Voltage Drop Calculator
Calculate the voltage drop across an electrical wire given the current, one-way run distance, and AWG wire gauge. Use this to verify that loads receive adequate voltage and meet NEC guidelines.
About this calculator
Voltage drop in a conductor depends on wire resistance, current, and total circuit length. This calculator uses: Voltage Drop = (Current × Distance × 2 × Resistance_per_1000ft) / 1,000, where Resistance_per_1000ft ≈ 1.02 × 1.26^(12 − AWG). The factor of 2 accounts for the complete current path (supply and return conductors). The resistance model 1.02 × 1.26^(12 − AWG) approximates the copper resistance in ohms per 1,000 feet for standard AWG sizes — each 3 AWG steps roughly doubles resistance, matching the AWG standard. Dividing by 1,000 converts feet-based resistance to the actual run length. The NEC recommends keeping voltage drop under 3% for branch circuits and under 5% combined for feeders plus branch circuits.
How to use
A 15 A circuit runs 75 feet (one-way) using 12 AWG copper wire on a 120 V system. Compute resistance: 1.02 × 1.26^(12 − 12) = 1.02 × 1.26^0 = 1.02 × 1 = 1.02 Ω/1,000 ft. Now apply the formula: Voltage Drop = (15 × 75 × 2 × 1.02) / 1,000 = (2,295) / 1,000 = 2.295 V. As a percentage of 120 V: 2.295 / 120 = 1.9% — well within the 3% NEC guideline. If the run were 150 feet, the drop would double to 4.59 V (3.8%), suggesting an upgrade to 10 AWG wire.
Frequently asked questions
What is an acceptable voltage drop percentage for residential electrical circuits?
The National Electrical Code recommends a maximum of 3% voltage drop on any individual branch circuit and no more than 5% total voltage drop when combining the feeder and branch circuit. Exceeding these limits causes lights to dim, motors to run hot and wear prematurely, and sensitive electronics to malfunction or fail. For critical loads like medical equipment or data centers, even lower drop targets — sometimes 1–2% — are specified. Calculating voltage drop before running wire helps you choose the right gauge and avoid costly rewiring later.
How does wire length affect voltage drop in an electrical circuit?
Voltage drop is directly proportional to the total conductor length, which is twice the one-way distance because current travels out through the hot wire and returns through the neutral. Double the run length and you double the voltage drop. This is why long wire runs — such as to a detached garage, barn, or landscape lighting — often require heavier gauge wire than a short run carrying the same current. Running the calculation for your specific distance ensures you select wire that keeps the load within the acceptable voltage window.
Why does lower AWG wire have less voltage drop than higher AWG wire?
Lower AWG numbers correspond to physically larger wire diameters with greater cross-sectional area, which means fewer electrons are squeezed through the same space — resulting in lower electrical resistance. Since voltage drop equals current multiplied by resistance (V = I × R), reducing resistance directly reduces voltage drop for the same current and distance. Each three-step decrease in AWG roughly halves the wire resistance. Choosing the correct gauge is a balance between minimizing voltage drop, meeting ampacity requirements, and managing material cost, since heavier copper wire is significantly more expensive per foot.