DC fuse & breaker size calculator
Pick a DC fuse or breaker size: at least 125% of continuous current, no larger than what your wire can safely carry. Covers 12 V, 24 V and 48 V circuits.
Results
| Recommended fuse size | 50 A |
|---|---|
| Minimum (125% of load) | 50 A |
| Maximum for this wire (ampacity) | 65 A |
| Largest standard fuse the wire allows | 60 A |
- A fuse protects the wire, so place it as close to the battery (or other source) as possible. The marine ABYC guideline is within 7 inches.
- Use fuses rated for DC at your system voltage. LiFePO4 banks can deliver very high fault currents, so main fuses should be class T or a high-interrupt-rating MRBF/ANL type rated for the bank.
About this calculator
A fuse protects the wire, not the device. It has to be big enough not to blow in normal use, which conventionally means at least 125% of the continuous current. It also has to be small enough to blow before the wire overheats, which means no more than the wire's ampacity.
Enter the load current and the wire you're using. The calculator returns the smallest standard fuse that won't nuisance-trip, and checks that it still protects the cable.
How to use it
- Enter the continuous current the circuit carries.
- Select the wire gauge already chosen. If you haven't chosen one, start with the DC wire size calculator.
- Use the recommended fuse size, or any standard size between the minimum and the maximum.
- Mount the fuse at the source end of the positive cable.
The formula
Minimum fuse = continuous current × 1.25, rounded up to a standard size.
Maximum fuse = the wire's ampacity, rounded down to a standard size.
Ampacity uses the NEC Table 310.16 copper 75 °C column as a general reference. Marine (ABYC E-11), automotive and bundled installations may require lower values.
Worked example (default values)
Inputs
| Continuous load current | 40 A |
|---|---|
| Wire gauge on the circuit | 6 AWG |
Results
| Recommended fuse size | 50 A |
|---|---|
| Minimum (125% of load) | 50 A |
| Maximum for this wire (ampacity) | 65 A |
| Largest standard fuse the wire allows | 60 A |
Tips and common mistakes
Standard sizes run 5, 7.5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 100, 125, 150, 175, 200, 225, 250, 300, 350 and 400 A, depending on the fuse family.
Breakers vs fuses: DC breakers add a convenient disconnect, but check the DC voltage and interrupt rating. Many cheap "DC breakers" are rated well below what a lithium bank can deliver into a short.
Frequently asked questions
What size fuse for a 2000W inverter on 12V?
About 2,000 ÷ (12 × 0.9) ≈ 185 A continuous, so 125% gives about 231 A and the next standard size is 250 A. That needs cable rated above 250 A, such as 4/0 AWG with high-temperature insulation. That's one reason larger inverters suit 24 V or 48 V.
Can I use a bigger fuse so it doesn't blow?
Not beyond the wire's ampacity. A fuse that's too big lets the cable overheat before it blows, which is a fire risk.
Do I need a fuse on the negative?
Generally no. Fuse the positive conductor at the source. Some installations add a main negative shunt, but that's for monitoring, not protection.
Related calculators
- DC wire size calculator (12 V, 24 V, 48 V)
Find the right copper wire gauge (AWG) for a 12 V, 24 V or 48 V DC circuit from current, cable length and allowed voltage drop, checked against ampacity.
- DC voltage drop calculator
Calculate voltage drop, percent drop, voltage at the load and watts wasted as heat for a copper DC cable of any AWG, length and current.
- Inverter size calculator
Size a power inverter from your appliance list: continuous watts, motor start-up surge, the next standard size and DC current from a 12, 24 or 48 V battery.
- Battery bank series & parallel calculator
Work out the voltage, amp-hours and total energy of a battery bank wired in series, parallel or series-parallel, with wiring tips for matched batteries.
Off-grid solar sizing, step by step
- Rough out the whole system · Off-grid solar system calculator
- Size the inverter from your loads · Inverter size calculator
- Plan the battery bank · Battery bank series & parallel calculator
- Check battery runtime · Battery bank runtime calculator
- Size the solar array · Solar panel size calculator
- Check panel string voltage · Solar string voltage calculator (series & parallel)
- Choose the charge controller · Solar charge controller size calculator
- Size the DC cables · DC wire size calculator (12 V, 24 V, 48 V)
- Pick the fuses (you are here)
Estimates for planning only, based on the assumptions shown. Real-world performance varies with equipment, temperature and installation. Updated October 19, 2026. See our methodology.