Fire alarm systems do more than ring bells and flash strobes. In a modern building, they reach into the mechanical room, the security closet, and the elevator machine room. They drive fans, drop maglocks, ground elevator cars, and feed life safety dashboards. When the alarm panel moves from normal to alarm, a web of relay contacts flips with it, and those dry contacts carry a simple message that other systems must obey: change state, now. Getting that wiring right is not glamorous work, but it is the difference between a code-compliant fire system and a tangle of gear that behaves unpredictably in a real emergency.
This is a field guide to alarm relay cabling across HVAC interfaces, access control, and elevator recall. It pairs code intent with job site pragmatism, focuses on what the wiring actually does, and flags the traps that cause punch-list headaches and failed inspections. It assumes you already know basic fire alarm installation tasks like device addressing and circuit supervision. The emphasis here is on integration, where mechanical and electrical scopes intersect and liability hangs on crisp interface design.
Why relay interfaces matter when things go wrong
When smoke builds on a floor, the detector does not shut fans by itself. The fire alarm control unit (FACU) interprets inputs and throws dedicated relays that command the air handler to stop, redirect, or purge. When a pull station is activated near an access-controlled lobby, the card readers do not magically unlock; they open because the fire panel drops power to the door hardware or directly opens the access control panel’s fire input. Elevators do not find their recall levels by luck; they are told to recall by a supervised pair of wires that bridge the fire panel and the elevator controller.
Every one of those operations rides on specific relay contacts and properly rated cabling. If those terminations are loose, mislabeled, or left to guesswork, the building will not behave as designed under stress. You get nuisance trips, stuck dampers, unlocked doors that should be secured during non-alarms, or worse, an elevator that fails to return to the recall floor when it should. An inspector will see the symptoms, not the painstaking wiring work, so the burden sits on the installation to be predictable, testable, and documented.
The wiring language: relays, supervision, and interlocks
Before diving into subsystems, align on terms you will find on shop drawings and in cut sheets:
A relay contact is a switch that changes state when a coil is energized. In the fire panel, relays can be addressable control modules or built-in output relays. They present as common (C), normally open (NO), and normally closed (NC). Dry contacts are preferred for third-party interfaces. Never export voltage from the fire panel into another system unless the design calls for it, and coordinate any power crossing early.
Supervision on output interfaces differs from input circuits. NACs and SLCs supervise themselves, but a dry contact to HVAC generally is not supervised unless a listed interface module provides end-of-line (EOL) monitoring. Where life safety depends on an output state, consider supervised control modules that watch for open circuits to the remote relay or damper actuator.
Fail-safe is not a moral statement, it is a chosen wiring behavior. A fail-safe unlock relay means if the fire panel loses power, doors unlock. A fail-safe fan shutdown means the fan is commanded off on loss of control signal. Make sure the mechanical and access contractor understands the intended fail behavior and that it matches code, not convenience.
Listed interfaces matter when the signal is part of the life safety function. If you use a non-listed relay pack to switch a smoke control signal, you risk a red tag. Look for UL 864 for fire alarm equipment, and follow the manufacturer’s published compatibility notes.
Codes and the boundaries they draw
The National Fire Alarm and Signaling Code (NFPA 72), the National Electrical Code (NFPA 70), the model building and fire codes, and system-specific standards like NFPA 92 for smoke control and ASME A17.1 for elevators shape the wiring. Jurisdictional amendments decide which edition applies. A few patterns hold across editions:
HVAC shutdown and smoke control wiring usually follows the mechanical code and NFPA 72 coordination language. The fire alarm provides signals, not necessarily the power to drive motors or dampers. Where smoke control is engineered, sequence logic must be documented and tested as a whole.
Access control is governed heavily by the building and fire codes. Door locks that impede egress must release upon fire alarm activation and upon loss of power, with very specific exceptions. The wiring must guarantee that release without relying only on software logic inside the access panel.
Elevator recall, shunt trip, and firefighter service derive from ASME A17.1, NFPA 72, and the electrical code. The fire alarm initiates recall and, where required, shunt trips the elevator main power under heat detector operation in the machine room, hoistway, or overhead. Those circuits require particular heat detector placement and wiring disciplines.
Cable ratings and separation rules still apply at every interface. Keep power-limited fire alarm circuits separate from non-power-limited circuits by barriers or spacing as required by NEC Article 760 and the equipment listings. If you land fire alarm conductors in another control cabinet, verify barriered compartments or install listed separation hardware.
HVAC interfaces: shutdown, smoke control, and damper logic
Nine out of ten HVAC integrations fall into two categories: simple shutdown on alarm, or coordinated smoke control sequences. The first is wiring-light; the second can consume weeks of review and test.
