Calculating Fuse Sizes for DC and Lithium Battery Systems

This article explains how to calculate the correct fuse size for 12 V, 24 V, and 48 V lithium battery systems. It covers continuous current, cable ratings, voltage requirements, and derating for temperature and installation environment.

Purpose of a Fuse

A fuse protects the cable, not the battery or inverter. The fuse must blow before the cable overheats during a short-circuit or overload.

1. Determine the Maximum Continuous Current

Identify the highest continuous current the circuit will draw. Examples:

• A 2000 W inverter on 12 V can draw about 200 A
• A DC-DC charger may draw 40 A on the input side
• A 48 V solar controller may output 30 – 100 A

Use the system’s worst-case continuous load.

2. Identify the Cable’s Current Rating

Every cable has a safe continuous amp rating. Typical values:

• 16 mm²: about 110–120 A
• 25 mm²: about 150–170 A
• 35 mm²: about 200–240 A
• 50 mm²: about 260–300 A

The fuse must always be less than or equal to the cable rating.

3. Apply Derating

Real installations operate above 25°C. Heat reduces the capacity of both fuses and cables.

Temperature derating for cables

Multiply the cable rating by a derating factor:

• Warm cabinet or van: around 0.9
• Bundled cables: 0.7–0.8
• Engine bay or enclosed conduit: 0.5–0.6

Example:
A 150 A cable in a hot cabinet with a factor of 0.85 becomes 127 A usable.

Temperature derating for fuses

Fuses also lose rating as temperature rises:

• At 50°C: reduce rating by around 10–15%
• At 70°C: reduce by around 20–25%

Example:
A 150 A fuse at 60°C may behave like a 120–135 A fuse.

4. Select a Fuse Above Load but Below Cable Limit

A correct fuse must:

1. Be higher than the system’s maximum continuous current

2. Be lower than or equal to the cable’s derated ampacity

Example:
• Load: 80 A
• Cable: 25 mm² rated at 160 A
• Derating factor: 0.85
• Derated cable rating: 160 × 0.85 = 136 A
Recommended fuse: 100 or 125 A

5. Consider Surge Loads

Some devices draw short surge currents. Inverters, pumps, fridges and chargers may have startup spikes. Fuses allow short surges based on their time-delay curve.

For inverters, choose a fuse around 125–150% of continuous current, provided it stays below the cable limit.

6. Check Voltage Rating

The fuse’s voltage rating must exceed the system’s maximum voltage.

• For 48 V LiFePO₄ batteries (which reach 57–59 V), use 58 V or 80 V rated fuses
• Do not use 32 V fuses in 48 V systems

Common types include MIDI, MEGA, ANL, and Class T.

7. Examples

12 V inverter

• 2000 W inverter = 166 A
• 50 mm² cable derated to 240 A
• Fuse above 166 A and below 240 A
Recommended: 200–225 A fuse

48 V solar controller

• Controller output: 75 A
• 16 mm² cable derated to 95 A
Recommended: 80–90 A fuse

Summary

Fuse sizing is based on cable protection:

• Calculate continuous load
• Check cable rating
• Apply temperature and bundling derating
• Choose a fuse above the load but below the derated cable rating
• Ensure the fuse’s voltage rating matches the system

Correct fuse sizing increases safety, reduces the risk of overheating, and prevents nuisance trips.