750 Feet of 16 AWG and a Misleading Commissioning Reading
One notification circuit leaves a 24 V addressable panel and runs 750 ft to the far end of a single-story distribution center before reaching the last appliance. The circuit uses 16 AWG stranded conductors in a multi-conductor cable. It carries eight wall-mounted horn/strobes. The manufacturer's data sheet lists each appliance at 0.075 A at 24 V, rated for 16–33 V, with published output ratings down to 16 V. The panel's listed NAC output is 24 V nominal and is listed to deliver not less than 20.4 V with the battery at end-of-discharge. At commissioning, with the battery at 27.0 V, the electrician measures 22.6 V at the last device and declares the circuit acceptable. The engineer's basis of design specified no more than 10% voltage drop to the last appliance.
1. Perform the worst-case voltage drop calculation and decide whether the circuit is acceptable.
Read the worked answer
Loop resistance. 16 AWG copper is about 4.89 Ω per 1,000 ft. Both conductors over 750 ft: R = 2 × 750 × (4.89 ÷ 1000) = 7.34 Ω
Load. 8 × 0.075 A = 0.60 A
Drop. 0.60 × 7.34 = 4.4 V
Worst case is not 27.0 V at the battery. Design is calculated with the panel's minimum NAC output at end-of-battery-discharge - 20.4 V here: 20.4 − 4.4 = 16.0 V at the last appliance
That is exactly the bottom of the appliance's 16–33 V range: zero margin. At 16 V the horn's sound output and the strobe's intensity fall below their published 24 V ratings, so the measured 22.6 V is misleading - it was taken from a fully charged battery that is not the worst-case condition.
The circuit also fails the engineer's criterion: 10% of 24 V is 2.4 V, and the calculated drop is 4.4 V.
2. Propose remedies and rank them. Which option may you not choose on your own authority, and why?
Read the worked answer
- Upsize the conductors. 14 AWG (≈3.07 Ω/1,000 ft) → R = 4.6 Ω, drop 2.76 V, 17.6 V at end of discharge. 12 AWG (≈1.93) → R = 2.9 Ω, drop 1.74 V, 18.7 V. Simplest fix on a new installation; costs a repull.
- Add a circuit extender / booster supply near the load. Best when the panel is near its NAC rating or the run is long; each appliance then draws locally and the drop problem largely disappears.
- Split into two circuits from the panel, halving current per circuit - only if spare NAC capacity exists.
- Reduce candela or remove appliances - the option you cannot take alone. Candela is set by space size and coverage, and audibility by ambient noise. Reducing either is a design change that must go back to the engineer of record with a re-check of coverage and sound levels.
Final caution: appliance current is not constant across voltage on all products. Use the manufacturer's voltage-drop tables (current at lowest expected voltage) rather than a single 24 V figure.
3. What must be measured and documented at acceptance?
Read the worked answer
- Measure voltage at the last appliance on each circuit, with all appliances on that circuit active, with the panel operating on battery (or with end-of-discharge simulated by disabling the charger).
- Record the measurements circuit by circuit, not as a single 'circuit OK' entry.
- Confirm the measured value is at or above the minimum in the appliance's published voltage-drop table and that the resulting candela and dBA still meet the design and code minimums.
- Verify supervision: open the circuit and ground it, and confirm the correct trouble annunciates.
- File the as-built voltage drop calculation and the field measurements in the record of completion. A commissioning reading taken at float voltage is not a substitute for either.