Plan and Specification Reading, System Layout, and Calculations
Domain 8 tests your ability to read a fire alarm drawing and prove the design will work. You need symbols, the riser diagram, the sequence of operations, and voltage-drop and battery calculations.
Core rules
- Read the symbol legend, floor plans, riser diagram, and sequence of operations together. The sequence tells you what activates what; the riser tells you circuit routing.
- Confirm device spacing and coverage against NFPA 72, and confirm circuit loading against panel and appliance ratings.
- Voltage drop formula: Vdrop = 2 times length times resistance per foot times current. Use the round-trip length for two conductors.
- Compare the result with the appliance minimum operating voltage, not the panel's nominal 24 V.
- Battery calculations follow the standby-plus-alarm method with derating.
Worked example
A notification circuit is 200 ft one way, carries 1.25 A, and uses 16 AWG copper with a resistance of about 4.1 ohms per 1,000 ft. Round-trip resistance is 2 times 200 times 4.1 divided by 1,000, or 1.64 ohms. Voltage drop is 1.64 times 1.25, or 2.05 V. Starting at 24 V, the last appliance sees about 21.95 V. Confirm that this exceeds the appliance minimum, typically 16 to 20 V, and adjust wire size or circuit loading if it does not.
Traps
- Using one-way length instead of round trip.
- Comparing voltage drop to nominal 24 V rather than the appliance minimum.
- Ignoring the sequence of operations when laying out circuits.
- Mixing battery and voltage-drop calculations.