Inputs and units
Length is in feet and current is in amps. Select a copper AWG size and nominal system voltage. Use the current that actually flows through the run, not simply the number printed on its breaker. The resistance values are fixed reference values; the tool does not ask for conductor temperature.
How the calculation works
Drop (V) = 2 × one-way feet × amps × resistance (Ω/1,000 ft) / 1,000
Drop (%) = 100 × drop / supply voltage
Load voltage = supply voltage − dropThe factor of two assumes the outgoing and return conductors have the same size and length. A larger conductor reduces resistance; a shorter run or smaller current reduces the loss. The same voltage loss takes a larger percentage of a 12 V supply than of a 48 V supply. The tool flags a drop above 3% as a comparison target, not as a universal pass/fail rule.
Worked example
For 20 ft one way, 10 A and 10 AWG copper, the table uses 0.999 Ω per 1,000 ft. Drop is 2 × 20 × 10 × 0.999 ÷ 1,000 = 0.3996 V. On a 12 V supply that is 3.33%, leaving about 11.60 V at the load. Changing to 8 AWG, at 0.628 Ω per 1,000 ft, reduces the estimate to 0.2512 V or 2.09%.
Assumptions and limits
This page covers DC resistance loss. The catalogue also offers higher voltage selections, but the method does not include AC reactance, power factor, three-phase geometry, connector resistance or a chassis return. Actual copper resistance rises as the conductor heats. The displayed ampacity column is a simplified reference: installation method, insulation, terminals, ambient temperature, bundling and applicable rules still matter.
References and calculation details
- Cable selection and voltage-drop tools — Southwire
- Ohm’s law — OpenStax
- View the calculator’s calculation code
Page prepared September 18, 2026. Examples describe this calculator’s implementation; reference links do not imply independent certification.