Security

Table 1 maps the parts to the four components. Subsections that cover one component carry an Applies to tag. An Informs tag marks the subsections behind each decision above. Smoke and carbon monoxide alarms are a code requirement, not a security choice; they are here because they share the walls, the wiring and often the panel.

1 · Openings: contacts, zones and glass-break

Bottom line. A contact tells the panel that a sash or leaf moved; it says nothing about the glass, which a glass-break detector covers. Wired contacts are installed with the frame; wireless ones carry a battery for their life.

A contact senses movement of the sash or leaf, not the glass

SensorsHardwareCamerasAlarms

Informs D1

A door or window contact is a magnetic reed switch on the frame and a magnet on the moving sash or leaf; separating them opens (or closes) the circuit. Recessed contacts sit in holes drilled in the head or jamb and the matching edge of the sash, invisible when closed; surface contacts mount on the face. A recessed contact on a window needs the hole and its wire path before the unit is installed, and many window and door makers offer factory-fitted contacts. Contacts on steel doors use a wider-gap magnet. Wireless contacts report on a supervision interval, so a silent sensor or a jammed channel shows as trouble, and carry their own transmitter and a battery lasting about 3–5 years (typical); twenty wireless sensors mean several battery changes a year. Radio paths from sensors at the facade cross the materials that attenuate most, coated glass and stucco on metal lath (Lighting · Radio links).

A supervised zone distinguishes normal, open and cut

Informs D1 D6

A wired zone runs from the panel to the contact and ends at an end-of-line resistor at the device. The panel reads the resistor value when the zone is normal, an open circuit when a contact opens or the wire is cut, and a short when the wire is bridged, so tampering shows as alarm or trouble rather than as normal (Figure 1). Resistance of the run itself does not limit it: at 22 AWG, about 16 Ω per 1,000 ft per conductor, a panel that accepts a few hundred ohms of loop resistance allows runs of thousands of feet. What limits a wired design is the number of zones on the panel and its expanders, and whether each opening gets its own zone, which identifies the opening that tripped, or shares one.

Glass-break detectors cover the glass a contact cannot, within a listed range and glass type

SensorsHardwareCamerasAlarms

Informs D1 D2

An acoustic glass-break detector listens for the low-frequency thud of an impact followed by the high-frequency sound of glass breaking; requiring both reduces false alarms from dropped objects. Listed ranges are typically 15–25 ft, with line of sight to the glass, and heavy drapes or shutters shorten them. Detectors are listed for particular glass types; laminated glass, used for sound and forced-entry resistance (Windows & Doors · Glazing build-ups), breaks with a different sound and is covered only by detectors listed for it. A shock sensor on the frame senses the impact instead and covers one opening each.

2 · Interior detection and false alarms

Bottom line. Motion sensors see a person by the change in infrared they cause crossing the sensor's zones; the signal scales with the temperature contrast between the person and what the sensor sees behind them, and false alarms come from moving warm sources and from use more than from sensitivity.

A PIR signal scales with the contrast between body and background

SensorsHardwareCamerasAlarms

Informs D2

S signal from a person crossing a zone · ε emissivity, near 1 for skin and fabric · σ Stefan–Boltzmann constant · T_body apparent surface temperature of a clothed person, about 86 °F (typical) · T_bg temperature of the surface behind the person

the sensor responds to the difference in radiated heat between the person and the background, which near room temperature is close to proportional to the temperature difference.

Over a 70 °F floor or wall a clothed person stands about 16 °F warmer than the background. A ceiling-mounted or high corner sensor looks down, so the floor is most of its background. A radiant floor's surface sits above the room by its output over its coefficient, about 1.94 Btu/h·sf·°F: the HVAC guide's reference ground floor, at about 17 Btu/h·sf on a design day (HVAC · Load by floor), runs near 79 °F and leaves about 45% of the signal; at half that load, a mild day, near 74 °F and 73%; at the 84 °F comfort limit (HVAC · Comfort limits) about 12% (Figure 2). Dual-technology sensors add a microwave Doppler channel that responds to motion regardless of temperature, and requiring both channels to agree also suppresses false alarms.

