Question: A fire alarm project specification requires a maximum 72 hours standby capacity for the system. NFPA 72 requires a minimum standby capacity of 24 hours. What is the maximum standby capacity permitted by NFPA 72?
Answer:
Chapter 1 of NFPA 72 identifies the codes as a minimum standard. 72-1.2.3 states “This Code establishes minimum required levels of performance…”. NFPA 72 identifies the minimum performance requirements of the fire alarm system.
However, Chapter 1 of NFPA 72 also states the exceeding the requirements of the code is also permissible. 72-1.5.1 states “Nothing in this Code shall prevent the use of systems, methods, devices, or appliances of equivalent or superior quality, strength, fire resistance, effectiveness, durability, and safety over those prescribed in this Code.”
NFPA 72 10.6.7.2 states that fire alarm systems are required to have adequate secondary power capacity to power the systems in quiescent (non-alarm condition) for a minimum of 24 hours. At the end of standby time, the system shall be capable of operating all alarm notification appliances for 5 minutes. Emergency voice alarm communications systems (voice evac) are required to provide 15 minutes of operation at maximum connected load.
NFPA 72 permits secondary power to be provides by storage batteries or an automatic starting, engine driven generator. If a generator is used, the system requires 4 hours of standby power. This 4 hours is to allow the system to function in case the generator does not automatically start.
Since the minimum requirement is 24 hours when using storage batteries stating a maximum capacity of 72 hours does not conflict with the Code. Remember, NFPA 72 only states the minimum requirements, and permits you to exceed them. If a system requires 18AH of battery capacity to achieve the minimum 24 hours standby time, going to 36 AH for 48 hours standby, or even 54AH for 72 hours standby would be acceptable by NFPA 72.
An important consideration when calculating battery requirements for the system is the requirements of NFPA 72-10.6.10.3.2, which requires that batteries be capable of being fully recharged by the fire alarm panel within 48 hours after a full discharge. Larger amp-hour batteries configurations require larger power supplies to meet this recharge time requirement.
Fire alarm control panel manufacturers state maximum battery capacity for the control units in their documentation. The primary driving factor in the maximum battery capacity of a control unit is the capability of the charger to meet the 48 hour recharge requirement.
Even if the charger is capable of meeting the 48 hour recharge requirement, the physical size of the larger amp-hour batteries may exceed the physical dimension for panel cabinet. This could result in an external battery requirement just to support the larger batteries