Alarm Cable Types and Which Run Gets Which

Why this matters

A house job uses five or six different cables, and most callbacks traced to cable come from the same two choices: a siren run in the same thin cable as the door contacts, so the siren is quiet on battery, and a run laid through an air-handling space in cable that is not listed for it, which an inspector or the next contractor finds. The first is a performance failure; the second is a code failure that can mean pulling the cable out. Both are settled when the cable schedule is written, before the first hole is drilled.

The schedule for a typical house

A two-story house with a basement panel, eleven door and window contacts, two motion detectors, two keypads, a wireless receiver, an interior and an exterior siren, four cameras, and two smoke detectors on the panel's fire zone. Gauges are common practice; power runs are checked by voltage drop, below.

CABLE SCHEDULE  12 ALDER CT
RUN                    CABLE        JACKET RATING
Door + window contacts 22/2 or 22/4 Class 2 (CL2/CM)
Motion detectors       22/4         Class 2 (CL2/CM)
Keypads, bus devices   22/4         Class 2; heavier
                                    if drop fails
Interior siren 40 ft   18/2         Class 2
Exterior siren 95 ft   18/2 min     Class 2, wet-rated
                                    where exposed
Panel transformer      18/2         per panel manual
Cameras x4             Cat6         CMR or CMP
Smoke detectors        fire alarm   FPL family, per
                       power-ltd    NFPA 72 + NEC 760
Any run in return air  plenum-rated CMP, CL2P, FPLP

The letters on the jacket are listings under the electrical code your jurisdiction has adopted. In NEC terms: CL2 is Class 2 power-limited cable (Article 725; the 2023 edition moved the cable listing and installation rules into the new Article 722), FPL is power-limited fire alarm cable (Article 760), and CM is communications cable (Articles 800 and 805 in the 2023 edition). A suffix raises the rating: R for riser (vertical runs between floors in a shaft), P for plenum. Communications cable of a matching rating is a permitted substitute for Class 2 cable under the substitution rules in the adopted code, which is why a lot of alarm cable on the truck is dual-marked. Check the current edition for the exact table.

Small-gauge four-conductor for everything that signals

Contacts, motion detectors and keypads carry tiny currents, so 22 AWG is the common choice and the reason is mechanical rather than electrical: it fits the terminals and the small cavity behind a recessed contact, and it pulls through a drilled jamb.

Why four conductors to a two-wire contact. The spare pair is the cheapest insurance in the trade. It becomes a second zone if a window is added, a tamper loop, or a replacement pair when a conductor breaks inside a door frame. Run 22/4 to anything you might ever expand from; run 22/2 only to a single contact at the end of a line.

Motion detectors need four: two for auxiliary power, two for the zone. A detector with a separate tamper switch on its own loop needs six, or double end-of-line wiring (see related: Double End-of-Line and Tamper on One Zone).

Keypads and other bus devices need four: power, common and two data. The current is small per device but adds up along a chain, which is the one place a 22 AWG signal cable becomes a power cable. The arithmetic is in The Keypad Data Bus and the Voltage at the Far End.

Stranded or solid. Stranded survives repeated flexing and is the usual alarm cable; solid lands cleanly in screw terminals and punch-downs. Either works in a wall. What does not work is twisting stranded conductors under a screw without tinning or a ferrule where the device's instructions call for one.

Heavier two-conductor for sirens and long power runs

A siren draws far more current than anything else on the panel, and only while it is sounding, which is also when the panel may be on battery. Voltage lost in the cable is volume lost at the siren.

Say the exterior siren's spec sheet lists 1.0 A at 12 V (illustrative; read your own). The run is 95 ft, so 190 ft of conductor round trip. Copper resistance per 1,000 ft of single conductor is about 16.1 ohms at 22 AWG, 6.4 ohms at 18 AWG and 4.0 ohms at 16 AWG.

• 22 AWG: 190 x 16.1 / 1,000 = 3.06 ohms. At 1.0 A, 3.06 V lost. From a 12.0 V battery-state output, the siren sees about 8.9 V.
• 18 AWG: 190 x 6.4 / 1,000 = 1.22 ohms, 1.22 V lost, about 10.8 V at the siren.
• 16 AWG: 190 x 4.0 / 1,000 = 0.76 ohms, 0.76 V lost, about 11.2 V at the siren.

