CCalclabhub

Voltage Drop Calculator

Voltage drop is the loss along a cable as current flows through its resistance. It is proportional to the length of the run, the current, and the resistance of the conductor, and it is counted twice because the current must return.

Voltage drop in volts

8.892

Drop as a percent of supply
3.8661
Voltage at the load
221.108
Loop resistance in ohms
0.4446

Values update as you type. This calculator covers the single scenario its formula assumes — see Common Mistakes for what it leaves out.

The formula this calculator uses

V_drop = 2 x length in km x resistance per km x current

L
One-way cable length, in kilometres for the resistance figure used
I
Current in amperes
R
Conductor resistance in ohms per kilometre
V_drop
Volts lost along the run, counted over both conductors

How to check the result by hand

  1. 1

    Convert the one-way length into kilometres

    Cable resistance is quoted per kilometre, so a 30 metre run becomes 0.03 kilometres. Keeping the length in metres while using a per-kilometre resistance understates the drop by a factor of a thousand.

  2. 2

    Double the length for the return path

    Current flows out along one conductor and back along another, so the loop is twice the one-way length. This factor of two is the most frequently forgotten part of the formula.

  3. 3

    Multiply by the resistance per kilometre

    0.03 kilometres at 7.41 ohms per kilometre is 0.2223 ohms per conductor, or 0.4446 ohms around the loop. The resistance figure comes from the cable datasheet for the conductor size and material.

  4. 4

    Multiply by the current to get the drop in volts

    Twenty amperes through 0.4446 ohms gives 8.892 volts. The drop scales linearly with current, so a heavier load loses proportionally more.

  5. 5

    Convert to a percentage and subtract from the supply

    8.892 divided by 230 and multiplied by 100 is 3.8660869565 percent, and subtracting the drop from 230 leaves 221.108 volts at the load. Both figures matter: one for compliance, one for the equipment.

For worked examples, common mistakes and the limits of this formula, read the full How To Calculate Voltage Drop page.