Estimate electrical cable size (AWG) from amps, voltage, run length, conductor material, derating factors, voltage drop, and simplified ampacity limits.
Inputs and method
Enter voltage, current, one-way run length, and candidate-conductor resistance in ohms per 1,000 feet. For a two-conductor path, voltage drop is 2 × current × resistance × one-way length ÷ 1,000; percentage drop divides that result by system voltage.
Calculations retain full floating-point precision until the labeled display step. Counts and packages use an explicit upward ceiling only where a whole purchase unit is required.
Interpreting the estimate
Use the result as a planning value and keep input precision consistent with the measurements or source data you have. Small displayed differences can come from final-place rounding; intermediate values are not rounded.
Assumptions and limits
This worksheet does not choose wire size, determine ampacity, or establish electrical-code compliance. Obtain resistance from current manufacturer data and have electrical work designed and checked for the applicable installation. To plan where cable runs through wall framing, see the stud spacing calculator; for the room or home dimensions behind a load estimate, use the square footage calculator; and for elevated installation work, review the ladder safety calculator.
Empty or invalid inputs, prohibited negatives, zero denominators, impossible relationships, and unsupported options do not produce a potentially misleading numeric result.
Sources
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NFPA 70 National Electrical Code — electrical installation and conductor safety requirements.
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Southwire, “Southwire Voltage Drop Calculator” — The two-conductor voltage-drop relationship (2 × current × resistance × one-way length ÷ 1,000) used by the worksheet, with resistance taken from conductor data.