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3 solve modes · 60/75/90°C · Cu / Al · AC / DC · ft / m · runs in your browser
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Voltage Drop Calculator

Three ways to solve it: find the drop on a given run, the minimum conductor size for a limit, or the maximum distance a conductor can carry. Copper or aluminum, 60/75/90°C, single- or three-phase, AC or DC.

Calculator

Solve a run

NEC voltage-drop limits appear as Informational Notes (recommended practice), not enforceable requirements. Results ignore reactance (steel-conduit effect on larger sizes is small at low voltage). Motor starting and harmonic loads need an engineered study. Quick-reference only — a licensed engineer dictates final values and confirms against the adopted NEC.

How it works

The math behind the result

Single-phase / DC
VD = 2 × K × I × D ÷ CM
Three-phase
VD = 1.732 × K × I × D ÷ CM
K at temperature
Cu: K(T) = 12.9 × (T + 234.5) ÷ 309.5  ·  Al: K(T) = 21.2 × (T + 228.1) ÷ 303.1
Three solve modes
Drop on a given run · smallest size that holds a limit · longest run a size can hold. Same formula, rearranged.
References checked
≤ 3% branch · ≤ 5% feeder + branch (NEC Informational Note practice)
Related tools
Feeder sizing (ampacity) · Conduit fill (raceway)
FAQ

Common questions

What is the formula for voltage drop?
Single-phase: VD = 2 × K × I × D ÷ circular mils. Three-phase: VD = 1.732 × K × I × D ÷ circular mils. DC uses the single-phase form. K is conductor resistivity at the insulation temperature — 12.9 for copper and 21.2 for aluminum at 75°C, scaled up or down for 60°C and 90°C ratings.
What voltage drop does the NEC allow?
The NEC states performance intent in Informational Notes, not enforceable limits. Common engineering practice: 3% maximum for a branch circuit and 5% total for feeder plus branch. The calculator checks both and labels each result against those references.
How does the temperature rating change voltage drop?
Conductor resistance rises with temperature. A 90°C-rated circuit (THHN/XHHW-2) runs about 5% more resistance than a 75°C circuit, and a 60°C circuit about 4% less. Pick the rating that matches your insulation; when in doubt, 75°C is the conservative default.
What is the minimum conductor size for a run?
The smallest AWG/kcmil size whose voltage drop stays within your limit for the given current, distance, and material. Choose ‘Minimum conductor size’ above, set the limit (3% or 5% is typical), and the tool walks the size ladder for you.
How far can I run a given conductor?
Maximum one-way distance = limit × voltage × circular mils ÷ (K × current), with the phase factor (2 for single-phase/DC, 1.732 for three-phase). Choose ‘Maximum circuit distance’ and the tool solves it directly. Ampacity is a separate check — run the feeder sizing calculator too.

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