Advanced Fire Alarm Calculator
Example Data Table
| Item |
Example Value |
Meaning |
| Panel standby |
180 mA |
Control unit load during normal monitoring |
| Detectors |
70 at 0.08 mA |
Initiating device standby demand |
| NAC appliances |
18 per circuit at 75 mA |
Alarm load on each notification circuit |
| Wire run |
320 ft using 14 AWG |
One way distance for voltage drop |
| Battery rule |
24 hours plus 5 minutes |
Required backup operating period |
Formula Used
Standby current equals panel standby plus detector standby plus module standby plus auxiliary standby.
Alarm current equals panel alarm plus device alarm plus total NAC alarm load.
Battery Ah equals standby amps times standby hours, plus alarm amps times alarm hours.
Required Ah equals battery Ah times safety factor times aging factor.
NAC design current equals appliance current plus extra load, multiplied by spare factor.
Round trip resistance equals wire ohms per foot times one way length times two.
Voltage drop equals NAC design current times round trip resistance.
End voltage equals system voltage minus calculated voltage drop.
How to Use This Calculator
Enter the control panel standby and alarm values first. Add detector counts and their current values. Add module counts and auxiliary loads. Enter the number of notification circuits, appliance count, and appliance current. Choose the wire gauge and one way wire length. Set standby hours, alarm minutes, safety factor, and aging factor. Press Calculate. Review the result above the form. Use CSV for spreadsheets. Use PDF for a compact record.
Simplex Fire Alarm Calculation Guide
Simplex fire alarm planning needs careful math. A panel must run during normal standby. It must also support a full alarm event. This calculator combines both needs. It helps estimate batteries, notification load, circuit headroom, and voltage drop.
The standby load covers the control panel, detectors, modules, and auxiliary devices. These parts draw small current all day. The alarm load adds horns, strobes, speakers, relays, and active modules. Alarm current is usually much higher. Good design separates these two states.
Battery capacity is one key result. The tool multiplies standby current by standby hours. It then adds alarm current for the selected alarm minutes. A safety factor and aging factor are applied. This gives a practical amp hour target. The selected battery should meet or exceed that value. Local rules may require more capacity.
Circuit loading is also important. Each notification circuit has a current limit. The calculator compares appliance demand with that limit. It also adds spare load when entered. Keeping spare capacity helps future changes. It also reduces nuisance troubles after field additions.
Voltage drop is another major check. Long wire runs create resistance. Higher current creates more drop. The calculator uses wire gauge resistance and round trip length. It estimates end of line voltage. Devices must still receive enough voltage during alarm. If the end voltage is low, use larger wire, shorter runs, fewer devices, or another circuit.
This page is useful during early design, review, and service estimates. It does not replace approved drawings. It also does not replace listed equipment data. Always confirm current values from product sheets. Always follow the authority having jurisdiction. Simplex systems can vary by model, options, and wiring method.
Use conservative numbers when unsure. Enter worst case appliance current. Include remote annunciators, door holders, relays, and interface modules. Check every circuit separately when loads are different. Export the result for project records. The CSV file supports spreadsheets. The PDF file supports quick review notes. Clear calculation records make coordination easier and safer.
For best results, compare each branch circuit separately. Record assumptions beside each output. Use installed cable length, not plan distance only. Small differences can change final voltage and required capacity during inspection later.
FAQs
What does this calculator estimate?
It estimates standby current, alarm current, battery capacity, notification circuit load, voltage drop, and end of line voltage from entered design values.
Can I use it for final approval?
Use it for planning and checking. Final approval should follow listed equipment data, local code, project drawings, and the authority having jurisdiction.
Why are standby and alarm currents separate?
Standby current runs for many hours. Alarm current runs for a shorter period but is larger. Battery sizing needs both conditions.
What is NAC design current?
It is the notification appliance circuit current after adding the entered spare percentage. It helps test circuit headroom before installation.
Why does wire gauge matter?
Smaller conductors have more resistance. More resistance creates higher voltage drop. Low end voltage can cause appliance operation issues.
What battery size should I choose?
Choose a listed battery size that meets or exceeds the required amp hour result. Also confirm panel cabinet and charger limits.
Why add a safety factor?
A safety factor allows margin for aging, tolerance, field changes, and uncertain current values. It supports conservative design review.
What should I do if voltage fails?
Reduce circuit load, shorten the run, use larger wire, add another circuit, or check whether appliances support the calculated voltage.