Simple AHU shutdown usually uses a fire alarm control module wired to the air handler’s shutdown input. Some mechanical controls want a dry contact closure to run and open to stop; others want the reverse. Wire the module’s common and NO or NC to match that logic, and label the control pair at both ends as Fire Alarm Shutdown. If multiple AHUs share one floor zone, a single relay might drive a bank of shutdown inputs through an interposing terminal block. Avoid daisy-chaining between panels unless the design explicitly calls for it, and use a supervised control module when the shutdown is a required life safety action.
Smoke control is a different animal. An engineered smoke control system might demand that the fire alarm sequence return air dampers closed, supply fans off, and exhaust fans on for certain zones, while pressurizing stairwells. The fire panel often acts as a command brain, issuing multiple relay outputs that feed the smoke control panel or direct HVAC controls. Here, the control points are usually supervised addressable modules that monitor wire integrity to remote relays. Each damper actuator may have a position end switch that must feed back to the fire alarm or smoke control panel. That feedback is important during acceptance testing when the inspector wants to see commanded state and actual state.
Duct detectors and their wiring present another nuance. Local code may still require return or supply duct smoke detectors on large air handlers, even if area detection exists. The duct detector can trip the AHU directly or signal the fire panel, which then commands shutdown through its control logic. If the duct detector has a local relay that cuts power, coordinate so that the fire alarm still sees the condition for annunciation and supervisory logic. Do not leave the building with rogue duct detectors cutting fans without the panel knowing why.
Be mindful of interlock latencies. For pressurization, a stairwell fan should spool up quickly, but dampers may take 10 to 30 seconds to travel. Some sequences intentionally delay fans to prevent damaging pressure differentials while blades are moving. Pick control modules with clear LED and address labeling, then commission with the mechanical team standing by. Plan for a two-person radio call during testing: one at the fire panel, one at the air handler or damper.
Practical cabling details for HVAC
Use fire alarm rated cable for control runs that leave the fire alarm riser, with the same survivability expectations as the rest of the life safety wiring. In smoke control projects that require 2-hour survivability, that may mean MI cable, CI cable, or cable in a 2-hour rated pathway. For simpler shutdown wiring, plenum-rated FPLP or mechanical room FPLR is often accepted, but confirm with the AHJ.
Terminate control pairs on labeled terminal blocks, not free-floating pigtails inside a VFD or MCC. When crossing from power-limited to non-power-limited enclosures, use listed interface relays and maintain spacing. Give the controls contractor a neat set of landing points and a point-to-point diagram, not a mystery bundle.
Provide tactile labeling that survives grime. A printed heat-shrink marker on each conductor at the far end saves an hour during troubleshooting months later when the ceiling grid is closed and nobody remembers which purple pair is which.
Access control: guaranteed egress under fire conditions
Door hardware design leaves very little gray area on egress. If a door has a maglock, the code expects it to release upon activation of the fire alarm system affecting that area, and upon loss of power to the locking system. Card readers and software cannot be the only path to egress. The wiring must embody that guarantee.
There are two typical patterns. For standalone maglocks powered by a dedicated 24 VDC supply, the fire alarm provides a normally closed relay in series with the lock power. When the panel goes into general alarm for that zone, the relay opens and drops power to the locks. The lock power supply itself should be listed for access control and equipped with fire alarm input terminals, which simplifies the connection and allows supervision of the release circuit. In both cases, the result is the same: the maglocks go dark, the door is free.
For doors controlled by an enterprise access control system, the lock outputs come from the access panel, which has its own fire input terminals. The fire alarm provides a dry contact to that input. When tripped, the access panel releases selected doors via its programming. This approach depends on software; many AHJs still require a hardwired drop of lock power in addition, especially for egress-critical openings. Clarify during submittals whether the jurisdiction requires both the software release and the power drop wired in hardware series.
Motion sensors and request-to-exit devices figure into the door’s local logic, but do not substitute for fire alarm release. A common mistake is assuming that a ceiling-mounted REX on a maglocked door guarantees egress. If smoke banks down and the sensor does not see a person, that door stays locked unless the fire relay cuts power.
From a pure wiring perspective, the cleanest result is a fire alarm relay per lock power supply or per building zone, not per door. That keeps wiring manageable and testing simple. The annunciator panel setup should include a clear zone map so that when Level 3 goes into alarm, the Level 3 doors tied to that relay open. During commissioning, watch the panel show which zone released and walk the door line to confirm behavior.
Practical cabling details for access control
Keep fire alarm conductors and access control conductors separated in shared cabinets unless the cabinet is listed for both and provides barriered compartments. Where you land a fire relay inside an access enclosure, use the designated fire input terminals, not a random input zone.
Use listed fire alarm cable for the relay run. If the cable leaves a rated shaft to reach the access power supply, honor the same survivability requirements as the rest of the life safety wiring for that building. Bond shielding per manufacturer instructions to avoid nuisance interference near door strikes and magnetic hardware.
