NEC Voltage Drop Chart

AWG and kcmil to circular mils chart

Circular mils (CM) describe a conductor's cross-sectional area. These are the exact NEC Chapter 9, Table 8 values (not the AWG-diameter approximation formula), the same table this site's voltage drop calculator uses internally.

Maximum one-way distance for 3% drop, 120V single-phase, copper

These figures are calculated with the same formula as the calculator (VD = 2 × K × I × L ÷ CM, K = 12.9 for copper), solved for the maximum one-way length L that keeps the drop at or under 3% of a 120V supply. Cells marked with a dash are current levels not typically paired with that wire size on a standard branch-circuit breaker (small-conductor overcurrent limits), they are left out to avoid implying a combination that would not be code-compliant regardless of voltage drop. Actual code-required ampacity depends on insulation rating and installation method and is a separate question from voltage drop, always confirm ampacity separately.

At 240V, the same wire size and current combination allows roughly double the distance above, because the same volts-dropped is a smaller percentage of a larger supply voltage. This is the same relationship covered in the calculator's "why supply voltage matters" logic.

How to read this chart

Pick the row for your wire size and the column closest to your load current, the distance shown is the farthest you can run that wire, one-way, before the drop reaches 3% at 120V. If your actual run is longer than the listed distance, either move up a wire size (a lower AWG number) or expect a drop above 3%, which may still be acceptable depending on the rest of the circuit, this is exactly what the voltage drop calculator works out precisely for your own current, distance, wire size, material and supply voltage rather than the fixed steps in this chart. See the voltage drop formula page for exactly how each distance in the table above was derived, including the algebra rearranged to solve for distance directly.

DC and low-voltage charts (12V, 24V)

Low-voltage DC systems, landscape lighting transformers, solar battery banks, RV and marine 12V circuits, are the case where this chart matters most in relative terms, because a few tenths of a volt of drop can already be a meaningful percentage of a 12V or 24V supply. The formula is identical (DC uses the same 2 × K × I × L ÷ CM single-phase-style formula), only the supply voltage changes, which is why a fixed chart at one voltage does not generalize well to 12V work, run your exact current and distance through the calculator instead of scaling a 120V chart by hand.

Why a fixed chart has limits

Any printed chart, including this one, has to fix most variables to fit on a page: one supply voltage, one conductor material, a handful of current steps. Real jobs rarely land exactly on a chart row. A run at 22A instead of 20A, a supply voltage of 208V instead of 120V or 240V, or an aluminum feeder instead of copper, all fall between the lines of a fixed table like this one. The calculator does not have that limitation, since it computes the exact voltage drop and percentage for whatever current, distance, wire size, material, circuit type and supply voltage you actually have, rather than rounding to the nearest chart row, entirely in your browser, see the privacy policy for details. Treat this chart as a fast sanity check or a way to compare a few wire sizes side by side, and use the calculator for the number you actually rely on.