What size fuse do I need? Start with one rule: a fuse protects the wire, not the device. It has to blow before the cable overheats, but it shouldn’t blow during normal operation. Here’s how to size fuses for battery cables, inverters, DC loads and solar strings.
Electrical work can be dangerous: follow the manual, fuse and size every cable correctly, and use a licensed electrician for anything connected to household wiring.
What Size Fuse Do I Need? The Three Checks
- At or below the wire’s ampacity. The fuse must not be larger than what the cable can safely carry (NEC Article 240).
- Above the normal load. For continuous loads (3+ hours), size wire and overcurrent protection at 125% of the load. That’s why a 16A continuous load needs a 20A circuit.
- Rated for the job: a DC voltage rating at or above your system voltage, and an interrupt (AIC) rating high enough for what your battery can deliver into a short.
Battery Main Fuse
Put it as close to the battery positive as practical. Size it for the system’s maximum continuous current × 1.25, without going over the cable’s ampacity. Lithium banks can deliver thousands of amps into a short, so use a high-interrupt fuse such as a Class T or the type the battery maker specifies.
Example math: a 2,000W inverter on 12V at 90% efficiency draws about 185A. × 1.25 = 231A, so a 250A fuse is the nearest standard size. That needs cable rated above 250A (4/0 copper at 230A in the NEC 75°C column isn’t enough, so check the inverter manual’s cable and fuse table).
Inverter Fuse
Follow the inverter manual first. Manufacturers usually specify an exact fuse and cable size. Where they don’t, use the battery main fuse method with the inverter’s maximum DC input current.
DC Load Circuits
| Load (example) | Current | Wire (example) | Fuse |
|---|---|---|---|
| LED lights | 3A | 16 AWG | 5A |
| 12V fridge | 5A | 12 AWG (for voltage drop) | 10A |
| Water pump | 8A | 12 AWG | 15A |
Examples only. Check the device manual and your wire’s ampacity, and keep the fuse at or below it.
Solar Panel Strings
NEC 690.8 sets PV source circuit current at 125% of the module’s short-circuit current (Isc), and 690.9 sizes overcurrent protection at 125% of that, which works out to 1.56 × Isc. String fuses are usually needed only when three or more strings run in parallel, but follow the module’s “maximum series fuse rating” and your controller manual.
Example: modules with Isc 11A need at least 11 × 1.56 = 17.2A, so a 20A fuse, provided the module’s max series fuse rating is 20A or more.
Fuse vs Breaker
Breakers can be reset and double as a disconnect. Fuses are often smaller, cheaper and available with higher interrupt ratings. Either way, it must be DC-rated for DC circuits.
Worked Example: Fusing a 12V Inverter System
Illustrative system: a 12V LiFePO4 battery, a 1,500W inverter, 2/0 AWG cable, and a 20A fused DC panel for lights and a fridge.
| Circuit | Max current | Wire | Fuse chosen | Why |
|---|---|---|---|---|
| Inverter | 1,500 ÷ 0.9 ÷ 12 ≈ 139A | 2/ | 175A or 200A | Above 125% of load (174A), at or below cable ampacity |
| DC panel feed | 20A loads | 10 AWG | 30A | Covers 125% of 20A, within 10 AWG ampacity |
| Fridge branch | 5A | 14 AWG | 10A | Protects the small wire |
| Battery main | Sum of above | 2/ | Class T, sized to cable | High interrupt rating close to the battery |
Example sizing only. Use the inverter manual’s recommended fuse and the cable maker’s ampacity. The cable choice comes from the wire gauge chart for 12V, 24V and 48V, and the current from watts to amps.
Fuse Types You’ll See
| Type | Typical range | Common use |
|---|---|---|
| Blade (ATO/ | up to about 40A | DC load circuits, fuse blocks |
| MIDI/ | about 30–200A | Charge controllers, mid-size feeds |
| ANL | about 35–750A | Inverter feeds |
| MRBF (terminal fuse) | about 30–300A | Mounted right on the battery post |
| Class T | about 1–1,200A | Battery main on lithium banks, high interrupt rating |
| PV fuse (gPV) | about 1–30A | Solar strings in a combiner box |
Ranges are typical catalog spans; check the specific fuse datasheet for voltage and interrupt (AIC) rating. Lithium batteries can deliver very high short-circuit current, which is why the interrupt rating matters. See fuses vs circuit breakers.
Placement Rules of Thumb
- Put the main fuse as close to the battery positive as practical. ABYC E-11 for boats calls for overcurrent protection within 7 inches of the battery, or up to 72 inches when the conductor is sheathed along its length.
- Fuse every branch where wire size drops.
- Fuse both ends of a cable connecting two current sources, such as two battery banks.
- Use fuses rated for the system’s DC voltage.
Solar String Fusing Math
NEC 690.9 generally requires string overcurrent protection when three or more strings are in parallel. Size the fuse at 156% of the panel’s short-circuit current (Isc × 1.25 × 1.25), and don’t exceed the “maximum series fuse rating” on the panel label.
Example math: Isc 11A × 1.56 = 17.2A, so the next standard size is 20A, valid only if the label allows 20A or more. See solar panel specs explained.
FAQ
Can I use a bigger fuse to stop it blowing?
No. If it blows during normal use, either the wire is too small or the load is bigger than you think. Fix the cause.
Where should the fuse go?
On the positive conductor, as close as practical to the power source (battery or bus bar).
Do I need a fuse on a solar panel?
Often not for one or two strings, but check NEC 690.9 and the module label.
What size fuse for a 100Ah battery?
It depends on the cable and load, not the Ah. Size to the cable’s ampacity and the largest load current.
What size fuse for a 2,000W inverter at 12V?
About 185A of DC current means a 250A fuse is common, provided the cable is rated for it. Follow the inverter manual.
Sources: NFPA 70 (NEC) 2023: Article 240, 210.20, 690.8, 690.9, Table 310.16; NFPA 70 (NEC) 690.9; ABYC E-11. Accessed Oct 2026.
Related: Tadeto Circuit Breaker Finder Review, Allwei 1500W Pure Sine Wave Inverter Review, LiTime 12V 50Ah LiFePO4 Battery Review
2 replies on “What Size Fuse Do I Need? Fuse Sizing for Batteries, Inverters and Solar”
[…] Fuse the cable at or below its ampacity; see what size fuse do I need. […]
[…] Install a fuse close to the battery. See what size fuse do I need. […]