Voltage drop calculator

Enter the current, the length and the cross-section of the cable, and we show the voltage drop in volts and percent, the smallest suitable cross-section and the loss.

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How do we calculate the voltage drop?

The calculator works for copper conductors at operating temperature, taking the resistance and reactance of the conductor (the full impedance) into account. On a single-phase network it takes the there-and-back path (two conductors), on a three-phase network the factor √3:

ΔU % = k · L · I · (R′ · cos φ + X′ · sin φ) / U · 100

where L is the one-way length of the cable in meters, I is the current, U is the nominal voltage (230 V or 400 V), and k is 2 for single-phase and √3 for three-phase. R′ = ρ / A is the resistance of the conductor per meter (the resistivity ρ of copper at operating temperature is 0.0225 Ω·mm²/m, and A is the cross-section in mm²), and X′ is the reactance per meter (0.08 mΩ/m). At cos φ = 1 the reactance term drops out. In the project sizing, L is the longest path from the board to the farthest device.

What does the result show?

• The voltage drop in percent and in volts.
• An indication of whether the value is within the selected limit: 3 % for lighting, 5 % for other circuits. A value between the two limits is a "borderline case": acceptable only for a non-lighting circuit.
• The smallest suitable cross-section from the sizes in the table, for the selected limit.
• The loss in the cable in watts.

What to watch out for

The calculator takes the cable length as one-way, so do not double it. Leave the power factor (cos φ) at 1 for resistive loads such as heating; enter a smaller value for motors and many kinds of electronics. This calculator does not check the current-carrying capacity of the cable: use the cable cross-section and MCB calculator for that.

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