Do I need a combiner box for two strings?

Usually not. With two parallel strings, one healthy string generally cannot push enough current into a faulted one to exceed the module's series fuse rating, so string fusing is typically not required. Many inverters and controllers accept two strings directly on separate MPPT inputs, which is cleaner. Confirm against your panel's series fuse rating.

Can I make my own combiner box?

The components are available and the wiring is straightforward, but a listed pre-built combiner removes any question at inspection about enclosure rating, fuse holders and busbar suitability. Given a combiner is a modest share of a system's cost and it is an outdoor enclosure carrying live DC, this is a poor place to improvise.

Does a combiner box need surge protection?

Not required, but genuinely worth it on ground mounts, long cable runs and in lightning-prone regions — a surge arriving down a long DC run has your inverter at the far end. Many combiners include it or have space for it, and it is far cheaper than an inverter.

Solar Combiner Boxes Explained: Do You Need One?

With one or two strings, probably not. From three parallel strings up, the healthy strings can backfeed a faulted one — and that is what a combiner prevents.

A combiner box joins several PV strings into a single output, giving each string its own fuse on the way through. Whether you need one comes down to a single threshold: how many strings you have in parallel. With one or two, usually not. With three or more, it is a safety requirement rather than a convenience.

The three-string threshold

A solar module can only push so much current — its short-circuit current, which is barely above its normal operating current. That is why a single string cannot overload itself, and why fuses are usually unnecessary with one string.

Now put three strings in parallel. If one develops a fault, the other two can push current backwards into it. Two healthy strings feeding a fault can exceed the panel's series fuse rating, which is the point at which the module's own wiring becomes the thing at risk. Each string therefore needs its own fuse, and a combiner box is where those fuses live.

With two strings, one healthy string generally cannot exceed the other's series fuse rating, so protection is typically not required. Three is the number that changes the answer — though confirm against your panel's stated series fuse rating and your adopted code.

What is inside one

String fuse holders, one per string, DC-rated for your array voltage
A busbar joining the fused outputs into a single positive conductor
A negative busbar
A grounding busbar for the equipment grounding conductor
Often a DC disconnect, so the combiner doubles as an isolation point
Sometimes surge protection, worth having on long runs or in lightning-prone areas

The practical benefits beyond the fuses

Even where fusing is not strictly required, a combiner is often worth it for reasons that show up during the twenty-five years after installation:

One conductor pair back to the inverter instead of one pair per string, which means less conduit, less roof clutter and lower cost on long runs.
A single place to isolate the array, rather than pulling connectors at height.
Fault isolation. When output drops, you can test each string at the combiner instead of tracing wiring across a roof.
Somewhere to put surge protection, which is genuinely valuable on ground mounts with long cable runs.

When you can skip it

A microinverter system does not need one at all — each panel converts to AC at the module, so there are no DC strings to combine. Small systems with one or two strings feeding a controller or inverter with multiple MPPT inputs also generally have no need, and neither does a small off-grid setup with a couple of panels going into a charge controller.

In those cases the combiner solves a problem you do not have, and adds an enclosure and connections to maintain.

If you buy one

Match it to your array: enough string inputs with room to grow, a DC voltage rating above your coldest-day open-circuit voltage, and an enclosure rated for outdoor exposure — typically NEMA 4 or better if it lives outside, which it usually does. Fuses must be DC-rated and sized to roughly 1.56 × the module's short-circuit current, as covered in overcurrent protection explained.

Mount it somewhere you can actually reach — a combiner on a roof peak is a combiner you will never use for fault-finding. And it needs bonding into the grounding system like every other enclosure: see grounding a DIY solar system.