The combiner box is chosen from the string layout and the inverter's input arrangement, not from its own data sheet. Decide how many strings are being grouped, whether they stay separate or combine, and at what voltage — the box then follows. Protection components are the last item, not the first.
Step 1 — Count the strings and decide whether they combine
This is the decision that sets the configuration. If one string is being protected and connected on its own, a 1-in-1-out box is the match. If three strings are being merged into a single inverter input, the 3-in-1-out box does that in one enclosure with a fuse per input. If three circuits must remain electrically separate — typically because they feed different inverter inputs or MPPT channels — the 3-in-3-out box routes them through one enclosure without merging them.
The difference between 3-in-1-out and 3-in-3-out is not capacity, it is whether the outputs are combined. Choosing the combined version where the circuits had to stay separate is not correctable by changing a component.
Step 2 — Set the voltage class from the array
The range is offered at DC 500 V and DC 1000 V. Match the box to the array's maximum system voltage; a 500 V box in a 1000 V array is a misapplication regardless of the current ratings involved. Low-voltage DC projects and smaller residential or off-grid arrays generally sit in the lower class, while larger arrays need the 1000 V class.
Step 3 — Check the protection the circuit actually needs
Each box integrates a photovoltaic fuse on the input, a DC surge protective device, and a DC circuit breaker on the output for isolation during installation and maintenance. The fuse rating and breaker rating are the parts most often customised, because they follow the string's short-circuit current and the inverter's requirements rather than the box.
Have the string's short-circuit current and the inverter's maximum input current available before you enquire — those two figures determine the fuse and breaker ratings, and they cannot be derived from the box alone.
The range at a glance
| Configuration |
Rated voltage |
Input protection |
Surge protection |
Output protection |
| 1-in / 1-out |
DC 500 V |
PV fuse |
DC SPD |
DC circuit breaker |
| 1-in / 1-out |
DC 1000 V |
PV fuse |
DC SPD |
DC circuit breaker |
| 3-in / 3-out |
DC 500 V |
PV fuse per circuit |
DC SPD |
DC circuit breaker per circuit |
| 3-in / 1-out |
DC 1000 V |
PV fuse, rating optional |
DC SPD |
DC circuit breaker |
What to verify before you order
- How many strings are grouped, and whether the outputs combine or stay separate.
- The array's maximum system voltage.
- The string short-circuit current, for the fuse rating.
- The inverter's maximum input current.
- The mounting location and whether the enclosure will be exposed.
- The standard the installation must be certified to.
Common selection mistakes
- Choosing 3-in-1-out when the circuits had to stay separate. Combining is irreversible in the field.
- Sizing the box but not the fuse. The fuse must be selected against the string's short-circuit current, not the box's rating.
- Assuming the voltage class is flexible. It is set by the array, not by the box's other ratings.
- Forgetting that the installation must be certified. The standard determines acceptable component ratings, and it varies by market.
