Voltage Drop Calculator

About the Voltage Drop Calculator

A voltage drop calculator determines how much voltage is lost along a length of electrical conductor due to the wire's resistance, helping electricians and engineers select appropriate wire gauge to maintain safe, efficient electrical systems. The NEC (National Electrical Code) recommends keeping branch circuit voltage drop below 3% of source voltage, and combined feeder plus branch drop below 5%. Excessive voltage drop causes incandescent and LED lights to dim, electric motors to run hotter and wear out faster, electronics to malfunction or overheat, and overall system efficiency to decline. Our calculator works for both AC (single-phase and three-phase) and DC systems, handles all standard AWG and metric wire sizes, and accepts any conductor material (copper, aluminium, or copper-clad aluminium).

How It Works

Voltage drop formula for single-phase AC and DC: V_drop = (2 × L × I × R) / 1000, where L = one-way length in feet, I = load current in amps, and R = conductor resistance in ohms per 1,000 feet (from NEC Chapter 9 Table 9). Copper resistance values: 14 AWG = 3.14 Ω/1000ft; 12 AWG = 1.98; 10 AWG = 1.24; 8 AWG = 0.778; 6 AWG = 0.491; 4 AWG = 0.308; 2 AWG = 0.194; 1/0 AWG = 0.122. Example: 15-amp circuit, 12 AWG copper wire, 75-foot run (one-way): V_drop = (2 × 75 × 15 × 1.98) / 1000 = 4,455 / 1000 = 4.455 V. On a 120V circuit: % drop = 4.455/120 × 100 = 3.71% — slightly over the recommended 3% maximum. Solution: upgrade to 10 AWG.

Tips & Best Practices

• ✓Doubling the wire size (decreasing by 2 AWG steps, e.g., 12 AWG → 10 AWG) reduces resistance by approximately 38% and proportionally reduces voltage drop.
• ✓For long runs over 100 feet, always calculate voltage drop — it frequently requires upsizing from the minimum permitted wire gauge for ampacity.
• ✓Solar PV DC wiring: the NEC recommends even tighter voltage drop limits of 2% for DC circuits to maximise energy harvest. The higher the voltage (96V or 120V string vs. 12V/24V), the lower the current and the smaller the drop.
• ✓Three-phase voltage drop: the formula uses √3 (≈1.732) instead of 2 as the multiplier, because in balanced three-phase systems the return current flows through the other two phases — not a separate neutral conductor.
• ✓Aluminium wire: aluminium has higher resistivity than copper (about 61% of copper's conductivity). Aluminium feeders must be 2 AWG sizes larger than equivalent copper for the same voltage drop. Always use anti-oxidant compound on aluminium connections.
• ✓EV charging circuits: a 48-amp Level 2 EVSE on a 100-foot run typically requires 4 AWG or larger copper to keep voltage drop under 3% on a 240V circuit.
• ✓Voltage drop during motor starting: motors draw 6-7× running current during start-up (locked rotor current). Size wiring for running load but verify starting current doesn't cause excessive momentary drop that trips protective devices.
• ✓Low-voltage lighting: 12V and 24V landscape and display lighting is very sensitive to voltage drop — a 3% drop on 12V is only 0.36V, but the current is proportionally higher than 120V systems, making voltage drop calculations critical even for short runs.

Who Uses This Calculator

Licensed electricians sizing branch circuit wiring for new construction and renovations. Solar panel installers calculating DC wire sizes between array, combiner, and inverter. Electrical engineers designing industrial power distribution systems. Homeowners planning EV charging station installations with correct wire sizing. Marine electricians designing boat electrical systems where excessive voltage drop is a reliability and safety concern. Low-voltage lighting designers specifying cable sizes for landscape and display lighting runs.