Class A SLC Specified but Wired Class B with T-Taps
A new six-story office building has an addressable signaling line circuit serving 1,200 devices. The engineer's sealed plans specify Class A signaling line circuits. The installing contractor wired the SLC as Class B and used T-taps on floors 3 and 5 to save two home runs. The panel is listed for Class B and shows no trouble. During the acceptance test, the contractor proposes to test 10 percent of the smoke detectors because the rest are the same model, and to use only the magnet test for duct detectors. The AHJ has not yet signed off, and the building owner wants to occupy the building next week.
1. Is the Class B wiring with T-taps acceptable when the plans specify Class A?
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No. The approved plans specify Class A signaling line circuits, and the contractor cannot substitute Class B because the panel happens to be listed for Class B and shows no trouble. Class A provides a redundant path so that a single open or ground fault does not disable the circuit; T-taps create branches that are not part of the supervised return path and defeat that redundancy. Even where a panel permits some T-tapping on Class B circuits, the installation does not match the sealed design. A field change of this magnitude requires written approval from the engineer of record and the AHJ, supported by a revised design and a code analysis. The contractor should rewire the SLC as Class A and remove the T-taps. The absence of a trouble indication is not proof of compliance, because the panel may be operating as designed for a Class B circuit.
2. Is the proposed 10 percent sampling and magnet-only testing acceptable for acceptance?
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No. Acceptance testing under NFPA 72 requires testing of all initiating devices, notification appliances, and system functions; there is no 10 percent sampling allowance for a new addressable system. The fact that many detectors are the same model does not excuse testing each device for annunciation, location, and function. The magnet test checks the detector's alarm circuit and the panel's response, but it does not verify smoke entry, sensing chamber performance, sensitivity, or contamination. Duct detectors must be tested with smoke or the manufacturer's approved aerosol method, and the associated relay, fan shutdown, and supervisory signals must be verified. The contractor must test 100 percent of the devices and document each result. Testing only 10 percent would leave most of the 1,200 devices unverified and would not support the record of completion.
3. What must the contractor do before the AHJ will accept the system?
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The contractor must correct the SLC to the specified Class A configuration, remove the T-taps, and provide revised as-built drawings showing the actual wiring. The engineer of record should review and seal the change if any deviation remains. The contractor must then perform a complete acceptance test: 100 percent of smoke detectors with smoke or approved test method, 100 percent of duct detectors with smoke/aerosol and verification of relay and fan shutdown, all manual stations, all notification appliances for audibility and visible coverage, all alarm and supervisory functions, and all trouble and power-failure conditions. Battery and voltage-drop calculations for the as-installed system should be available. The completed record of completion, device test records, and as-builts should be submitted to the AHJ. Given the size of the system and the pending occupancy, the contractor should request an inspection sequence rather than ask for a waiver of testing.