This voltage drop calculator computes the voltage lost along a wire run — single-phase AC/DC with the standard K-factor formula — and flags it against the 3% branch-circuit guideline.

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Long Runs Tax Voltage — Wire Size Is the Rebate

Sizing the backup supply at the far end of such runs is our Generator Size Calculator. Part of our Construction Calculators collection.

Worked Example (Verified)

  • 20 A at 120 V over 100 ft on 12 AWG copper (defaults): VD = 2×12.9×20×100 ÷ 6,530 = 7.90 V = 6.58% → only 112.1 V arrives — a workshop subpanel feeder this is not; 10 AWG improves it to 4.97 V (4.14%), 8 AWG to 3.13 V (2.60%, inside the line)
  • Why barns and well pumps are the classic victims: distance × current conspire, and motors draw MORE current as voltage sags, worsening the drop that caused it
  • DC/solar note: at 12 V the same 7.9 V drop is catastrophic in percent (66%) — low-voltage systems use fat, short cables for this arithmetic reason

Drop Discipline

  • NEC guidance (informational, not a hard limit): ≤ 3% branch, ≤ 5% feeder + branch combined.
  • Ampacity and voltage drop are separate checks: a wire can be legal on heat and still too small for the distance.
  • Three-phase drops use √3 instead of 2 as the multiplier and line-to-line CM conventions — same physics, different factor.
  • Aluminum conductors need roughly two AWG sizes larger for the same drop (K ≈ 21.2).

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Frequently Asked Questions

How do you calculate voltage drop?

Single-phase: VD = 2 × K × I × L ÷ CM, with K = 12.9 for copper, I in amps, L the one-way length in feet, and CM the wire's circular-mil area (12 AWG = 6,530). The defaults give 7.90 V — 6.58% of 120 V. This calculator also states the delivered voltage.

What voltage drop is acceptable?

The NEC recommends (as fine-print guidance) no more than 3% on a branch circuit and 5% total including the feeder. Motors, compressors and electronics are happiest inside those bands; lighting merely dims. Long rural runs are where designs blow the budget.

Why does wire gauge matter so much?

Resistance is inverse to cross-section: each AWG step changes area by ~26%, and three steps double/halve it. Going 12 → 8 AWG in the example cuts the drop from 7.90 V to 3.13 V — copper is cheaper than burned-out pump motors.

Does voltage drop waste energy?

Yes: the dropped volts × current becomes heat in the wire (7.9 V × 20 A ≈ 158 W warming the conduit). It's both a performance problem and a small continuous energy bill — upsizing pays back on long, loaded runs.

Is the formula different for DC?

The same 2·K·I·L/CM applies to DC circuits (no reactance to speak of at these scales) — and matters more, because low-voltage DC systems (12/24/48 V) suffer huge percentage drops. Solar and RV wiring live and die by this page's arithmetic.

Is this voltage drop calculator free?

Yes — free, instant, no sign-up.