Energy & Workshop Math

Voltage Drop Calculator

Estimate conductor voltage drop from length, current, supplied resistance and power factor.

Units

Your measurements

Results update as you type.

Circuit type

One-way circuit length

Load current (A)

Supply voltage (V)

Three-phase: line-to-line RMS voltage.

Reference conductor (AWG)

Southwire SPEC10010 copper or SPEC10015 aluminum. DC resistance adjusted to 75°C; AC resistance at 75°C and reactance at 60Hz. Edit the data for your actual cable.

Resistance per conductor (Ω/m)

Reference product data can be edited. Match resistance temperature and AC reactance to the actual cable and installation.

Extra options & cost (optional)

Fractions and number formats

Use decimals (12.5), fractions (12 1/2 or 12-1/2), or explicit lengths (12ft 6in, 12′ 6″, 25 mm). A comma followed by one or two digits is a decimal comma: 12,5 = 12.5. Grouped commas mean thousands: 1,200 = 1200. Use a decimal point for ambiguous values. Scientific notation is not accepted.

Enter your measurements

Results update as you type.

About this calculator

Compare volts and percentage lost along a DC, single-phase or balanced three-phase run, using the displayed per-conductor data. The tool makes no gauge recommendation. Named reference data is product- and condition-specific; actual installation design is separate.

How to measure

Measure the one-way route length from supply to load; the model applies the appropriate circuit factor.
Supply current, supply voltage and conductor resistance in the selected resistance-per-length unit.
AC modes additionally use reactance and lagging power factor. Use matched conductor data; this is not wire-size or code-compliance advice.

How to use this calculator

Choose DC, single-phase or balanced three-phase.
Enter the one-way length, current and supply voltage.
Enter the per-conductor resistance — and reactance for AC.
For AC, enter the lagging power factor.
Calculate for drop in volts and percent.

Formula

DC drop = 2 × length × current × resistance

AC single phase ≈ 2 × length × current × (resistance × PF + reactance × sin(acos(PF)))

AC three phase ≈ √3 × length × current × (resistance × PF + reactance × sin(acos(PF)))

Drop % = 100 × drop ÷ supply

A drop at or above the supply voltage is outside the model

Assumptions

Reference data is for the named Southwire product series at 75°C. Its 60 Hz reactance assumes the cited non-ferrous conduit and conductor spacing; other cables, frequencies and arrangements require matched data.
AC reactance is at 60Hz; AC estimates use a balanced, lagging-power-factor approximation.
The simplified AC projection is supported only when the total series-voltage magnitude is at most 10% of supply. This is a software model limit, not a code or safety limit. Outside it, full circuit/phasor analysis is required.

Reference copper examples

Reference copper examples
Gauge	AC resistance at 75°C (Ω/1,000 ft)
14 AWG	3.265
12 AWG	2.063
10 AWG	1.291

Southwire SPEC10010 Table 2, checked 8 November 2026. This table does not certify ampacity.

Common mistakes

Entering round-trip length where one-way length is requested.
Mixing resistance temperature or reactance conditions.
Treating a voltage-drop gauge comparison as an ampacity check.

Worked example

Try it: press “Load example” above the form.

20 m one-way at 10 A on the reference 12 AWG copper AC data (2.063 Ω/1000 ft), PF 1, drops about 2.707 V: 1.177% of 230 V.

Limitations

Educational voltage-loss estimate only. Use cable data matched to the stated conditions. This tool does not select a conductor or check ampacity, protective devices or installation compliance. The simplified AC projection is supported only when the total series-voltage magnitude is at most 10% of supply. This is a software model limit, not a code or safety limit. Outside it, full circuit/phasor analysis is required.

Frequently asked questions

Does this select a safe wire gauge?

No. This calculator reports voltage loss for the conductor data supplied. It does not select a conductor or check ampacity, protective devices, temperature, terminations or local installation requirements.

Why does DC use different resistance from AC?

The reference data provides DC resistance at 25°C and AC resistance at 75°C. This tool adjusts the DC value to 75°C with the manufacturer’s temperature formula; AC mode uses the cited AC resistance and 60 Hz reactance.

What does the worked example show?

20 m one-way at 10 A on the reference 12 AWG copper AC data (2.063 Ω/1000 ft), PF 1, drops about 2.707 V: 1.177% of 230 V.

Which assumptions should I check before using the result?

Educational voltage-loss estimate only. Use cable data matched to the stated conditions. This tool does not select a conductor or check ampacity, protective devices or installation compliance.

Reviewed by Codex AI editorial agent (Editorial, 2026-11-12) · Codex independent specialist-review AI agent (educational arithmetic/source review; not licensed human approval) (Electrical, 2026-11-12).

Sources

Sources explain the methods and any named product presets. Check your product label where coverage, yield or density differs.

ABB: electrical handbook
Voltage-drop mathematical context, section 2.2.2. Historical handbook, not current code authority or wire-sizing guidance.
OpenStax: electrical power and energy
DC power and energy relationships. AC/runtime assumptions are stated separately and require appropriate domain review.
Southwire SPEC10010: copper electrical data, Table 2
Reference copper product series. DC resistance at 25°C, AC resistance at 75°C, inductive reactance at 60 Hz. Checked 8 November 2026.
Southwire SPEC10015: aluminum electrical data, Table 2
Reference aluminum product series. Listed sizes only. Not a generic NEC ampacity or code approval. Checked 8 November 2026.
Southwire: Power Cable Manual, fourth edition
Historical manufacturer reference. General resistance-versus-temperature discussion: chapter 2, printed pages 2-16–2-17. The cited product data and operating assumptions do not establish current local-code compliance or complete cable design.

Calculation documentation · Updated 2026-11-12 · Version 1.6.4. Estimates are planning aids. Confirm quantities, coverage and prices with your supplier before purchasing. Found an error? Report it.

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