Voltage Drop Calculator
Every electrical conductor has resistance, and that resistance causes a voltage drop when current flows through it. On long wire runs, this loss can be significant enough to affect the performance of motors, lights, and electronic equipment. This calculator computes voltage drop using the standard formula: VD = 2 x L x R x I / 1,000, where L is the one-way wire length in feet, R is the conductor resistance per 1,000 feet, and I is the current in amps. The factor of 2 accounts for both the outgoing and return conductors in a single-phase or DC circuit. Results include the voltage drop in volts, the percentage voltage drop relative to the supply voltage, and whether the run meets the NEC informational note recommending no more than 3% for branch circuits and no more than 5% combined for feeders plus branch circuits. Resistance values are sourced from NEC Table 9 for copper and aluminum conductors. Select copper or aluminum and enter your wire gauge, one-way run length, current, and supply voltage to get an instant result.
Voltage drop formula
For a single-phase or DC circuit, voltage drop accounts for both the outgoing and return conductors:
VD (V) = 2 × L (ft) × R (Ω/1,000 ft) × I (A) / 1,000
VD (%) = VD (V) / Vsupply × 100
Aluminum R = Copper R × 1.61
Worked example: AWG 12 copper, 100 ft one-way, 20 A, 120 V
- R for AWG 12 copper = 1.98 Ω/1,000 ft
- VD = 2 × 100 × 1.98 × 20 / 1,000 = 7.92 V
- VD% = 7.92 / 120 × 100 = 6.60% (exceeds NEC 3% recommendation)
- To meet 3%, use AWG 6 or shorten the run to under 46 ft
NEC voltage drop guidance
NEC 2023 informational note to Section 210.19(A)(1) recommends branch circuit conductors be sized so the voltage drop does not exceed 3%. The informational note to Section 215.2(A)(3) recommends that the combined voltage drop for feeders and branch circuits not exceed 5%. These are recommendations, not mandatory code requirements in most US jurisdictions, but represent best practice.
Frequently asked questions
What is voltage drop?
Voltage drop is the reduction in voltage that occurs as current flows through a wire. All conductors have resistance, and when current flows through resistance, some electrical energy is lost as heat. The voltage available at the end of a long wire run is therefore lower than at the source. Excessive voltage drop causes lights to dim, motors to overheat and run inefficiently, and electronic equipment to malfunction.
Why does the NEC recommend a maximum of 3% voltage drop?
The NEC (National Electrical Code) does not mandate a maximum voltage drop but includes a fine print note (informational note) in Section 210.19 and 215.2 recommending that branch circuit conductors be sized so the total voltage drop does not exceed 3%, and that the combined voltage drop for feeders and branch circuits does not exceed 5%. These figures represent good engineering practice rather than a hard code requirement in most jurisdictions. Keeping voltage drop below these thresholds ensures equipment operates within its rated voltage range.
How can I reduce voltage drop?
The most effective ways to reduce voltage drop are: (1) use a larger wire gauge (lower AWG number), which reduces resistance; (2) shorten the wire run by relocating the panel or subpanel closer to the load; (3) increase the supply voltage where permissible (e.g., use 240V instead of 120V for long runs to large loads, since for the same power the current is halved and voltage drop is halved); (4) use multiple parallel conductors for very high-current runs.
What is the difference between copper and aluminum conductors?
Copper has lower resistivity than aluminum, meaning a copper conductor has less resistance per foot than the same-size aluminum conductor. Aluminum's resistivity is approximately 61% higher than copper. For the same ampacity, aluminum conductors must be one or two sizes larger than copper. Aluminum is lighter and less expensive per foot, making it common for large service entrance and feeder conductors. For branch circuits, copper is standard. Connections to aluminum conductors require special care to prevent oxidation and use connectors rated for aluminum.
What happens if voltage drop exceeds 5%?
Voltage drop above 5% on a combined feeder and branch circuit can cause motors to draw excess current and overheat, reduce lighting output, cause variable-speed drives and sensitive electronics to malfunction or fault, and in extreme cases void equipment warranties. For motors, NEC Article 430 requires that the terminal voltage be within the nameplate range. Many motor nameplates allow plus or minus 10% of rated voltage, but some equipment is more sensitive. Consult equipment specifications and an electrician if you have a long run with significant loads.
Sources
- NFPA 70: National Electrical Code (NEC) 2023, Table 9 and Informational Notes to Sections 210.19 and 215.2: nfpa.org.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. For reference only. Consult a licensed electrician for installation decisions. See our methodology.