Check it from the other side: the 22 AWG drop is roughly 2.5 times the 18 AWG drop, which matches the ratio of their resistances (16.1 / 6.4 = 2.5). The exterior siren gets 18/2 as a floor, and 16/2 when the run is long or the siren draws more. The interior siren at 40 ft on 18/2 loses 80 x 6.4 / 1,000 x 1.0 = 0.51 V at the same draw, so 18/2 is plenty there. Whether a given voltage at the siren still meets its output rating is in the siren's documentation and in How to Install an Alarm Siren and Check Its Output.

The transformer run from a plug-in transformer to the panel carries the panel's whole charging and operating current. Most panel manuals state a minimum gauge and a maximum length for it. Use the manual's figures, not the leftover 22/4.

Cat6 for cameras, and why it is not "alarm cable"

IP cameras ride on Cat6 carrying both data and power over Ethernet. The cable is a communications cable with its own performance category, termination rules and length limit, and it is chosen and terminated to those rules rather than to alarm habits: no splices mid-run, no untwisting beyond what the termination standard allows, no stapling flat. Category and termination detail is in Ethernet Cable Categories and Terminations; power budget and the effect of a long run on PoE are in Power over Ethernet for Cameras and Access Points.

Outdoor camera drops need cable listed for the location: an exterior wall run in sunlight wants a UV-resistant jacket, and any run in buried conduit is a wet location and wants cable listed for wet locations.

Fire-rated power-limited cable for fire circuits

Smoke and heat detectors on a panel's fire zone are a fire alarm circuit, and the cable is power-limited fire alarm cable in the FPL family under NEC Article 760 as adopted, installed to NFPA 72 in the edition your jurisdiction has adopted. Red jackets are a convention, not the listing: the marking on the jacket is what the inspector reads. Fire circuits in many jurisdictions are separately licensed work, and a burglary installer adding them may be outside their license; see related: Combination Fire and Burglary Panels and the License Line.

Plenum-rated cable where the air moves

The space that catches installers is not the obvious duct, it is a space used as part of the air path:

• A drop ceiling used as a return plenum, common in finished basements and light commercial.
• Panned joists: a joist bay with sheet metal across the bottom, used as a return duct.
• The return-air side of an air handler in a closet or attic.

Wiring in a space used for environmental air falls under NEC 300.22 as adopted, which in substance limits what may run there to plenum-rated cable or cable in metal raceway. The adopted code carries a dwelling-unit exception, in substance, for wiring that passes through a joist or stud space at right angles to its long dimension; running ALONG a panned return is not that. Check the current edition. In practice: if your route runs along a panned bay or above a return-air ceiling, either pick a different bay or put plenum-rated cable in that run (CMP for Cat6, CL2P for Class 2, FPLP for fire). Plenum cable costs more per foot, which is why it goes on the schedule for that run rather than on the whole job.

Separation from power

Keep every one of these cables away from line voltage in boxes, bored holes and long parallel runs, per the separation rules for Class 2, fire and communications circuits in the adopted code. Low-voltage cable sharing a bored hole with a 120 V cable fails inspection, invites induced noise on keypad data, and puts line voltage on your conductors if the other cable's insulation is damaged. Where you cannot route around power, cross it at right angles. Any new 120 V receptacle for a transformer or a camera supply is electrician's work where local rules require it; alarm and low-voltage licensing itself varies by state and city (see related: Alarm and Low-Voltage Licensing Varies by State and City).

How to verify you got this right

Before the pull: every run on the schedule has a gauge, a conductor count and a jacket rating, and every siren and power run longer than a short hop has a voltage-drop figure beside it. After the pull: read the printed marking on each cable where it enters a plenum, a fire zone device and the outdoor entry, and confirm it matches the schedule. A cable that cannot be read is a cable that cannot be shown to an inspector. Keep the schedule in the job record, so whoever adds a camera or a smoke detector next year knows what is already in the walls.

References

• NFPA 70, National Electrical Code, in the edition adopted by your jurisdiction (Article 725, Article 722 in the 2023 edition; Articles 760, 800, 805; Section 300.22)
• NFPA 72, National Fire Alarm and Signaling Code, in the edition adopted by your jurisdiction
• Panel and siren manufacturers' installation manuals (transformer gauge and length, siren current)
• See related: Ethernet Cable Categories and Terminations; The Keypad Data Bus and the Voltage at the Far End; Power over Ethernet for Cameras and Access Points