Wire Voltage Drop

Free Wire Voltage Drop calculator with clear step-by-step results.

About Wire Voltage Drop

Voltage drop is current times the resistance of the whole circuit path, which for a single-phase or DC run means twice the cable length because current returns through the second conductor. Three-phase circuits use a root-three factor instead. Aluminium's higher resistivity increases drop by about 64% for the same cross-section.

Formulas

Voltage drop

Drop = k x resistivity x length / CSA x current, where k = 2 single phase or root 3 three phase

Power lost

Watts = voltage drop x current

Worked example

With the default values below, Wire Voltage Drop returns Voltage drop of 2.243%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Load currentminimum 0.01 A: 20 A
One-way cable lengthminimum 0.1 m: 30 m
Conductor cross-sectional areaminimum 0.3 mm2: 4 mm2
System voltageminimum 6 V: 230 V
Conductor materialone of Copper, Aluminium: Copper
Circuit typeone of Single phase or DC, Three phase: Single phase or DC

Results

Voltage drop: 2.243% %
Voltage drop: 5.160 V
Voltage at the load: 224.84 V
Power dissipated in the cable: 103.20 W

Step by step

1Values used
2Voltage drop
3Power lost
4Voltage drop= 2.243 %
5Voltage drop= 5.160 V
6Voltage at the load= 224.84 V
7Power dissipated in the cable= 103.20 W

Assumptions and limits

The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Cable sizing must satisfy voltage drop, current-carrying capacity after derating, and fault-loop impedance requirements simultaneously. Designs must comply with the applicable wiring regulations and be verified by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Cable sizing must satisfy voltage drop, current-carrying capacity after derating, and fault-loop impedance requirements simultaneously. Designs must comply with the applicable wiring regulations and be verified by a qualified electrician.

Frequently Asked Questions

What drop is acceptable?

Commonly 3% for lighting and 5% for other circuits measured from the origin of the installation, though specific limits vary by code and by circuit type.

Why does drop matter beyond efficiency?

Low voltage at the load causes motors to draw more current and overheat, dims lighting, and can prevent contactors from holding in. The dissipated power also heats the cable.