False alarms come from moving warm sources and from use more than from sensitivity

SensorsHardwareCamerasAlarms

Informs D2 D6

A PIR alarms on a warm source that moves across its zones: sun patches crossing a floor through west or south glass, a forced-air register or space heater cycling in view, curtains moving in a draft, and pets. Pet-immune optics mask the lowest 2–3 ft of the field and are rated by pet weight, at the cost of missing a person crawling below the mask. Placement that looks across a room toward an interior wall rather than toward a window removes the solar case. User error, arming with a door open or entering without disarming, is the commonly reported leading cause (typical, per industry false-alarm programs); entry delays and cross-zoning (two sensors before dispatch) are controls ANSI/SIA CP-01 sets for panels, and video verification of an alarm is the subject of a separate standard, AVS-01.

3 · Entry hardware: resisting forced entry

Bottom line. A door is forced at its weakest link, which is usually the frame at the strike rather than the lock; the lock, strike, screws and frame act as one assembly, and the egress code limits what can be added to the inside.

A kicked door usually fails at the strike, where short screws hold only the jamb

SensorsHardwareCamerasAlarms

Informs D3

W reference withdrawal design value, lb · G specific gravity of the wood · D screw diameter, in · p threaded penetration, in. Average ultimate withdrawal is about five times the reference value

a screw holds in proportion to how far it threads into the wood and to the wood's density squared.

A deadbolt's 1 in bolt is stronger than the wood it engages. Builder strikes are held by two screws about ¾ in long that reach only the ¾ in jamb, which is shimmed off the stud by up to ½ in; the jamb splits along its grain at the mortise and the door swings open with the bolt still thrown. Screws 3 in long cross the jamb and the shim space and thread about 1.7 in into the stud, carrying about 2.8 times as much with two screws and 5.6 times with four (Figure 3, Figure 4). A reinforced strike box or a full-length strike plate spreads the load along the jamb. A hinge held only in the jamb fails the same way; a long screw at each hinge reaches the stud. Values are ultimate withdrawal estimates for comparison; dynamic impact also splits wood around short screws before they withdraw.

Lock grades rate the hardware; they do not rate the frame

SensorsHardwareCamerasAlarms

Informs D3

ANSI/BHMA A156.40 grades residential deadbolts A to C separately for security, durability and finish (AAA is A in all three), with security tests for bolt strength, impact on the door and resistance to picking and prying; A156.2 grades bored locksets 1 to 3. ASTM F476 tests a whole swinging door assembly, door, frame and hardware, against impact and forced entry in grades 10 to 40 (Windows & Doors · Heat, sound, security). A high-grade deadbolt in a builder frame is governed by the frame. A multi-point lock engages the frame at three or more points, which spreads the load the strike would carry alone.

Sliding doors and windows are forced by lifting and prying, not by impact

SensorsHardwareCamerasAlarms

Informs D3

A sliding panel hangs on rollers in a track and can be lifted out of the track if the head clearance allows; anti-lift blocks or screws in the head channel close that gap. Its latch is a hook into the frame, which a pry bar can bend. A foot bolt or a second lock at the meeting stile, a bar in the track, and laminated glass in the panel each address one path. ASTM F842 rates sliding doors, and ASTM F588 rates windows, against these attacks.

The egress code sets what can be added to the inside of a door or window

SensorsHardwareCamerasAlarms

Informs D3 D5

The required egress door is readily openable from inside without a key, special knowledge or effort (CRC R311.2), which excludes a double-cylinder deadbolt on it, so glass within reach of the thumb-turn, a sidelite or a door light, is the path a thumb-turn leaves; laminated or rated glass there resists it. Bars, grilles or screens over an emergency escape opening release from inside without a key, tool or force greater than needed to open the window (R310.4). Window opening control devices for fall protection release the same way (R312.2). These are treated in full with the openings (Windows & Doors · Life safety, Escape openings).

4 · Cameras: coverage, detail, mounting and recording

Bottom line. A camera's useful range is the distance at which it still puts enough pixels on the subject for the task, which falls with a wider lens; its mounting height sets the angle to a face, and its recording sets storage and network load. California law treats audio separately from video.

Detail at the subject is pixels per foot, which falls with distance and lens width

SensorsHardwareCamerasAlarms

Informs D4

ppf pixels per foot across the scene at distance d · N horizontal pixels · FOV horizontal field of view · W width of scene covered at d

a camera spreads a fixed number of pixels across a width that grows with distance and with lens angle, so detail per foot falls with both.

