Class A SLC Sabotaged and Riser Run Without Survivability in a 78 ft Building
A 6-story, 78 ft office building in Wilmington is receiving a new addressable system during a core-and-shell renovation. The drawings specify a Class A signaling line circuit, 2-hour pathway survivability for the riser serving floors 4 through 6, and installation per NEC Article 760. On the rough-in walk you find: (1) about 40 ft of the floor 5 SLC shares a 1 in. EMT with the 120 V corridor lighting circuit, with splices made in a shared 4-11/16 in. box; (2) at the last device on floor 5, the contractor landed the outgoing and return legs of the loop on the same two terminals inside the same backbox "to close the loop"; (3) the riser to floors 4 through 6 is FPLR cable in the exit stair with no 2-hour protection; (4) the NAC circuits are Class B and the owner asks whether that is acceptable. The electrical inspector and the State Fire Marshal's office will witness acceptance.
1. Evaluate findings (1), (2), and (3) in order of severity, citing the governing requirement for each.
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Finding 2 - most severe in effect. A Class A loop exists so that a single open or a single device removal does not disable the circuit; both legs must be physically and electrically separated so a fault at one point cannot take out both. Landing the outgoing and return legs on the same terminals in the same box collapses the loop back into a single-path circuit at that point, so a fault there isolates everything downstream. That defeats the design intent and the drawings, and it must be corrected.
Finding 3 - serious, and code-driven. Where the design requires pathway survivability (and a 78 ft building is the classic case, with the riser serving multiple floors), the wiring must be protected by a listed circuit protective system, be installed in a 2-hour rated enclosure, or use circuit integrity cable. FPLR cable alone is only a riser fire-resistance rating for vertical runs and provides no 2-hour survivability. Replace with CI cable or provide the rated enclosure.
Finding 1 - violation with a fire risk. Power-limited fire alarm conductors may not share a raceway or enclosure with electric light and power conductors unless separated by a barrier or by the separation the installation rules permit. Mixed conductors plus a splice point in the same box means a 120 V fault can energize the SLC. Separate raceways and boxes.
2. Describe exactly how finding (2) must be corrected and what test at acceptance proves the circuit is truly Class A.
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Correction:
- Route the return leg in its own raceway path so that the outgoing and return conductors do not share a box, conduit, or cable jacket at the last device. The last device is the end device of the loop; the loop returns from it independently back to the panel.
- Terminate the return leg at the panel's loop return terminals, not at the last device's field terminals.
- Keep the outgoing and return conductors separated throughout - the common failure is that they are separated at the panel and then run in the same conduit for the whole floor, which returns the circuit to Class B behavior in practice.
- Label the loop ends and update the as-built and the point list so the addresses on the return leg are documented.
Acceptance proof - do not simply look at the as-builts:
- With the system normal, open the loop at a mid-point device (remove the device head or open a terminal). The panel should annunciate a trouble, and all devices on both sides must remain communicating. Record which devices stayed in service.
- Repeat the open at a second location on the other side of the loop, ideally near the far end.
- Restore and confirm all troubles clear and all devices return.
If devices on one side drop out when a single open is introduced, the circuit is not Class A regardless of the wire in the ceiling.
3. Answer the owner's Class B question for the NACs, and outline how you handle the AHJ witness test given the rework.
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Class B NACs. Class B notification circuits are permitted for many occupancies; the code requires that a single open on a Class B circuit not prevent alarm notification throughout the protected area, which is achieved by circuit design and by supervising the circuit for opens. What matters here is whether the drawings and the AHJ's requirements call for Class A notification - high-rise and partial-evacuation designs frequently do, because a single fault must not silence part of a floor. Do not decide it in the field: check the approved drawings and the AHJ's requirement. If Class A notification is required, the riser rework is the moment to fix it, not after the ceiling closes.
AHJ witness test. Do not schedule it until the rework is complete and self-tested. Sequence: correct the shared raceway and splices; correct the Class A return; replace the riser; then run an internal pretest of every device, the Class A open tests, NAC audibility and synchronization, and battery load test. Provide the AHJ with as-builts, battery and voltage-drop calculations, the record of completion, and a written list of the corrected deficiencies, including who performed the work and under whose license. Expect the inspector to select the fault locations for the Class A test himself - which is exactly why the return leg must be genuinely separated.