Voltage drop calculator
Calculate voltage drop and verify the NEC 3% guideline for any circuit. Enter wire length, current, gauge, and system voltage — results update live.
How to calculate voltage drop
Voltage drop is the reduction in voltage across a conductor caused by its resistance. Every wire has resistance, and when current flows through it, some voltage is consumed before reaching the load. The formula for single-phase circuits is: VD = (2 × L × R × I) ÷ 1000, where L is the one-way wire length in feet, R is the resistance in ohms per 1,000 feet, and I is the load current in amps. The factor of 2 accounts for both the hot and neutral conductors carrying current over the full run.
Aluminum wire has about 61% higher resistance than the same gauge copper conductor. A 12 AWG copper wire has 1.98 ohms per 1,000 ft; 12 AWG aluminum has 3.18 ohms per 1,000 ft. For this reason, when aluminum conductors are used (common in large feeders and service entrances), upsizing by 1–2 AWG is typically required to achieve the same voltage drop performance as copper.
NEC voltage drop guidelines
NEC 210.19(A) contains an informational note recommending that voltage drop on branch circuits not exceed 3% and that total voltage drop from service entrance to the farthest outlet not exceed 5%. While these are recommendations rather than hard requirements, most electrical inspectors and engineers treat 3% as the de facto standard for branch circuit design. Equipment manufacturers often specify that their warranty requires voltage within 5% of rated voltage.
The most common reason contractors need to upsize wire gauge is when running circuits to detached structures (shops, garages, barns), HVAC equipment at the back of a building, or any load at the end of a long residential run. A 20A circuit to a garage 100 ft away on 12 AWG copper at 120V will see about a 3.3% drop — just over the guideline. Upsizing to 10 AWG brings that to 2.1%, well within spec.
Frequently asked questions
What is the maximum voltage drop allowed by NEC?
NEC 210.19(A) recommends maximum 3% voltage drop on branch circuits and 5% total from the service entrance to the load. These are recommendations, not hard code requirements, but most inspectors and engineers treat 3% as the practical limit for branch circuits. Higher voltage drop reduces equipment efficiency and can cause motors to overheat.
How do I calculate voltage drop for a long circuit?
The formula for single-phase voltage drop is: VD = (2 × L × R × I) / 1000, where L is the one-way wire length in feet, R is the wire resistance in ohms per 1000 feet, and I is the current in amps. For three-phase, replace the factor of 2 with 1.732. Then divide by system voltage to get the percentage. This calculator does all that math automatically.
What wire gauge should I use for a 100 amp subpanel?
For a 100A subpanel, NEC requires minimum 2 AWG copper or 1/0 AWG aluminum conductors (based on ampacity). However, for runs over 100 feet, voltage drop may require you to upsize to 1 AWG copper or 2/0 AWG aluminum to stay within the 3% drop guideline. Use this calculator with your specific run length to verify.
Does voltage drop matter for short runs?
For runs under 50 feet at typical residential currents (15–20A), voltage drop is rarely a concern — it stays well under 1%. Voltage drop becomes important for long runs to detached garages, outbuildings, well pumps, HVAC equipment, and commercial spaces. Any run over 100 feet or carrying significant load should be verified with a voltage drop calculation.
Ready to send this estimate to your customer?
EstimateLock locks your price so they commit before they shop around.
Send a locked estimate →