NAC vs SLC: fire alarm circuits explained

The examples here use the Fire-Lite, Notifier and System Sensor manuals listed below. They do not establish wiring rules for another manufacturer's equipment.

NAC: check power, supervision and notification settings

A conventional DC NAC supplies compatible horns and strobes. A speaker circuit belongs to the notification system too, but carries amplifier audio; do not apply a horn/strobe DC calculation to a speaker circuit.

Fire-Lite's MS-9200UDLS manual, §2.5, shows its Class B and Class A NAC arrangements.

The Class B arrangement uses the specified end-of-line resistor at the field end of the circuit. In conventional reverse-polarity supervision, the standby polarity reverse-biases the appliances' blocking diodes, keeping the horns and strobes off while a small supervisory current travels through the wiring and end-of-line resistor. An open interrupts that supervision path and produces trouble. In alarm the supply changes polarity so the appliance diodes conduct and the appliances operate. This describes conventional polarized DC appliances; it is not a supervision model for every notification technology.

Locate the end-of-line resistor where the instructions require it; placing it at the panel does not supervise the field wiring in the same way. Do not assume the same termination arrangement for a return-wired circuit.

Size the circuit with each appliance's maximum listed current at the low end of its voltage range for the selected settings and waveform. Account for source capacity, synchronization and voltage at each appliance. System Sensor's L-Series instructions provide a manufacturer example of a 16–33 VDC window for the applicable 24 V nominal DC arrangement; the specified MDL3 combination uses a 16.5–33 V window. Those values are product-specific, not a universal NAC minimum. The same instructions specify 12–18 AWG input terminals for those appliances; a conductor that passes the voltage-drop calculation must still fit the terminals.

Use the voltage-drop guide for an out-and-back and segment-current calculation. Use the battery guide to account for notification loading on the bank that actually supplies it.

SLC: check communication and connected equipment

The Notifier SLC Wiring Manual describes compatible addressable detectors, monitor modules and control modules. The control equipment identifies devices by their addresses and supervises communication by polling: it requests device information and expects valid replies. A device that stops responding produces a communication trouble. Notifier's §1.5 distinguishes CLIP polling from FlashScan's faster group-based communication; polling does not mean that every protocol interrogates devices in the same sequence.

A monitor module can report a connected contact input; a control module can operate an output circuit. Identify the module's separately supplied power as well as its SLC connection.

A conventional initiating device circuit (IDC) is another circuit type. An addressable monitor module can connect an IDC to an SLC. The address then identifies the module; it does not automatically identify which conventional device on that IDC operated.

For an SLC, use the exact controller's protocol, device compatibility, resistance, capacitance, cable length and topology limits. A NAC wire-size result answers a power question and does not establish SLC data performance. The cable guide connects these limits to cable selection.

Manufacturer wiring limits are part of the circuit design

Notifier's SLC Wiring Manual, §2.1, recommends 12–18 AWG twisted unshielded pair for controllers including the NFS-320 and NFS2-640. For those Class B examples, its resistance limit is 50 Ω per branch, measured through the complete outgoing-and-return conductor path. The same section gives 0.5 µF per branch, and its associated wiring notes also cap total SLC wiring capacitance at 0.5 µF, including conductor-to-conductor and conductor-to-ground capacitance. Multiple branches cannot each consume the whole capacitance allowance.

Those are manufacturer examples, not generic SLC limits. Controller and detector combinations can have different limits, including lower resistance allowances on the manual's identified self-test configurations. The selected controller's complete wiring instructions establish the permitted circuit.

Class A, B, N and X: follow the fault's effect

Use NFPA 72 (2022), Chapter 12, §12.3 for the classification reference. The following are our practical explanations, not code definitions or a replacement performance table:

• Class B: a radial circuit has one outward route. An open drops every device beyond the break and produces trouble.
• Class A: trace the outgoing and return routes. The return provides another way to reach devices when one route opens. A return pair alone does not establish protection against a wire-to-wire short.
• Class X: trace continued operation around an open, then examine short isolation as well as the return route. In Notifier's implementation, isolator arrangements protect other devices from a short at one device connection. A Class A loop with occasional group isolators is not automatically Class X.
• Class N: examine the communications network and its endpoints. This is a network pathway concept, not another two-wire detector-loop diagram. Identify monitored communications and the required alternate network paths; ordinary Ethernet connectivity alone does not establish Class N performance. Use §12.3.6 for that review.

For the wired B/A/X implementations discussed here, a single ground can produce a trouble indication while devices remain operational. Continued communication is not evidence that the wiring is fault-free. Notifier's §§4–5 show its Class B, A and X SLC implementations. NFPA's SIG-PRO development record supplies Class N background. The final adopted edition supplies the complete criteria. Do not transfer a legacy style label into a current classification without examining the manual and actual wiring.

Pathway class and fire survivability are separate checks. Use §12.4 for survivability navigation. A return cable installed beside the outgoing cable also needs the applicable routing review; the class letter does not prove physical protection.

Apply the distinction on a drawing

For a fictional monitor module connected to a waterflow contact, trace the contact circuit to the module, the SLC to the control unit, and the programmed notification output to its supply and NAC. Record which devices become unavailable at each proposed fault location. This is a tracing exercise; it specifies no real equipment or program.

For one term at a time, see the short pages on the NAC circuit, the signaling line circuit, the initiating device circuit, the end-of-line resistor and Class A vs Class B wiring.

To practice predicting each fault's effect on a wiring diagram, run the fire alarm circuit class drill.

Use SLC loop troubleshooting for localization and submittals and layout for drawing review. Return to Alarm Ladder or take the free Level II check.

Sources

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• NAC Circuit Designs & Applications technical background, SIG-NAS meeting agenda, NFPA, technical discussion on PDF p. 30. Retrieved 17 Oct 2026; blocking diodes and polarity reversal, not adopted code requirements. No code text or diagrams reproduced.
• MS-9200UDLS Series Manual, 52750 Rev M, Fire-Lite, §2.5. Retrieved 17 Oct 2026; conventional NAC implementation and supervision.
• SLC Wiring Manual, 51253 Rev U9, Notifier, §§1.5, 2.1–2.3, 4–5 and 6–7. Retrieved 17 Oct 2026; polling, communication trouble, device roles, IDC interfaces, conductor sizes, branch resistance, branch/total capacitance and SLC implementations.
• L-Series wall appliance instructions, I56-5845-002, System Sensor, pp. 1–2. Retrieved 17 Oct 2026; DC appliance voltage, synchronization and 12–18 AWG terminal example.
• NFPA 72 (2022), NFPA, §§12.3, 12.3.6 and 12.4, referenced by number only. Access route retrieved 17 Oct 2026; final section text was not accessible in the public reader.
• NFPA 72 SIG-PRO second-revision statements, NFPA, Class N background at §A.12.3.6. Retrieved 17 Oct 2026; development context, not final code text. No figures or tables reproduced.