Calculating Fuse Sizes

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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.

Installation

Calculating Fuse Sizes