Voltage Drop Calculator
Calculate voltage drop across a wire run. Find out if your conductor size is adequate for the load and distance.
How it works
Voltage drop is calculated using the formula: VD = 2 × I × L × R / 1000, where I is the current in amps, L is the one-way length in feet, and R is the conductor resistance in ohms per 1,000 feet. The factor of 2 accounts for both the hot and neutral conductors. The NEC recommends keeping voltage drop at 3% or less for branch circuits (5% total including feeder).
Example
A 20A circuit runs 100 feet to a load using AWG 12 copper wire (resistance ≈ 1.98 Ω/1000ft) on a 120V system. Voltage drop = 2 × 20 × 100 × 1.98 / 1000 = 7.92V, which is 6.6% — too high. Upgrading to AWG 10 (1.24 Ω/1000ft) gives 4.96V drop, or 4.1% — better but still above the 3% recommendation for a 200ft run. AWG 8 (0.778 Ω/1000ft) gives 3.11V, or 2.6% — within limits.
Frequently asked questions
What is the NEC voltage drop recommendation?
The NEC recommends a maximum of 3% voltage drop for branch circuits and 2% for feeders, with a total of no more than 5% from the service panel to the furthest outlet. These are recommendations, not mandatory requirements — except for sensitive equipment like medical devices.
Does voltage drop apply to DC circuits too?
Yes. For DC circuits the formula is the same: VD = 2 × I × L × R / 1000. DC systems like solar PV, battery banks, and automotive wiring are often more sensitive to voltage drop than AC systems because there is no transformer to compensate.
What resistance value should I use for copper wire?
Common values (Ω per 1,000 ft): AWG 14 = 3.14, AWG 12 = 1.98, AWG 10 = 1.24, AWG 8 = 0.778, AWG 6 = 0.491, AWG 4 = 0.308. For aluminum, multiply by approximately 1.61.
How do I reduce voltage drop?
Use a larger wire gauge, shorten the run length, increase the system voltage (e.g. use 240V instead of 120V for long runs), or reduce the load current. Doubling the wire size roughly halves the voltage drop.
← Back to all calculators