Audibility and Voltage Drop on an Existing Nursing Home NAC
An existing 24 V panel protects a 96-bed skilled nursing facility. During a verification walk-through you record a 60-second average ambient of 71 dBA in the east activity room while a music program is running, and 45 dBA in the quiet corridor outside. The activity room has two wall-mounted 75 cd horn/strobes at 84 in above finished floor, 30 ft apart; the data sheet lists 84 dBA at 10 ft on the high horn setting. NAC 1 is rated 2.0 A and measures 23.4 V at the terminals with the circuit loaded. It feeds 12 horn/strobes drawing 0.145 A each through a 350 ft home run of 16 AWG. The manufacturer publishes 19.5 V at the NAC terminals when the 24 V battery reaches end voltage. Appliances are listed for 16 to 33 V.
1. Determine the required sound pressure level in the activity room and the corridor, and evaluate whether the installed appliances achieve it.
Read the worked answer
The rule is the greater of 15 dBA above average ambient or 5 dBA above the maximum ambient lasting at least 60 seconds.
- Activity room: 71 + 15 = 86 dBA versus 71 + 5 = 76 dBA, so 86 dBA governs.
- Corridor: 45 + 15 = 60 dBA versus 45 + 5 = 50 dBA, so 60 dBA governs, with the usual 45 dBA floor.
Now compare the hardware. The horn produces 84 dBA at 10 ft, which is already 2 dB short before any distance loss. At 30 ft the loss is about 20 log(30/10) = 9.5 dB, putting the far side of the room near 74 to 75 dBA, roughly 11 dB below the requirement. The corridor is likely satisfied but must be measured, not assumed.
Do not simply turn up the horn. Document the measured deficiency, then add appliances so the required level is met throughout the occupied area, or substitute higher-output devices, and re-measure at multiple points including the far corners and behind the door. Also confirm the sleeping-room criterion separately: 75 dBA at the pillow with the door closed, which this room does not address.
2. Evaluate whether NAC 1 will operate its appliances at end of battery life, and recommend a fix.
Read the worked answer
Use the worst case, not the comfortable one. At end of battery life the panel's NAC output falls to the published 19.5 V, and that is the condition the standard cares about.
- Load: 12 x 0.145 A = 1.74 A (87 percent of the 2.0 A rating, leaving no room to add devices).
- Loop resistance: 2 x 350 ft x 4.016 ohms/1000 ft = 2.81 ohms.
- Drop: 2.81 x 1.74 = 4.89 V.
- Voltage at last device: 19.5 - 4.89 = 14.6 V, below the 16 V listed minimum.
So the circuit fails, even though the 23.4 V measurement taken on site looked fine. Options:
- 14 AWG: 1.77 ohms loop, 3.08 V drop, 16.4 V at the last device. It passes, but with almost no margin for a dirty connection or an added appliance.
- Split into two circuits of six devices: 0.87 A per circuit, 2.45 V drop, about 17.0 V. This is the better answer, and it also restores spare capacity on the panel.
- NAC extender near the load where re-pulling is impractical.
Recommend splitting the circuit on 14 AWG, and record the calculation with the assumed supply voltage stated explicitly.