Voltage Drop Calculator
Calculate the percentage voltage drop along a conductor run based on current, one-way distance, wire gauge, and system voltage. Use it to ensure circuits meet the NEC's 3% voltage-drop guideline.
About this calculator
Voltage drop occurs because every conductor has resistance, and current flowing through that resistance dissipates energy as heat, reducing the voltage available at the load. The percentage voltage drop formula used here is: VD% = (2 × I × d / V) × resistivity_factor / cross_section_factor, where the resistivity factor is 12.9 for single-phase (copper) or 7.46 for three-phase, and the cross-section factor approximates AWG conductor area as 2^((AWG−4)/3.2). The factor of 2 accounts for the round-trip path (hot and neutral/return conductors). A higher AWG number means a smaller wire with more resistance and greater drop. The NEC recommends keeping voltage drop below 3% on branch circuits and 5% total for feeders and branch circuits combined. Excessive drop causes motors to overheat and lights to dim.
How to use
Example: single-phase circuit, 20 A load, 100 ft one-way run, 12 AWG copper wire, 120 V system. Step 1 — phases factor: single-phase → 12.9. Step 2 — AWG cross-section factor: 2^((12−4)/3.2) = 2^(8/3.2) = 2^2.5 ≈ 5.657. Step 3 — numerator: 2 × 20 × 100 / 120 = 33.33. Step 4 — multiply by resistivity: 33.33 × 12.9 = 430. Step 5 — divide by cross-section: 430 / 5.657 ≈ 76.0. Wait — the formula yields VD% ≈ 7.6%, indicating 12 AWG is undersized for this run; upgrading to 8 AWG (factor ≈ 16) drops the result to ≈ 2.7%, within the NEC 3% limit.
Frequently asked questions
What is an acceptable voltage drop percentage for residential and commercial wiring?
The NEC recommends a maximum of 3% voltage drop on any individual branch circuit or feeder, and no more than 5% combined from the service entrance to the farthest outlet. Keeping within these limits ensures motors, sensitive electronics, and lighting operate within their rated voltage range. Some applications such as data centers and medical equipment specify even tighter limits — as low as 1–2% — to protect equipment and maintain performance.
How does wire gauge (AWG) affect voltage drop over a long run?
Lower AWG numbers indicate larger conductors with less resistance per foot, resulting in less voltage drop for the same current and distance. For example, upgrading from 12 AWG to 10 AWG roughly halves the resistance and the voltage drop. When runs exceed 50–75 feet at typical branch-circuit loads, it is often necessary to upsize the wire by one or two gauges beyond the minimum ampacity requirement to keep voltage drop within acceptable limits.
Why does a three-phase circuit have lower voltage drop than a single-phase circuit with the same wire?
In a three-phase system, power is distributed across three conductors rather than two, and the return currents partially cancel each other in the neutral. The effective resistivity constant in the voltage drop formula drops from 12.9 (single-phase) to 7.46 (three-phase), roughly a 42% reduction. This means a three-phase circuit can deliver the same power over a longer distance with the same wire size while staying within the same voltage drop percentage — one reason three-phase distribution is preferred for long feeders.