Fire Wire Size Calculator

The Fire Wire Size Calculator helps electricians, fire safety engineers, and building inspectors determine the correct wire size for fire protection systems. It ensures that the selected wire can handle the required electrical current (ampacity) and that voltage drop stays within safe limits. Proper wire sizing reduces the risk of circuit failure, overheating, and non-compliance with electrical and fire codes. This tool is part of the Electrical Safety and Fire Protection Calculator category.

Formula of Fire Wire Size Calculator

1. Ampacity Check

First, confirm that the wire’s ampacity meets or exceeds the load current.

Load Current (I)
I = Power (W) / Voltage (V)

Required Ampacity ≥ Load Current

Always consider a safety margin based on local code requirements.

2. Voltage Drop Check

Keep the voltage drop within acceptable limits, typically no more than 5% for power wiring and even lower for sensitive fire alarm circuits.

Voltage Drop (%) = (2 × Length × Current × Resistance per unit length) / Voltage × 100

Where:

• L = one-way wire length (feet or meters)
• I = current (amps)
• R = resistance per unit length (ohms per unit)
• V = system voltage (volts)

If the calculated voltage drop exceeds the allowed percentage, choose a larger wire size.

3. Minimum Wire Size Selection

Pick a wire size that meets both:

• Sufficient ampacity for the current
• Acceptable voltage drop for the entire circuit length

Reference Table

Wire Size (AWG)	Typical Ampacity (Copper, 75°C)	Approx. Resistance (ohms per 1000 ft)
14 AWG	15 A	3.14 Ω
12 AWG	20 A	1.98 Ω
10 AWG	30 A	1.24 Ω
8 AWG	40 A	0.778 Ω
6 AWG	55 A	0.491 Ω

Note: Always check local standards and manufacturer data for precise values.

Example of Fire Wire Size Calculator

Scenario:

• Fire alarm load: 120 W
• System voltage: 24 V DC
• Cable run: 150 feet one way

Step 1: Calculate load current
I = 120 W / 24 V = 5 A

Step 2: Calculate voltage drop for a candidate wire size (e.g., 14 AWG, 3.14 Ω per 1000 ft)

VD = (2 × 150 ft × 5 A × 3.14 / 1000) / 24 V × 100
= (2 × 150 × 5 × 0.00314) / 24 × 100
= (4.71) / 24 × 100 ≈ 19.6%

This exceeds the typical limit (5% for sensitive circuits). Try a larger wire.

Step 3: For 10 AWG (1.24 Ω per 1000 ft)

VD = (2 × 150 × 5 × 1.24 / 1000) / 24 × 100
= (1.86) / 24 × 100 ≈ 7.8%

Still high — consider 8 AWG for longer runs or reduce run length.

Most Common FAQs