9.1 Notification Appliance Circuits and Circuit Classes

Why the pair leaving the panel is a testable skill

The New York Department of State (DOS) Security or Fire Alarm Installer written exam lists control devices among the bulletin topics, and Module 2 of the 81-hour course (19 NYCRR 196.8) is where those devices become copper in the building. Independent OpenExamPrep material in this chapter is study help for that sitting. It is not a DOS publication and it does not claim NFPA review. Chapter 8 covered what lives inside the fire alarm control unit (FACU). This section covers what leaves the FACU: notification appliance circuits (NACs), how they differ from initiating device circuits (IDCs) and signaling line circuits (SLCs), and the Class A versus Class B behavior you can prove with one lifted wire and a trouble LED.

A fire NAC is not a burglar-siren output. Fire notification appliances are listed, polarized, and supervised, and the run is sized for voltage drop. A 12 VDC security siren on an auxiliary output can be loud and still be the wrong mental model for a 24 VDC horn-strobe circuit. This license covers security or fire. Name the circuit by function, not by jacket color.

Polarized horns and strobes, end-of-line, and supervision

A typical 24 V fire horn, strobe, or horn-strobe is polarized. The plus terminal (a “+” mark or a red lead) lands on NAC positive; the minus terminal lands on NAC negative. Reverse the pair and the appliance will not operate as listed even when a meter still shows about 24 V. The same diode or polarized electronics explain a classic supervision method: reverse polarity in standby. Supervision current can still flow through the end-of-line (EOL) device while polarized appliances stay silent. Alarm forwards the polarity and the horns and strobes operate. Newer FACUs may supervise NACs with other listed methods — read the installation instructions — but polarity marks exist for this reason.

The EOL belongs at the electrically last appliance, not on the FACU door and not in the truck. An EOL at the panel supervises only as far as the door. A field cut then looks healthy. On a conventional Class B NAC, that EOL is how the panel sees a continuous pair. On a Class A NAC, both ends of the pair typically return to panel terminals, so a discrete EOL resistor is often omitted because the panel already watches two supervised legs.

NAC appliances are parallel loads. Each device adds its listed current. Chapter 6 used a teaching nameplate of 75 mA at 24 V for one horn-strobe. Ten of those draw 0.750 A. The pair must carry that current without pushing the last device outside its listed voltage window.

Voltage drop — stay inside the listed window

Voltage drop is still V = I × R using round-trip pair resistance. The pass/fail question is not “did we pull 18 AWG.” It is whether every appliance still sees a voltage inside its listed operating range.

Many 24 V notification appliances publish a typical manufacturer window near 16–33 VDC. Treat 16–33 VDC as a typical manufacturer window, not as a universal law and not as a substitute for the cut-sheet number in the submittal. If the last strobe is listed 16–33 VDC and you deliver 15.4 V, that appliance is out of range even though someone still calls the system “24 volt.”

Check the low end on secondary power, not on charger float. A 24 V sealed-lead-acid pair on float may sit near 27.6 V, which is still under a typical 33 V maximum, but float is the wrong number for a depleted-battery drop study. Check the high end so NAC voltage (including any boost circuit) does not exceed the listed maximum.

Worked example — 300 ft NAC

• Voltage used for the check: 24.0 V at the NAC terminals under the secondary-power condition you are analyzing.
• Load: 10 appliances at 75 mA = 0.750 A.
• Wire: uncoated copper 18 AWG ≈ 6.39 Ω per 1,000 ft (the about-20 °C teaching value from Chapter 6; NEC Chapter 9, Table 8’s 75 °C column is higher and would give a larger drop).
• One-way length 300 ft, so round-trip = 600 ft.

1. R_pair = 6.39 Ω/1,000 ft × 0.600 = 3.834 Ω.
2. V_drop = 0.750 A × 3.834 Ω = 2.88 V.
3. V_last = 24.00 V − 2.88 V = 21.12 V.

