NFPA in USA

NFPA 72 calculator

Wire Resistance Calculator

Lookup and calculate circuit resistance from AWG size and one-way circuit length.

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

Wire Resistance Calculator

NFPA 72 ยท Fire Alarm Systems

nfpainusa.comGenerated record

Project record

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

Wire size
18 AWG
Circuit current
1.5 A
One-way length
300 ft
Source voltage
24 V
Minimum 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
Rtotal = R per 1000 ft 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

Rtotal = R per 1000 ft 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 total circuit resistance as the basis for voltage drop and circuit review.

Check before use
Confirm AWG size, one-way length, current, source voltage, and minimum device voltage.
Common pitfall
Using conductor resistance without round-trip length understates voltage drop.
Next step
Fire Alarm NAC Voltage Drop Calculator

Input evidence

Confirm these values before using the result.

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

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

FAQ

What is this Wire Resistance 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.