Every cable in an addressable system, what terminates where, what voltage you should measure at each point, and what the signals look like on the wire.
A single loop panel needs six distinct cable types. Getting one of them wrong is the most common reason a system fails its first inspection.
| Circuit | Cable | Size | Screen | Notes |
|---|---|---|---|---|
| SLC loop | Fire resistant 2 core FP200 Gold, MICC, FPLR / FPLP |
1.5mm² 16 AWG |
Usually yes | Screen required by most protocols. Check the panel manual, some Apollo installs run unscreened. |
| NAC / sounder | Fire resistant 2 core | 1.5–2.5mm² 14–16 AWG |
No | Sized by voltage drop, not by current rating. See the drop calculation below. |
| Initiating device circuit | Fire resistant 2 core | 1.5mm² | No | Conventional zone hung off a monitor module. EOL resistor at the far device. |
| Mains supply | Standard building wiring | 2.5mm² 12 AWG |
No | Dedicated, unswitched, labelled circuit. Never on a socket ring or a lighting circuit. |
| Battery | Flexible, fused at the battery | 2.5mm² | No | Short run inside the enclosure. Two 12V SLA in series gives 24V. |
| Panel network | Fire rated screened pair, or fibre | 1.5mm² | Yes | Only on multi-panel jobs. Fibre if the run crosses a building or a lightning risk. |
Fire rating is a survival time, not a brand. BS 5839-1 standard grade means 30 minutes of circuit integrity, enhanced means 120 minutes. NFPA and the NEC classify by installation location instead: FPL general, FPLR risers, FPLP plenum spaces. Specify the survival requirement first, then pick a cable that carries the matching certification.
Notification devices stop working reliably below about 16V. The panel supplies 24V nominal but battery standby drops that to roughly 20.4V at the terminals, so the whole cable run has to lose less than about 4V.
Drop in volts equals 2 × L × I × R, where L is the one way run in metres, I is the total alarm current on that circuit in amps, and R is the resistance per metre. For 1.5mm² copper that is about 0.0121 Ω/m. A 60m run carrying 1.2A drops 2 × 60 × 1.2 × 0.0121 = 1.74V, which is comfortable. Double the current and you are at 3.5V and out of margin.
The panel gives you four terminals per loop. Two send the loop out, two receive it back. Every device in between passes the pair through rather than tapping it.
Keep polarity consistent all the way round. Positive from the panel goes to the positive input of every device, and out of its positive output to the next. One reversed device drops everything downstream of it.
Bond the screen at the panel only. Grounding both ends creates a loop between the panel earth and the building steel, and any mains cable running nearby will induce current into it. That shows up as random comms errors that move from address to address.
No T-taps on a Class A loop. The panel measures the loop as a single electrical path, and a spur hanging off the middle breaks that assumption.
| Limit | Typical | What happens if you exceed it |
|---|---|---|
| Loop resistance, per leg | 40–50 Ω | Devices at the far end brown out during alarm and drop off the poll |
| Loop capacitance | 0.3–1.0 µF | Data edges round off, panel reports intermittent comms faults |
| Devices per loop | 99–159 | Protocol dependent hard ceiling, the panel simply will not enrol more |
| Devices per isolator | 20–32 | One short takes out more of the building than the code allows |
Put a scope across the loop pair and you will not see a clean digital bus. The panel and the devices talk in two completely different ways on the same two wires.
| Point | Quiescent | Alarm | Meaning |
|---|---|---|---|
| SLC loop, across the pair | 24–40V DC | unchanged | Loop voltage does not drop in alarm. Devices signal by current, not by shorting the line. |
| NAC, at the panel | −24V DC | +24V DC | Polarity reverses. Blocking diodes keep sounders silent while the EOL is still supervised. |
| NAC, at the last device | n/a | ≥ 20.4V DC | Below about 16V the sounder is out of spec, which is the whole point of the drop calculation. |
| IDC through a monitor module | EOL value | near 0 Ω | The contact shorts across the EOL. Panel reads the resistance change, not a voltage. |
| Battery, on float charge | 27.2–27.6V DC | ≥ 21V DC | Below 21V under load the batteries are done, whatever the date on the label says. |
| Screen to earth | < 1 Ω at panel | open at far end | Continuous screen, earthed once. Any reading at the far end means it is bonded twice. |
A fire alarm circuit has to know the difference between quiet and broken. Without an end of line resistor a cut cable and a healthy standby circuit look identical to the panel. The resistor makes the panel measure a specific value at all times, so any change is detectable.
