Sheet CALC-20 · Design Guide & Calculator
Fire Alarm System Design: Detection, Loops, Battery and Alarm Circuits
Estimate detector quantities, test geometric coverage, check addressable loop capacity, size standby batteries, screen alarm-circuit voltage drop and document key cause-and-effect interfaces. Use it to develop the design, then verify every final value against the adopted standard, fire strategy and certified manufacturer data.
The Guide
What the Calculator Is Actually Checking
A fire alarm design is not a detector-count exercise. It is a chain. The chosen detection strategy must suit the fire risk, the devices must cover the spaces, manual initiation must be available where required, alarm notification must reach the occupants, the control equipment must have capacity and resilience, the standby supply must carry the system through loss of mains, and every interface must perform the correct action when a fire signal occurs.
CALC-20 turns that chain into separate checks. It deliberately keeps several limits editable because the adopted standard, authority, project category and manufacturer can change them.
The Design Sequence
Define the fire strategy
Life safety, property protection, sleeping risk, evacuation method, building use and authority requirements decide what the system must do.
Choose detection
Select smoke, heat, multi-sensor, beam, aspirating, flame or special detection because it suits the fire and environment, not because one type is cheapest.
Lay out devices
Test both area coverage and the maximum distance from any protected point. Then inspect ceilings, beams, HVAC, obstructions and inaccessible voids.
Check loops and circuits
Keep spare capacity. Check isolator strategy, circuit topology, cable type, voltage drop and the effect of a single fault on the protected area.
Size standby power
Use actual panel and device currents. Battery capacity is driven by standby hours, alarm duration, ageing allowance and charger capability.
Write cause and effect
Define what each alarm condition does to lifts, AHUs, smoke control, access doors, suppression, remote monitoring and voice alarm systems.
Current Standards Basis
| Reference | Use in the design |
|---|---|
| BS 5839-1:2025 | Current UK code of practice for design, installation, commissioning and maintenance of fire detection and fire alarm systems in non-domestic premises. |
| NFPA 72:2025 | Current National Fire Alarm and Signaling Code where the NFPA framework is adopted. |
| ISO 7240-14:2013 | Current ISO system-design standard for fire detection and alarm systems in and around buildings. The 2013 date is the current ISO edition. |
| ISO 7240-2:2017 | Control and indicating equipment. |
| ISO 7240-3:2020 | Audible alarm devices. |
| ISO 7240-4:2017 | Power supply equipment. |
| ISO 7240-5:2018 | Point-type heat detectors. |
| ISO 7240-7:2023 | Point-type smoke detectors using scattered light, transmitted light or ionization. |
| ISO 7240-27:2025 | Point-type fire detectors using smoke, carbon monoxide and optionally heat sensors in combination. |
Choose one governing system-design framework and record it clearly. Do not combine detector-spacing, audibility, standby, zoning or fault-tolerance rules from BS 5839, NFPA 72 and ISO 7240 as though they were interchangeable. Product standards and the governing system code should be identified separately in the specification.
Battery Sizing
The Simple Equation, and Why the Inputs Matter
The screening model uses C = (Iq × ts + Ia × ta) × (1 + margin). Here C is required ampere-hours, Iq is total quiescent current, ts is standby duration, Ia is alarm current and ta is alarm duration in hours.
The equation is easy. The difficult part is using the correct currents and reserve assumptions. Catalogue averages are not enough for construction. Use the selected panel's quiescent current, the actual detector and module loads, notification-device current at the selected tone and volume, battery ageing allowance, low-temperature performance where relevant, and the panel manufacturer's maximum supported battery and charging current.
In Kenyan Practice
What Usually Causes Trouble on Site
Detector quantities without a fire strategy. A bill of quantities can contain hundreds of devices and still fail to explain what is being protected, why each detector type was chosen or how evacuation is initiated.
Wrong detector in the right place. Smoke detectors in dusty workshops and kitchens can create persistent false alarms. Heat detection can be too slow for a sleeping risk. Match the sensing principle to the environment and expected fire.
No allowance for ceilings and HVAC. Flat-plan drawings hide beams, bulkheads, supply diffusers, high atria and ceiling voids. Detector siting must be checked against the coordinated reflected ceiling plan and mechanical drawings.
Battery sizing by panel nameplate. The battery supports the installed system, not an empty panel. Count every loop device and every notification load using certified current data.
Cause-and-effect left until commissioning. Lift homing, AHU shutdown, magnetic door release, smoke control and suppression interfaces must be agreed before tender. Otherwise every contractor assumes somebody else owns the logic.
Related Tools
Where This Goes Next
The fire alarm panel supply belongs in the load schedule. Mains and auxiliary circuits can then be checked in the cable sizing calculator and protective device calculator. Coordinate the alarm system with emergency lighting, access control, lifts, mechanical smoke control and the building fire strategy before issue for construction.
Beyond the Calculator
Need a Fire Alarm Design or System Review?
We can develop detector layouts, loop drawings, cause-and-effect matrices, alarm schematics, battery calculations and specifications as part of a coordinated electrical and fire-safety design package.