Voltage Drop Calculator - NEC Compliant Wire Sizing Tool

The NEC recommends a maximum 3% voltage drop for branch circuits and 5% total from service panel to the final load. Most electricians aim to keep branch circuit drop under 3% for optimal performance and energy efficiency.

Voltage drop is calculated using: V_drop = 2 × L × R_per_ft × I, where L is the one-way cable length in feet, R_per_ft is the resistance per foot of the wire (from conductor tables), and I is the current in amperes. The factor of 2 accounts for both the outgoing and return conductors.

Copper wire has lower resistivity (~1.72 μΩ·cm) compared to aluminum (~2.82 μΩ·cm), meaning copper produces significantly less voltage drop for the same gauge and length. Aluminum is lighter and cheaper, and is commonly used in utility and large feeder applications.

To reduce voltage drop: (1) Use a larger wire gauge (lower AWG number), (2) Shorten the cable run, (3) Increase the supply voltage, or (4) Reduce the load current. Increasing wire gauge by 3 AWG sizes (e.g., 12 AWG to 9 AWG) approximately halves the resistance.

For 12V DC systems, voltage drop is especially critical because 3% equals only 0.36V. The formula is the same: V_drop = 2 × L × R × I. Because the source voltage is low, even a small drop becomes a large percentage. This calculator automatically handles 12V, 24V, 48V, and any other voltage you enter.

NEC Article 210.19 (informational note) recommends a maximum 3% voltage drop on branch circuits. NEC 215.2 recommends 5% maximum total voltage drop from the service panel to the final load. These are recommendations, not hard requirements, but they represent best practice for safe and efficient installations.

Yes. As wire temperature increases, resistance increases. The resistance values used in this calculator are at 20°C (68°F) standard conditions. At higher operating temperatures (e.g., 75°C conductor temperature), actual resistance and voltage drop will be higher — typically by 15–25% for copper at full rated temperature.

Voltage drop is caused by the inherent electrical resistance of conductors. Every wire has resistance based on its material, length, and cross-sectional area. As current flows through this resistance, some energy is converted to heat (following P = I²R), resulting in a lower voltage at the load end of the circuit.