Electrical Theory and Code
Electrical Theory and Code
This domain tests Ohm's law, power, voltage drop, overcurrent protection, grounding, and NEC Article 760 rules for fire alarm circuits. You must be able to calculate a circuit and then decide whether the Code permits the wiring method.
Core rules
- V = I × R; P = V × I. For DC voltage drop, use VD = 2KIL/CM, where K ≈ 12.9 for copper, I is amperes, L is one-way feet, and CM is circular mils.
- Article 760 separates non-power-limited fire alarm circuits (NPLFA) from power-limited fire alarm circuits (PLFA). PLFA must be listed, marked, and supplied by a listed power source; field reclassification is not allowed.
- Conductors and cables must be listed for fire alarm use. Separation from power and lighting conductors, support, and protection are tested because a misapplication can induce noise or fault the circuit.
- Overcurrent protection protects the conductor, not the device. Grounding and bonding must follow the manufacturer's instructions and the NEC; an isolated NAC return is not a substitute for an equipment ground.
- Voltage drop matters most on NACs: the last appliance must see its listed minimum voltage at the end of battery discharge, not at nominal 24 V.
Worked example
A 24 VDC NAC draws 2 A and runs 200 ft one way in 14 AWG copper (4,110 CM). VD = 2 × 12.9 × 2 × 200 / 4,110 = 2.51 V. The appliance sees 21.49 V at nominal. If the panel's low-battery voltage is 20.4 V, the actual end voltage is lower: 20.4 − 2.51 = 17.89 V. Recalculate from low-battery, not 24 V; if 17.89 V is below the appliance listing, increase conductor size or reduce load.
Traps
Using one-way length in the drop formula; calculating from 24 V when the panel is on battery; ignoring inrush; assuming any 14 AWG cable is acceptable because it is low voltage; and confusing equipment grounding with conductor identification.