Voltage Drop Calculator
Calculate voltage drop across any wire run. Enter current, length, and wire gauge — get voltage drop, power loss, and remaining load voltage for electrical compliance.
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A
mm²
V
Enter your values above to see the results.
Tips & Notes
- ✓Voltage drop formula: V_drop = 2 × I × R_wire for two-conductor runs (factor of 2 accounts for both hot and neutral/return conductors). For DC circuits: V_drop = I × R_total where R_total = 2 × ρ × L / A.
- ✓NEC (National Electrical Code) limits voltage drop to 3% for branch circuits and 5% for feeders plus branch circuits combined. For a 120 V branch circuit: maximum 3% drop = 3.6 V. Exceeding this causes equipment malfunction and energy waste.
- ✓Conductor resistivity: copper ρ = 1.68×10⁻⁸ Ω·m (most common); aluminum ρ = 2.65×10⁻⁸ Ω·m (57% more resistance per same area — requires larger gauge). Use copper for most wiring; aluminum for utility service entrances.
- ✓Doubling the wire cross-section area halves resistance and halves voltage drop. Going from AWG 14 (2.08 mm²) to AWG 12 (3.31 mm²) — about 1.6× area — reduces drop by 37%. Going up two AWG numbers (e.g., 14 to 10) roughly doubles the area.
- ✓Temperature increases resistance: copper resistance coefficient = 0.00393/°C. A copper wire at 75°C has 20.4% higher resistance than at 20°C. NEC wire ampacity tables already account for standard temperature ratings.
Common Mistakes
- ✗Forgetting to double the wire length for the return path — voltage drop occurs on both the outgoing and return conductor. For a 10 m run, total conductor length is 20 m. The formula V_drop = I × R uses total conductor resistance, which requires 2× the one-way distance.
- ✗Using wire ampacity instead of voltage drop for sizing — ampacity limits prevent overheating; voltage drop limits prevent equipment malfunction. In long runs, voltage drop often requires a larger wire than ampacity alone would dictate.
- ✗Ignoring voltage drop for DC systems — DC systems (12 V automotive, 24 V solar, 48 V telecom) are more sensitive to voltage drop because the absolute drop is a larger percentage of a lower supply voltage. 1.2 V drop on 12 V is 10% — much more significant than 1.2 V on 120 V.
- ✗Calculating voltage drop only at rated load — starting current for motors can be 6-8× running current. Voltage drop at startup may far exceed running-load calculations, causing motors to fail to start or breakers to trip.
- ✗Not accounting for conductor temperature rating — NEC limits the assumed temperature for voltage drop calculations to the conductor temperature rating (60°C, 75°C, or 90°C). Using room-temperature resistivity overestimates available capacity.
Voltage Drop Calculator Overview
Voltage drop in conductors is an invisible energy tax that grows with current and distance. In residential wiring, excessive voltage drop means dimmer lights and hotter motors. In automotive systems, it means sluggish winches and dim headlights. In solar and battery systems, it means lost energy and shorter run times.
Voltage drop formula:
V_drop = 2 × I × ρ × L / A | R_wire = 2 × ρ × L / A | P_loss = I² × R_wire
EX: 20 A, 25 m run, copper AWG 12 (A = 3.31 mm² = 3.31×10⁻⁶ m²) → R = 2 × 1.68×10⁻⁸ × 25 / 3.31×10⁻⁶ = 0.254 Ω → V_drop = 20 × 0.254 = 5.07 V → 5.07/120 = 4.2% — exceeds 3% NEC limit. Use AWG 10 (5.26 mm²) → V_drop = 3.19 V = 2.66% ✓Required conductor area for target voltage drop:
A_required = 2 × I × ρ × L / V_drop_max
EX: 30 A, 50 m run, limit 3% of 240 V = 7.2 V → A = 2 × 30 × 1.68×10⁻⁸ × 50 / 7.2 = 7.0×10⁻⁶ m² = 7.0 mm² → AWG 8 (8.37 mm²) meets requirementAWG wire properties — copper:
| AWG | Area (mm²) | Resistance (Ω/100m) | NEC Ampacity (75°C) | Max run at 15A, 3% drop on 120V |
|---|---|---|---|---|
| AWG 14 | 2.08 | 1.62 | 15 A | 7.4 m (24 ft) |