21.12 V sits inside a typical 16–33 VDC window.

Same load, 800 ft one-way (1,600 ft round-trip):

1. R_pair = 6.39 × 1.6 = 10.224 Ω.
2. V_drop = 0.750 × 10.224 = 7.67 V.
3. V_last = 24.00 − 7.67 = 16.33 V.

That is barely above a 16 V minimum on paper. After 24 hours on batteries the FACU voltage is lower than 24.0 V, and some strobes draw more current as voltage falls. 16.33 V is not a comfortable design. Split the NAC, upsize to 16 AWG or 14 AWG, or shorten the run. Use the listed current at the voltage you are analyzing, including strobe inrush or RMS if the sheet gives it — not a memorized 75 mA for every candela rating.

Synchronization of strobes in a field of view

NFPA 72 treats visible notification in a shared field of view as a timing problem: strobes that can be seen together must be synchronized. Unsynchronized flashes in one view can appear to flash faster than either strobe alone and can affect people with photosensitive epilepsy. Raising candela does not remove the need to sync. Synchronization is a control requirement, not a brightness requirement.

Installer rules of thumb:

• Use appliances and a NAC output or sync module that share a listed sync protocol. Two devices that both claim “1 Hz” but speak different protocols often produce a visible double-flash.
• A corridor that looks into a lobby, or a glass storefront that sees two wall strobes, is still one field of view.
• Do not confuse strobe sync with audible temporal-three coding. Temporal-three is the public-mode horn or speaker pattern. Sync is the visible flash timing.

Speaker circuits for emergency voice/alarm communications

Emergency voice/alarm communication systems (EVACS) use speaker circuits, not 24 V horn-strobe NACs, for voice and tones. Typical distribution is 25 VRMS or 70.7 VRMS. Each speaker sits on a wattage tap (¼ W, ½ W, 1 W, 2 W, and similar). Amplifier sizing is the sum of taps plus spare capacity, not a DC milliamp nameplate.

Speaker circuits are still supervised. An open or short must show trouble. Class A speaker loops are common in large buildings because a single open must not kill a whole floor of voice messages. Do not land 24 VDC horn-strobes on a speaker pair, and do not land speakers on a DC NAC. If the circuit uses an EOL, it is the listed EOL for that audio circuit. Battery calculations for EVACS use a longer alarm duration (Section 9.2).

IDC versus NAC versus SLC

An IDC is a listening circuit. A NAC is a power circuit for appliances. An SLC is a conversation. T-tapping a conventional IDC or NAC the way some SLCs allow hides an open from a single EOL. Chapter 8.3 covered addressable architecture; this table is the field pair you land with a screwdriver.

Circuit class — Class B versus Class A on a single open

NFPA 72 defines several pathway classes, including letters you will meet later (Class N for certain networks, Class X for dual-fault isolation on some SLCs). Do not dump the whole table into a job. What you must wire and test here is Class B versus Class A when one conductor opens.

Class A does not mean “no trouble.” If a Class A circuit opens and the panel stays normal, supervision failed. Class B does not mean “unsupervised.” Class B is supervised; it simply cannot keep far appliances working through an open.

Grounds and shorts have extra pathway rules. For this sitting, own the single-open story and the EOL versus return-pair landing. Two-hour riser survivability is a different requirement (later fire-technology chapter). Do not call Class A “2-hour cable.”

Exam traps

• Landing polarized strobes backwards and calling the NAC dead.
• Parking the EOL on the FACU terminals.
• Using one-way footage in a drop calculation.
• Treating a typical 16–33 VDC manufacturer window as optional because the panel still says “24 V.”
• Mixing strobe protocols in one field of view.
• Landing speakers on a 24 V NAC, or DC horns on a 70.7 V speaker riser.
• Saying Class A hides an open with no trouble indication.
• Calling an SLC a zone that shorts to alarm.

When you can name the pair, land the EOL or the return, keep voltage inside the listed window, and synchronize strobes that share a view, you have the installer-testable NAC story. Section 9.2 turns the same milliamperes into battery ampere-hours.