How do I size a fuse for a 12V circuit?

Size the fuse at roughly 1.25x the continuous current of the load, but never above the wire's safe ampacity rating — the fuse's job is to protect the wire, not the appliance. Place it as close as practical to the power source, in the positive conductor only.

What's the difference between a blade fuse, an ANL fuse, and a Class T fuse?

Blade fuses (ATC/ATO) handle small branch circuits (under ~30A) in a fuse block. ANL/MIDI fuses handle larger DC runs like inverters and DC-DC chargers (50-300A). Class T fuses are required for the main fuse on a LiFePO4 battery bank — they have a much higher interrupt rating needed to safely break the very high short-circuit current lithium batteries can deliver.

Where should the main battery fuse be placed?

Within about 7 inches (under a foot) of the battery's positive terminal, on the positive conductor only. This protects the entire length of cable from the battery to the bus bars — any fault in that cable run is covered by this single fuse.

RV & Van Fuse Sizing Guide: How to Protect Every Circuit

Every fuse in a van or RV electrical system exists for one reason: to protect the wire from overheating in a fault. Get the sizing logic right and the rest follows.

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The one rule

A fuse protects the wire, not the appliance. Every conductor needs overcurrent protection sized at or below its safe ampacity, placed as close as practical to the power source.

Fuse size = at least 1.25× the continuous load current, but never more than the wire's rated ampacity.

If a wire is rated for 30A and your load draws 20A continuously, the fuse should be roughly 25A — comfortably above the load, but at or below the wire's 30A limit.

Fuse types for a van/RV system

Fuse type	Typical use	Current range
Blade (ATC/ATO)	Branch circuits — lights, pumps, fans, USB	1-30A
ANL / MIDI	Inverter, DC-DC charger, larger DC runs	50-300A
Class T	Main battery fuse on LiFePO4 banks	100-400A
Mega fuse	Alternative to ANL for some main-circuit setups	100-300A

Why LiFePO4 needs a Class T main fuse

LiFePO4 batteries can deliver extremely high short-circuit current — far more than a lead-acid battery of similar size. A standard blade or ANL fuse may not have a high enough interrupt rating to safely clear that fault current; it can arc internally and fail to open the circuit. A Class T fuse has a high interrupt rating (20,000A+) specifically designed for this.

Fuse the positive conductor only

Always place fuses in the positive (hot) wire, never the negative/ground. Fusing the negative would leave the rest of the system live to the chassis even after the fuse blows — defeating the purpose entirely.

Placement: as close to the source as possible

The main battery fuse should sit within about 7 inches of the battery's positive terminal. Every branch circuit off the bus bars gets its own fuse, sized to that specific circuit's wire and load — not a single fuse covering multiple circuits.