Voltage Drop Calculator
Use this voltage drop calculator to estimate the volts lost along a copper or aluminum cable. For DC and single-phase, VD = 2 × I × R × L ÷ 1000; for three-phase, use √3 instead of 2 (L is one-way length in feet, R in ohm per 1000 ft). Pick DC, single-phase or three-phase, enter voltage, current (or watts), one-way length and wire size, then check the percent drop against your limit and see the smallest wire that passes. Enter one-way length only—the formula already accounts for the return path on DC and single-phase.
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Voltage drop (V)
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Voltage drop (%)
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Voltage at load (V)
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Result
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Current used (A)
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Resistance used (ohm per 1000 ft)
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Smallest listed size within the limit
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Estimate only. Uses DC resistance at 75 °C for stranded uncoated copper and aluminum from NEC Chapter 9, Table 8. It ignores reactance, power factor, temperature correction and conduit effects, and is not a substitute for the NEC, local code or a licensed electrician. VD = 2 × I × R × L ÷ 1000 (DC, single-phase); VD = √3 × I × R × L ÷ 1000 (three-phase); L is one-way length in feet, R in ohm per 1000 ft. Power to current: I = W ÷ V (÷ √3 for three-phase, unity power factor). Common guidance is 3% on a branch circuit and 5% overall.
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How it works
- What is the voltage drop formula?
- For DC and single-phase AC, VD = 2 × I × R × L ÷ 1000, where I is current in amps, R is the conductor resistance in ohms per 1000 ft and L is the one-way length in feet. For three-phase, replace 2 with √3 (about 1.732). Percent drop = VD ÷ source voltage × 100. Example: 120 V, 20 A, 100 ft one-way, 12 AWG copper (1.98): 2 × 20 × 1.98 × 100 ÷ 1000 = 7.92 V, which is 6.6%. Enter one-way length only; the 2 (or √3) already covers the return path.
- What is an acceptable voltage drop?
- A common guideline, found in NEC informational notes rather than as a hard requirement, is a maximum of 3% on a branch circuit or feeder and 5% for feeder and branch circuit combined. Some equipment, local rules or project specs are stricter, so this calculator lets you change the limit (default 3%). Motors and sensitive electronics often need a tighter limit than the 3% guideline.
- How does cable length affect voltage drop?
- Drop grows in direct proportion to length when wire size and current stay the same. A 20 A load on 12 AWG copper drops 3.96 V (3.3% of 120 V) over 50 ft one-way and 7.92 V (6.6%) over 100 ft. Halving length halves the drop; doubling current doubles it. To keep drop in check on long runs, use a larger wire.
- Is copper or aluminum better for voltage drop?
- Copper has lower resistance. At 12 AWG, the table value is 1.98 ohm per 1000 ft for copper and 3.25 for aluminum, so aluminum drops about 64% more at the same size and current. Aluminum generally needs to be about two AWG sizes larger to match copper's resistance. Matching copper AWG one-for-one with aluminum is a common mistake—check both ampacity and voltage drop.
- Why is the one-way cable length doubled in the formula?
- In a DC or single-phase circuit, current flows out on one conductor and returns on the other, so the current travels twice the one-way distance. That is the 2 in 2 × I × R × L. In a balanced three-phase circuit the line-to-line drop uses √3 instead of 2, which reflects the phase relationship between the conductors. If your measured length is already round-trip, halve it before entering it here or you will double-count.