Shared Neutral and Voltage Drop on a New Notification Appliance Circuit
A contractor is adding a notification appliance circuit (NAC) for a new wing. The existing panel has a 24 VDC NAC output rated 3 A. The new circuit will feed 14 horn/strobes along a 350 ft run of 14 AWG stranded copper. The contractor proposes to tap the new NAC off an existing 120 VAC lighting circuit's neutral 'to save a wire,' and to use the same 14 AWG for the 24 VDC NAC because 'it's the same size as the lighting wire.' Each horn/strobe draws about 0.15 A at 24 VDC in alarm. The contractor says the 3 A panel rating covers 14 devices at 0.15 A (2.1 A) with margin, so voltage drop is 'not a concern.'
1. Evaluate the proposal to share the lighting circuit neutral with the 24 VDC NAC.
Read the worked answer
Reject it. This is a fundamental wiring error.
Reasoning:
- The NAC is a 24 VDC circuit derived from the fire alarm panel's power supply. It does not use a 120 VAC lighting circuit neutral. Tapping a 120 VAC neutral is not a 'shared neutral' in the intended sense - it is connecting two unrelated systems and creates a shock, backfeed, and ground-fault hazard.
- Fire alarm circuits must be dedicated and electrically supervised and must not share conductors with other systems. Mixing the NAC return with a lighting neutral defeats supervision and can introduce AC noise, ground faults, and a path for the lighting circuit to energize fire alarm wiring.
- The correct arrangement is a dedicated 24 VDC NAC circuit from the panel, with its own supervised return, and the panel's power supply/battery sized for the load.
So the 'save a wire' proposal is not a code nuance - it is a safety defect and must be corrected before any testing.
2. The contractor claims voltage drop is not a concern. Check the current math and explain why voltage drop still matters.
Read the worked answer
The current math is roughly right but incomplete - and the conclusion is wrong.
Current:
- 14 x 0.15 A = 2.1 A total, under the 3 A NAC rating. That part is fine.
- But the panel's NAC rating is a total-load limit, and the battery/standby calculation must also cover the added load for the required duration. Adding 2.1 A to an existing system may exceed the battery capacity even if the NAC output itself is within rating.
Voltage drop:
- Voltage drop depends on current x resistance of the run, and 14 AWG over 350 ft (700 ft round trip) has meaningful resistance. At 2.1 A the drop can pull the far-end voltage below the listed operating range of the horn/strobes (typically ~16–33 V for 24 V regulated appliances). Devices at the end of the run may fail to sound or flash reliably even though the panel 'sees' the load.
- The correct method is to compute drop for the worst-case (end-of-line) device at full alarm load, not just total current, and to size conductors (or split the circuit / use a booster) so every appliance stays within its listed voltage range.
So 'same size as lighting wire' is irrelevant; the NAC conductor must be sized for the 24 VDC load and the run length.