V
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Supply voltage at the source (120, 240, 48, 12 V, etc.)
A
Please enter a valid current.
Current drawn by the load in amps
ft
Please enter a valid wire length.
Distance from source to load (one way — return calculated automatically)
Single-phase: factor 2 (go + return). Three-phase: factor 1.732
Voltage Drop
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Sources & Methodology
Wire resistance values from NEC Chapter 9 Table 9. NEC limits from Articles 210.19 and 215.2. Formulas from IEEE and NFPA 70.
NFPA 70 — National Electrical Code (NEC) 2023
NEC Articles 210.19(A) and 215.2(A) define the 3% branch circuit and 5% total system voltage drop recommendations. Chapter 9 Table 9 provides conductor AC resistance values.
Southwire Voltage Drop Calculator Reference
Industry-standard wire manufacturer reference for conductor resistance values at 75°C operating temperature and voltage drop calculation methodology.
Mike Holt Enterprises — NEC Electrical Reference
Authoritative NEC code interpretation reference covering voltage drop limits, conductor sizing rules, and practical application guidelines for electricians.
Methodology: DC/Single-phase: VD = 2×I×L×R/1000 (L in ft, R in Ω/1000ft). Three-phase: VD = 1.732×I×L×R/1000. Resistance values at 75°C from NEC Ch.9 Table 9. Aluminium = 1.61× copper resistance. Metric: R per metre = R_per_1000ft / 304.8. VD% = VD/Vs×100. End voltage = Vs − VD.
⏱ Last reviewed: March 2026
Voltage Drop Formula & NEC Guidelines
Voltage drop is the reduction in voltage that occurs as current flows through a conductor due to the wire’s resistance. Every wire has resistance, and by Ohm’s law (V = I×R), current flowing through that resistance creates a voltage loss. The longer the run and the higher the current, the greater the drop.
Voltage Drop Formula
DC / Single-Phase: VD = 2 × I × L × R / 1000
Three-Phase: VD = 1.732 × I × L × R / 1000
VD = voltage drop (V) • I = current (A) • L = one-way length (ft)
R = conductor resistance (Ω per 1000 ft) • Factor 2 = go + return conductors
Example: 120V, 20A, 100 ft one-way, 12 AWG copper (R = 1.98 Ω/1000ft):
VD = 2 × 20 × 100 × 1.98 / 1000 = 7.92V
VD% = 7.92 / 120 × 100 = 6.6% ⚠️ Exceeds NEC 3% limit
Use 8 AWG (0.778 Ω/kft): VD = 2×20×100×0.778/1000 = 3.11V = 2.6% ✅
R = conductor resistance (Ω per 1000 ft) • Factor 2 = go + return conductors
Example: 120V, 20A, 100 ft one-way, 12 AWG copper (R = 1.98 Ω/1000ft):
VD = 2 × 20 × 100 × 1.98 / 1000 = 7.92V
VD% = 7.92 / 120 × 100 = 6.6% ⚠️ Exceeds NEC 3% limit
Use 8 AWG (0.778 Ω/kft): VD = 2×20×100×0.778/1000 = 3.11V = 2.6% ✅
NEC Voltage Drop Limits
| Circuit Type | NEC Limit | Max Drop on 120V | Max Drop on 240V |
|---|---|---|---|
| Branch circuit alone | 3% | 3.6V | 7.2V |
| Feeder alone | 3% | 3.6V | 7.2V |
| Combined (feeder + branch) | 5% | 6.0V | 12.0V |
| Sensitive electronics (recommended) | 1–2% | 1.2–2.4V | 2.4–4.8V |
AWG Copper Wire Resistance at 75°C (NEC Chapter 9 Table 9)
| AWG | Cu Ω/1000ft | Al Ω/1000ft | Cu Area (mm²) | Ampacity (Cu) |
|---|---|---|---|---|
| 14 | 3.14 | 5.06 | 2.08 | 15A |
| 12 | 1.98 | 3.19 | 3.31 | 20A |
| 10 | 1.24 | 2.00 | 5.26 | 30A |
| 8 | 0.778 | 1.25 | 8.37 | 50A |
| 6 | 0.491 | 0.790 | 13.3 | 65A |
| 4 | 0.308 | 0.496 | 21.2 | 85A |
| 2 | 0.194 | 0.312 | 33.6 | 115A |
| 1/0 | 0.122 | 0.196 | 53.5 | 150A |
| 2/0 | 0.0967 | 0.156 | 67.4 | 175A |
| 4/0 | 0.0608 | 0.0982 | 107 | 230A |
💡 Rule of thumb for long runs: For every doubling of wire length, voltage drop doubles. For every step up in AWG (e.g. 12 to 10), resistance drops by about 20%, reducing voltage drop by 20%. Going up two AWG sizes (12 to 8) reduces resistance by about 38%. If your calculated drop exceeds 3%, go up 1–2 AWG sizes and recalculate.
