Fuse Size Calculator

Calculating the correct fuse size is essential for electrical safety in any 12V, 24V, or 48V DC system. The fuse size calculator uses a straightforward formula that accounts for your load wattage, system voltage, and a safety margin to prevent nuisance tripping.

The fuse sizing formula: Fuse Size = (Watts / Volts) x 1.25. This gives you the minimum fuse rating needed to safely handle your load while providing a 25% safety margin for voltage fluctuations and temporary current spikes.

For example, to calculate what size fuse you need for a 1500W inverter on a 12V system: divide 1500W by 12V to get 125A, then multiply by 1.25 to get 156A. Round up to the next standard fuse size, which is typically 175A or 200A ANL fuse.

Important: The fuse must also be sized to protect the wire connecting the load to the battery. Even if your load only requires a 50A fuse, if your wire is only rated for 40A, you must use a 40A fuse or upgrade the wire. The fuse protects the weakest link in the circuit.

Choosing the right fuse type is just as important as selecting the correct amperage. Different fuse styles are designed for different current ranges and applications in 12V fuse sizing scenarios.

ATC/ATO blade fuses are the standard automotive fuse style, available from 1A to 40A. They are inexpensive, widely available, and easy to replace. Use these for low-power 12V accessories like LED lights, USB chargers, fans, and small electronics. They plug into standard fuse blocks found in most vehicles and RVs.

MIDI and ANL fuses are bolt-down style fuses designed for high-current DC applications. MIDI fuses cover the 30A to 200A range, while ANL fuses extend from 35A up to 750A. These are the go-to choice for inverter connections, battery bank interconnects, and charge controller wiring. They mount in dedicated fuse holders that bolt directly to battery terminals or busbars.

Class T fuses offer the highest level of protection for lithium battery systems. They feature extremely fast response times and can safely interrupt fault currents exceeding 20,000A. While more expensive than ANL fuses, Class T fuses are recommended for LiFePO4 battery installations where fault currents can be dangerously high due to the low internal resistance of lithium cells.

Glass tube fuses are older-style fuses still found in some applications. They are available in various sizes but are generally being replaced by blade and bolt- down styles in modern installations. If you encounter glass tube fuses in an older system, consider upgrading to a more robust fuse type during any renovation.

Proper fuse placement is critical for fire prevention in DC electrical systems. The fundamental rule is simple: install fuses as close to the battery positive terminal as possible, within 12 inches (30cm).

Why close to the battery? The unprotected wire between the battery and fuse is the most dangerous section of any circuit. If this wire shorts to ground before reaching the fuse, the fuse cannot protect it—potentially causing the wire to melt and start a fire. By keeping this distance minimal, you minimize the risk of an unprotected short circuit.

In a typical solar installation, you need fuses in several locations: between the battery positive and inverter, between the battery positive and charge controller DC input, between solar panels and charge controller (using a DC-rated breaker or fuse), and on any branch circuits feeding DC loads. Each positive wire leaving your main battery bank should pass through a fuse or breaker.

Fuse blocks and distribution panels simplify wiring by providing a central location for multiple fused circuits. A typical setup uses a high-amperage main fuse (such as a Class T) between the battery and a busbar, then individual fuses for each circuit leaving the busbar. This provides both system-level and circuit-level protection.

The negative side typically does not require fusing in most 12V systems because the chassis or common ground serves as the return path. However, in marine applications or systems with isolated grounds, both positive and negative may require protection per ABYC or other electrical codes.

Fuses are your last line of defense against electrical fires. When properly sized and installed, they protect your wiring by interrupting the circuit before wires can overheat to dangerous temperatures.

How fuses prevent fires: When current flow exceeds the fuse rating, the fuse element heats up and melts, breaking the circuit. This happens in milliseconds for severe faults (short circuits) or over several seconds for moderate overloads. Without a fuse, excessive current would continue flowing, heating the wire until insulation melts and ignites surrounding materials.

DC systems require DC-rated fuses. Unlike AC power which crosses zero voltage 120 times per second (at 60Hz), DC power maintains constant voltage. This makes DC arcs harder to extinguish because there is no natural zero-crossing point. DC- rated fuses and breakers are designed with arc suppression features specifically for direct current applications.

Never bypass or upsize a fuse that keeps blowing. A repeatedly blowing fuse indicates a problem— either an overloaded circuit that needs to be split, undersized wiring that needs upgrading, or a fault that needs repair. Installing a larger fuse masks the problem while creating a fire hazard.

Lithium battery safety: LiFePO4 and other lithium batteries can deliver extremely high fault currents due to their low internal resistance. A short circuit on a lithium battery bank might produce 5,000A to 20,000A or more—enough to vaporize thin wires instantly. Class T fuses are specifically designed to safely interrupt these extreme currents and are highly recommended for all lithium battery installations.

Even experienced DIY solar installers make fusing mistakes that compromise safety. Here are the most common errors and how to avoid them when sizing fuses for your 12V, 24V, or 48V system.

Mistake: Oversizing fuses for surge handling. Some installers use oversized fuses to prevent tripping during motor startup surges. This is dangerous because the fuse can no longer protect the wiring. Instead, use slow-blow or time-delay fuses that tolerate brief surges while still protecting against sustained overcurrent conditions.

Mistake: Using automotive fuses for high currents. Standard ATC blade fuses are not designed for currents above 40A. Using multiple blade fuses in parallel or oversized blade fuses for high-current circuits is unsafe. Always use the appropriate fuse type for your current requirements: MIDI for 30-200A, ANL for 35-750A.

Mistake: Not matching fuse to wire rating. The fuse must be sized to protect the wire, not just the load. If you have 10 AWG wire (rated for 30A) feeding a 20A load, use a 30A fuse—not a 25A fuse that might nuisance trip, and not a 40A fuse that fails to protect the wire. The fuse rating should never exceed the wire ampacity.

Mistake: Installing fuses too far from the battery. A fuse installed 5 feet from the battery leaves 5 feet of unprotected wire that could short circuit. National Electrical Code (NEC) and best practices require fuses within 12 inches of the battery terminal. Use a Class T fuse holder that mounts directly to the battery post if space is limited.

Mistake: Using AC-rated breakers for DC circuits. Circuit breakers must be specifically rated for DC voltage. An AC breaker may fail to interrupt a DC fault, allowing dangerous arcing to continue. Always verify the DC voltage rating before installing any breaker—most DC breakers are rated for 32V, 48V, or 125V DC.

Mistake: Forgetting the solar panel fuse. While a single solar panel may not need overcurrent protection, parallel panel strings require fuses on each string. If one string shorts, the other parallel strings can backfeed current through the fault, potentially causing a fire. Fuse each parallel string at 1.25x the panel short-circuit current (Isc).