Voltage Drop Calculator
Voltage drop for copper or aluminum wire from 18 AWG to 1000 kcmil or 1.5–240 mm²: drop in volts and percent, voltage at the load, wire loss, minimum wire size, maximum run length and an NEC ampacity check.
How it works
V_drop = k × L × (ρ ÷ A) × I ÷ n (k = 2 for DC / single-phase, √3 for three-phase; L one-way length; ρ = 0.017241 Ω·mm²/m copper, 0.028264 aluminum at 20 °C, × (1 + α(T − 20)); n parallel sets) · % drop = V_drop ÷ V × 100 · Minimum area A = k × L × ρ × I ÷ (V_allowed × n) · Maximum length L = V_allowed ÷ (k × R × I)
Current flowing through a wire’s resistance loses voltage along the way. On a DC or single-phase circuit the current goes out on one conductor and back on the other, so the one-way length is counted twice; on a balanced three-phase circuit the factor is √3 and the result is the line-to-line drop. Resistance comes from the conductor’s cross-section and metal — copper at 0.017241 Ω·mm²/m, aluminum (61% IACS building wire) about 64% higher — and rises with temperature, about 22% between 20 °C and 75 °C. Pick a mode: the voltage drop for a given size and length, the minimum wire size that stays within your allowed drop (both AWG/kcmil and metric mm² are reported), or the maximum run length for a size. Load can be entered in amps or watts, and under More options you can set parallel conductors, the conductor temperature (75 °C reproduces NEC Chapter 9 Table 8 values) and the ampacity column used for the NEC Table 310.16 check. Each answer comes with the worked steps, a comparison table of every size and a chart of the sizes around yours. Electricians usually keep branch circuits under about 3% and feeder plus branch under 5%; ampacity, which is about heating, is a separate limit and both must be met.
Worked example
A 20 A load on a 100-foot single-phase run of 12 AWG copper: the conductor is 3.309 mm², so its resistance is 0.017241 ÷ 3.309 = 0.00521 Ω/m (1.588 Ω per 1,000 ft). The drop is 2 × 30.48 m × 0.00521 Ω/m × 20 A = 6.35 V, which is 5.29% of 120 V, leaving 113.65 V at the load. That is over 3%, and the minimum-size mode shows the minimum area is 5.84 mm², so 8 AWG (8.37 mm², 2.51% drop) is the smallest AWG size that meets the 3% target; 10 AWG (3.33%) narrowly misses. At the conservative 75 °C basis, the same 12 AWG run drops 7.73 V (6.44%).
Sources: NFPA 70, National Electrical Code — Table 310.16 and Chapter 9 Table 8 · NIST — Copper wire tables (Handbook 100)
Frequently asked questions
Do I use one-way or round-trip length?
Enter the one-way distance from source to load. The factor of 2 (or √3 for three-phase) already accounts for the current travelling out and back through the conductors.
What voltage drop is acceptable?
Informational notes in the US National Electrical Code suggest no more than 3% on a branch circuit and 5% for feeder and branch circuit together. They are recommendations for efficiency and performance rather than hard rules, but sensitive equipment, long runs and motors make them worth following.
How do I find the right wire size for a long run?
Choose “Minimum wire size”, enter the current, one-way length, voltage and allowed drop. The calculator works out the minimum cross-section, then reports the first AWG/kcmil and metric sizes that are large enough, plus the smallest AWG size that also meets the NEC ampacity for the current.
Why does aluminum wire need to be bigger?
Aluminum conducts about 61% as well as copper, so for the same voltage drop it needs roughly 1.6 times the cross-section — usually about two AWG sizes larger. It is lighter and cheaper, which is why it is common for feeders and service entrances.
Why do different calculators give slightly different answers?
Mostly the resistance basis. Many use DC resistance at 20 °C (this calculator’s default); NEC Chapter 9 Table 8 lists values at 75 °C, which are about 22% higher; and AC calculators for large conductors add reactance from NEC Table 9. Choose the conductor temperature under More options to match the method you need.
Does voltage drop affect ampacity?
No — they are separate checks. Ampacity is how much current a conductor can carry without overheating its insulation (NEC Table 310.16); voltage drop is about the voltage that reaches the load. A short run is usually limited by ampacity and a long run by voltage drop, so check both.
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This calculator is for educational purposes. Double-check important results before acting on them.