What is the recommended transformer sizing guideline for an alarm panel and connected devices?

• A. The transformer should be sized to cover all devices at 100% output
• B. The transformer should be sized to output 120% of the expected load
• C. The transformer should be sized to 50% of the expected load
• D. The transformer should only need 80% of its output to cover all needs

Answer: (D)

Alarm panels are a continuous-load situation, meaning the power supply runs almost all the time. The recommended approach is to size the transformer so the expected load uses only about 80% of the transformer's rated output. That 20% margin provides headroom for startup surges from devices like keypads or sirens, voltage variation as the supply ages, and any small future additions to the system. It helps keep the transformer from overheating and ensures reliability over time. If you sized to 100% of the transformer, there’d be little to no room for peaks or aging; sizing much smaller than the load would be wasteful and underutilize the capability. Using about 80% of the rating strikes the right balance.