How this calculation works
Multiply the one-way length by two for outgoing and return conductors. Convert kcmil to circular mils before dividing. The approximate coefficients 12.9 for copper and 21.2 for aluminum are stated assumptions in ohm·circular-mil per foot. Compare the calculated drop with the actual entered supply voltage rather than a hidden fixed 120 V reference.
Resistive drop V = 2 × K × current A × one-way feet / circular-mil areaA worked example
At 10 A, one-way distance 100 ft and copper coefficient 12.9, a 10 kcmil conductor gives 2×12.9×10×100/10000 = 2.58 V. Relative to 120 V, that is 2.15%. Entering 10 circular mils instead of 10 kcmil would be a thousand-fold area error.
Good to know
A resistive approximation, not a conductor-sizing, ampacity or wiring-compliance tool. AC mode does not model reactance or non-unity power factor. Actual resistance varies with temperature and conductor construction. No safe-size recommendation or threshold approval is inferred.
Reference: Southwire: voltage drop resources and calculator scope (opens in a new tab) ↗
One-way distance becomes a two-conductor path
The entered distance describes one direction from source to load. A two-conductor circuit includes an outgoing and a return path, so the formula uses twice that distance. If you enter the entire round-trip length as though it were one-way, the calculation doubles the path again and overstates the model drop.
A circular mil is an area unit, not a millimetre. One kcmil is one thousand circular mils. The original page blurred those labels; the rebuilt form requires the area unit explicitly. The Ohm’s law calculator explains the voltage-current-resistance relation underneath this path calculation.
An approximate coefficient is not a wiring approval
The K values are approximate resistivity assumptions in ohm·circular-mil per foot. Material temperature, construction and connections affect actual resistance. In AC mode this page still uses a resistive approximation rather than calculating complex impedance or power factor. The plotted line varies distance while holding those assumptions fixed.
The LED series resistor calculator estimates a different resistor problem and also distinguishes arithmetic dissipation from rated hardware. Here, even a small percentage drop does not establish allowable current, protection or code compliance. If the model drop exceeds the entered source voltage, the negative subtraction is displayed as a sign that the assumed fixed-current operating model is not physically useful.
A few questions, answered
Should distance include both directions?
No. Enter one-way distance; the formula supplies the factor of two.
Is 10 kcmil equal to 10 circular mils?
No. It equals 10,000 circular mils.
Does a small drop approve a conductor size?
No. Ampacity, protection and installation requirements are separate.