Signing Off a Long Remote NAC on 16 AWG
An FAS-I technician is asked to sign off the notification circuit for a new 24 VDC addressable panel serving a detached maintenance shop. NAC 1 is rated 3.0 A and leaves the panel in the main building, runs 700 ft of 16 AWG THHN in EMT, and feeds ten 24 V horn/strobes drawing 0.25 A each at 24 V. The panel's listed operating range for appliances is 16–33 V. With the circuit in alarm, the installer measured 23.9 V at the panel terminals but did not measure at the far end, saying the wire is new and the run is short. The manufacturer's voltage-drop worksheet was left blank. The shop has no other notification appliances, the panel has no booster, and the general contractor wants the final inspection on Thursday. You have wire resistance tables, a true-RMS multimeter, and the appliance data sheets.
1. Determine whether NAC 1 meets the appliances' listed minimum voltage at the last device, using the distributed-load method, and state whether you would sign off the circuit. Show your work.
Read the worked answer
Total load is 10 × 0.25 A = 2.5 A. A 700 ft one-way run is 1,400 ft of conductor. 16 AWG copper is about 4.89 ohms per 1,000 ft, so R = 1.4 × 4.89 = 6.85 ohms. With the loads spread along the run, the end-of-line drop is approximately I × R/2 = 2.5 × 3.42 = 8.6 V, leaving about 23.9 − 8.6 = 15.3 V at the last appliance. That is below the 16 V listed minimum. The concentrated worst case (all load at the far end) is 23.9 − 17.1 = 6.8 V. Horn current also tends to rise as voltage falls, which makes the drop worse, and on battery-only operation the source is lower still, since the panel's low-battery cutoff is typically around 20.4 V. The installer's panel-terminal reading proves nothing about the end of the circuit. I would not sign it.
2. Give two compliant remedies and the verification you would require before the inspection.
Read the worked answer
Remedy 1: replace the 16 AWG with 12 AWG. 12 AWG is about 1.93 ohms/1,000 ft, so R = 1.4 × 1.93 = 2.70 ohms and the distributed drop is 2.5 × 1.35 = 3.4 V, leaving about 20.5 V at the last device - comfortably above 16 V. Remedy 2: keep the wire and install a listed remote power supply or booster at the shop with its own supervised secondary supply, so the panel NAC carries only the control signal or a much smaller load; splitting the NAC into two circuits is a variant of the same idea. Reducing appliance count or using lower-current appliances is a third option, but it changes the design and must be shown on the drawings. Verification: measure voltage at the last appliance with every appliance on the circuit operating, first on primary power and then with AC off and the panel on battery near low-battery, using a true-RMS meter; record the readings on the test form; and have the designer complete the voltage-drop worksheet.
3. The electrical contractor argues that because 23.9 V is measured at the panel and the code only requires 16 V, the circuit passes. How do you respond, and what would you put in writing?
Read the worked answer
The 16 V figure applies at the appliance terminals, not at the panel. The panel terminal is the source; the conductors between the panel and the last device consume voltage, so the only number that matters is what the last appliance sees under full alarm load. A 23.9 V source reading with no load current flowing, or with the meter at the panel, is not evidence of compliance. In writing, I would state the calculated end-of-line voltage, the method and resistance values used, the fact that the measurement was taken at the panel rather than at the last device, and that the circuit does not meet the listed minimum. I would note that I cannot sign the record of completion for that circuit until it is corrected and re-measured, and I would copy the owner's representative and the designer so the deficiency is documented before the inspection date.