How the calculator works

Every number on the calculator comes from the formula below. Here is each term, where it comes from, and what is left out.

Formula

V = 2 × K × I × L ÷ CM (single-phase and DC)

Percent drop = V ÷ supply voltage × 100. Voltage at the load = supply voltage − V.

Smallest size = the first size in the table whose CM is at least 2 × K × I × L ÷ (target ÷ 100 × supply voltage).

Constants

Constant	Value	Basis and source status
K, copper	12.9	Ohm·CM per foot at 75 °C, the constant used in the NEC voltage drop informational note. Verify against NEC Chapter 9 before launch.
K, aluminum	21.2	Same basis, aluminum. Verify before launch.
Return conductor	2	The loop is twice the one-way length.

Conductor areas

AWG areas are computed from the AWG diameter formula (diameter in mils = 5 × 92^((36 − n) ÷ 39), area = diameter²). kcmil sizes use their nominal circular-mil value. The calculator does not copy any code table. Sizes covered: 14 AWG, 12 AWG, 10 AWG, 8 AWG, 6 AWG, 4 AWG, 3 AWG, 2 AWG, 1 AWG, 1/0 AWG, 2/0 AWG, 3/0 AWG, 4/0 AWG, 250 kcmil, 300 kcmil, 350 kcmil, 400 kcmil, 500 kcmil.

Targets

The 3% and 5% figures are the NEC informational-note guidance for branch and total drop. They are not mandatory limits unless your authority adopts them. The custom option accepts any target from 0.1% to 10%.

What the calculator does not check

Ampacity against code tables. You enter the value from your code book.
Temperature derating and conduit fill.
Power factor and motor starting current.
Three-phase circuits.
Code compliance, permits or local amendments.

Review status

The formula, constants and worked examples have not yet been reviewed by a licensed electrician. This page will name the reviewer and the date once a review is done. Until then, treat results as a check of your own working, not as a design.

Changes

Date	Change
2026-11-05	New design: the answer and a diagram of the run come first, voltage presets, copy result and share link. The formula and constants did not change.
2026-11-05	First version published.

How to calculate voltage drop · Ampacity vs voltage drop · The calculator