Label at the door, at the power supply, and at the fire panel. On multi-tenant floors, someone will change a lock power supply in a few years. Give them the information to reconnect the fire relay without guesswork.
Elevator recall and shunt trip: wiring without shortcuts
Elevators demand disciplined wiring and clear division of responsibility. The elevator contractor controls the car and the machine. The fire alarm contractor provides initiation and command signals. The electrical contractor often owns the shunt trip breaker and power feeds. Inspectors judge the assembly against A17.1 and the local fire code. That means your relay cabling must align with the elevator controller’s specific terminal designations and with the fire alarm sequence of operations.
Primary and alternate recall are driven by smoke detection in the elevator lobby and machine room or control space. A smoke detector at the designated recall floor sends a signal that brings all cars to the alternate recall level. A detector at any other floor drives cars to the primary recall level. That logic lives in the fire alarm system, which in turn closes a dry contact pair to the elevator controller’s recall input. In practice, you run an addressable input module at each lobby detector, program the logic inside the panel, then provide an addressable control module that ties to the elevator controller. The control module’s C and NO or NC wires land on clearly labeled recall terminals. Supervise the run if the listing and the AHJ require it.
Firefighter service Phase I and Phase II interfaces are generally internal to the elevator controller and car. The fire alarm’s role is to command recall and to annunciate status. Do not wire into the car unless the elevator contractor directs and takes responsibility for interface points.
Shunt trip is separate from recall. Heat detectors in the machine room, hoistway, or overhead must operate a shunt trip device to remove power to the elevator before sprinklers discharge onto live equipment. That signal must be heat-based, not smoke-based, and typically initiated by properly listed, rate-of-rise and/or fixed temperature heat detectors placed per code. The fire alarm panel can provide the power to the shunt trip coil through a listed relay, or the electrical contractor may provide a shunt trip power supply with a fire input. Verify the control voltage, coil current, and contact ratings. If your control module cannot directly switch the trip coil load, install a listed interposing relay within a barriered enclosure. Never drive a shunt https://www.losangeleslowvoltagecompany.com/blog/ trip coil from a general-purpose relay without confirming ratings and UL compatibility.
Cabling must respect the elevator machine room boundaries. Where conductors enter the controller, follow the manufacturer’s termination practices and separation requirements. Keep power-limited fire alarm cable away from high-voltage elevator wiring. Provide ferrules or ring lugs as required. Some elevator inspectors will not accept bare-stranded landings in screw clamps.
As an operational note, shunt trip timing interacts with sprinkler waterflow. A good acceptance test demonstrates that heat detectors actuate the trip before the sprinkler flows. On the day of test, set detectors in test mode or use heat guns with the elevator technician present. The building owner will not appreciate a surprise elevator outage caused by a miswired shunt trip circuit.
Annunciation and the operator’s mental model
In a crisis, building staff read the annunciator first. If the annunciator panel setup is cryptic, they will waste seconds translating. Map zones and labels to human geography, not just device loops. A label that reads Level 8 Lobby Smoke east makes sense, but Level 8 Duct Return AHU-5 is only helpful if the staff knows where AHU-5 lives. Tie annunciation to the integration points. If an alarm zone releases stair pressurization and drops east wing maglocks, say so in the event messaging. Some panels allow custom text per output activation. Use it.
From a wiring standpoint, annunciators often daisy-chain on a data bus, with local power for backlighting. Keep their cable run protected and separate from general tenant cabling. Test annunciators under alarm and loss-of-power scenarios. If the annunciator goes dark under UPS switchover, adjust the power design. The safety communication network depends on those panels staying up when utility power fails.
Documentation that inspectors actually use
Most problems with emergency evacuation system wiring show up not because the design is wrong, but because nobody can tell what was actually built. A single drawing sheet that gathers the interfaces is worth its weight. It should show the fire alarm control unit, each control module address, the destination equipment tag, the contact function, and the cable identifier. Add a simple matrix that ties alarm inputs to outputs: which detectors command which dampers, which zones drop which maglocks, which heat detectors shunt which elevator line.
During testing, keep a copy printed and mark it with the inspector’s initials as items pass. That document becomes part of the closeout package and a gift to the maintenance crew years later. When they need to trace a dampers-not-closing complaint back to a miswired relay, your diagram saves hours.
Coordination with other trades before the first wire is pulled
Integration succeeds or fails in coordination meetings. HVAC controls teams often plan to use their own relays and want the fire alarm to give only a signal. Access control vendors may want a software release and resist a hardwired power drop. Elevator teams bring their own standards and have strict rules about who touches the controller. Get those preferences on the table early, then write them into the submittals so everyone bids and builds the same thing.