IEC 62676-4 sets the detail needed for four tasks: detect about 7.6 ppf, observe 19, recognize a known person 38 and identify an unknown one 76. A 4 MP camera, 2,560 px wide, with a 90° lens covers a 30 ft wide approach at 15 ft and identifies faces only to about 17 ft (Figure 5); an 8 MP camera extends that to 25 ft, and a 4 MP camera with a 50° lens to 36 ft over a narrower width (Table 2). A wide camera therefore watches a yard and a narrow one at the entry identifies who comes to the door.

Pixel densities per IEC 62676-4 (25, 62.5, 125 and 250 px/m). Compression, low light and motion blur reduce effective detail below these geometric values.

Mounting height sets the angle to a face, which bounds identification from below

SensorsHardwareCamerasAlarms

Informs D4

h camera height, ft · h_face face height, about 5.5 ft · θ_max steepest useful angle to a face, about 15° (typical)

a camera looking down steeply sees the top of a head; the higher it is, the farther away a face must be to be seen from the front.

A camera at 9 ft sees a face at less than 15° only beyond 13 ft; at 12 ft only beyond 24 ft. The identification range of the reference 4 MP, 90° camera ends at 17 ft, so above about 10.0 ft it has no distance where a face is both steep enough and detailed enough (Figure 6). An eave camera at 10–12 ft watches movement; identification at an entry comes from a camera near head height, which a doorbell camera at about 4 ft is. The result rests on the 15° limit: many reviewers accept up to about 30° for a face a person examines, and at 30° the reference camera's window stays open to about 15 ft of height.

Storage and upload follow bitrate times recording time

SensorsHardwareCamerasAlarms

Informs D4 D6

Mbps average bitrate of one stream

a stream's bits per second times the seconds in a day, in gigabytes.

A 4 MP stream in H.265 averages about 4 Mb/s (typical, 2–6 depending on scene and night noise), 43 GB a day. Four cameras recording continuously for 30 days fill about 5.2 TB; recording on motion at a fifth of the time, about 1.0 TB. A local recorder holds this on the network at the distribution point (Low-Voltage · Heat at the enclosure); a cloud service sends it upstream, 16 Mb/s continuous for four cameras, which some connections carry easily and others only partly; upload capacity varies widely by plan and technology. Recording that stops when power or the network does misses the events that cause outages; the recorder, switch and modem on backed-up power or a UPS close that gap.

Night images depend on infrared range and on light that does not glare into the lens

SensorsHardwareCamerasAlarms

Informs D4

Built-in infrared illuminators light the scene to a rated distance, typically 60–100 ft for a bullet camera, in monochrome; anything close to the lens, an eave, a wall or a spider web, reflects the infrared back and washes out the image. Outdoor lighting on motion or photocell control (Lighting · Code) gives color images and visible deterrence; a fixture in the camera's view, or one shining toward it, flares the image; a fixture beside or behind the camera lights the scene without flare.

California law treats recorded audio differently from video

SensorsHardwareCamerasAlarms

Informs D4

Recording a confidential communication without the consent of all parties is a crime under Penal Code 632, and a civil claim under 637.2 for the greater of $5,000 per violation or three times actual damages; a conversation at a front door or on a driveway can be confidential to the people having it. Video of an area visible to the public carries no such consent rule. Penal Code 647(j) makes it a crime to use a camera to view the interior of a bedroom, bathroom or changing area of another, with intent to invade privacy, and Civil Code 1708.8, a constructive-invasion statute for offensive capture of private activity, together with the common-law intrusion tort, can reach a camera aimed into a neighbor's windows or yard. Many cameras and doorbells record audio by default. An HOA's CC&Rs can also limit exterior devices.

5 · Electronic access: locks, doorbells and the garage

Bottom line. Electronic access devices replace a key with a code, a phone or a schedule; each needs power and a failure mode, and a code-required mechanical path remains on the inside.

