When to use: Use this when sizing the DC power supply for a PLC rack so it can carry the processor backplane draw plus every digital input/output, analog channel, communications card, and field device powered from the supply. Sum the module and field loads, apply a 25% spare-capacity margin, convert to amps, then round up to the next standard PSU rating. Undersizing leads to brownouts and erratic I/O; this confirms the supply has adequate continuous current and wattage.