Two details reduce friction. First, agree on point naming that matches across systems. If the fire alarm calls a relay FA-AHU-3-SHDN, make sure the mechanical controls point list uses AHU-3 Shutdown from FA, not something ambiguous. Second, agree on test scripts. A short method of procedure that lists, step by step, the alarm, the expected HVAC/door/elevator response, and the acknowledgement path makes the acceptance test efficient.
Cable selection and physical routing that hold up over time
Cable matters as much as logic. Pick insulation and rating for the environment. In plenums, use FPLP. In risers, FPLR. Where the cable must survive fire for a specified duration, use CI cable or route within a rated pathway. Anchor conduits and cable trays so a damper closure or fan vibration does not damage runs. Protect conductors at sharp edges with bushings. Bond shields at one end unless the manufacturer requires otherwise. Route away from VFDs and high-current feeders that can induce noise into control wiring, particularly for long runs to rooftop equipment.
Where external equipment provides terminals that do not accept fine-strand cable, crimp ferrules to avoid stray whiskers and intermittent faults. Do not mix copper and aluminum under the same lug without listed adapters. Provide drip loops outdoors and at rooftop penetrations so water does not travel into enclosures. When tying into NEMA 3R boxes, seal penetrations, but let the enclosure breathe as intended to avoid condensation.
Commissioning habits that prevent callbacks
The best teams build a rhythm around repeatable checks:
- Verify contact state labeling on every control module with a meter before landing on external equipment. Confirm which side is common, NO, and NC, and mark the correct pair on the as-built. Simulate alarm conditions at the initiating device, not only at the panel. Pull a station, aerosol a smoke detector, apply heat carefully to a heat detector. Then watch the external equipment react and confirm the panel shows the expected event text. Document feedback where available. If a damper provides open/closed end switches, tie them to monitor modules and include them in the test. Seeing the green light on a damper actuator is not as persuasive as the panel recording Damper D-3 Closed. Prove fail-safe behavior. Cut power to the fire panel to see that doors unlock if that is the design. Kill the access power supply and verify release. Drop the HVAC control supply and observe that the air handler remains off, not mysteriously on. Photograph terminations and labels before closing panels. Those images save return trips when a later contractor claims the fire alarm never brought a signal to their cabinet.
Edge cases worth planning for
Older buildings with partial upgrades often have legacy relay boards driving house systems. A new addressable panel might control an old relay bank. In those cases, consider replacing the intermediate relays with listed control modules rather than daisy-chaining through tired hardware. Each added relay introduces contact resistance and failure points.
Campus-style facilities sometimes want global actions: trigger in Building A drops maglocks in Building B. That is not a casual design choice. Cross-building life safety control must use a robust safety communication network with deterministic behavior and must be acceptable to the AHJ. In many cases, you will be better off limiting automatic actions to the affected building and relying on mass notification cabling or messaging for adjacent structures rather than remote lock control.
Atriums and large-volume spaces create nuanced smoke control pressure relationships. If your control relays are set up to sequence exhaust and supply simultaneously, you might see doors that will not open under pressure. The fix might be a time-stagger on the control logic or different fan speeds, not just rewiring, but field discovery starts with seeing that your relays are firing in the intended order and that position feedback agrees.
Mixed-use buildings stack retail over parking. Fire alarm outputs may need to coordinate with CO systems, gas detection, and roller grilles or fire curtains. Those curtains often have their own controllers and battery backups and expect a fire input. Run a dedicated, labeled contact pair to each curtain controller, and test local manual release stations too.
Training the people who will live with the system
Once the wires are tidy and the labels are crisp, the job is only half done. Building staff must understand what to expect when an alarm hits. Walk them through a few real scenarios. Stand on a floor, pull a station, and show how the doors release and which fans stop. Go to the elevator lobby and watch recall. Move to the annunciator and see the text. People remember demonstrations better than manuals.
Leave behind a slim, useful guide: a one or two page quick-reference with the alarm panel connection map, the most critical life safety wiring design points that affect daily operations, and contact info. Call out any planned future integration points, such as a placeholder relay for a future tenant build-out, so changes later do not compromise the system.
The quiet craftsmanship of reliable integration
Most of alarm relay cabling is quiet work: tightening lugs, reading schematics, and pulling pairs through chases sized more for plumbers than for fine control wire. Yet those small tasks are what makes a building behave during emergencies. The craft shows in choices like placing a supervised control module at the damper rather than in the fire riser, so a kicked conduit will alarm instead of silently failing. It shows in the decision to give access control both a software release and a hardwired power drop, even if the minimum code could be interpreted more generously. It shows in the patient coordination with the elevator technician to land on the right terminals and label them in their language.
Do the fundamentals well. Keep the circuits simple, supervise where life safety depends on the path, align the fail states with code intent, and write documentation a stranger can follow. Pair that with a thorough test and a habit of teaching the operators what the panel is really saying. The next time the panel flips its relays for real, the building will do what it should, and most occupants will never know how much thought sat behind a handful of dry contacts.