A smart lock adds a motor and a radio to a mechanical lock

SensorsHardwareCamerasAlarms

Informs D5

A battery smart lock drives the bolt with a motor and keeps a mechanical thumb-turn inside and, usually, a key cylinder outside; batteries last about 6–12 months (typical) and the lock warns before they fail. It joins a control network over Z-Wave, Zigbee, Thread or Wi-Fi, or a phone over Bluetooth (Lighting · Radio links); Wi-Fi locks draw more and change batteries sooner. A door position sensor, a contact in the frame, tells whether the door is closed before the bolt throws, which a lock alone cannot. Hardwired electric strikes and mortise locks need a low-voltage run and a power supply and are set fail-secure (locked without power) on exterior doors; the inside lever or thumb-turn still opens the door mechanically, as R311.2 requires.

A wired video doorbell needs a transformer sized for it

SensorsHardwareCamerasAlarms

Informs D5

Wired video doorbells run from the chime transformer, typically 16–24 V AC, and need roughly 10–40 VA by product (typical). Older chime transformers are often 10 VA at 16 V, about 0.62 A. Pressing the button routes current to the chime solenoid, so for that moment the transformer carries the camera, its infrared and the chime together; a small transformer sags and the doorbell reboots. A 16–24 V, 30 VA transformer or a PoE doorbell on its own run (Low-Voltage · PoE types) removes the limit. A doorbell at about 4 ft sits near face height (Part 4).

A garage opener is an entrance with its own state law and listing

SensorsHardwareCamerasAlarms

Informs D5

Since 1 July 2019 a residential garage door opener sold or installed in California includes a battery backup that operates it in an outage, and a replacement garage door may not be connected to an existing opener without one (Health and Safety Code 19892; civil penalty $1,000 per opener). Openers are listed to UL 325, which requires entrapment protection, the photo-eyes at the door bottom; an automatic close on a timer is permitted only where the opener has monitored entrapment protection and gives an audible and visual warning before the door moves (UL 325; 16 CFR 1211). A tilt or contact sensor reports whether the door is open, and the door between garage and house is part of the garage separation and, in California, closes and latches on its own (Windows & Doors · Life safety).

6 · Panel, power and communication

Bottom line. The panel collects zones, decides when to alarm, and reports; its battery has to carry it through the outage and the alarm, and its path to a monitoring service or a phone has to survive the loss of the network or the power that a burglary or a fire may cause.

Standby battery is current times standby hours plus alarm current times alarm time, with margin

SensorsHardwareCamerasAlarms

Informs D6 D7

I currents, A · T hours · 1.2 a 20% margin for aging and temperature

the battery has to carry the system idling for the required time and then sounding for the required time, with a fifth to spare.

For a panel, keypad and wired sensors drawing about 0.35 A and a siren adding 1.2 A, household burglary duty under UL 1023, 4 hours of standby and at least 5 minutes of alarm, needs about 1.8 Ah. Household fire alarm duty under NFPA 72 is 24 hours of standby and 4 minutes of alarm, about 10 Ah (Table 4). The common 7 Ah battery carries the burglary case with room and the fire case only if the standby current is about 0.24 A or less; this is the battery arithmetic behind combining smoke detection with the security panel (Part 7). Network equipment and cameras are powered separately and stop in an outage unless they are on backed-up power (Low-Voltage · Continuous load; Solar & Battery · Backup scope).

Each reporting path fails with different events

SensorsHardwareCamerasAlarms

Informs D6

Copper telephone lines, the original alarm path, are being retired. Panels now report over cellular radio, over the internet through the house network, or both. A network path fails when the modem or the house power fails, or when a cable outside is cut; a cellular path survives both and is supervised by periodic check-ins, so its loss is itself reported. A panel in a metal enclosure inside stucco on metal lath can need its cellular antenna moved outside the enclosure. With dual paths, either failing produces a trouble signal rather than silence.

Monitoring decides who responds and how an alarm is verified

Informs D6

Self-monitoring sends alerts to the occupants' phones; a monitoring center listed to UL 827 receives the signal, attempts to reach the occupants and dispatches. Many police departments require the alarm to be registered and charge for repeated false dispatches, and some give priority to alarms verified by video, audio or a second zone. ANSI/SIA CP-01 panels default to an entry delay, an exit delay that restarts when a door is reopened, cross-zoning options and a cancel window, which reduce dispatches from user error. Which agency registers alarms and its fee schedule belong in the House Project Guide.

7 · Smoke and carbon monoxide alarms in a remodel

Bottom line. A permitted interior remodel brings the whole dwelling's smoke alarms, and its CO alarms where there is an attached garage or fuel-burning appliance, up to the rules for new construction; the work opened sets whether they must also be hardwired and interconnected.

