Voltage Drop Calculator
Estimate DC, single-phase or three-phase voltage drop from current, conductor length, material and cross-sectional area, with a minimum-area target.
- DC/single-phase and three-phase modes
- Copper or aluminum conductor resistivity
- Shows drop %, load voltage, loss and minimum conductor area
Calculator workspace
Enter the known values, calculate, and review the supporting figures and formula instead of relying on a single unexplained number.
What this Voltage Drop Calculator does
Voltage drop occurs because real conductors have resistance. As current flows through a cable, some supply voltage is lost along the conductor and energy is dissipated as heat. This calculator provides a transparent planning estimate for DC/two-wire or single-phase circuits and balanced three-phase circuits using conductor material, cross-sectional area, one-way length and current. It also reverses the calculation to estimate the minimum conductor area needed to stay within a chosen voltage-drop percentage.
The workspace is designed for quick checking as well as repeatable planning. Inputs remain editable, the supporting figures are shown with the headline result, and the calculation runs locally in the browser. That makes it easy to change one assumption at a time, compare scenarios and copy or save a result together with the values that explain it.
Formula and calculation method
R = ρL/A; DC/single-phase drop ≈ 2IR; balanced three-phase drop ≈ √3IR.
Conductor resistance is estimated from R = ρL/A using resistivity at approximately 20°C: about 0.017241 Ω·mm²/m for copper and 0.028264 Ω·mm²/m for aluminum. For a DC or single-phase two-conductor path, voltage drop is approximated as 2IR. For balanced three-phase circuits it is approximated as √3IR using one-way conductor resistance. The minimum-area estimate algebraically solves the same expression for A at your chosen maximum drop.
How to use the calculator
- Choose the calculation mode, shape, unit or known-value combination when the page provides one.
- Enter values that belong to the same problem or measurement scenario and keep units consistent with the field labels.
- Select Calculate and review both the main answer and the supporting metrics shown with it.
- Change one assumption at a time when comparing scenarios; this makes the effect of each input easier to understand.
- Use Copy Result, Share Result or Save Result Image when you need a portable record of the calculation.
Worked example
For a 230 V single-phase circuit carrying 20 A through 30 m of 4 mm² copper conductor, the one-way resistance is roughly 0.129 Ω. The two-conductor voltage drop is therefore about 5.17 V, or 2.25%, leaving roughly 224.8 V at the load under the simplified resistive model.
How to interpret the result
Compare the calculated percentage with the design criterion appropriate to the equipment and installation. A conductor that satisfies voltage drop may still be too small for ampacity, short-circuit protection, installation temperature, bundling or mechanical requirements. Conversely, a code-compliant conductor can still produce undesirable voltage drop over a long run. Use both checks, not one as a substitute for the other.
A calculator result is only as useful as the values entered. For measurements, use appropriately precise source data; for costs, use prices that match the same currency and period; for technical work, compare the output with drawings, equipment data or other authoritative information. The supporting metrics on this page are included specifically to make cross-checking easier.
Important assumptions and limitations
This simplified model ignores AC reactance, power factor, conductor operating temperature, skin effect, harmonic currents, connection resistance and installation-specific correction factors. The “next common metric size” is a convenience list, not a code recommendation. Always verify conductor sizing with applicable electrical standards, manufacturer data and a qualified professional when required.
This calculator is provided for planning, checking, education and general informational use. It makes the formula and assumptions visible, but it does not replace project specifications, professional engineering, financial or medical advice, contractual documents, equipment ratings or applicable standards.
Frequently asked questions
Why is one-way length entered if current returns through another conductor?
The calculator applies the appropriate two-conductor factor for DC/single-phase circuits, so you enter only the physical one-way run length.
Does a 3% result mean the cable is code compliant?
Not necessarily. Voltage drop is only one design consideration; ampacity, protection, temperature and local electrical rules must also be checked.
Why do copper and aluminum give different results?
Their electrical resistivities differ, so the same length and cross-sectional area produce different resistance and voltage drop.