Volt Drop Calculator

Alarm load is the maximum current demand placed on the control equipment’s power supply and outputs, with sounders, beacons, relays and auxiliary outputs all operating together. That is the condition in which the supply is delivering its highest current and its output voltage is at its lowest, so it is the reading that shows how well the supply regulates under full demand. Every sounder, beacon and detector is rated to operate across a supply voltage range published in its datasheet (17 to 28V for Apollo XP95 devices, for example), and a device is only assured to meet its declared performance, including its sound or light output, while the voltage at its terminals stays within that range. A system can read correctly at standby and still run outside the rated range of its devices at full load, which is the condition it has to perform in during a fire, so the output voltage is verified under load as well as at rest.

Any reduction in voltage at the control equipment output carries through to every device connected to it, before any cable loss is added, and it comes out of the same margin the cable run has to work within. A panel output that already falls under load leaves less allowance for cable volt drop, so the two checks on this page need to be read together.

How we test. We take a reading of the steady output voltage on the control equipment PCB, at a point such as a sounder or auxiliary output. We do not take it from the battery charge terminals, because the standby battery charge voltage is a separate check. We then take a second reading at exactly the same point while the system is under load, with the sounders and other outputs activated, and work out the percentage drop between the two.

Our limit. We require that the drop is no more than 5%. That is our own acceptance limit. BS 5839‑1:2025 (clause 43.2.6) asks for the power supply output to be checked with the battery disconnected and the alarms activated, so that it can be confirmed as close to the nominal voltage, with the full load simulated where that isn’t practicable. Note 2 to that clause says it would be reasonable to expect at least 85% of the nominal voltage; the 2017 edition said 95%. The manufacturer documents we reviewed state the requirement as keeping every device within its rated voltage range, rather than as a set percentage. A limit of 5% between standby and loaded readings is stricter than either figure.

Once the control equipment output is confirmed, the next check is what the cable takes off it. This matters most on the critical signal paths: the sounder and device circuits, the detector or loop circuits and the power supply wiring that have to keep working during a fire.

Cable volt drop is the there‑and‑back conductor resistance multiplied by the current, so it grows with the length of the run and with the number of devices drawing current from it:

Volt drop (V) = 2 × length (km) × resistance (Ω/km) × current (A)

Every sounder, beacon or loop device added to a circuit increases the current, and every extra metre adds resistance. Enter the lowest voltage the control equipment will deliver in the condition you are checking, for example the loaded reading from Part 1, and the result is compared with the minimum operating voltage on the device datasheet.

Cable resistance. The resistances are the maximum conductor values at 20°C from BS EN 60228 (the same figures apply to solid and stranded copper at these sizes). Copper resistance rises by roughly 0.4% for every degree, so a conductor at 70°C has about 20% more resistance than the 20°C figure. The voltage drop tables in BS 7671 Appendix 4 assume the conductor is at its maximum permitted operating temperature, which is why a temperature option is provided here.

What the standards require. BS 5839‑1:2025 doesn’t set its own percentage limit for cable volt drop. Clause 25.1 says the electrical characteristics of the cables, such as voltage drop for the ELV supply, should be in accordance with BS 7671. BS 7671 Regulation 525.1 requires the voltage at the terminals of fixed equipment to be greater than the lower limit in the product standard for that equipment, so the minimum used here should come from the device datasheet. Clause 25.2 requires the critical signal paths, the ELV supply from an external power supply unit and the final circuit supplying the mains to the system to use fire‑resisting cable.

The 5% option. Where the datasheet figure isn’t to hand, the tick box applies a limit of 95% of the supply voltage. That is a strict rule of thumb of our own, not a requirement. Many 24V devices are rated to work well below 95% of nominal, so the datasheet figure will often allow a longer run.

Loop length and device totals. On addressable systems the loop limits are set by the manufacturer, and they take priority over any calculation here. BS EN 54‑13 requires the manufacturer of the system to declare and have assessed the maximum cable length and impedance, the maximum number of devices and the maximum electrical loading for each transmission path, and the supply voltage must stay within each device’s specification at the lowest supply voltage with the maximum load. In practice that means limits such as these, which vary by manufacturer and panel, so always check the installation manual:

Apollo XP95 and Discovery protocols allow up to 126 devices per loop, with devices operating from 17 to 28V. Hochiki ESP protocol allows up to 127 devices per loop. Advanced’s MxPro 5 manual gives a maximum recommended loop cable length of 1500 to 2000 metres and a maximum of 500mA per loop, and warns that the voltage drop at full load has to stay within the detector rating. Loop‑powered sounders and beacons draw far more current than detectors, so they use up loop capacity quickly and shorten the length a loop can run.

For fault tolerance, BS 5839‑1:2025 clause 11.2.2 also limits how much protection a single fault can disable. A single open‑circuit or short‑circuit fault on a detector circuit should not disable protection in an area of more than 2,000 m² or on more than one floor (plus five devices on the floors immediately above and below), and in practice this is what sets how many devices can sit between short‑circuit isolators. For control equipment with more than 512 detectors and manual call points, clause 11.2.4 says to follow the manufacturer’s instructions.

What this doesn’t cover. The calculation is for a conventional circuit fed from a panel or power supply output, such as sounders and beacons, with the whole circuit current treated as drawn at the far end, which is the worst case. For addressable loops use the manufacturer’s loop calculator. It doesn’t check the cable’s current rating or the voltage with a fault present on the circuit.