CircuitMix Tools › PV String Fuse Calculator
PV String Fuse Calculator
Calculate the recommended solar PV string fuse size using module Isc, number of parallel strings, safety factor, conductor ampacity and module maximum series fuse rating.
☀️ Field Rule: PV string fuses protect a faulted string from reverse current coming from other parallel strings. Check this together with Solar String Sizing, PV Voc Calculator, Solar Panel Series/Parallel Calculator and Solar Cable Loss Calculator.
🧯 Parallel PV Strings → String Fuses → Combiner / MPPT
String 1 String 2 String 3+ FUSE FUSE FUSE COMBINER — A fuse Enter Isc and parallel strings to calculate fuse size.
Module Isc
Parallel Strings
Safety Factor
Module Max Series Fuse
String Cable Ampacity
Fuse Standard
Fuse Rule
Design Note
Result Type
Many designs use 1.25 × Isc as a minimum fuse current, then select the next standard DC fuse size that does not exceed the module maximum series fuse rating or cable ampacity.
Presets:modern modulesmall stringshigh Isc module
Existing Fuse Size
Module Isc
Safety Factor
Max Series Fuse
Cable Ampacity
Module Isc
Parallel Strings
Output Safety Factor
Combiner Output Cable Ampacity
This tab estimates combined array current after paralleling strings. Use it as a planning aid for combiner output cable, breaker and isolator checks.

📐 Formula Reference

Minimum Fuse Current
Minimum fuse current ≈ module Isc × safety factor
Common factor = 1.25
Selected Fuse
Select next standard fuse size ≥ minimum current
But ≤ max series fuse and cable ampacity
Parallel Strings
Combined Isc = module Isc × number of parallel strings
Reverse Current
Faulted string can receive current from other parallel strings. Fuse need depends on module rating and code.

📋 Quick Reference

Common PV Fuses
Small modules10–15A
Modern modules20–25A
Large modules25–30A
Always Check
Max series fusedatasheet
DC voltage ratingrequired
Cable ampacitylimit
Related Design
String voltageVoc
Cable lossvoltage drop
Combiner outputtotal Isc

📚 Engineering Notes

String fuses protect against reverse currentWhen strings are paralleled, other strings can feed current into a faulted string. A PV fuse limits that current path.
Use the module datasheetThe fuse size must not exceed the module maximum series fuse rating. Also confirm the fuse voltage rating is suitable for the PV DC voltage.
Check voltage and wiring togetherUse the Solar String Sizing Calculator and PV Voc Calculator before selecting DC fuse voltage rating.
Complete the protection workflowUse this with the Solar Panel Series/Parallel Calculator, Solar Cable Loss Calculator and MPPT Charge Controller Calculator.

What is a PV String Fuse Calculator?

A PV string fuse calculator helps estimate the correct fuse size for solar panel strings connected in parallel. It uses module short-circuit current, a safety factor, maximum series fuse rating and cable ampacity to recommend a practical DC fuse size.

When are PV string fuses needed?

String fuses are commonly used when multiple PV strings are connected in parallel and fault current from other strings could backfeed into a single faulted string. Exact requirements depend on local code, module datasheet, combiner design and inverter or charge controller input configuration.

Solar protection workflow

Use the Solar Panel Series/Parallel Calculator to calculate array current, the Solar String Sizing Calculator for string voltage, the PV Voc Calculator for cold voltage correction, the Solar Cable Loss Calculator for cable checks and the MPPT Charge Controller Calculator for controller input limits.

Important fuse selection notes

Use a PV-rated DC fuse and holder with a voltage rating suitable for the corrected string Voc. The selected fuse must protect the conductor and should not exceed the module maximum series fuse rating listed in the panel datasheet.

❓ Frequently Asked Questions

Not always. Fusing depends on the number of parallel strings, module reverse-current capability, module maximum series fuse rating and local electrical code.
Use a properly rated DC PV fuse and holder. DC arcs are harder to interrupt than AC, so voltage and DC interrupt rating matter.
No. The selected string fuse should not exceed the module maximum series fuse rating from the datasheet.
🔗 Related Tools
☀️Solar String Sizing ❄️PV Voc Calculator 🔆Solar Series/Parallel 📉Solar Cable Loss 🔋MPPT Controller 📉Voltage Drop