Why Voltage Drop Matters
- Motors: Torque is proportional to voltage squared. A 5% drop reduces available torque by ~10% and increases current, causing overheating.
- LED lighting: LEDs are constant-current devices and regulate well, but LED drivers may flicker or shut down with excessive voltage drop.
- Electronics: Computers, PLCs, and sensitive equipment require stable voltage. Low voltage can cause random resets, data errors, or damage.
- Heating elements: Power = V²/R. A 5% voltage drop reduces heating power by ~10%, extending process times.
- Solar/battery systems: In 12V and 24V DC systems, even a 2% drop (0.24–0.48V) is significant. Use heavier wire or higher system voltage.
Frequently Asked Questions
DC / Single-phase: VD = 2×I×L×R/1000 (L in feet, R in Ω/1000ft). The factor 2 accounts for current flowing both ways (go and return). Three-phase: VD = 1.732×I×L×R/1000. VD% = VD/Vs×100. For example, 20A through 100ft of 12 AWG copper (1.98Ω/kft): VD = 2×20×100×1.98/1000 = 7.92V = 6.6% on 120V — exceeding NEC 3%.
NEC 210.19(A) recommends 3% maximum for branch circuits. NEC 215.2(A) recommends 3% for feeders. The combined system (feeder + branch) should not exceed 5% total. These are recommendations, not hard code violations, but AHJ (Authority Having Jurisdiction) may require compliance. For motors, computers, and sensitive equipment, target 1–2% for best performance.
Four methods: (1) Increase wire gauge (one AWG size up = ~20% less resistance). (2) Shorten the run where possible. (3) Use 240V instead of 120V — same wattage at half the current means quarter the voltage drop. (4) For DC systems, increase system voltage from 12V to 24V or 48V. On solar/battery systems going from 12V to 48V reduces current by 75%, dropping voltage drop by 75% for the same wire.
12 AWG copper at 75°C: 1.98 Ω per 1000 feet = 0.00649 Ω per foot = 0.0213 Ω per metre. At 20°C (room temperature), it is slightly lower: 1.62 Ω/kft. NEC calculations use 75°C values because conductors operate at elevated temperature under load. At full 20A ampacity, 12 AWG copper is the correct minimum size for a 120V branch circuit.
Three-phase VD = 1.732×I×L×R/1000 vs single-phase VD = 2×I×L×R/1000. The ratio is 1.732/2 = 0.866, so three-phase has 13.4% less voltage drop than single-phase for the same current and wire size. Three-phase also delivers more power per conductor, making it more efficient for industrial loads over long distances.
12 AWG gives VD = 2×20×100×1.98/1000 = 7.92V = 6.6% — too high. 10 AWG gives 2×20×100×1.24/1000 = 4.96V = 4.1% — still above 3%. 8 AWG gives 2×20×100×0.778/1000 = 3.11V = 2.6% ✅ within 3%. Use 8 AWG copper for this 100ft, 20A, 120V circuit to comply with NEC 3% recommendation.
Yes. Aluminium conductivity is about 61% of copper, so aluminium wire has about 1.61× higher resistance for the same AWG. To match copper voltage drop, use aluminium two AWG sizes larger: copper 10 AWG (1.24Ω/kft) vs aluminium 8 AWG (1.25×1.61 ≈ 1.25Ω/kft — actually this is 0.778×1.61 = 1.25, equivalent to copper 10 AWG). Aluminium requires special connectors and anti-oxidant compound but costs less per pound.
Use the same formula: VD = 2×I×L×R/1000. For a 12V system, 10A load, 20ft one-way with 10 AWG copper: VD = 2×10×20×1.24/1000 = 0.496V = 4.1% on 12V — too high. Use 8 AWG: VD = 2×10×20×0.778/1000 = 0.311V = 2.6% ✅. Low-voltage DC systems (12V, 24V) are much more sensitive to voltage drop than 120V AC. Keep DC runs as short as possible.
Excessive voltage drop causes: (1) Motors drawing excess current, overheating, and tripping overloads. (2) Lights appearing dim or flickering. (3) Electronic equipment malfunctioning or failing to start. (4) Increased wire heating (wasted energy as heat). (5) Reduced appliance life. (6) Failed NEC inspection if drop is excessive. The solution is almost always to increase wire size, shorten the run, or increase the system voltage.
Rearrange the formula: R_max = VD_max×1000/(2×I×L). For 3% on 120V = 3.6V max drop: R_max = 3600/(2×I×L). Then find the AWG size whose resistance at 75°C is at or below R_max. Example: 15A, 50ft: R_max = 3600/(2×15×50) = 2.40Ω/kft. 12 AWG = 1.98Ω/kft — within limit ✅. This calculator shows the NEC compliance result automatically.
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