Permitted interior alterations engage the whole dwelling

SensorsHardwareCamerasAlarms

Informs D7

Where alterations, repairs or additions requiring a permit occur, or a sleeping room is added, the dwelling is equipped with smoke alarms located as required for new dwellings (CRC R314.2.2; California's text adds a valuation threshold, commonly cited as $1,000 of permitted work), and with CO alarms where the dwelling has a fuel-burning appliance or an attached garage with an opening into it (R315.2.2). Work on exterior surfaces, such as roofing, siding, windows and doors, and installation or repair of plumbing or mechanical systems are excepted. Interconnection and hardwiring are not required in existing areas where the work removes no interior wall or ceiling finish exposing the structure, unless an attic, crawl space or basement gives access without removing finishes; alarms installed under the alteration rule may then be battery powered (California's R314.6), and a battery-only smoke alarm sold in California since 1 July 2014 has a sealed 10-year battery (Health and Safety Code 13113.7) (Figure 9). An all-electric house with an attached garage is in the CO case.

Smoke alarms go in and outside sleeping rooms and on each story; CO alarms outside sleeping areas and on each level

SensorsHardwareCamerasAlarms

Informs D7

Smoke alarms go in each sleeping room, outside each separate sleeping area in the immediate vicinity of the bedrooms, and on each story including basements and habitable attics (R314.3); where more than one is required they are interconnected so that one sounds all, by wire or by listed wireless interconnection (R314.4). The 2024 model code removed the fixed distances from cooking appliances, relying on alarms listed to the current UL 217 for resistance to cooking nuisance alarms; placement near kitchens follows the listing and NFPA 72, and R314.3 keeps alarms at least 3 ft horizontally from the door of a bathroom with a tub or shower (Figure 10). NFPA 72 has smoke alarms replaced 10 years after manufacture, so alarms of the house's original vintage are past it. CO alarms go outside each separate sleeping area in the immediate vicinity of the bedrooms, on every occupiable level, and in a bedroom with a fuel-burning appliance in it or its bathroom (R315.3), listed to UL 2034. A combination smoke and CO alarm serves both where listed for both. Smoke alarms are not listed for garages; a heat alarm there, interconnected, is required where energy storage is installed in the garage (Solar & Battery · Storage location).

A household fire alarm system may replace interconnected smoke alarms

SensorsHardwareCamerasAlarms

Informs D6 D7

Smoke alarms are self-contained: detector, sounder and battery backup in each, interconnected. A household fire alarm system under NFPA 72 Chapter 29 instead uses system smoke detectors on a listed control unit, which may be the security panel if it is listed for fire, and is permitted in place of smoke alarms where it gives the same coverage and becomes a permanent fixture of the house owned by the homeowner (R314.7); the model text carries no supervising-station requirement. The system then has the 24-hour standby battery of Part 6, and its wiring is power-limited fire alarm circuitry under Article 760, with its own cable types. Monitoring the house for fire is the gain; the panel, its battery and its listing become part of the life-safety equipment. Keeping 120 V interconnected alarms and adding a listed relay or wireless listener that reports them to the security panel monitors them without that change.

8 · Code, law, permits and registration

Which codes and laws apply

No building code requires an intrusion alarm, cameras or a minimum of locks in a California dwelling. What binds the security layer is the egress code for the hardware, the electrical code for its wiring, state law for recording and for garage openers, and local alarm registration. The smoke and CO requirements of Part 7 are building code. Table 5 lists them.

Permits and documentation

Smoke and CO alarms are inspected under the remodel's building permit: at rough-in, boxes and the interconnection cable; at final, the alarms in their locations, interconnected and powered, often by testing one to hear all sound. The 120 V circuit for hardwired alarms and any receptacle for a panel transformer is electrical work under the permit. Intrusion wiring is low-voltage, Class 2, and many building departments do not permit it separately (Low-Voltage · Permits). Alarm registration is with the police department, not the building department.

