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Voltage drop calculator

7.88 V drop (6.57%)112.12 V at the load · 12 AWG copper
Over 3% — voltage drop needs 8 AWG or larger (check ampacity too)
Wire
12 AWG copper · 6,530 cmil
Resistance per 1,000 ft
1.9701 Ω
Voltage drop
7.88 V (6.57%)
Volts at the load
112.12 V
Smallest size for 3% drop (check ampacity separately)
8 AWG (2.60%)

These sketches are here to help you order material and price the work quickly. They are not engineered or shop drawings — always build from your approved project drawings.

System voltage
Phase
Conductor
Target drop

DC resistance at 75 °C, stranded wire. This checks voltage drop only — it does not size wire for ampacity (NEC 310.16).

Show the math▾
  1. Wire area (circular mils)
    (5 × 92^((36 − AWG) ÷ 39))² circular mils
    12 AWG = 6,530
  2. Resistance per 1,000 ft
    K ÷ circular mils × 1.02 stranding × 1,000
    12.61 ÷ 6530 × 1.02 × 1,000 = 1.9701
  3. Voltage drop
    2 × one-way ft × amps × Ω per 1,000 ft ÷ 1,000
    2 × 100 × 20 × 1.9701 ÷ 1,000 = 7.88
  4. Percent drop
    drop ÷ source volts × 100
    7.88 ÷ 120 × 100 = 6.57%
  5. Volts at the load
    source − drop
    120 − 7.88 = 112.1

Where the numbers come from

  • Wire area (AWG): d = 0.005" × 92^((36 − AWG) ÷ 39); circular mils = (d in mils)²; 1/0 = AWG 0, 4/0 = AWG −3; kcmil sizes are thousands of circular milsThe AWG definition (ASTM B258). 12 AWG = 6,530 cmil, 4/0 = 211,600 cmil — the same areas NEC Chapter 9 Table 8 lists.
  • Resistance at 75 °C: K = 12.61 Ω·cmil/ft copper, 20.77 aluminum; × 1.02 for strandingWorked out from the conductivities NEC Chapter 9 Table 8 says it used (Note 4: copper 100% IACS, aluminum 61%): 10.371 Ω·cmil/ft at 20 °C ÷ IACS × (1 + α × 55 °C), α = 0.00393 copper, 0.00403 aluminum. Every size lands within 0.5% of Table 8's stranded uncoated-copper and aluminum columns (e.g. 12 AWG Cu 1.97 vs 1.98; 4/0 Al 0.1001 vs 0.100). DC resistance only — reactance is ignored, which understates drop on big AC feeders at low power factor (Table 9 covers that).Source
  • Voltage drop: Single-phase: 2 × L × I × R ÷ 1,000 · Three-phase: √3 × L × I × R ÷ 1,000 (L = one-way feet, R = Ω per 1,000 ft)Ohm's law over the wire. Single-phase current goes out and back (2 × L); balanced three-phase line-to-line drop is √3 × the drop in one wire.
  • 3% / 5% target: 3% on a branch circuit or feeder, 5% total for both — a recommendation, not a requirementNEC 210.19(A) Informational Note No. 4 (branch circuits) and 215.2(A) Informational Note No. 2 (feeders) in the 2017–2020 editions; the numbering moves in later editions. Informational notes are not enforceable code. This calculator does NOT size wire for ampacity (NEC 310.16), terminal temperature or derating — check those, and have a licensed electrician verify.

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Volts lost in a run of copper or aluminum wire, and the smallest size that keeps the drop under 3% or 5%.

Read this first

This is a planning estimate from DC resistance at 75 °C. It ignores reactance and power factor, so it is fine for small branch circuits but reads low on large AC feeders. It does not size wire for ampacity (NEC 310.16), terminal ratings or derating. Always check ampacity, follow the code your area has adopted, and have a licensed electrician verify the design.