NFPA in USA

NFPA 72 calculator

Fire Alarm NAC Voltage Drop Calculator

Calculate round-trip voltage drop and end-of-line voltage for notification appliance and signaling circuits.

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

Conductor resistance

6.385 ohm/1000 ft

Calculated output

Round-trip length

600 ft

Calculated output

Total circuit resistance

3.831 ohm

Calculated output

Wire size18 AWG
Conductor resistance6.385ohm/1000 ft
Round-trip length600ft
Total circuit resistance3.831ohm
Voltage drop5.75V
End-of-line voltage18.25V
Voltage screenPass

NFPA in USA calculation workpaper

Fire Alarm NAC Voltage Drop 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

Wire size
18 AWG
Circuit current
1.5 A
One-way circuit length
300 ft
Source voltage
24 V
Minimum device voltage
16 V

Results

Wire size
18 AWG
Conductor resistance
6.385 ohm/1000 ft
Round-trip length
600 ft
Total circuit resistance
3.831 ohm
Voltage drop
5.75 V
End-of-line voltage
18.25 V
Voltage screen
Pass

Calculation basis

Formula
Vdrop = I x R x 2L / 1000
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

Vdrop = I x R x 2L / 1000

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 end-of-line voltage to decide whether the circuit needs larger wire, shorter length, or lower load.

Check before use
Confirm one-way route length, conductor size, current, source voltage, and listed minimum voltage.
Common pitfall
Entering round-trip length as one-way length doubles the voltage drop.
Next step
Wire Resistance Calculator

Input evidence

Confirm these values before using the result.

  • Wire size
  • Circuit current (A)
  • One-way circuit length (ft)
  • Source voltage (V)
  • Minimum device voltage (V)

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

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FAQ

What is this Fire Alarm NAC Voltage Drop 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.