NFPA in USA

NFPA 72 calculator

Fire Alarm Battery Capacity Calculator

Calculate NFPA 72 style standby and alarm battery capacity with optional aging margin and standard battery size recommendation.

Fire Alarm SystemsNFPA 72Workpaper

Calculation workpaper

Enter inputs, review outputs, then use the guidance below before carrying the result forward.

US customary units

Calculation inputs

Enter project screening values in the units shown.

Ready

Worksheet header

Optional project fields for keeping the calculation screen traceable.

Results

Ready

Required capacity

7.35 AH

Calculated output

Final capacity

8.82 AH

Calculated output

Suggested battery

12 AH

Calculated output

Required capacity7.35AH
Final capacity8.82AH
Suggested battery12AH

NFPA in USA calculation workpaper

Fire Alarm Battery Capacity Calculator

NFPA 72 ยท Fire Alarm Systems

nfpainusa.comGenerated record

Project record

Project ID
Not entered
Area
Not entered
System
Not entered
Revision
Not entered
Prepared by
Not entered
Checked by
Not entered

Inputs

Standby current
300 mA
Standby duration
24 hr
Alarm current
1800 mA
Alarm duration
5 min
Apply 20% aging margin
Yes

Results

Required capacity
7.35 AH
Final capacity
8.82 AH
Suggested battery
12 AH

Calculation basis

Formula
AH = standby current x hours + alarm current x alarm hours
Reference standard
NFPA 72
Professional boundary
Verify adopted code, project criteria, listed equipment data, and AHJ requirements before relying on this record.
NFPA in USAnfpainusa.comThis is a calculation record, not a signed or sealed engineering document.

Calculation basis

Use the formula and linked reference data after the result is visible. Keep input selection separate from the calculation screen.

Formula

AH = standby current x hours + alarm current x alarm hours

Copper Wire Resistance

Wire SizeOhms per 1000 ft
12 AWG1.588
14 AWG2.525
16 AWG4.016
18 AWG6.385
20 AWG10.15
22 AWG16.14

Use this result

Use final amp-hour capacity to select a standard battery size for review.

Check before use
Confirm standby current, alarm current, durations, and aging margin match the panel schedule.
Common pitfall
Mixing device current from different alarm states can oversize or undersize the battery.
Next step
NAC Circuit Load Calculator

Input evidence

Confirm these values before using the result.

  • Standby current (mA)
  • Standby duration (hr)
  • Alarm current (mA)
  • Alarm duration (min)
  • Apply 20% aging margin

Authority basis

Check jurisdiction, AHJ, submittal, and edition records before using this result as project evidence.

AHJ Contact ChecklistFire Protection Submittal ChecklistInspection and Testing Forms WorkflowNFPA 13, 72, and 25 Edition Basis Worksheet

Record retention

Save the PDF after project fields, inputs, and results are set.

PDF includes

  • Project header and calculator inputs
  • Computed result rows and warnings
  • Formula, reference basis, and professional boundary

Check before filing

  • Input traceability
  • Method review
  • Output reasonableness
  • Professional boundary

Open next

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Engineering disclaimer
NFPA in USA provides engineering workpapers, calculation checks, and reference tables for project screening and coordination. Final design decisions, code interpretations, submittals, and acceptance records must be completed by qualified professionals and verified against the adopted code, project criteria, listings, and Authority Having Jurisdiction requirements. This site does not issue signed or sealed engineering documents.

Related tools and tables

NFPA 72 tool hubAWG Wire Resistance Table for Fire Alarm CircuitsFire Alarm Battery Capacity Calculator

FAQ

What is this Fire Alarm Battery Capacity calculator for?+
Use it to compute a defined fire protection value from documented inputs, then retain the inputs, result, formula, and warnings with the project record.
Are the results suitable for permit submission?+
Use the saved record only after it is checked against adopted code, project criteria, equipment listings, and AHJ requirements.