Adding Horn/Strobes to an Existing Fire Alarm NAC
An existing addressable fire alarm panel in a Greensboro K-8 school is being modified during a media-center renovation. The panel has one NAC rated 24 VDC, 2.0 A maximum. The existing NAC already supplies 8 horn/strobes that draw 90 mA each at 24 V. The contractor proposes to add 12 wall-mounted horn/strobes that draw 110 mA each at 24 V on the same NAC. The new devices are 420 ft from the panel, one-way, on existing 16 AWG stranded cable. The panel battery is 24 V, 7 Ah; standby load is 110 mA, and alarm load excluding the NAC is 250 mA. The design requires 24 h standby plus 5 min alarm. The contractor says the panel's 2 A NAC rating is enough because the new devices total 1.32 A. The AHJ has asked for a load and voltage-drop calculation before accepting the shop drawings.
1. Calculate total NAC current and compare it with the panel rating. Is the contractor correct?
Read the worked answer
Existing load: 8 × 0.090 A = 0.72 A. New load: 12 × 0.110 A = 1.32 A. Total NAC current: 0.72 + 1.32 = 2.04 A. The panel NAC is rated 2.0 A maximum, so the proposed arrangement exceeds the rating by 0.04 A. The contractor is not correct. Even a small exceedance is not acceptable because the panel listing and NAC rating are limits, not targets. The fix is to split the devices onto a second NAC, add a listed NAC extender/power supply, or reduce the number of devices on this circuit. The calculation must be shown on the shop drawings and coordinated with the panel manufacturer's compatibility listing.
2. Calculate worst-case voltage drop to the end device and evaluate it against a typical 24 V fire alarm appliance rating.
Read the worked answer
Use the full loop length: 420 ft one-way means 840 ft of conductor. For 16 AWG stranded copper, Chapter 9 Table 8 gives about 4.09 ohms per 1,000 ft. Loop resistance is 0.840 × 4.09 = 3.44 ohms. At the calculated 2.04 A, the drop is 2.04 × 3.44 = 7.02 V, leaving about 16.98 V at the end device from a 24 V source. That is at or near the bottom of many appliance listings, which often start at 16 V. If the panel is operating on battery at a lower voltage, the end-device voltage will be lower still. The calculation is conservative because not all current flows through the entire cable, but it shows there is little margin. The load exceedance alone requires redesign, and the voltage drop supports splitting the NAC or adding a power supply closer to the devices.
3. Does the 7 Ah battery appear adequate for 24 h standby plus 5 min alarm? Show the calculation and state any limitations.
Read the worked answer
Standby: 0.110 A × 24 h = 2.64 Ah. Alarm current: 0.250 A non-NAC + 2.04 A NAC = 2.29 A. Five minutes is 5/60 = 0.0833 h, so alarm energy is 2.29 × 0.0833 = 0.191 Ah. Total before derating is 2.64 + 0.191 = 2.831 Ah. Applying a 1.25 aging/derating factor gives about 3.54 Ah. A 7 Ah battery has enough calculated capacity for this load, so the battery is not the immediate problem. However, the battery must be a listed fire alarm battery, the charger must be capable of recharging it, and the calculation must use the actual panel alarm current. The NAC overload and voltage drop still require correction before approval.