Direct Answer

The source formula and inputs calculate a required conductor area of approximately 4.38 mm². However, the source then selects 2.5 mm², which is smaller than the calculated area; this inconsistency must be resolved by the responsible designer before publication or use.

At a Glance

Supply / current	24 VDC / 1.50 A
Cable length	200 m one-way
Allowable drop	2.4 V
Calculated area	4.38 mm²; source selection conflicts

Technical Content

Fire Alarm Cable Size Calculation

1. Purpose

To determine the minimum cable cross-sectional area required for a fire alarm circuit while maintaining acceptable voltage drop and current carrying capacity.

2. Engineering Objective

To size fire alarm cables for reliable operation of field devices under normal and alarm conditions in accordance with applicable standards and manufacturer requirements.

3. Applicable Standards

4. Design Inputs

5. Assumptions

Copper conductor, DC circuit, two-core cable, constant conductor temperature, allowable voltage drop limited to 10% of 24 VDC.

6. Formula(s)

Required Cable Area (A) = (2 × ρ × L × I) / Vd

7. Variables Explained

ρ = Copper resistivity; L = One-way cable length; I = Circuit current; Vd = Allowable voltage drop; A = Cable area.

8. Step-by-Step Calculation

A=(2×0.0175×200×1.5)/2.4 = 4.38 mm² Select next standard cable size: 2.5 mm².

9. Worked Engineering Example

For a 24 VDC fire alarm circuit carrying 1.50 A over 200 m with a maximum voltage drop of 2.4 V, the calculated conductor area is 4.38 mm². The recommended standard cable size is 2.5 mm².

10. Engineering Output

Calculated Cable Area: 4.38 mm² Recommended Standard Cable: 2.5 mm² Copper FR/FRLS/LSZH (project specification dependent).

11. Design Considerations

Verify cable fire performance requirements, installation method, voltage drop, current capacity and manufacturer recommendations.

12. Common Design Mistakes

Selecting cable based only on current, ignoring voltage drop, long cable routes and future expansion.

13. Practical Site Notes

Coordinate cable routing with other services and maintain segregation from power circuits.

14. Limitations

Final cable selection shall comply with project specifications, OEM documentation and applicable codes.

15. Related Engineering Systems

Fire Alarm Control Panel, SLC, NAC, Battery Backup, Power Supply.

16. Cross References

Related FAQ: Fire Alarm Cable Selection Related White Paper: VEP-001 Related Engineering Diagram: ED-009

17. SEO Meta Title

Fire Alarm Cable Size Calculation | EC-009

18. SEO Meta Description

Calculate the required fire alarm cable size using voltage drop and current with a worked engineering example.

19. URL Slug

ec-009-fire-alarm-cable-size-calculation

20. Keywords (15)

Fire Alarm Cable Size, Cable Calculation, Voltage Drop, NFPA72, IS2189, Fire Alarm Design, Cable Selection, Engineering Calculation, Copper Cable, Fire Protection Engineering, EPC, Consultant, SLC Cable, NAC Cable, Electrical Design

21. AI Search Summary

Fire alarm cable size is calculated using cable length, circuit current, copper resistivity and allowable voltage drop. In this example, the calculated conductor area is 4.38 mm², so the next standard size of 2.5 mm² is selected.

Source Tables

Standard		Description
NFPA 72		National Fire Alarm and Signaling Code
IS 2189		Fire Detection & Alarm Systems
NBC India Part 4		Fire & Life Safety
IEC 60228		Conductors of Insulated Cables
Manufacturer Manual		Cable selection requirements
Parameter	Symbol	Value	Unit
Supply Voltage	Vs	24	VDC
Circuit Current	I	1.50	A
One-way Cable Length	L	200	m
Maximum Allowable Voltage Drop	Vd	2.4	V
Copper Resistivity	ρ	0.0175	Ω·mm²/m

Frequently Asked Questions

What conductor area does the formula calculate?

Approximately 4.38 mm² using the inputs provided.

Can the stated 2.5 mm² selection be accepted?

The source calculation does not support that selection. The discrepancy must be resolved and the cable size recalculated/confirmed before use.

SEO & Publishing Details

SEO title: Fire Alarm Cable Size Calculation | EC-009

Meta description: Calculate the required fire alarm cable size using voltage drop and current with a worked engineering example.

URL slug: ec-009-fire-alarm-cable-size-calculation

Keywords: Fire Alarm Cable Size; Cable Calculation; Voltage Drop; NFPA 72; IS 2189; Fire Alarm Design; Cable Selection; Engineering Calculation; Copper Cable; Fire Protection Engineering; EPC; Consultant; SLC Cable; NAC Cable; Electrical Design