Fire Alarm Circuit Loading After Tenant Fit-Out
You are the low voltage contractor for a 4-story office building. The existing addressable fire alarm control panel (FACP) is a 24 VDC system with a notification appliance circuit (NAC) rated at 3.0 A per circuit and a total panel NAC capacity of 6.0 A. A tenant adds 14 new horn/strobes to NAC 1. Each new device draws 145 mA at 24 VDC in alarm. NAC 1 already served 18 devices totaling 1.9 A. The panel's secondary (battery) supply is 7.0 Ah, and the engineer's design documents show 24 hours standby plus 5 minutes alarm. The tenant wants the work done and inspected this week. Your crew has already pulled the additional cable and terminated the new devices in a daisy chain on NAC 1.
1. Calculate the new NAC 1 alarm current and determine whether the circuit and panel can support the added devices as wired. Show your arithmetic and state the governing limit.
Read the worked answer
New device load: 14 × 0.145 A = 2.03 A. Existing load: 1.9 A. Total NAC 1 alarm current: 1.9 + 2.03 = 3.93 A.
Governing limits:
- Per-circuit rating: 3.0 A → 3.93 A exceeds by 0.93 A.
- Panel total NAC capacity: 6.0 A → if other NACs are lightly loaded the panel total may be fine, but the circuit itself is over its 3.0 A rating.
Conclusion: the circuit as wired is non-compliant and will likely cause voltage drop, dim/failing strobes, or a trouble condition. The added load cannot stay on NAC 1. Options: split the new devices onto a spare or underloaded NAC, add a notification appliance circuit extender/booster supply with its own battery, or reduce device count. Do not simply raise a breaker or ignore the rating - the 3.0 A figure is the circuit's design limit, not a nuisance trip point.
2. The panel's battery is 7.0 Ah. Using 24 h standby plus 5 min alarm, explain what you must verify before energizing, and whether the added alarm load alone is the deciding factor.
Read the worked answer
Battery sizing must cover standby current for 24 h plus alarm current for 5 min (0.083 h), with a derating factor (commonly 1.2 for aging/temperature) applied to the calculated capacity.
Rough check of the alarm contribution only:
- Added alarm current 2.03 A × 0.083 h ≈ 0.17 Ah.
- That alone looks trivial against 7.0 Ah.
But the added alarm load is not the deciding factor. The real driver is the total standby current of the panel (FACP quiescent, addressable devices, annunciators, communicator) multiplied by 24 h, plus the total alarm current of all NACs, then derated. If standby draw is, say, 0.20 A, that is 0.20 × 24 = 4.8 Ah before derating; with 1.2 derating that is 5.76 Ah, leaving little margin once alarm load is added. You must obtain the panel's published standby and alarm current tables, compute the full load, and confirm the 7.0 Ah battery (derated) covers it. If not, the battery must be upsized - a code/listing requirement, not optional.
3. The tenant pressures you to energize now and 'fix the paperwork later.' What is the correct professional response, and what documentation should exist before the system is returned to service?
Read the worked answer
Do not energize a circuit known to exceed its rating. Returning an over-loaded NAC to service risks devices failing to operate in a fire - a life-safety failure, not a cosmetic defect.
Correct response:
- Stop and inform the tenant and building owner in writing that NAC 1 exceeds its 3.0 A rating and cannot be energized as wired.
- Redesign (split circuits, add a booster/extender, or reduce devices) and get the change reflected in the design/permit documents.
- Recalculate battery and voltage drop after the redesign.
Documentation before return to service:
- Updated, as-built riser/point-to-point drawings showing device counts and circuit assignments.
- NAC current calculations per circuit and panel total.
- Battery calculation (standby + alarm, derated).
- Records of inspection/testing of every device (smoke, horn/strobe) per the applicable installation and testing standards, signed by the qualified tester.
- Any required permit/inspection sign-off from the authority having jurisdiction.
Verbal assurances and 'later paperwork' are not acceptable for life-safety work.