A user of a computer aided design (CAD) system graphically places fire alarm appliances, such as smoke detectors and sirens, in a drawing area. After elements are placed and physical paths are determined, a series of electrical circuit connections between alarm source elements and alarm appliance elements are determined. The electrical circuit connections define wire gauges and lengths passing through the physical paths. The wire types may be determined by predefined rules, and multiple wires may be placed in a single physical path. Subsequently, the CAD system performs electrical calculations associated with the alarm system. The electrical calculations may be based on a supervisory mode of operation as well as an alarm mode of operation. The resistance of wires in the alarm system may be based on a scaled wire length and/or a schematic (non-scaled) wire length. In one example, a backup battery capacity requirement is calculated.