Fuse Rating Calculator

Understanding Fuse Ratings

What is a Fuse?

A fuse is a safety device that protects electrical circuits from damage caused by excessive current. It contains a metal wire or strip that melts when too much current flows through it, breaking the circuit and preventing fire, equipment damage, or injury.

Why Correct Fuse Sizing Matters

Undersized fuse: Blows frequently during normal operation, causing nuisance trips and inconvenience.

Oversized fuse: May not protect the circuit adequately. Wiring can overheat and cause fires before the fuse blows. This is extremely dangerous.

Correct fuse: Protects the circuit and equipment while allowing normal operation. The fuse should be rated at 125% of the continuous load, or match the wire's ampacity, whichever is lower.

Fuse Rating Calculation Formula

From Power: Current (A) = Power (W) ÷ Voltage (V)

Fuse Rating: Fuse = Current × Safety Factor (typically 1.25 to 2.0)

Example: 600W device on 12V system: 600W ÷ 12V = 50A. With 125% safety factor: 50A × 1.25 = 62.5A. Use next standard size: 80A fuse.

Safety Factors Explained

125% (1.25): Standard safety margin for resistive loads (heaters, lighting). Minimum recommended.

150% (1.5): Recommended for general use and electronic equipment. Provides good protection with minimal nuisance blowing.

200% (2.0): Required for inductive loads (motors, compressors) due to high inrush current on startup.

Common Fuse Types

Blade Fuses (Automotive):

• Mini Blade: 2-30A (most common in modern cars)
• Standard Blade: 3-40A (older vehicles, general use)
• Maxi Blade: 20-80A (high current automotive)

Midi Fuses:

• Range: 30A-150A
• Used for: High current circuits, battery feeds, alternator charging
• Common in: Trucks, commercial vehicles, aftermarket installations

Strip/Link Fuses:

• Range: 30A-300A+
• Bolted connections for very high current
• Main battery protection, inverters

Glass Tube Fuses:

• Range: 1A-30A typically
• Common in: Household electronics, audio equipment
• Fast-blow (F) or Slow-blow (T/SB)

Automotive Fuse Applications

• 5-10A: Interior lights, radio, clock, low power accessories
• 15-20A: Horn, cigarette lighter, heated mirrors, power windows
• 25-30A: Cooling fans, heated seats, high-power accessories
• 40-60A: Air conditioning compressor, large cooling fans
• 80-100A: Battery charging circuit, main power distribution
• 150A+: Alternator output, main battery feed, high-power inverters

Wire Gauge and Fuse Relationship

The fuse must never exceed the wire's current-carrying capacity (ampacity). Here are maximum fuse sizes for common wire gauges:

• 22 AWG (0.33mm²): Max 3A fuse
• 18 AWG (0.82mm²): Max 10A fuse
• 16 AWG (1.3mm²): Max 15A fuse
• 14 AWG (2.1mm²): Max 20A fuse
• 12 AWG (3.3mm²): Max 25A fuse
• 10 AWG (5.3mm²): Max 40A fuse
• 8 AWG (8.4mm²): Max 60A fuse
• 6 AWG (13mm²): Max 80A fuse

Fast-Blow vs Slow-Blow Fuses

Fast-Blow (F): Reacts quickly to overcurrent. Use for: sensitive electronics, circuits with no inrush current, protection of delicate equipment.

Slow-Blow (T/SB): Tolerates brief current surges. Use for: motors, transformers, capacitive loads, anything with high startup current.

When to Replace Fuses

• Fuse has blown (visual inspection shows broken element or darkened glass)
• Frequent nuisance blowing indicates undersized fuse or circuit fault
• Upgrading electrical equipment to higher power
• Installing new circuits or accessories
• Corrosion on fuse holder contacts

Critical Safety Rules

• NEVER use a higher rated fuse than calculated: This defeats the fuse's protective function
• NEVER bypass a fuse with wire or foil: Extremely dangerous - fire risk
• Always identify and fix the fault: If a fuse blows repeatedly, there's an underlying problem
• Use correct fuse type: Don't substitute fast-blow for slow-blow or vice versa
• Ensure fuse holder is rated adequately: Holder must support the current rating
• Check voltage rating: 32V fuses for automotive, higher ratings for mains