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Calculator · Fire alarm · BS EN 60228 cable data

Fire alarm sounder circuit calculator

Enter the lowest voltage the panel puts on the sounder circuit and the lowest voltage the sounders work at, both from the data sheets. Then enter how many sounders there are, the current of each, and the cable. You see the voltage at the last sounder, whether there is enough, and the longest cable for that load.

Your figures

V

From the panel's data sheet, when running on battery.

V

From the sounder's data sheet.

sounders
Enter the current in
mA

The alarm current, from the sounder'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 panel to the last sounder.

Result

Enter the two voltages, the sounders and the cable to see the result.

Working

V_end = V_panel − I × R, where R = r × 2 × L × k

  • V_panel is the lowest voltage the panel can supply to the circuit. Use its data sheet figure for running on battery, not the mains figure.
  • I is the circuit current: the number of sounders times the current of each.
  • r is the resistance of one conductor per metre (BS EN 60228, plain copper, at 20 °C), L the run length and k the hot-cable factor (1.20 at 70 °C).
  • With every sounder at the far end, all the current runs the full length. That is the worst case. If the sounders are spread evenly along the cable, the drop is about half.

BS 5839-1 does not set a fixed voltage drop for you to use. The two voltages come from the panel and sounder data sheets, and this tool has no default for either.

This checks voltage only. It does not check the panel's total output current or the battery size. For the battery, use the fire alarm battery calculator.

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.