Voltage drop calculator

Estimate the voltage lost along a copper cable run — DC or AC, single-phase or three-phase — and see what share of your line voltage that represents.

Cable length

One-way cable run

Load current

Expected current draw

Conductor area

Cross-sectional area of the conductor

…or AWG size

e.g. 10, 4/0 or 0000

Enter an area directly or an AWG size — the AWG value is converted to mm² and used as the area.

Circuit type

V_drop = 2 × ρ × L × I ÷ A

Line voltage (optional)

e.g. 230 V single-phase or 400 V three-phase

Voltage drop: 0.690V
Share of line voltage: --%

Keep the drop low

Heavy-current runs call for heavy-duty connectors and high-current terminals that keep contact resistance — and heat — down across the whole circuit.

Frequently asked questions

What is an acceptable voltage drop?

NEC 210.19 recommends keeping branch-circuit drop under 3% and total (feeder + branch) under 5%. Long low-voltage runs (12 V, 24 V) are almost always voltage-drop-limited before ampacity.

Why is the drop doubled in single-phase circuits?

In a single-phase circuit the current travels out on the live conductor and back on the neutral — two one-way lengths — so the drop is 2 × I × R. Three-phase drops use √3 × I × R instead.

How does conductor size affect voltage drop?

Resistance per metre falls as the conductor area rises, so the drop is inversely proportional to mm². Doubling the cross-section halves the drop. This calculator solves the required area directly from current, length, voltage and the allowed percentage.

How it works

The calculator uses the copper resistivity of ρ = 0.01724 Ω·mm²/m. Single-phase drop is V = 2 × ρ × L × I ÷ A (out and return conductor); three-phase uses V = √3 × ρ × L × I ÷ A. This is a simplified model — inductance, skin effect and operating temperature are not included, so treat the result as an estimate, not a code calculation.

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