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AS/NZS 5033-2014
What Is a DC Combiner Box?
In systems with multiple parallel strings, a DC combiner box (also called a string combiner or array junction box) aggregates individual string cables into a single main DC cable before the inverter. Beyond reducing cable runs, the combiner box is the location for string-level overcurrent protection, string monitoring fuses/breakers, and surge protection devices.
When Is a Combiner Box Required?
AS/NZS 5033 mandates string fusing when three or more strings are paralleled and the cable rating could be exceeded by fault current from adjacent strings. In practice, any residential system with more than two strings benefits from a combiner box for both compliance and fault-finding capability.
Number of Parallel Strings | String Fusing Required? | Combiner Box Recommended? |
|---|---|---|
1–2 strings | Not required (if cable rated adequately) | Optional — SPDs still beneficial |
3–4 strings | Yes — DC-rated fuses or MCBs | Yes — simplifies wiring and protection |
5+ strings | Yes — mandatory | Yes — essential for fault isolation |
String Monitoring — Finding Faults Fast
Modern combiner boxes include current sensors on each string input. When one string produces measurably less current than its peers under the same irradiance, the monitoring system flags a fault — typically a failed panel, shading, or open-circuit connector. Without string monitoring, a single failed string in a 6-string system loses 17% of array output but shows no obvious external sign at the inverter display.
String monitor data should be logged at 5–15 minute intervals
Alert thresholds: string current deviating >10% from median of all strings on clear days
Combined with thermal imaging, string monitoring pinpoints faults to a specific panel or connector
Labelling Requirements for Combiner Boxes
Every combiner box must be labelled per AS/NZS 5033 Amendment 2: "CAUTION — PV ARRAY — Do not disconnect under load" plus the maximum system voltage at the enclosure terminals. The enclosure must also show the number of strings and fuse rating for each string.
Engr. Jason Morales — Founder, SolarEnergyPH
