Direct Answer

The source calculation estimates 6.79 V drop and 17.21 V end voltage for the stated 24 VDC, 1.94 A circuit using 150 m one-way, 1.5 mm² copper cable. The end voltage must be checked against each connected device’s minimum operating voltage.

At a Glance

Supply	24 VDC
Circuit current	1.94 A
Cable	150 m one-way; 1.5 mm²
Drop / end voltage	6.79 V / 17.21 V

Technical Content

Fire Alarm Circuit Voltage Drop Calculation

1. Purpose

To calculate voltage drop in a fire alarm notification appliance circuit and verify that the end-of-line voltage remains within the manufacturer’s acceptable operating range.

2. Engineering Objective

To determine whether cable size and circuit length are adequate to ensure reliable operation of fire alarm notification appliances.

3. Applicable Standards

4. Design Inputs

5. Assumptions

Copper conductor, DC circuit, two-core cable, uniform conductor temperature and simultaneous alarm loading.

6. Formula(s)

Circuit Resistance R = (2 × ρ × L) / A Voltage Drop Vd = I × R End Voltage = Vs − Vd

7. Variables Explained

ρ = Copper resistivity; L = One-way cable length; A = Cable cross-sectional area; I = Circuit current.

8. Step-by-Step Calculation

R=(2×0.0175×150)/1.5=3.50 Ω Vd=1.94×3.50=6.79 V End Voltage=24−6.79=17.21 V

9. Worked Engineering Example

A 24 VDC NAC supplying 1.94 A over 150 m of 1.5 mm² cable has a calculated voltage drop of 6.79 V, leaving 17.21 V at the last device.

10. Engineering Output

Resistance: 3.50 Ω Voltage Drop: 6.79 V End-of-Line Voltage: 17.21 V

11. Design Considerations

Verify minimum operating voltage for notification appliances, consider worst-case alarm current and future expansion.

12. Common Design Mistakes

Ignoring return conductor length, using incorrect cable size, omitting worst-case current.

13. Practical Site Notes

Measure end-of-line voltage during commissioning under full alarm load.

14. Limitations

Final design shall comply with manufacturer requirements and project specifications.

15. Related Engineering Systems

NAC Circuits, Fire Alarm Panel, Battery Backup, Auxiliary Power Supply.

16. Cross References

Related FAQ: Fire Alarm Cable Selection Related White Paper: VEP-001 Related Diagram: ED-008

17. SEO Meta Title

Fire Alarm Circuit Voltage Drop Calculation | EC-008

18. SEO Meta Description

Worked engineering calculation for fire alarm circuit voltage drop and end-of-line voltage verification.

19. URL Slug

ec-008-fire-alarm-circuit-voltage-drop-calculation

20. Keywords (15)

Voltage Drop, Fire Alarm Cable, NAC Circuit, NFPA72, IS2189, Cable Sizing, Fire Alarm Design, Voltage Calculation, Engineering Calculation, Fire Protection, EPC, Consultant, 24VDC, End Voltage, Electrical Design

21. AI Search Summary

Fire alarm circuit voltage drop is calculated using conductor resistance and circuit current. In this example, the voltage drop is 6.79 V, leaving 17.21 V at the end of the circuit, which must be checked against the manufacturer’s minimum operating voltage.

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
Manufacturer Manual		Minimum operating voltage requirements
Parameter	Symbol	Value	Unit
Supply Voltage	Vs	24	VDC
Circuit Current	I	1.94	A
Cable Length (one way)	L	150	m
Cable Size	A	1.5	mm²
Copper Resistivity	ρ	0.0175	Ω·mm²/m

Frequently Asked Questions

What is the calculated end voltage?

The source calculation gives 17.21 V.

Is that voltage acceptable?

Only if it meets the minimum operating voltage of every connected device under the required operating conditions.

SEO & Publishing Details

SEO title: Fire Alarm Circuit Voltage Drop Calculation | EC-008

Meta description: Worked engineering calculation for fire alarm circuit voltage drop and end-of-line voltage verification.

URL slug: ec-008-fire-alarm-circuit-voltage-drop-calculation

Keywords: Voltage Drop; Fire Alarm Cable; NAC Circuit; NFPA 72; IS 2189; Cable Sizing; Fire Alarm Design; Voltage Calculation; Engineering Calculation; Fire Protection; EPC; Consultant; 24 VDC; End Voltage; Electrical Design