Installation, Wiring, and Circuit Calculations
Article 760 governs the wiring
Classify circuits first: Class 1 (power-limited or non-power-limited, installed like Chapter 3 wiring) versus Class 2/3 (power-limited, using Chapter 9 tables, i.e., PLFA). NPLFA uses 14 AWG minimum and Chapter 3 methods; PLFA may use smaller conductors as listed. IDCs and NACs must be supervised. Class B uses an end-of-line device and signals a trouble on an open; Class A returns the circuit to the panel for survivability. Know separation from power conductors, support intervals (within 12 in. of boxes, every 10 ft), box fill, and that cable must be listed for fire alarm (FPL, FPLR, FPLP).
Calculations: NAC voltage drop is Vd = 2 x L x I x R / 1000, where R is ohms per 1,000 ft for the gauge. Battery standby is total standby current x standby hours plus alarm current x alarm hours, multiplied by a 1.25 aging/safety factor. Typical commercial standby is 24 hours plus 5 minutes of alarm (60 hours in some occupancies), per NFPA 72 and the adopted code.
Traps: using one-way length in the drop formula (you must double for the return); omitting the 1.25 factor; mixing amp-hours with amps; assuming an EOL resistor cures a ground fault; and using non-listed cable because "it's in conduit."
Worked example: a 1 A NAC on 500 ft of 14 AWG (about 3.1 ohms/1,000 ft). Vd = 2 x 500 x 1 x 3.1 / 1000 = 3.1 V. If the panel supplies 24 V, the far end sees about 20.9 V - verify against the appliance's listed minimum voltage (often 16-20 V). Rule of thumb: calculate the worst-case device at the end of the longest circuit, not the first device.