Circuit Design, Voltage Drop, and Battery Calculations
Voltage drop
Use the loop length, not the one-way distance: Vd = I × R × 2L, where R is the conductor resistance per unit length. A common design limit is 10% drop on a NAC, but the controlling rule is that each appliance must receive at least its listed minimum operating voltage. Calculate at alarm current, including strobe inrush where applicable.
Battery calculation
For a protected-premises system, standby is typically 24 hours followed by 5 minutes of alarm. Convert minutes to hours: 5 min = 5/60 = 0.0833 h. Add standby and alarm loads, then apply a 20% aging margin by multiplying by 1.2. Select the next standard battery size above the result.
Worked example
A 24 V system draws 0.150 A standby and 3.0 A alarm. Standby: 0.150 × 24 = 3.6 Ah. Alarm: 3.0 × 0.0833 = 0.25 Ah. Total = 3.85 Ah. With 20% margin: 3.85 × 1.2 = 4.62 Ah. A 5 Ah battery is the calculated minimum; a 7 Ah battery is commonly selected for margin. For voltage drop, a 2 A load on 300 ft of 16 AWG (4.89 Ω/1000 ft) gives 2 × 300 × 4.89/1000 = 2.93 V, or 12.2% of 24 V - too high. 14 AWG (3.07 Ω/1000 ft) gives 1.84 V, or 7.7%, which is acceptable.
Traps
Do not use one-way length. Do not forget the 1.2 aging factor. Do not mix ampere-hours with amperes. Do not calculate voltage drop at quiescent current. Do not ignore appliance minimum voltage while chasing a 10% rule.