A fuse must protect the conductor and suit the DC voltage, available fault current and equipment requirements; load current alone does not determine it.
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 | High-interrupt main battery protection where specified | Check device data |
| Mega fuse | Alternative to ANL for some main-circuit setups | 100-300A |
When a LiFePO4 system may call for Class T
LiFePO4 batteries can deliver high short-circuit current. The main protective device needs a DC voltage rating and interrupt rating suitable for the available fault current. Class T is a common high-interrupt option, but it is not a universal rule for every battery or architecture. Use the battery maker's requirements and the chosen fuse holder's documented ratings.
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
Place the main battery protection as close to the source as the applicable guidance and battery maker require. Every branch circuit off the distribution point needs coordinated protection for that circuit's conductor, load and installation conditions.
Worked example
A DC-DC charger rated for 50A continuous, wired with 6 AWG cable (rated for ~75A in this application):
Fuse size ≈ 50A × 1.25 = 62.5A, rounded to a standard size — a 60A or 65A ANL/MIDI fuse, which is below the cable's 75A rating. ✓
Worked planning check
A hypothetical 12A device may have startup demand above 12A. Record the maker's fuse recommendation, cable ampacity after derating, connector limit and fuse time-current behaviour. If they cannot be coordinated, redesign the circuit rather than automatically applying a percentage multiplier.
The figures or dimensions in this example are explicit planning assumptions, not a product test or an installation specification. Replace them with your equipment data and actual measurements.
Planning questions
Can I choose a fuse just from the appliance current?
No. Check cable ampacity under the installed conditions, equipment instructions, terminal limits, DC voltage and interrupt capacity. Protection must coordinate with the complete circuit.
Does a wiring diagram prove the installation is safe?
No. It is a planning record. Actual routing, termination, protection and commissioning results must agree with the design and applicable requirements.
When should I involve an electrician?
Before work that exceeds your competence, particularly mains design, source transfer, bonding, high-current faults and final inspection or testing. Agree the scope and evidence of verification in advance.
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Jack Kennedy is a self-builder who has built two vans. He created Roam Wired to simplify conversion planning and share free tools, supported by affiliate links.
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