Click through the four states below. The current reading is what you would measure on a 24V circuit with a 10kΩ EOL.
EOL values are manufacturer specific and there is no safe default. Notifier initiating circuits commonly use 47kΩ and their NACs 10kΩ. Apollo monitor modules are often 20kΩ. Hochiki is frequently 10kΩ. Fire-Lite NACs are often 4.7kΩ. Read the device data sheet every single time, because fitting the wrong value gives you a circuit that reads normal and never detects a break.
At the far end of the circuit, inside the last device on the run. Not in the panel, not in a junction box halfway, not across the terminals at the panel end. The whole function is to prove the cable is intact along its entire length, and a resistor sitting anywhere other than the end only proves the cable up to that point.
Detectors and call points only need the loop pair. Modules need a second circuit, and the two families wire in opposite directions.
Addressable sounder bases draw their power from the loop instead of a NAC. Convenient, but each one takes several milliamps in alarm and a loop has a fixed current budget. Fit thirty of them and the far end of the loop browns out the moment everything sounds at once. Run the manufacturer's loop calculator before you commit to the design, not after the devices are on the ceiling.
Standby requirement is 24 hours of quiescent load plus a period in full alarm. NFPA 72 asks for 24 hours plus 5 minutes of alarm. EN 54 and BS 5839-1 ask for 24 hours plus 30 minutes. Take the quiescent current, multiply by 24, add the alarm current multiplied by the alarm period in hours, then add roughly 25 percent for ageing.
A panel drawing 0.35A quiescent and 2.1A in alarm under EN 54 needs (0.35 × 24) + (2.1 × 0.5) = 9.45 Ah, plus derating gives about 12Ah. Fit 12Ah cells, not 7Ah, and write the calculation on the inside of the door.
If you are building the RS485 training loop rather than wiring a real building, this is the equivalent connection table. Same topology, safe voltages, and the faults behave the same way.
| ESP32 | MAX485 | Function | Real world equivalent |
|---|---|---|---|
| GPIO16 | RO | Receive | Loop receive side of the panel line driver |
| GPIO17 | DI | Transmit | Loop transmit, the voltage modulation in Fig 3 |
| GPIO4 | DE + RE | Direction | Half duplex turnaround, no equivalent, real loops are always driven |
| 3V3 | VCC | Supply | Loop supply, 24V on a real system |
| GND | GND | Common | Loop negative |
| — | A / B | Bus pair | The SLC pair itself, daisy chained board to board |
| 120 Ω | A to B | Termination | First and last node only. Omit it and you get the same intermittent comms faults as an over-capacitance loop |
Do not connect the bench rig to real fire alarm equipment. It is a training tool. Fire alarm installation and commissioning is a licensed activity in Egypt and almost everywhere else, and the commissioning certificate has to be signed by a certified engineer.
Disconnect the circuit from the panel before measuring resistance. Leave it connected for voltage readings.
| Reading | You measure | Cause |
|---|---|---|
| IDC resistance, panel disconnected | ∞ | Break in the cable, or the EOL was never fitted |
| IDC resistance, panel disconnected | near 0 Ω | Short between conductors, usually a nicked cable in a metal back box |
| IDC resistance, panel disconnected | half the EOL | Two EOL resistors fitted in parallel, someone left one at a previous device |
| Loop leg resistance, end to end | > 50 Ω | Run too long, conductor too thin, or a corroded joint in a junction box |
| Conductor to earth | < 1 MΩ | Insulation damage. Megger at 500V with all devices removed to confirm |
| Screen continuity, panel to far end | open | Screen not carried through a joint. Supervision of the screen is now useless |
| Screen to earth at far end | < 1 Ω | Screen bonded at both ends. Lift the far end |
| NAC at last device in alarm | < 20V | Voltage drop. Increase conductor size or split the circuit |
| Battery under load | < 21V | Batteries at end of life, replace both as a pair |