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Calculator · BS EN 60228 cable data

DC voltage drop calculator

Enter the supply voltage, the current the load draws, the cable size and the length of the run. You get the voltage lost in the cable, the voltage left at the load, and the longest run for a drop you choose. It shows every step. It uses the maximum resistance for copper cable, so check the answer against the equipment's data sheet.

Your figures

V
Enter the current in
mA

The total the cable carries, from the equipment's data sheet.

Copper, per conductor. Resistance is the maximum BS EN 60228 allows, so real cable is usually a little better.

m

One way, from the supply to the load. The sum allows for the return conductor.

Longest run for a drop I choose (optional)
Give the allowed drop as
%

Leave blank to skip. The equipment's data sheet says how much it can lose.

Result

Enter the supply voltage, current and cable details to see the result.

Working

V_drop = I × R, where R = r × 2 × L × k

  • r is the resistance of one conductor per metre. It comes from the cable size (BS EN 60228, plain copper, at 20 °C).
  • L is the length of the run. The current goes out and back, so the sum uses twice this.
  • k is 1 for a cable at 20 °C. For a hot cable at 70 °C it is 1.20, taking 0.4% more resistance for each degree.
  • I is the load current in amps.

For a fire alarm sounder circuit or a door lock, use the sounder circuit calculator or the lock cable calculator. They ask for the minimum voltage the equipment needs and say whether the circuit passes.

For guidance only. Always check against the current edition of the standard and the manufacturer's data.

Built by the engineer behind Kerova, certificate software for fire and security firms.