Definition
A combiner box is an electrical enclosure that consolidates multiple PV strings into a single DC output for transmission to the inverter. Each string input includes an OCPD (typically a fuse), and the combiner output may include a disconnect, surge protection device (SPD), and monitoring.
Quick Facts
| Field | Detail |
|---|---|
| Term | Combiner Box |
| Category | Solar Engineering / Components |
| Engineering Discipline | Electrical Design, Solar Components |
| Standards | UL 1741, NEC 690.9 |
| Typical Sizes | 6, 8, 12, 16, 20, 24 strings |
| Voltage Ratings | 600 V, 1000 V, 1500 V |
| Difficulty Level | Beginner to Intermediate |
What’s Inside a Combiner Box
- String input terminals with per-string fuses (or fuse holders).
- Bus bars combining all string positives and all string negatives.
- Output feeder lugs for the trunk cable.
- Optional load-break disconnect (DC disconnect switch).
- Surge protection device (SPD) Type 2 to ground.
- Monitoring (current sensors per string, IoT logger).
- NEMA 4X / IP65 enclosure for outdoor weather protection.
Sizing Considerations
String count
Number of inputs must match the number of parallel strings expected per combiner.
Voltage rating
Match the system voltage class. 1500 V combiners are dimensionally larger than 1000 V to maintain creepage and clearance.
Current rating
Maximum aggregate current ≤ rated bus and feeder ampacity.
Fuse selection
Per NEC 690.9: fuse rating = 1.25 × Isc (round up to standard ratings: 15, 20, 25, 30, 35, 40 A).
Worked Example
12-string combiner, 1000 V system:
- Module Isc = 13.5 A.
- Per-string fuse = ceil(1.25 × 13.5) = 17 A → 20 A standard.
- Total parallel current = 12 × 13.5 = 162 A.
- 1.25 × 162 = 202.5 A → bus and output sized for 250 A.
- Output cable: 4/0 AWG copper (260 A at 90°C, derated typically 200–220 A).
Manufacturers
- Bentek, GroenWerk, AmtecSolar (US).
- SMA Combine Box, Sungrow PVC (manufacturer-paired).
- Solareast, Suntellite (India).
Permitting Implications
The SLD must show:
- Combiner location.
- Number of inputs and per-string fuse rating.
- Output feeder size and material.
- Disconnect type if integrated.
Common Mistakes
- Fuse rating < 1.25 × Isc.
- Voltage rating below system voltage.
- Bus ampacity exceeded at full string parallel.
- Missing SPD in lightning-prone area.
- Mismatched enclosure rating (NEMA 1 outdoor — should be 4X).
Best Practices
- Match combiner voltage rating to system (don’t undershoot).
- Pre-install Type 2 SPD even if not “required” by code.
- Provide individual string monitoring for fault detection.
- Specify load-break disconnects integrated in the combiner to simplify the SLD.
- Mount near the array to minimize DC wiring losses.
Standards & Certifications
- UL 1741 — Inverter and combiner listing.
- NEC 690.4 (B) — Equipment listing.
- NEC 690.8 — Conductor sizing.
- NEC 690.9 — OCPD.
- IEC 61730 — Module safety (referenced).
Key Takeaways
- A combiner box consolidates multiple PV strings into one DC output via fuses and bus bars.
- Per-string fuses sized at 1.25 × Isc; common ratings 15–30 A.
- Match voltage class (600 / 1000 / 1500 V) to the system.
- Include SPD in lightning-prone regions.
- The SLD must show combiner location, fuse ratings, output cable size, and integrated disconnect if present.
Frequently Asked Questions
10 commonly searched questions about Combiner Box.
What is a combiner box?
An electrical enclosure that consolidates multiple PV strings into a single DC output. Each string enters through a fuse, with optional disconnects, surge protection, and monitoring.
When do I need a combiner box?
When connecting 3+ strings in parallel to a single inverter input. Most residential systems with string inverters don't need separate combiners; commercial systems with 6–24 strings per inverter typically do.
How many strings per combiner?
Common sizes: 6, 8, 12, 16, 20, 24 strings. Select based on inverter MPPT count and project layout.
What OCPD goes in a combiner?
Per NEC 690.9, each string fuse rated for 1.25 × Isc, typically 15–30 A depending on module Isc and parallel string count.
Are combiner boxes UL listed?
Yes. Specify UL 1741 listed combiners with the appropriate voltage rating (600 V, 1000 V, or 1500 V system).
What is a recombiner box?
A second level of consolidation in utility-scale plants — combines outputs of multiple combiners into a larger trunk feeder to the central inverter.
Where should combiners be located?
Near the array to minimize DC wiring; protected from weather; accessible for maintenance. Typically on a pole, equipment pad, or skid attached to the racking.
Do all combiners include SPD?
Modern combiners often include Type 2 SPDs, especially for lightning-prone areas. Verify specification before purchase.
Does the combiner box include a disconnect?
Many do, but not all. NEC 690.13 requires a DC disconnect at the inverter or accessible point; the combiner's load-break disconnect (if present) satisfies that requirement.
What's the voltage rating?
Match to system: 600 V (residential), 1000 V (commercial), 1500 V (utility). Don't mix — 1500 V combiners on a 1000 V system is acceptable; the reverse is not.
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