DC Fuse Size Calculator: Don’t Burn Your Van
DC Fuse Size Calculator: Don’t Burn Your Van
In electrical design, the fuse is the sacrificial lamb. It must die so your expensive equipment (and your family) can live. But sizing it is misunderstood. Rule #1: The fuse protects the wire, not the appliance. Use this tool by ShockTrail to find the safe fuse range for your build.
The Fuse Sizer
Enter your load current and your wire’s ampacity rating to find the “Sweet Spot” for your fuse.
Fuse Size Calculator
Safety Logic by ShockTrail0 Amps
Class-T Recommended
The “Lithium Danger”: Why ANL Fuses Are Not Enough
If you have a Lead-Acid battery, a standard ANL or Mega fuse is fine. But if you have Lithium (LiFePO4), you have a problem called “AIC” (Ampere Interrupting Capacity).
Lithium batteries have very low internal resistance. In a dead short, they can dump 20,000 Amps instantly. A cheap fuse might try to blow, but the electricity will arc across the gap and melt the fuse holder, starting a fire. This is why Blue Sea Systems recommends Class-T Fuses for all main Lithium battery banks.
| Fuse Type | Interrupt Rating (AIC) | Safe For Lithium? |
|---|---|---|
| Glass / Blade (ATC) | ~1,000 Amps | NO (Only for small accessories) |
| ANL / Mega | ~3,000 – 5,000 Amps | RISKY (Only small banks) |
| Class-T | 20,000 Amps | YES (Required for Safety) |
The 125% Rule (NEC)
According to the National Electrical Code, your fuse should be sized at least 125% of your continuous load to prevent “nuisance blowing” (when the fuse blows for no reason during normal operation).
Formula: Load Amps × 1.25 = Minimum Fuse Size
How to Place the Fuse
The main fuse must be placed as close as possible to the battery positive terminal. ABYC standards suggest within 7 inches. If the wire shorts out before the fuse, the fuse cannot save you.
Frequently Asked Questions (FAQ)
Can I use a breaker instead of a fuse?
For small circuits, yes. But most DC breakers (like the cheap ones on Amazon) have very low AIC ratings. For the main battery protection, a Class-T fuse is superior to almost any breaker.
My inverter is 2000W. What fuse do I need?
2000W / 12V = ~166 Amps. Apply the 1.25x rule: 166 * 1.25 = 208A. You should use a 225A or 250A Class-T fuse (assuming your wire is 2/0 AWG or thicker).
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