Voltage Drop
Calculator
Calculate voltage drop for DC, single-phase, and three-phase circuits. Uses AWG wire gauge, checks NEC 3% and 5% limits, and recommends minimum wire size.
3
CIRCUIT TYPES
AWG
WIRE SIZES
NEC
COMPLIANCE
Cu/Al
MATERIALS
Voltage Drop Calculator
NEC-compliant · AWG · Cu/Al
Enter source voltage, load current, and wire length above to see your voltage drop.
WIRE REFERENCE
AWG Wire Gauge — Resistance & Ampacity
Resistance values at 75°C (167°F) per NEC. Ampacity for copper in conduit.
| AWG | Area (CM) | Ω/1000ft (CU) | Ω/1000ft (AL) | Max Ampacity (CU) | Typical Use |
|---|---|---|---|---|---|
| 14 AWG | 2,580 | 3.14 | — | 15 A | General lighting/outlets |
| 12 AWG | 6,530 | 1.98 | — | 20 A | Kitchen, bathroom circuits |
| 10 AWG | 10,380 | 1.24 | 2.04 | 30 A | Dryers, A/C units |
| 8 AWG | 16,510 | 0.778 | 1.28 | 40 A | EV chargers (Level 2), ranges |
| 6 AWG | 26,240 | 0.491 | 0.808 | 55 A | Sub-panels, large A/C |
| 4 AWG | 41,740 | 0.308 | 0.508 | 70 A | Sub-panels, hot tubs |
| 2 AWG | 66,360 | 0.194 | 0.319 | 95 A | Service entrance, feeders |
| 1/0 AWG | 105,600 | 0.122 | 0.201 | 125 A | Large feeders |
| 2/0 AWG | 133,100 | 0.0967 | 0.159 | 145 A | Service entrance |
| 4/0 AWG | 211,600 | 0.0608 | 0.1 | 195 A | Large service entrance |
NEC Recommends Max 3% Drop on Branch Circuits
The National Electrical Code recommends no more than 3% voltage drop on individual branch circuits and feeders, and 5% total from service entrance to load. This is a recommendation (not a code requirement), but exceeding it causes equipment inefficiency, overheating, and shortened equipment life. For motor circuits and sensitive electronics, stay under 3%.
Single-Phase vs Three-Phase
Single-phase (1Ø): Two conductors carry current. Current flows out on one wire and returns on the other — doubling the effective wire length. Formula uses multiplier 2. Used in homes (120V, 240V) and small commercial applications.
Three-phase (3Ø): Three conductors, 120° apart. Uses √3 (≈1.732) multiplier instead of 2, because the phases partially cancel. More efficient for long runs and heavy loads. Used in commercial and industrial buildings (208V, 480V).
Copper vs Aluminum Wiring
Copper (K=12.9): Standard for residential. Lower resistivity, more flexible, more reliable connections, compatible with all devices. Preferred for everything under 1 AWG.
Aluminum (K=21.2): About 64% higher resistance than copper. Used for large feeders (1/0 AWG and up) and service entrance where its lighter weight and lower cost justify use. Requires anti-oxidant compound and compatible connectors rated AL/CU. Always 1-2 gauge larger than copper for same ampacity.
How to Calculate Voltage Drop
Voltage drop is calculated using Ohm's Law applied to the conductor resistance. The standard NEC-based formula uses the K-constant method, which incorporates conductor resistivity, current, and length per circular mil area.
DC and Single-Phase AC Formula
Vdrop = (2 × K × I × L) / CM Where: K = conductor constant (Cu: 12.9, Al: 21.2 at 75°C) I = current in amperes (A) L = one-way conductor length in feet CM = conductor cross-section in circular mils The 2× multiplier accounts for both conductors (out + return). Voltage at load = Source voltage − Vdrop % Voltage drop = (Vdrop / Source voltage) × 100 Example: 120V, 15A, 50 ft, 12 AWG copper (CM = 6,530): Vdrop = (2 × 12.9 × 15 × 50) / 6,530 Vdrop = 19,350 / 6,530 = 2.96 V % drop = (2.96 / 120) × 100 = 2.47% ✓ (under 3%)
Three-Phase Formula
Vdrop = (1.732 × K × I × L) / CM The √3 (1.732) replaces 2× because three phases partially offset each other, reducing the effective return path voltage. Example: 208V 3Ø, 30A, 100 ft, 10 AWG copper (CM = 10,380): Vdrop = (1.732 × 12.9 × 30 × 100) / 10,380 Vdrop = 66,988 / 10,380 = 6.45 V % drop = (6.45 / 208) × 100 = 3.10% (just over 3% — use 8 AWG)
Finding Minimum Wire Size for Target % Drop
Rearranging for CM: CM = (2 × K × I × L) / Vdrop_max (1Ø/DC) CM = (1.732 × K × I × L) / Vdrop_max (3Ø) Where Vdrop_max = Source voltage × (max % drop / 100) Example: Stay under 3% on 120V, 20A, 75 ft copper (1Ø): Vdrop_max = 120 × 0.03 = 3.6 V CM = (2 × 12.9 × 20 × 75) / 3.6 CM = 38,700 / 3.6 = 10,750 CM → 10 AWG (10,380 CM) just fits
When Does Voltage Drop Matter?
Voltage drop becomes a practical concern whenever wire runs are long relative to the load current, or when the load is sensitive to voltage variation.
Common Problem Scenarios
EV Chargers: A Level 2 charger at 48A on a 240V circuit, located 80 feet from the panel, can easily exceed 5% drop on 6 AWG wire. Many EV chargers will throttle charge rate or fault if supply voltage drops below 208V on a 240V circuit.
Lighting Circuits
LED drivers are generally tolerant of 5-10% voltage variation, but incandescent and fluorescent lights show visible dimming above 3% drop. Long runs for landscape lighting or string lights (often low-voltage 12V or 24V DC) suffer dramatically from voltage drop — even a small resistance causes a large percentage drop on low-voltage circuits.
Motor Circuits
Motors are the most sensitive loads to voltage drop. A 10% voltage reduction causes approximately a 19% reduction in torque (torque ∝ V²), forcing the motor to draw more current to maintain load. This increases winding temperature and dramatically shortens motor life. NEC recommends staying under 3% for motor branch circuits.
Low-Voltage DC Systems
Voltage drop is proportionally much worse at low voltages. A 1V drop on a 120V circuit is 0.83%. The same 1V drop on a 12V DC system is 8.3% — a significant problem. Solar and battery systems, automotive wiring, and LED lighting on 12V/24V DC require very careful wire sizing to keep percentage drop under 3%.