CircuitMix Tools › Solar Combiner Box Calculator
Solar Combiner Box Calculator
Size a PV combiner box for string current, fuses, output breaker, busbar rating, SPD voltage and output cable checks.
☀️ Field Rule: Combiner box sizing starts from module Isc, parallel string count and corrected PV voltage. Use this with the PV String Fuse Calculator, Solar Cable Loss Calculator, Solar String Sizing Calculator and PV Voc Calculator.
🔆 PV Strings → Combiner Box → Inverter / MPPT
String 1 String 2 String N COMBINER BOX — strings — A output MPPT — A Enter PV string data to size combiner box components.
Module Isc
Parallel Strings
Current Safety Factor
Module Max Series Fuse
String Cable Ampacity
Combiner Output Cable Ampacity
Combiner Busbar Rating
Fuse Standard
Combiner Type
For multiple parallel strings, each string often needs a PV-rated fuse or breaker. Always confirm with local code and the module datasheet.
Presets:3 strings4 strings6 strings
Combined Isc
Safety Factor
Output Breaker / Disconnect
Output Cable Ampacity
Busbar Rating
Temperature / Derating Margin
Corrected String Voc
Inverter Max DC Voltage
Combiner Voltage Rating
SPD Rating
Required Voltage Margin
System Grounding
Use corrected cold Voc from the PV Voc Calculator. Select PV-rated DC components suitable for the maximum voltage.

📐 Formula Reference

Combined Isc
Combined Isc = module Isc × parallel strings
Design Current
Design current = combined Isc × safety factor
String Fuse
Minimum fuse ≈ module Isc × 1.25, rounded to next standard size
Voltage Rating
Combiner, SPD and disconnect ratings must exceed corrected cold string Voc

📋 Quick Reference

Combiner Components
String protectionfuse/breaker
Output devicebreaker/isolator
Surge protectionPV SPD
Current Checks
String fuseIsc × factor
Output cablecombined current
Busbarrated current
Voltage Checks
Cold Vocmaximum
SPD voltagePV-rated
DisconnectDC-rated

📚 Engineering Notes

Combiner current comes from parallel stringsSeries panels increase voltage. Parallel strings increase current. Combiner output current is mainly the sum of string currents.
Use PV-rated DC componentsPV DC arcs are difficult to interrupt. Fuses, holders, breakers, isolators and SPD must be rated for DC PV voltage and current.
Check string fuse firstUse the PV String Fuse Calculator to confirm string fuse size before selecting the combiner box.
Check cable and voltage togetherUse the Solar Cable Loss Calculator, PV Voc Calculator and Solar String Sizing Calculator.

What is a Solar Combiner Box Calculator?

A solar combiner box calculator helps size the components used to combine multiple PV strings into one output circuit. It estimates combined current, string fuse size, output breaker or disconnect rating, output cable ampacity, busbar rating and voltage rating checks.

How to size a PV combiner box

Start with module short-circuit current and the number of parallel strings. Multiply Isc by the number of strings to get combined Isc, then apply a safety factor for output current planning. String fuse size should also respect the module maximum series fuse rating.

Solar combiner workflow

Use the Solar Panel Series/Parallel Calculator to define strings, the PV String Fuse Calculator for string fuses, the PV Voc Calculator for cold voltage, and the Solar Cable Loss Calculator for output cable checks.

Important safety note

This calculator is a planning aid. Final combiner box selection must follow local electrical code, module datasheet, inverter manual and manufacturer ratings for DC voltage, current, temperature and interrupting capacity.

❓ Frequently Asked Questions

A combiner box is commonly used when multiple PV strings are connected in parallel before going to an inverter, MPPT controller or disconnect.
Not always. It depends on parallel string count, module reverse-current capability, maximum series fuse rating and local code.
Use PV-rated DC protection devices. AC breakers may not safely interrupt DC PV arcs unless specifically rated for the DC voltage and application.
🔗 Related Tools
🧯PV String Fuse 📉Solar Cable Loss ☀️Solar String Sizing ❄️PV Voc Calculator 🔆Solar Series/Parallel 🔋MPPT Controller