ok so secondary power. every fire alarm submittal ive ever put together has a battery calc page in it, and most techs i talk to can do the math just fine, they just dont know why the numbers are 24 and 5. so lets clear that up first, then get into the part that actually trips people up, generator vs battery, and where voice systems change the rule.
why 24 hours
NFPA 72 (section 10.6.7 in the editions ive worked under, the numbering shifts a little edition to edition so check yours) wants the system to survive a full day on secondary power in the normal supervising state before anyone's touched the panel. nothing alarming, no trouble being worked, just sitting there watching the building. the logic is simple. if the building loses utility power friday night, nobody's walking in to look at it until monday. 24 hours covers a weekend ish gap with a little margin. its not the true worst case, just the number the code landed on.
some occupancies get bumped to 60 hours. hospitals, certain high rises, places where a longer unattended gap is realistic. those are the exception. the overwhelming majority of panel swaps and new installs i run are plain 24/5, so thats what ill walk through here.
and then 5 minutes of alarm (mostly)
after the standby period, the code wants the battery to still have enough left to run a full alarm condition, everything activated at once, for 5 minutes. thats the worst case load, notification appliances, communicator, everything the panel can throw, all at the same time, right at the tail end of a battery thats been sitting on standby for a full day.
heres the part people miss. if the job includes an in building voice evacuation setup, an actual emergency voice/alarm system with speakers and live or recorded messages, that alarm period usually jumps to 15 minutes instead of 5. i think its table 10.6.7.2.1 in the newer editions, dont quote me on the exact table number, ive seen it move around. if youre on a voice job, go pull the actual number for whatever edition your AHJ has adopted. dont just assume 5 because thats what you always use on conventional jobs.
how this feeds the battery calc
standby and alarm periods are literally the two numbers you multiply by. standby amps times 24, alarm amps times however many minutes that job requires (5 or 15) divided by 60, add them, then derate. i already went through the full formula and a real panel swap job, number by number, in my battery calc walkthrough, so i wont re type the whole thing here. short version though, the derating factor almost every panel maker's form uses is 1.2x, which comes from the code not wanting you to discharge a sealed lead acid battery past 80 percent of its rated capacity.
a small office job, actual numbers
different job than the one in that other article. this one was a small single tenant office, one Fire-Lite conventional panel, pretty light load.
| Item | Current |
|---|---|
| Panel quiescent | 0.080 A |
| 2 NAC circuits, supervising only | 0.004 A |
| 4 SLC devices, supervising | 0.028 A |
| Total standby | 0.112 A |
| NAC1, 3 horn/strobes | 0.303 A |
| Panel alarm draw | 0.070 A |
| Total alarm | 0.373 A |
standby term: 0.112 x 24 = 2.688 Ah alarm term: 0.373 x (5/60) = 0.031 Ah raw total: 2.719 Ah derated: 2.719 x 1.2 = 3.26 Ah
next standard battery size up is 4 Ah. thats it, tiny job, tiny battery. compare that to the 7 Ah result on the panel swap in my other article and you can see how much the standby draw, mostly quiescent current plus whatever comms gear is riding along, drives the whole outcome. the alarm term barely moves the needle on either job, because 5 minutes is such a small slice of 24 hours.
generator instead of batteries?
this comes up more than you'd think, especially on bigger jobs. yes, NFPA 72 does allow an engine driven generator to serve as secondary power instead of, or alongside, batteries. its not a free pass though. from what ive seen it still wants some amount of battery capacity as a bridge, so the system isnt dead in the gap between losing utility power and the generator actually picking up and stabilizing. the generator also needs to be dedicated or have priority transfer for life safety loads, with fuel sized to cover the standby period. the exact minimums have moved around by edition, so im not going to throw out a number id have to walk back later. pull the current requirement for whatever edition is adopted where you're working.
honestly, for the small stuff, single tenant office, small retail, a church, youre basically always doing batteries. generators show up on campuses, hospitals, big mixed use buildings with mass notification, places that already have a generator for other life safety loads and it's just easier to tie the fire alarm secondary power into that. if youre not sure which way a specific job should go, thats a conversation with the AHJ and probably a fire protection engineer, not something to guess at from a blog post.
what actually gets flagged
a few things ive seen kicked back on the secondary power side specifically, separate from the battery calc math itself. AHJs check that the standby and alarm periods you used on the form actually match the occupancy on the plans, not just whatever the panel maker's template defaulted to. if the plans call out a voice evac system and the calc still shows 5 minutes of alarm, thats an easy catch and an easy correction letter, and it makes the whole submittal look less careful than it probably was.
battery age is another one, not really a submittal item but its part of the same picture so ill mention it. sealed lead acid batteries in these panels are usually good for somewhere around 3 to 5 years before capacity starts dropping, and inspection standards generally want an actual load test, not just a resting voltage check. a battery that reads fine sitting on the shelf can still sag under a real alarm load if its old. thats more of a chapter 14 testing thing than a design thing, but worth keeping in mind when youre sizing a replacement on an existing system, not just a new install.
anyway
full disclosure, i built FireDeck because filling this stuff out by hand, panel after panel, AHJ after AHJ, got old. it runs the battery calc on the actual manufacturer form for Fire-Lite, Notifier, Potter or Silent Knight and packages it with the rest of the submittal. theres a free 30 day trial if you want to poke at it, no card needed. but the math above is the math either way, generator or battery, 5 minutes or 15.