| AWG 12 | 3.31 | 1.02 | 20 A | 11.8 m (39 ft) |
| AWG 10 | 5.26 | 0.640 | 30 A | 18.8 m (62 ft) |
| AWG 8 | 8.37 | 0.403 | 50 A | 29.8 m (98 ft) |
| AWG 6 | 13.3 | 0.253 | 65 A | 47.4 m (155 ft) |
| AWG 4 | 21.2 | 0.159 | 85 A | 75.5 m (248 ft) |
| Material | Resistivity (Ω·m at 20°C) | Relative to Copper | Common Use |
|---|---|---|---|
| Silver | 1.59×10⁻⁸ | 0.95× | Contacts, RF connectors |
| Copper (annealed) | 1.68×10⁻⁸ | 1.0× (reference) | All general wiring |
| Gold | 2.24×10⁻⁸ | 1.33× | Plating, connectors |
| Aluminum | 2.65×10⁻⁸ | 1.58× | Utility feeders, overhead lines |
| Tungsten | 5.60×10⁻⁸ | 3.33× | Light bulb filaments |
| Nichrome | 1.10×10⁻⁶ | 65× | Heating elements, resistance wire |
Frequently Asked Questions
V_drop = 2 × I × ρ × L / A (for round-trip two-conductor run), where I is current in amperes, ρ is resistivity (copper = 1.68×10⁻⁸ Ω·m), L is one-way length in meters, and A is cross-sectional area in m². Example: 15 A circuit, 30 m run (copper AWG 14, A = 2.08×10⁻⁶ m²) → V_drop = 2 × 15 × 1.68×10⁻⁸ × 30 / (2.08×10⁻⁶) = 2 × 15 × 0.000242 = 7.27 V. At 120 V supply, drop is 6.1% — exceeds the 3% NEC limit. Use AWG 12 or larger.
The National Electrical Code (NEC) recommends (not mandates) maximum 3% voltage drop on branch circuits and 5% total for the combined feeder and branch circuit. At 120 V: 3% = 3.6 V max drop. At 240 V: 3% = 7.2 V max. Exceeding these limits causes: equipment operating below rated voltage (motors run hotter and lose efficiency at lower voltage); lighting appears dim; electronic equipment may malfunction or fail to start; sensitive equipment like medical devices, computers, and PLCs may fault. Energy is also wasted as heat in the conductor — the power lost = I² × R_wire.
Required cross-sectional area A = 2 × I × ρ × L / V_drop_max. Example: 20 A circuit, 40 m run, limit 3% of 120 V = 3.6 V → A = 2 × 20 × 1.68×10⁻⁸ × 40 / 3.6 = 7.47×10⁻⁶ m² = 7.47 mm². The next standard AWG size with area ≥ 7.47 mm² is AWG 8 (8.37 mm²). Verify ampacity: AWG 8 is rated 50 A (90°C) or 40 A (60°C) — adequate for 20 A. When voltage drop requires larger wire than ampacity, voltage drop governs the selection.
Approximate voltage drop per 100 feet (30.5 m) of round-trip copper at rated NEC ampacity: AWG 14 (15 A load): 3.14 V; AWG 12 (20 A): 3.94 V; AWG 10 (30 A): 3.74 V; AWG 8 (40 A): 3.95 V. These examples all exceed 3% at 120 V (3.6 V limit), showing that long runs at full ampacity frequently require upsizing. For 240 V circuits, the same drop is a lower percentage: 3.94 V on 240 V = 1.64%, which is within limits.
DC low-voltage systems are far more sensitive to voltage drop than 120/240 V AC systems because the same absolute drop represents a much higher percentage. On a 12 V system, 1.2 V drop = 10% — enough to cause motors to run sluggishly, lights to be noticeably dim, and electronics to malfunction. General rule for 12 V automotive: limit voltage drop to 0.5 V (4.2%) for high-current accessories like winches, light bars, and audio amplifiers. Size wiring using the same formula: larger wire, shorter runs. Car audio rule of thumb: 1/0 AWG (53.5 mm²) for amplifiers over 1,000 W.
Voltage drop refers to the resistive loss in conductors (wires, cables, busbars) between the source and load. It increases linearly with current: V_drop = I × R_wire. Voltage regulation refers to how well a power supply, transformer, or regulator maintains its output voltage under varying load. A poorly regulated supply may drop 5 V from no-load to full-load due to internal impedance and feedback limitations. Both effects reduce load voltage, but they have different causes and solutions: larger conductors fix voltage drop; better regulation or higher source voltage fixes regulation issues. Both must be analyzed for systems with long wiring and varying loads.