About the Combiner Box (AJB) Sizing
Array Junction Box (AJB) sizing determines the appropriate enclosure, busbar rating, and protection device ratings for the AJB — the combiner box that aggregates multiple solar strings into a single DC output feeding the inverter or DCDB. Correct AJB sizing prevents overheating, arcing, and string-level faults from propagating to the inverter.
How to calculate it
- 1Count the number of strings to be combined: each string connects one pair of conductors (+ and −) to the AJB.
- 2Total AJB output current = Isc per string × number of strings × 1.25 safety factor.
- 3Select busbars rated for this total current. Copper busbar current density: 1.5–2 A/mm² for enclosed busbars.
- 4Select string fuses: each string fuse rated at 1.5–2× Isc of that string (typically 10–15 A for 60-cell panels with Isc ~10 A).
- 5Select output MCCB or DC isolator rated for the total output current at the system voltage.
- 6Enclosure IP rating: IP54 minimum for outdoor/rooftop; IP65 for dusty or coastal sites.
Worked example
A 50 kWp array with 10 strings of 5 kWp each, Isc = 10 A per string. Total AJB output current = 10 × 10 × 1.25 = 125 A. String fuses: 10 × 15 A (one per string). Output busbar: 125 A copper busbar. DC isolator: 150 A, 1,000 V DC rated. Enclosure: IP65 for rooftop exposure.
What you can calculate
- String count
- Fuse sizing
- Type 2 SPD
- Busbar capacity
Standards & references
Frequently asked questions
Do all solar installations need an AJB?
Small residential systems (1–10 strings) often feed strings directly into the inverter's MPPT inputs — no separate AJB needed. AJBs become necessary when: (1) the number of strings exceeds the inverter's direct input capacity, (2) the plant is large enough to require a central inverter, or (3) long string cable runs make it more economical to combine strings in a field AJB before running a single large DC cable to the inverter room.
What is the difference between AJB and DCDB?
AJB (Array Junction Box) is in the field — it combines multiple strings into one output. DCDB (DC Distribution Board) is at the inverter room — it receives outputs from multiple AJBs and distributes them to inverter inputs, with surge protection and isolation devices. Large plants have multiple AJBs in the field feeding one DCDB at the inverter station.
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