NFPA 72 §12

What it requires

Defines pathway Classes A, B, C, D, E, N and X by how the circuit behaves under fault conditions. NFPA 72 does not require a particular class: the designer selects it from the system's purpose, the device count on the pathway, the occupant load, and the consequence of a fault. Chapter 23 (23.4-23.7) governs the performance of initiating device, signaling line and notification appliance circuits at a protected premises and refers back to this chapter for the class.