Where it goes wrong

1. Windows and doors replaced before contact holes and wires are placed, leaving surface or wireless contacts as the only option (Part 1).
2. Zones wired without an end-of-line resistor at the device, so a cut or bridged cable reads as normal (Part 1).
3. Glass-break detectors over laminated glass they are not listed for (Part 1).
4. Downward-looking PIR sensors over radiant floors, which lose half or more of their signal on design days (Part 2).
5. Motion sensors facing sunlit glass or a cycling heat source, the false alarms that lead to systems being left off (Part 2).
6. A high-grade deadbolt in a strike held by ¾ in screws; the frame splits first (Part 3).
7. A double-cylinder deadbolt on the egress door, which the egress rule excludes and an inspector flags (Part 3).
8. Wide-angle eave cameras expected to identify faces; mounted high and wide they cannot (Part 4).
9. Doorbells and cameras recording audio by default (Part 4).
10. A video doorbell on a 10 VA chime transformer, which reboots when pressed (Part 5).
11. A replacement garage door hung on an opener without battery backup, contrary to H&S Code 19892 (Part 5).
12. Smoke detection added to a security panel on a burglary-sized battery (Part 6).
13. An interior remodel finaled without whole-house smoke and CO alarms, or with battery alarms where walls were opened or the attic gives access (Part 7).

Reference

Glossary

Cross-zoning

Requiring two zones to trip before an alarm is dispatched.

DORI

Detect, observe, recognize, identify: the IEC 62676-4 detail levels for video.

Dual-technology sensor

A motion sensor combining passive infrared and microwave Doppler.

End-of-line resistor

A resistor at the far end of a zone that lets the panel tell normal from open and short.

Fail-secure, fail-safe

An electric lock that stays locked, or unlocks, when power is lost.

Household fire alarm system

System smoke detectors and notification on a listed control unit (NFPA 72 Chapter 29), in place of smoke alarms.

PIR

Passive infrared: a motion sensor responding to changes in received infrared across lens zones.

Smoke alarm, smoke detector

An alarm contains its own sounder and power; a detector signals a control unit.

Strike

The plate and pocket in the frame that receive the latch or bolt.

Formula sheet

Lens and resolution for each task

Table 6 gives the width of approach a camera can identify across; at the identification density every camera covers the same width per pixel, about 34 ft per 2,560 pixels.

Figure color legend

Red: deficient, or fails the stated criterionAmber: marginal or conditionalGreen: meets the criterionGray: existing or baselineTeal: neutral quantity, reference line or annotation; never a grade

Sources

• California Residential Code (2025, from 1 January 2026): R302.5.1, R310.4, R311.2, R312.2, R314, R315; ICC, IRC R314 code change text (alterations, interconnection and power exceptions); City of Antioch, Residential smoke and carbon monoxide alarms (2025 CRC handout, 2026); The Building Code Forum, Smoke alarm installation changes in the 2024 IRC.
• NFPA 72, Chapter 29 (household fire alarm systems, secondary power; 10-year replacement); UL 217, UL 268, UL 2034, UL 325 and 16 CFR 1211, UL 827, UL 1023 (ELK M1 listing summary); ANSI/SIA CP-01, AVS-01.
• California Penal Code 632, 637.2, 647(j) (Justia, 2024 code); Civil Code 1708.8; Health and Safety Code 13113.7 and 19892 (SB 969, 2018; City of Foster City summary); Humboldt County, CRC R302 (California self-latching amendment); Mono County and Rancho Cucamonga 2025 smoke and CO alarm handouts (valuation threshold).
• IEC 62676-4 (video surveillance application guidelines), DORI densities; CCTV Design Tool, DORI standard.
• ANSI/BHMA A156.40 and A156.2; ASTM F476, F842, F588.
• American Wood Council, NDS (2018), wood screw withdrawal; specific gravities by species group.
• Companion guides: Low-Voltage & Data, Windows & Doors, Lighting, HVAC, Solar & Battery, Framing (set v2).

Set v2 draft 1, 2026-10-23: rewritten as a generic theory guide to spec set v2 from the v1 guide (archived), raised from Light to Moderate depth. Adds zone supervision, PIR contrast over radiant floors, strike-screw withdrawal, camera pixel density and mounting angle, storage and upload, doorbell power, the garage-opener law, panel battery duty, and the smoke and CO remodel trigger now owned here; house values moved to the House Project Guide. After independent review: radiant-floor contrast tied to the HVAC reference load, 50° lens rows, ASTM F588, UL 1023 alarm time, California self-latching and valuation threshold, 637.2 damages and the 30° sensitivity added. Values from one script.

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