load, percentage, and NEC compliance.
How to Reduce Voltage Drop
If your calculation shows excessive voltage drop, use one or more of these strategies:
1. Increase Wire Size (Most Effective)
Going from 12 AWG to 10 AWG reduces resistance by 37%. Each gauge step (e.g., 12→10→8) roughly halves the resistance and voltage drop. Use the calculator to test different gauges until the drop percentage is acceptable.
2. Reduce Run Length
Relocate the panel closer to the load, or add a sub-panel in a distant building. Voltage drop is directly proportional to distance — cutting the run in half halves the drop.
3. Use Higher Voltage
Running 240V instead of 120V delivers the same power at half the current, reducing voltage drop by 50% (since drop is proportional to current). The percentage drop is also halved.
4. Use Three-Phase Power
Three-phase distribution uses √3 ≈ 1.73 instead of 2 as the multiplier, which results in ~13% less voltage drop versus single-phase for the same wire and current. Ideal for long commercial runs.
Worked Examples
Example 1: Outlet 100ft from Panel (120V, 20A, 12 AWG)
- Vdrop = 2 × 20 × 1.98 × (100/1000) = 7.92 V
- Vload = 120 − 7.92 = 112.08 V
- % drop = (7.92 / 120) × 100 = 6.6% — exceeds NEC 3% and 5%
- Solution: Upgrade to 10 AWG → 2 × 20 × 1.24 × 0.1 = 4.96V (4.1%) or 8 AWG → 3.11V (2.6% ✓)
Example 2: HVAC Unit 50ft from Panel (240V, 30A, 10 AWG)
- Vdrop = 2 × 30 × 1.24 × (50/1000) = 3.72 V
- Vload = 240 − 3.72 = 236.28 V
- % drop = (3.72 / 240) × 100 = 1.55% — within NEC 3% ✓
Example 3: Three-Phase Motor 150ft Away (208V, 40A, 8 AWG)
- Vdrop = √3 × 40 × 0.778 × (150/1000) = 1.732 × 40 × 0.778 × 0.15 = 8.09 V
- Vload = 208 − 8.09 = 199.91 V
- % drop = (8.09 / 208) × 100 = 3.89% — within NEC 5% combined limit