Cable Size and Voltage Drop Calculator

What is voltage drop?

A cable is a resistance. When current flows, part of the supply voltage is lost along the cable and the load sees a lower voltage. If the drop is too large, motors run hot, lamps dim and electronics shut down. This is why regulations and specifications set a limit on voltage drop.

Formula

• Single phase and DC: ΔV = 2 × L × I × (ρ / S × cos φ + x × sin φ)
• Three phase: ΔV = √3 × L × I × (ρ / S × cos φ + x × sin φ)

L is the one way length (m), I the current (A), S the conductor size (mm²), ρ the resistivity (Ω·mm²/m) and x the reactance per unit length (Ω/m). With zero reactance and a power factor of 1 this becomes the familiar hand formula ΔV = 2 × L × I / (k × S), where k is the conductivity: 56 for copper and 35 for aluminium.

Example

A 16 A load 25 m away on a 230 V single phase circuit is fed through 2.5 mm² copper. ΔV = 2 × 25 × 16 / (56 × 2.5) = 5.71 V, which is 2.48 percent of 230 V.

Which resistivity?

A conductivity of 56 is close to cold copper. A loaded cable warms up and its resistance rises. IEC 60364-5-52 Annex G uses 1.25 times the 20 °C resistivity for operating temperature: 0.0225 for copper and 0.036 Ω·mm²/m for aluminium. The same example then gives 7.2 V, or 3.13 percent. Use the IEC option to stay on the safe side.

What the size suggestion covers and what it does not

The "Suggest a size" tab returns the smallest standard size that meets the voltage drop limit you enter. It does not mean the cable can carry that current. Current carrying capacity depends on the installation method (in conduit, on tray, buried), ambient temperature, the number of grouped cables and the insulation type, and is read from the tables of IEC 60364-5-52 or the applicable national standard. On short runs ampacity usually governs the size, on long runs voltage drop does. Check both and take the larger. The protective device and short circuit withstand are verified separately as well.

Limits

The calculation assumes the load is concentrated at the end of the run and balanced on three phase. Skin effect in large sizes, harmonics and motor starting current are not included.