NAC Tap in a Shared Raceway: Load, Voltage Drop, and Article 760 Separation
A four-story occupied office building in Paramus has an addressable 24 V dc fire alarm control panel with four notification appliance circuits rated 3.0 A each. A third-floor tenant fit-out adds 14 listed horn/strobes, each drawing 85 mA at 24 V dc, on one new notification circuit. The electrical contractor proposes to pull two 18 AWG THHN conductors from the existing riser through a 3/4 in. EMT that already carries a 120/208 V multiwire branch circuit serving corridor lighting, then tap the nearest existing NAC terminal in a junction box. The one-way circuit length is 180 ft. The existing load on that NAC is 1.2 A. The contractor argues that because the panel is power-limited, Article 760 permits 18 AWG and sharing the raceway is acceptable as long as the conductors are insulated for 600 V. The electrical inspector has flagged the submittal, and the fire alarm contractor needs to know whether to order 14 AWG and a separate raceway.
1. Determine whether the proposed circuit is within the panel NAC rating and whether 18 AWG conductors will operate the appliances at the end of the line. Show the load calculation and the voltage-drop calculation, and state which supply voltage governs.
Read the worked answer
Load: 14 appliances x 0.085 A = 1.19 A. Adding the existing 1.2 A gives 2.39 A, which is under the 3.0 A circuit rating (about 0.6 A of spare capacity). The current check passes, but it is not the governing test.
Voltage drop: The circuit must be evaluated at the lowest voltage the panel can deliver when operating on batteries at end of discharge, not at the 24 V nominal or the 27.6 V charger float. For a 24 V system that low-battery value is 20.4 V (85 percent of nominal).
18 AWG stranded copper is about 8.1 ohms per 1,000 ft. Round trip = 2 x 180 ft = 360 ft = 0.36 kft, so R = 0.36 x 8.1 = 2.9 ohms. Drop = 2.39 A x 2.9 ohms = 6.9 V. End-of-line voltage = 20.4 - 6.9 = 13.5 V.
Listed 24 V appliances typically have a minimum operating voltage of 16 V. At 13.5 V the far end of the circuit is outside the listed range, so the design fails on battery even though the current is within the NAC rating.
With 14 AWG (about 3.1 ohms/kft): R = 1.13 ohms, drop = 2.7 V, end-of-line = 17.7 V. That works, but the margin is only about 1.7 V. Verify with the appliance manufacturer's voltage-drop worksheet and check whether 12 AWG, a shorter route, or splitting the load across two NACs gives a healthier margin. Also confirm that the added tap does not strand supervision at the original junction box and that box fill and splicing are acceptable.
2. Evaluate the proposal to share the 3/4 in. EMT with the 120/208 V lighting circuit under Article 760. Is the 18 AWG size itself an NEC violation? What must the contractor do instead?
Read the worked answer
Separation is the violation, not the wire size.
- The circuit the contractor is creating is a power-limited fire alarm (PLFA) circuit, governed by Article 760, Part III.
- NEC separation rules require PLFA conductors to be kept at least 2 in. from electric light and power conductors, or separated by a listed barrier. The practical effect is that the PLFA conductors and the 120/208 V branch-circuit conductors cannot occupy the same raceway unless a listed barrier physically divides them. Running both in one 3/4 in. EMT does not satisfy that requirement, and 600 V insulation on the fire alarm conductors does not fix it.
- 18 AWG is the minimum size permitted for PLFA conductors, so size alone is not a code defect. The defect is electrical performance (see voltage drop) and the shared raceway.
What to do: Install a separate raceway or listed fire alarm cable pathway for the new NAC, keeping the required separation from the lighting circuit. If a common wireway is unavoidable, use a listed barrier system approved for the purpose. Bond and ground per Article 250 and keep the fire alarm conductors identifiable at both ends. Do not tap an existing NAC terminal in a box that was not intended for the splice without verifying box fill, accessibility, and that supervision of the extended circuit is maintained.