Free calculator · NFPA 72 · NEC Chapter 9
NAC Voltage Drop Calculator
Check the end-of-line voltage on a notification appliance circuit and find the longest run your wire size can carry. There is no 10% rule: the last appliance just has to see its listed minimum voltage.
End-of-line voltage
- Wire resistance
- Loop resistance
- Voltage drop
- Maximum one-way length
Worst case: full load at the far end. The panel manufacturer's calculation for the installed equipment governs.
How the voltage drop calculation works
Rloop = (Ω/1000 ft) × 2 × one-way length ÷ 1000
Vdrop = I × Rloop
Vend = Vstart − Vdrop ≥ appliance minimum
The circuit goes out and back, so the wire length is twice the one-way distance. Example: 1.5 A over 200 ft of 14 AWG solid (about 3.07 Ω per 1000 ft) is a 1.23 Ω loop and a 1.84 V drop, leaving 18.56 V at the end. That passes a 16 V appliance, even though it's a 9% drop.
Wire resistance used (NEC Chapter 9, Table 8)
| Size | Solid (Ω/1000 ft) | Stranded (Ω/1000 ft) |
|---|---|---|
| 18 AWG | 7.77 | 7.95 |
| 16 AWG | 4.89 | 4.99 |
| 14 AWG | 3.07 | 3.14 |
| 12 AWG | 1.93 | 1.98 |
| 10 AWG | 1.21 | 1.24 |
Uncoated copper, DC resistance at 75°C.
Read more: NAC Voltage Drop: There Is No 10% Rule in NFPA 72.
Voltage Drop Questions
Is there a 10% voltage drop rule in NFPA 72?
No. NFPA 72 does not set a flat percentage. The requirement is that every notification appliance receives at least its listed minimum operating voltage under worst-case conditions.
Why start at 20.4 volts instead of 24?
At the end of the required standby period, a nominal 24 V battery supply can be down to about 20.4 V (85 percent). Calculating from 20.4 V checks the circuit when it is weakest, which is when it still has to work.
What minimum voltage should I use?
The listed minimum operating voltage on each appliance's datasheet. Many 24 V regulated notification appliances are listed down to 16 V, but check the actual model.
Is the lumped-load method too conservative?
It assumes the whole circuit load sits at the far end, which is the worst case. If a circuit fails this check, a point-to-point calculation that places each appliance at its real distance may pass. The panel manufacturer's calculation still governs.
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This calculator is a planning aid and does not replace a full design review, the manufacturer's calculation, or the requirements of your authority having jurisdiction. Not affiliated with NFPA.