Burglar alarm wiring is the dedicated low-voltage cabling infrastructure that connects sensors, keypads, and sirens to a central control panel, utilizing specific gauges and supervised loop topologies to detect both intrusions and wire tampering. In a real installation, this wiring changes a simple mechanical switch loop into a supervised, tamper-resistant network where wire resistance and topology dictate whether the panel sees a fault, an alarm, or a healthy zone. Installers commonly confuse burglar alarm wiring with standard HVAC thermostat wire or low-voltage landscape cable, assuming any thin copper wire will carry a 12V signal reliably over long distances without degrading the data bus or triggering false alarms.
The Core Concept: Supervised Loops and Wire Topology
At the heart of any hardwired security system is the zone loop. Most intrusion sensors (like magnetic door contacts and glass break detectors) use a Normally Closed (NC) circuit. Think of an NC security loop like a drawbridge: if the bridge is down (closed circuit), traffic flows and the panel is happy. If a car stops on it (sensor trips) or someone cuts the bridge cables (wire cut), traffic stops and the alarm sounds.
However, a simple open/closed loop is vulnerable. If an intruder cuts the wire before opening the door, the panel just sees an open circuit and might not differentiate between a fault and an intrusion. This is where End-of-Line (EOL) resistors come in. By placing a resistor (typically 2,000Ω or 5,600Ω depending on the panel brand) at the very last device on the wire run, the panel constantly monitors the exact resistance of the circuit.
- Normal (Secure): Panel reads the exact EOL resistance (e.g., 2,000Ω).
- Alarm (Open): Sensor trips, breaking the circuit. Panel reads infinite resistance.
- Alarm (Short): Wires touch together, bypassing the resistor. Panel reads ~0Ω.
- Trouble/Tamper: Panel reads a resistance value outside the expected tolerance (e.g., 4,000Ω if resistors are wired incorrectly or wire is degrading).
Where You Meet This in Practice: Keypads, Sensors, and Sirens
On the jobsite, you will encounter three distinct wiring environments, each with different physical and electrical demands:
- Sensor Zones: These are usually daisy-chained (run from one window to the next) or home-run (every sensor back to the panel). They carry almost zero current, just a 12V reference signal measuring resistance. Wire gauge matters less here than physical topology.
- Keypad Data Buses: Keypads (like the DSC WT5500 or Honeywell 6160) require four wires: two for 12V power and two for data (UART/RS-485). The data wires are highly susceptible to voltage drop and electromagnetic interference (EMI). Running these parallel to 120V AC Romex will induce noise, causing keypad dropouts.
- Sirens and Strobes: These are heavy inductive loads. An interior siren might draw 300mA, while an exterior strobe/siren combo can pull 1.5A or more during an alarm event. This is where wire gauge and voltage drop calculations become critical to ensure the siren actually sounds at full volume.
The Math That Matters: Voltage Drop and Wire Gauge
The most common mistake in alarm wiring is using 22 AWG wire for high-current devices like sirens. Let us run a worked numeric example to prove why 18 AWG is mandatory for power runs.
Using 22 AWG Copper Wire:
- Resistance of 22 AWG: ~16.14 ohms per 1,000 feet.
- Resistance for 400 feet: (400 / 1000) * 16.14 = 6.45 ohms.
- Voltage Drop (V = I × R): 0.5A × 6.45Ω = 3.22V drop.
- Voltage at Siren: 12.0V - 3.22V = 8.78V. (Result: The siren will sound weak, or the panel will reset due to a brownout on the auxiliary power bus).
Using 18 AWG Copper Wire:
- Resistance of 18 AWG: ~6.38 ohms per 1,000 feet.
- Resistance for 400 feet: (400 / 1000) * 6.38 = 2.55 ohms.
- Voltage Drop (V = I × R): 0.5A × 2.55Ω = 1.27V drop.
- Voltage at Siren: 12.0V - 1.27V = 10.73V. (Result: Perfect operation; most 12V sirens operate flawlessly down to 10.5V).
Decision Tree: Choosing the Right Cable and Resistor
Use this decision matrix to select the correct materials for your next rough-in. Always verify the specific EOL resistor value required by your panel manufacturer (DSC typically uses 5,600Ω, while Honeywell/Resideo and Interlogix often use 2,000Ω).
| Device Type | Wire Gauge & Type | Topology / EOL Requirement | Concrete Pick |
|---|---|---|---|
| Magnetic Door/Window Contact | 22 AWG 2-conductor | Series EOL at the *last* sensor on the daisy chain. | 22/2 Solid Copper + 2KΩ Resistor |
| Motion Detector (PIR) | 22 AWG 4-conductor (or 6-conductor for tamper) | Home run preferred. EOL wired in series across the NC loop terminals. | 22/4 Solid Copper + 2KΩ Resistor |
| Keypad (Data & Power) | 18 AWG 4-conductor (or 22/4 if run is under 100ft) | Home run only. Never daisy-chain data buses. No EOL required. | 18/4 Solid Copper (Shielded if near AC) |
| Interior Siren / Speaker | 18 AWG 2-conductor | Home run. Observe polarity if using a piezo driver. No EOL. | 18/2 Stranded or Solid Copper |
Common Confusions: Security Wire vs. Thermostat Wire
Walk into a big-box hardware store, and you will see spools of 18/5 or 20/4 'thermostat wire' next to the security cable. While they look identical, they are not interchangeable for professional alarm work.
First, thermostat wire is often unjacketed or uses a PVC jacket not rated for pulling through rough framing alongside AC lines. More importantly, many cheap thermostat wires use Copper-Clad Aluminum (CCA) instead of pure bare copper. CCA has a higher resistance per foot and is brittle; it will snap if you strip it with standard coaxial/UTP strippers and will corrode rapidly under screw terminals, leading to phantom 'Zone Open' troubles three years after installation.
Always buy cable explicitly labeled as Security Wire or Alarm Cable (typically marked as CL2 or CL3 rated by UL for in-wall use). For modern systems utilizing high-speed data buses or IP integrations, ensure the data pair is twisted to reject common-mode EMI from nearby fluorescent lights or microwave ovens.
FAQ: Troubleshooting and Code Requirements
Q: Do I need a license to pull burglar alarm wiring in my own home?
A: In most US jurisdictions, homeowners can pull low-voltage (under 50V) security wiring in their own primary residence without a low-voltage contractor's license. However, if you are connecting the system to a central monitoring station that dispatches police, the Electronic Security Association (ESA) and local AHJs often require the final panel programming and sign-off to be done by a licensed alarm contractor to prevent false dispatch fines.
Q: Can I run alarm wire in the same conduit as 120V AC power?
A: No. The NFPA 72 (National Fire Alarm and Signaling Code) and standard NEC practices strictly prohibit running low-voltage signaling circuits in the same raceway, box, or conduit as Class 1 (120V/240V) power circuits unless separated by a permanent physical barrier. Induced voltage can fry your panel's data bus and poses a severe shock hazard.
Q: My keypad says 'Comm Failure' but the wiring looks fine. What is wrong?
A: Check your wire gauge and run length. Data buses (like the DSC Keybus or Honeywell Keypad Bus) suffer from capacitive loading and voltage drop. If you used 22 AWG for a 150-foot keypad run, the data signal edges are likely degrading. Splice in a heavier gauge (18 AWG) for the power legs, or install a localized power supply module near the keypad to boost the 12V bus locally.
