Voltage Drop Calculator

Calculate voltage drop in volts and percent for DC, single-phase and three-phase circuits from wire size, material, current and one-way length.

Checks the result against the NEC 3% and 5% guidance and finds the smallest wire that stays under 3%.

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Results will appear here once you enter your values.

The formula this calculator uses

Voltage drop is Ohm's law applied to the wire itself. The calculator uses the DC resistance at 75°C from NEC Chapter 9, Table 8, expressed in ohms per 1,000 feet:

  • DC and single-phase: VD = 2 × I × R × L ÷ 1,000
  • Three-phase: VD = 1.732 × I × R × L ÷ 1,000

L is the one-way length. The 2 accounts for current flowing out on one conductor and back on the other; in a balanced three-phase circuit the phase currents are 120° apart, so the multiplier becomes √3. With parallel sets, the drop is divided by the number of sets. Percent drop is VD ÷ source voltage × 100.

Many textbooks use the equivalent K-method, VD = 2 × K × I × L ÷ CM, with K ≈ 12.9 Ω·cmil/ft for copper and 21.2 for aluminum. K is just resistance × circular mils, so the two give nearly the same answer.

Worked example: 16 A at 120 V over 100 feet

A 120 V, 16 A load sits 100 ft from the panel on 12 AWG copper. Table 8 gives 1.93 Ω per 1,000 ft (6,530 circular mils).

  • VD = 2 × 16 A × 1.93 Ω × 100 ft ÷ 1,000 = 6.18 V
  • Percent drop = 6.18 ÷ 120 = 5.15%, so the load sees about 113.8 V
  • The K-method gives 2 × 12.9 × 16 × 100 ÷ 6,530 = 6.32 V; using AC resistance from Chapter 9 Table 9 (2.0 Ω/kft) gives about 6.4 V. All three put the drop over 5%.

Both methods fail the 3% target. Stepping up to 10 AWG (1.21 Ω/kft) gives 3.87 V or 3.23%, still over. 8 AWG (0.764 Ω/kft) gives 2.44 V or 2.04%, which passes. On a 240 V circuit the same 8 AWG run drops only 1.02%, one reason long runs to well pumps and outbuildings are usually 240 V.

What the 3% and 5% numbers actually mean

The NEC recommends, in informational notes to 210.19(A) and 215.2(A), that branch circuits drop no more than 3% and that feeder plus branch circuit drop no more than 5% combined. Informational notes are not enforceable, so an inspector generally cannot fail a job on them. Enforceable limits do exist for sensitive electronic equipment (647.4(D)) and fire pumps (695.7), and many engineering specifications write 3%/5% in as a contract requirement.

The recommendation matters anyway. Motors and compressors draw more current at low voltage and run hotter, LED drivers may flicker, and every volt lost is energy paid for and turned into heat in the wire.

Common mistakes

  • Entering the round-trip length. The formula already doubles the length. Entering 200 ft for a 100 ft run doubles the answer.
  • Sizing by drop alone. A wire that passes voltage drop must also carry the current. Check ampacity under Table 310.16 and the 240.4(D) limits of 15, 20 and 30 A for 14, 12 and 10 AWG copper.
  • Ignoring reactance on big AC feeders. Resistance-only math is close for small conductors but understates drop for large conductors in steel conduit at low power factor. Use NEC Chapter 9 Table 9 impedance for those.
  • Low-voltage DC. 12 V and 24 V systems lose a large percentage over short runs. 10 A over 25 ft of 12 AWG copper drops 0.97 V, which is 8% of 12 V.

Frequently Asked Questions

Common questions about the Voltage Drop Calculator

Use the one-way distance from the source to the load. The formula multiplies by 2 (or by 1.732 for three-phase) to account for the return path.

Sources & References

NFPA 70: National Electrical Code (Chapter 9 Table 8, Table 310.16, 210.19, 215.2)

The NEC publishes conductor areas and DC resistance (Chapter 9 Table 8), allowable ampacities (Table 310.16) and the informational notes recommending 3% branch and 5% total voltage drop.
→Source: NFPA

Voltage Drop Calculations

Worked examples of VD = 2 x K x I x D / CM for single-phase and 1.732 x K x I x D / CM for three-phase circuits, including the 6 AWG copper single-phase and 1 AWG aluminum three-phase examples used to check this calculator.
→Source: Mike Holt Enterprises

Southwire Voltage Drop Calculator

Manufacturer voltage-drop and conductor-sizing calculator based on NEC resistance and reactance data, useful for cross-checking large AC feeders.
→Source: Southwire Company

Motor Systems

DOE Industrial Efficiency and Decarbonization Office resources on motor-system efficiency, including the effect of supply-voltage quality on motor performance and losses.
→Source: U.S. Department of Energy

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