If panel requirements are 40 amps, which transformer should be used?

• A. A 40 amp transformer
• B. A 50 amp transformer
• C. A 60 amp transformer
• D. A 30 amp transformer

Answer: (B)

The main idea is sizing for a continuous load. Alarm systems are typically on continuously, so the transformer needs to be rated larger than the panel’s continuous current to provide headroom and avoid overheating or nuisance trips.

Multiply the panel requirement by 1.25 (125%): 40 amps × 1.25 = 50 amps. A fifty-amp transformer gives enough capacity for the continuous load while not being unnecessarily large. A forty-amp transformer would be undersized for a continuous 40-amp load, and a thirty-amp transformer is clearly too small. A sixty-amp transformer is overkill for this scenario when fifty amps meets the required headroom.

So, a fifty-amp transformer is the appropriate choice.