Adding Horn/Strobes to an Existing NAC in a Hotel Corridor
A three-story, 96-room hotel in a first-ring Minneapolis suburb is adding eight horn/strobes (65 mA each at 24 VDC, 15 cd) to an existing notification appliance circuit serving a new first-floor corridor. The NAC leaves the fire alarm control panel in the first-floor electrical room and runs 240 ft to the first new appliance, then 120 ft more to the last one. All existing and proposed wiring is 16 AWG FPLR in 3/4 in. EMT. The panel's NAC is rated 2.0 A at 24 V filtered DC and already carries 1.10 A of alarm load. The panel has a 24 V, 18 Ah battery set, an existing calculated standby load of 0.35 A, and a low-battery cutoff that presents 20.4 V to the NAC at end of discharge. Appliance cut sheets list an operating range of 16 to 33 V. The corridor ceiling is a return-air plenum and the riser passes through two floor assemblies. The contractor wants to energize Friday.
1. Can the eight appliances be added to this NAC as designed? Show the current and voltage-drop math at both nominal and end-of-discharge voltage.
Read the worked answer
Capacity first: 8 x 0.065 = 0.52 A added, so the circuit carries 1.62 A against a 2.0 A rating, about 81 percent. That passes the panel rating but leaves only 0.38 A of headroom, which is thin for a corridor that will likely grow.
Voltage drop is the real problem. 16 AWG runs about 4.02 ohms per 1000 ft per conductor, so the loop is about 8.04 ohms per 1000 ft. Segment 1 is 480 ft of loop, 3.86 ohm, carrying 1.62 A: a 6.25 V drop. Segment 2 is 240 ft of loop, 1.93 ohm, carrying 0.52 A: a 1.00 V drop. Total about 7.25 V.
At a nominal 24 V the last device sees about 16.75 V, only 0.75 V above the 16 V minimum, and that assumes the supply is exactly 24 V. At the 20.4 V end-of-discharge condition the last device sees about 13.15 V, below its listed range, so it will drop out or fail to sync during a real battery event. Do not energize as designed. Upsizing the new run to 12 AWG (about 1.93 ohms per 1000 ft) gives roughly 1.5 V in segment 1 and 0.24 V in segment 2, about 1.74 V total, leaving 18.6 V at end of discharge. Splitting the corridor onto a second NAC also works and buys back capacity.
2. The contractor says the cable is already FPLR in EMT, so the plenum and the riser are both covered. Is that correct?
Read the worked answer
Half right, and the half that is wrong is the exposed portion. In the vertical shaft through the two floor assemblies, a riser-rated cable (FPLR) is the correct listing, provided the penetrations are firestopped as the rated assembly requires.
In the corridor, the governing rule is 300.22(C) for space used for environmental air. Where the wiring method is EMT, the raceway itself is permitted, so cable inside continuous EMT can be acceptable. Where the cable leaves the raceway - for example, an exposed whip from a box to a ceiling-mounted horn/strobe, or any section where the EMT is omitted for convenience - the cable must be plenum listed (FPLP) for that exposed length. Review the actual routing, not the spec sheet: the failure mode in these jobs is a short exposed whip above the lay-in ceiling that everybody assumes is 'in conduit.'
Also confirm that the added devices are listed for the mounting height and candela required by the adopted code edition for the corridor size, and that the plans examiner has accepted the added appliance locations before the ceiling grid goes in.
3. Does the battery need to be replaced? Support the answer with a standby calculation and identify the controlling constraint.
Read the worked answer
Run the standard two-part calculation. Standby: 0.35 A x 24 h = 8.4 Ah. Alarm: 5 minutes at (0.35 + 1.62) = 1.97 A, which is 1.97 x 5/60 = 0.16 Ah. Total 8.56 Ah. Apply a 20 percent aging/derating factor and the requirement is about 10.3 Ah against an 18 Ah battery, so capacity is not the limiting item.
What should be checked next is the charger's ability to recharge the battery within the manufacturer's stated time after a full alarm, and that the panel's overall power supply load (not just this NAC) stays within its listed rating. The controlling constraint for this job is the 20.4 V end-of-discharge voltage, not ampere-hours: that is the number that kills the last appliance once the 16 AWG drop is applied. Fix the wiring, then re-run both the battery and the low-battery-cutoff numbers and record them for the acceptance test file.