T-Tapping an SLC and Sharing a Raceway in a Rated Stair
A contractor is adding a monitor module for a new sprinkler flow switch on the third floor of a five-story, fully sprinklered office building with a voice evacuation system. The existing signaling line circuit is Class B, installed in EMT, and passes through the two-hour rated exit stair enclosure on its way up from the panel on the first floor. The contractor proposes to T-tap the new module off the nearest SLC device in a non-rated lay-in corridor ceiling, and to share a 1 in. conduit with the 120 V corridor lighting circuit for a 40 ft section inside the stair. The new module is 210 ft from the panel and the SLC currently carries 78 devices.
1. Evaluate the proposed T-tap.
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Reject it.
An SLC is not a bus you can branch at will. Essentially every listed addressable system prohibits T-taps on the signaling line circuit, and the installation must follow the manufacturer's published instructions and listing. The technical reasons:
- A T-tap creates a branched path that defeats the intended fault response of the Class B circuit - a single open can leave part of the circuit operating while the panel's supervision model assumes it does not.
- Fault isolation, device counts, and voltage/wattage budgets are calculated on a single continuous path.
- The T-tap point itself is frequently inaccessible or unlisted as a splice point.
Correct approach: extend the circuit from an existing device in the normal daisy-chain fashion, or run a new SLC from the panel's next available loop/terminal. Then update the circuit schedule: 79 devices plus the new module must be checked against the loop's maximum device count and current budget, and the added 210 ft of conductor must be included in the loop resistance/voltage calculation. Re-verify the device's address and the flow switch's reporting (alarm versus supervisory) before programming.
2. Evaluate running the SLC in the same conduit as the 120 V lighting circuit through the stair.
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Not acceptable as proposed.
Two separate problems:
Circuit separation. Fire alarm SLC and NAC conductors are power-limited (Class 2/3) circuits. They may not occupy the same raceway, box, or enclosure with power conductors unless the installation uses a listed barrier or a wiring method specifically listed for the combination. A shared 1 in. conduit with 120 V lighting is a straightforward violation and also creates a shock and noise/induction problem. Run a separate raceway.
Pathway survivability. The stair is a two-hour rated enclosure, and the building is a high-rise with a voice system. Code-required survivability levels typically demand that the circuit either be installed in a two-hour rated enclosure, be a listed two-hour circuit integrity cable, or be protected by a listed equivalent. A 40 ft run through a rated stair is exactly where the AHJ will look. Whatever method is chosen - embedded in the rated construction, listed CI cable, or a listed enclosure - it must be continuous and documented.
Resolve both before rough-in. After the change, the acceptance test must include the new module and the altered circuit, and the as-built riser must reflect the actual pathway.
3. What documentation and testing does this change trigger?
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Treat it as a design change, not a field fix:
- Revised drawing / RFI response showing the new circuit routing, conductor sizes, and the splice/termination points - sealed by the designer of record if the original submittal was sealed.
- Updated calculations for the SLC device count, loop loading, and voltage drop; plus an updated battery calculation if standby current changed.
- Sequence of operations update identifying the flow switch as alarm or supervisory, its annunciation, and any output it drives.
- Test and acceptance of the new device per the same standard as the rest of the system, including a signal received at the supervising station.
- As-built and record of completion revisions, with the change noted so the next inspector is not comparing the panel to an obsolete drawing.
If any of this is skipped, the system may work on the day of the test but the documentation will not match the building - which is the condition that fails the next inspection.