Notification Circuit Voltage Drop and Sleeping-Room Audibility
A new three-story motel has one notification appliance circuit supplying 18 horn/strobes on the second-floor corridor and in the sleeping rooms. The panel is rated 24 VDC and the circuit is protected at 2.0 A. At the panel, the circuit current with all appliances operating is measured at 1.8 A. The farthest appliance is 400 ft of 16 AWG copper cable from the panel, one-way. The appliance manufacturer lists an operating voltage range of 16 V to 33 V and a rated current of 0.10 A per appliance at 24 V. The corridor ambient sound level is 52 dBA, and the measured alarm level in the corridor is 68 dBA. In a sleeping room with the door closed, the measured alarm level at the pillow is 70 dBA. The owner wants to use the existing 16 AWG cable to avoid pulling new wire. The designer must determine whether the circuit works and whether the audible notification meets the code.
1. Calculate the voltage drop on the 16 AWG circuit at the measured current and determine the voltage at the farthest appliance. Is the circuit acceptable?
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Use the voltage-drop formula for a two-wire circuit: VD = 2 x L x R x I, where L is the one-way length, R is the resistance per foot of the conductor, and I is the circuit current. For 16 AWG copper, use approximately 4.02 ohms per 1000 ft, or 0.00402 ohms per ft. VD = 2 x 400 ft x 0.00402 ohms/ft x 1.8 A = 5.79 V. The voltage at the farthest appliance is approximately 24 V - 5.79 V = 18.21 V.
The appliance is listed to operate down to 16 V, so 18.21 V is above the minimum operating voltage. However, the circuit is operating with about a 24% drop and only about 2.2 V above the minimum. That leaves little margin for a low panel battery, additional appliances, or a higher actual current at low voltage. A 20% design margin would require at least 19.2 V at the farthest appliance, and 18.21 V fails that margin. The circuit is therefore not a robust design even though it may operate on a bench test. The measured current should also be compared with the panel's 2.0 A circuit rating; at 1.8 A the circuit is loaded to 90% of its rating, which is another reason to avoid adding appliances. The designer should not accept the circuit as-is without a documented voltage calculation and the manufacturer's minimum-voltage data.
2. What are the options to correct the voltage drop, and what factors should govern the choice?
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The most direct option is to increase the conductor size. Replacing the 16 AWG with 14 AWG reduces the resistance to about 2.525 ohms per 1000 ft. The new drop would be 2 x 400 x 0.002525 x 1.8 = 3.64 V, giving about 20.36 V at the farthest appliance, which provides a reasonable margin. Another option is to reduce the load by using lower-current appliances or by splitting the circuit into two notification appliance circuits, if the panel has spare circuits and the zone/notification plan permits it. A third option is to move the panel or add a notification appliance circuit extender or power booster closer to the load, provided it is listed for the system and supervised correctly.
The choice should consider the panel's circuit rating, the battery calculation, the required synchronization, the wiring class, and the owner's construction constraints. Reducing the load by changing appliances may affect the candela rating and the audible output, so the replacement must still meet the coverage and sound requirements. Splitting the circuit may require additional wiring and a revised sequence of operations. The designer should document the selected option with a point-by-point voltage calculation at the minimum battery voltage, not only at normal 24 V, and should verify that the panel can supervise the revised circuit.
3. Evaluate the audible notification results. Does the corridor and sleeping-room sound level meet the applicable public-mode and sleeping-area criteria?
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The corridor result is acceptable on its face. Public-mode audible notification generally must be at least 15 dBA above the average ambient sound level or 5 dBA above the maximum sound level lasting at least 60 seconds, whichever is greater, and not less than 65 dBA. With a 52 dBA corridor ambient, 15 dBA above ambient is 67 dBA, and the measured 68 dBA meets that threshold by 1 dBA. That is a thin margin, so the designer should verify the measurement location, the meter weighting, and whether the ambient level varies.
The sleeping-room result is not acceptable. In sleeping areas, the alarm must be at least 75 dBA at the pillow with the door closed, or 15 dBA above the ambient, whichever is greater, and it must be capable of waking occupants. The measured 70 dBA at the pillow is 5 dBA short of the 75 dBA criterion. The designer should add a listed notification appliance inside the sleeping room or increase the output of the existing appliance, then retest at the pillow with the door closed. The corridor level should not be used to justify the sleeping-room level, because the closed door attenuates the sound. After correction, both the corridor and each sleeping room should be measured and recorded.