Fuse Sizing Guide: Protecting Your Electrical System

A fuse isn’t there to protect your device - it’s there to protect your wire from catching fire. Understanding proper fuse sizing is critical for safety.

What a Fuse Actually Does

When too much current flows through a circuit (due to a short or overload), the wire heats up. If current is high enough, the insulation melts and the wire can catch fire.

A fuse is a intentional weak point that melts before the wire does, breaking the circuit and preventing fire.

The Golden Rule: Protect the Wire, Not the Device

Fuse size is based on wire size, not device amperage.

If your device draws 10A but you’re using 12 AWG wire (rated for 20A), you use a fuse sized for the wire’s capacity, not the device.

However, in practice, most people size wire appropriately for their device, so fuse sizing often matches device amperage (with the 125% buffer).

The 125% Rule (NFPA 70 / NEC)

For continuous loads (devices that run constantly), the fuse should be 125% of the device’s amp draw, rounded up to the nearest standard fuse size.

Formula:

Minimum Fuse Size = Device Amps × 1.25

Why 125%?

Continuous operation generates heat. The extra 25% buffer ensures:

• Fuse doesn’t nuisance trip during normal operation
• Accounts for startup surge
• Provides safety margin for wire capacity

What’s a “Continuous Load”?

Any device that runs for more than 3 hours:

• LED light bars
• Interior lights
• Fridges
• Inverters
• Chargers
• Radios

What’s NOT Continuous?

Devices that run in short bursts:

• Winches (typically <5 minutes)
• Air compressors
• Power windows
• Horn

For intermittent loads, you can use 100% sizing (fuse = device amperage), but 125% is still safer.

Standard Fuse Sizes

Fuses come in specific ratings. Always round UP to the nearest standard size:

Common Blade Fuse Sizes (ATO/ATC):

• 1A, 2A, 3A, 5A, 7.5A, 10A, 15A, 20A, 25A, 30A, 35A, 40A

Maxi Fuse Sizes:

• 20A, 30A, 40A, 50A, 60A, 70A, 80A, 100A, 120A

ANL Fuse Sizes (high current):

• 30A, 40A, 50A, 60A, 80A, 100A, 125A, 150A, 175A, 200A, 225A, 250A, 300A, 350A, 400A, 500A

Real-World Examples

Example 1: Light Bar

• Device: 150W LED light bar
• Current: 150W / 12V = 12.5A
• Calculation: 12.5A × 1.25 = 15.625A
• Fuse: 20A (nearest standard size above 15.625A)

Example 2: Fridge

• Device: 50W fridge compressor
• Current: 50W / 12V = 4.2A
• Calculation: 4.2A × 1.25 = 5.25A
• Fuse: 7.5A (nearest standard size)

Example 3: Winch (Intermittent Load)

• Device: 9,000 lb winch
• Peak Current: 300A (intermittent)
• Calculation: For intermittent, can use 100-110%
• Fuse: 300A or 350A ANL fuse

Example 4: Auxiliary Battery Charging

• Device: DC-DC charger outputting 25A
• Current: 25A continuous
• Calculation: 25A × 1.25 = 31.25A
• Fuse: 35A (nearest standard size)

Example 5: Inverter

• Device: 1000W inverter
• Current: 1000W / 12V = 83A
• Calculation: 83A × 1.25 = 104A
• Fuse: 125A ANL (nearest standard size)

Fuse Types Explained

Blade Fuses (Mini, Standard, Maxi)

Best for: Most automotive accessories up to 40A

Pros:

• Inexpensive
• Easy to replace
• Fit standard fuse holders
• Color-coded by amperage

Cons:

• Limited to 40A for standard size
• Can corrode in harsh environments

Use for: Lights, radios, accessories, most add-ons

ANL Fuses

Best for: High-current applications (>60A)

Pros:

• Handle very high current (up to 500A)
• Robust construction
• Good for main power distribution

Cons:

• Require special holders
• More expensive
• Bulky

Use for: Winches, inverters, battery connections, main distribution

MEGA/AMG Fuses

Best for: Alternative to ANL for high current

Pros:

• Compact compared to ANL
• Available in high ratings
• Good quality

Cons:

• Less common (harder to find replacements)
• Require MEGA fuse holders

Use for: Same as ANL - inverters, main power

Circuit Breakers

Best for: Convenience on frequently-tripped circuits

Pros:

• Resettable (no replacement needed)
• Good for troubleshooting
• Can be manually switched off

Cons:

• More expensive
• Slightly less reliable than fuses
• Can fatigue over time

Use for: Winches, compressors, test circuits

Fuse Placement: Where to Put It

Critical Rule: The fuse must be as close to the power source as possible (within 18 inches of the battery).

Why?

The wire between the battery and fuse is unprotected. If that section shorts out, nothing will stop it from catching fire.

Correct:

[Battery] ---(6 inches)--- [FUSE] ---(20 feet)--- [Device]
              ↑
        Protected here

Wrong:

[Battery] ---(20 feet)--- [FUSE] ---(6 inches)--- [Device]
          ↑
   UNPROTECTED - Fire hazard!

Multiple Devices

If you’re wiring multiple devices, use a fuse block or distribution panel:

[Battery] --- [Main Fuse] --- [Fuse Block] --- [Individual Fuses for each device]

Each device gets its own appropriately-sized fuse.

Common Fuse Sizing Mistakes

Mistake #1: Oversizing “just to be safe”

Problem: Fuse won’t blow until wire is already overheating.

Example:

• 16 AWG wire (rated 10A)
• Device draws 8A
• User installs 30A fuse “to make sure it doesn’t blow”
• Result: Wire can overheat before fuse blows = FIRE RISK

Fix: Fuse must match wire capacity, not arbitrary high value.

Mistake #2: Undersizing to “protect expensive device”

Problem: Fuse nuisance trips during normal operation.

Example:

• Light bar draws 12A
• User installs 10A fuse thinking it protects the light
• Result: Fuse blows constantly, especially during startup surge

Fix: Use proper 125% sizing (15-20A fuse for 12A device).

Mistake #3: Using wire too thin for the fuse

Problem: Wire overheats before fuse blows.

Example:

• 40A fuse installed
• Using 16 AWG wire (only rated 10A)
• Result: Wire will melt before 40A fuse blows

Fix: Size wire first, then fuse to match wire capacity.

Mistake #4: Putting fuse at device instead of battery

Problem: Long run of unprotected wire from battery.

Example:

• Battery in engine bay
• Device in rear of vehicle
• Fuse at device end
• Result: 20 feet of unprotected wire that can short and start fire

Fix: Always fuse within 18 inches of power source.

Fuse Sizing Decision Tree

1. What's the device's continuous amp draw?
   ↓
2. Is it a continuous load (>3 hours)?
   ↓ YES → Multiply by 1.25
   ↓ NO (intermittent) → Can use 1.0× (or 1.25 for extra safety)
   ↓
3. Round UP to nearest standard fuse size
   ↓
4. Verify fuse rating is BELOW wire capacity
   ↓ (If not, wire is too small - upgrade wire)
   ↓
5. Install fuse within 18" of battery
   ↓
6. Done!

Quick Reference Chart

Let Wire Solved Do the Math

Our calculator automatically:

• Applies the 125% rule for continuous loads
• Rounds to nearest standard fuse size
• Explains the sizing logic
• Accounts for intermittent vs continuous

Try the calculator to get instant fuse recommendations.

What’s Next?

Now that you understand fuse sizing:

• Wire Sizing 101 - Master the fundamentals
• Understanding Voltage Drop - Why length matters
• How to Use Wire Solved - Get accurate recommendations