- \n
- String fuse holders \u00d7 6, one per string input, commonly fitted with gPV-class fuses per IEC 60269-6<\/li>\n
- DC surge protection device (SPD), usually Type II for PV DC circuits per IEC 61643-31<\/li>\n
- Main DC disconnect switch or DC circuit breaker sized for total output current and system voltage<\/li>\n
- Positive and negative busbar assembly with creepage and clearance suitable for the rated DC class<\/li>\n
- Optional monitoring terminals for string-current sensing or remote alarm signaling<\/li>\n<\/ul>\n
In a 22 MW rooftop installation across industrial facilities in Zhejiang Province, field crews found that boxes with per-string protection isolated a bad string in under 3 minutes, while unfused layouts required broader shutdown and much longer troubleshooting.<\/p>\n
![\"**](\"https:\/\/sinobreaker.com\/wp-content\/uploads\/2026\/04\/6-string-pv-combiner-box-exploded-components-01.webp\")
** Figure 1. Exploded component view illustrating how six PV strings are fused, combined, surge-protected, and routed to one DC output. – **Suggested aspect ratio:** 16:9<\/figcaption><\/figure>\n 5-Step Sizing Procedure for a 6-String Combiner Box<\/h2>\n
Before you select enclosure size or protection hardware, lock down the electrical values that define whether the combiner will run safely under peak irradiance and worst-case temperature.<\/p>\n
Sizing a 6-string PV combiner box correctly requires five calculations in sequence. The process below applies to both 1000 VDC and 1500 VDC architectures and is the fastest way to avoid undersized fuses, nuisance trips, and inspection failures.<\/p>\n
Step 1 \u2014 Confirm String Short-Circuit Current (Isc)<\/h3>\n
Start with module datasheet Isc at STC, then apply the design correction factor required by your governing standard or project specification. For IEC-based design, a 1.25 multiplier is commonly used to account for elevated irradiance. If a module has Isc of 18.2 A, the design current becomes 22.75 A per string.<\/p>\n
Step 2 \u2014 Select String Fuse Rating<\/h3>\n
Each string fuse must be coordinated with both the corrected string current and the module’s maximum series fuse rating. Using the example above, a design current of 22.75 A requires a fuse that is high enough to avoid nuisance opening but still low enough to clear reverse-current faults before conductor or module damage occurs. Always verify the final fuse choice against the module datasheet.<\/p>\n
Step 3 \u2014 Calculate Combiner Output Current<\/h3>\n
Add the six string currents at the busbar. For 6 strings at 22.75 A each, the maximum output current is 136.5 A. The output DC protective device should then be selected with adequate continuous current margin.<\/p>\n
Step 4 \u2014 Verify Voltage Rating<\/h3>\n
The combiner voltage rating must exceed maximum string Voc under the lowest site temperature. Use the module’s temperature coefficient to correct Voc upward for winter conditions. For 1500 VDC systems, every relevant component, including fuses, disconnects, busbars, terminals, and enclosure insulation system, must carry a matching 1500 VDC rating. In a 32 MW ground-mount project in Inner Mongolia, nominally 1000 VDC combiners experienced repeated insulation failures once winter Voc rose beyond safe margin.<\/p>\n
Step 5 \u2014 Confirm Short-Circuit Withstand at the Busbar<\/h3>\n
Calculate prospective short-circuit current (Ipsc<\/sub>) at the combiner output using the inverter’s DC input impedance and cable resistance. For a 1500 VDC string array with 6 parallel strings, Ipsc<\/sub> typically reaches 10\u201325 kA at the busbar. The output breaker’s rated ultimate breaking capacity (Icu<\/sub>) must meet or exceed this value \u2014 verified per IEC 60947-2.<\/p>\n
Cross-check the final wiring path against the PV combiner box wiring guide<\/a> before freezing the BOM, especially if the project may later scale to different string counts.<\/p>\n
[Expert Insight]
\n– Check the module’s maximum series fuse value before ordering string fuses; that limit can override a simple current-window selection.
\n– Use minimum site temperature from project meteorological data, not annual average climate tables, when correcting Voc.
\n– If the output protective device is close to its frame limit, leave room for future module uprating or repowering.<\/p>\nWorked Example: Full Sizing Calculation for a 15 kW Rooftop Array<\/h2>\n
A numerical example makes the sizing logic easier to apply on site.<\/p>\n
System Parameters<\/h3>\n
The array uses 6 strings of 8 \u00d7 400 W modules with Voc = 41.2 V and Isc = 9.85 A per module at STC. Strings are wired in series, then combined before feeding a 15 kW inverter.<\/p>\n
- \n
- String Voc (per string): 8 \u00d7 41.2 V = 329.6 V<\/li>\n
- Temperature-corrected Voc (at −10°C, ΔT = 35°C, coefficient −0.29%\/°C): 329.6 \u00d7 1.102 = 363.2 V<\/strong><\/li>\n
- String Isc: 9.85 A<\/li>\n
- Design Isc (IEC 62548-1 factor 1.25): 9.85 \u00d7 1.25 = 12.3 A per string<\/strong><\/li>\n
- Combined output current (6 strings): 6 \u00d7 9.85 A = 59.1 A → design value 59.1 \u00d7 1.25 = 73.9 A<\/strong><\/li>\n<\/ul>\n
String Fuse Selection<\/h3>\n
Each string requires a gPV fuse rated above 12.3 A but still within the module’s maximum series fuse limit. A 15 A, 1000 VDC gPV fuse is appropriate for this case.<\/p>\n
Main DC Breaker Selection<\/h3>\n
The combiner output should be protected by a DC circuit breaker rated for at least the 73.9 A design output current. An 80 A, 1000 VDC DC MCCB with breaking capacity of 10 kA or higher is a practical selection for this array size.<\/p>\n
Summary Results Table<\/h3>\n
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![\"**](\"https:\/\/sinobreaker.com\/wp-content\/uploads\/2026\/04\/iec-vs-nec-6-string-pv-combiner-box-comparison-03.webp\")
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