semkuVoltage drop calculator
Estimate voltage drop for your wire and run.
Calculation rules
For DC and single-phase, drop = 2 × current × resistance × one-way feet / 1,000; for balanced three-phase, line-to-line drop = √3 × current × resistance × one-way feet / 1,000. Drop percent = drop / supply voltage × 100; load voltage = supply voltage − drop.
Resistance at 75 °C is calculated as ρ₂₀ / circular mils × (1 + α₂₀ × 55) × 1,000 × construction factor. Copper uses ρ₂₀ = 10.371 Ω·cmil/ft and α₂₀ = 0.00393 from NBS Handbook 100, Copper Wire Tables (1966); aluminum uses 17.002 and 0.00403 from NBS Handbook 109, Aluminum Wire Tables (1972). Solid uses factor 1; stranded uses 1.02. The copper adjustment follows Handbook 100; applying the same adjustment to aluminum is this calculator's assumption.
The model uses DC resistance at 75 °C, a resistive load with power factor 1, and balanced loading for three-phase. It excludes AC reactance, conduit and installation effects; actual drop can be greater for inductive loads, larger conductors or unbalanced three-phase loads.
The size search checks unrounded drop against the entered target, including exact equality. It searches 14 AWG through 4/0 AWG for the selected metal, using the selected construction through 8 AWG and stranded construction for larger sizes. It checks voltage drop only, not ampacity, protection or installation requirements. The 3% and 5% choices are comparison targets, not compliance decisions.
Volts and percentages display two decimal places; resistance displays four significant digits. Halfway values round up in magnitude, once from unrounded values. Rounded resistance is not reused in calculations. The prefilled circuit is an example.
Frequently asked questions
- Is the length one-way or round-trip?
- Enter one-way length; the DC and single-phase formula includes the return path.
- What voltage should I enter for three-phase?
- Enter line-to-line voltage; the result assumes balanced loading and reports line-to-line drop.
- Does the smallest size mean the wire is suitable to install?
- It meets only the entered voltage-drop target in this model; ampacity, protection, installation conditions and code compliance are not checked.
- Why can another calculator give a different result?
- Resistance data and AC or installation assumptions can differ; the resistance used here is shown with the result.
Sources
NBS Handbook 100, Copper Wire Tables (1966)
NBS Handbook 109, Aluminum Wire Tables (1972)
NBS conductor basis · checked
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Calculations run in your browser without sending your calculation inputs or results to the server. See “Privacy on this page” for information about access data.