Wire & voltage

Voltage Drop Calculator

Voltage drop equals current multiplied by total conductor-path resistance. Resistance rises with length and resistivity and falls as conductor area increases.

Calculate resistive voltage drop, delivered voltage, conductor resistance, and I²R loss for DC, single-phase, or three-phase circuits.

Calculated result

Calculated voltage drop

Calculating…

Calculated locally in your browser

Planning math only. Verify equipment specifications and installation requirements separately.

Voltage Drop Calculator relationship diagram. The illustration supports the text result; it is not a wiring or installation drawing.

What this calculator returns

Compare percent drop with your editable planning target, then verify ampacity, protection, insulation, terminals, installation method, and local rules separately.

Formula and variables

The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.

Formula

Vdrop = I × Rpath; R = ρ × L / A. For a balanced three-phase resistive circuit, the path factor is √3 instead of 2.

Vdrop: Voltage lost across the conductor path, in volts.
I: Current through the conductor, in amperes.
R: Electrical resistance of the modeled conductor path, in ohms.
ρ: Material resistivity at the modeled temperature, in ohm-metres.
L: Conductor length used by the selected path model.
A: Metal cross-sectional area of the conductor.

I is current, ρ is material resistivity adjusted for temperature, L is one-way length, and A is conductor area.

Worked example

Worked example inputs
Input	Value
System type	DC
Source voltage	120 V
Load current	20 A
Conductor material	Copper

For a 120 V DC circuit drawing 20 A through 50 ft one-way of 10 AWG copper at 20°C, the round-trip resistive drop is about 2 V and the load sees about 118 V.

Reference table

Calculated conductor resistance by AWG, material, and temperature. Values use the AWG geometry equation and the resistivity constants documented on Sources; they are not copied from an electrical code table.
AWG	Cu 20°C Ω/1000 ft	Cu 75°C Ω/1000 ft	Al 20°C Ω/1000 ft	Al 75°C Ω/1000 ft	Cu 20°C Ω/km	Al 20°C Ω/km
14	2.5254	3.0712	4.1400	5.0576	8.2853	13.5825
12	1.5882	1.9315	2.6036	3.1807	5.2107	8.5421
10	0.9988	1.2147	1.6374	2.0004	3.2770	5.3722
8	0.6282	0.7640	1.0298	1.2581	2.0609	3.3786
6	0.3951	0.4805	0.6476	0.7912	1.2961	2.1248
4	0.2485	0.3022	0.4073	0.4976	0.8152	1.3363

Frequently asked questions

What voltage-drop percentage should I enter?

Use the target required by your project criteria. The default is an editable planning value, not a code-compliance threshold.

Why does a larger conductor reduce voltage drop?

A larger conductor has more cross-sectional area and therefore less resistance for the same material, length, and temperature.

Why is distance entered one way?

The calculator applies the return-path factor for the selected system so the field value matches a measured one-way run.

Assumptions and limitations

Resistive planning model; reactance and harmonics are not modeled.
The default 3% is editable and is not a compliance determination.
Conductor dimensions are mathematically derived from AWG.

Method and sources

Review the AWG reference, calculation methodology, and standards status for the references most relevant to this calculation. The broader technical sources index records source scope and verification. Last reviewed 2026-08-13.