Solar Panel Fuse Size Calculator (NEC/IEC)

To size a solar fuse, multiply the panel’s short-circuit current (Isc) by 1.56 (which is 1.25 × 1.25) to account for continuous operation and safety factors per NEC 690.8 and IEC 62548.

The result is the minimum fuse current rating. Always round this value up to the next standard fuse size, but ensure it never exceeds the panel’s maximum series fuse rating found on its datasheet.

The 80% rule generally states that a fuse or circuit breaker should only carry up to 80% of its rated current continuously.

This provides a safety margin, ensuring the device doesn’t operate near its thermal limit, which helps prevent overheating and extends its lifespan, especially in applications with sustained loads like solar power systems.

The 20% rule (or 120% factor) in solar design acknowledges that panels can produce up to 20% more current or power than their rated output under specific conditions, like cold, sunny days (high irradiance).

Designers must account for this potential increase when sizing components like wires and fuses to prevent overheating and ensure system safety and reliability.

For a Victron MPPT 100/50 charge controller, which handles a maximum output current of 50 amps, the fuse on the battery/output side needs to be sized slightly larger.

Typically, a 60 A fuse is used. This is based on the common practice of sizing the fuse at 125% of the max continuous current (50 A × 1.25 = 62.5 A), then selecting the next available standard size down (60 A) to provide protection without causing nuisance trips.

A typical 400W solar panel (often designed for higher voltage systems) might have a Short Circuit Current (Isc) around 22 amps (assuming ~18V Vmp, though this varies).

Using the 1.56 safety factor: 22 A × 1.56 ≈ 34.3 A. You would round this up to the next standard fuse size, which is 35 A. Always confirm the actual Isc from the panel's datasheet.

A common 200W panel (often used in 12V or 24V systems) typically has an Isc of about 11 amps.

Applying the safety factor: 11 A × 1.56 ≈ 17.2 A. Rounding up to the next standard size gives a recommended fuse of 20 A per string.

A standard 100W panel (frequently used for 12V applications) usually has an Isc around 5.5 amps.

Calculating the minimum fuse size: 5.5 A × 1.56 ≈ 8.6 A. The next standard fuse size up is 10 A, making it the appropriate choice.

A 300W panel often has an Isc near 17 amps (depending on its voltage design).

Using the standard calculation: 17 A × 1.56 ≈ 26.5 A. Rounding this value up leads to a recommended fuse size of 30 A.

For fusing a DC load or circuit in a 12V system (not the solar panel string itself), first find the current: divide the total power (Wattage) by the voltage (12V), so I = P / V.

Then, apply a safety factor, typically 1.25x the calculated current (I × 1.25). Round this result up to the next standard fuse size. For example, a 200W load at 12V draws 16.7A; 16.7A × 1.25 = 20.9A, so use a 25A fuse.

Yes, absolutely. Solar PV systems require fuses specifically designed and rated for DC circuits and the high voltages often encountered (up to 1500Vdc). Standard AC fuses or automotive fuses are not suitable and can be dangerous.

Look for fuses explicitly marked for "PV," "Solar," or meeting standards like UL 248-19 (for NEC compliance, often marked "UL PV Fuse") or IEC 60269-6 (for IEC compliance, often marked "gPV").