Understanding combiner box sizing
Why a combiner box exists at all
Once an array has more strings than an inverter's MPPT inputs can directly accept, or when strings need to be combined before reaching a single feed to a disconnect or inverter, a combiner box brings multiple strings together into one output. Each incoming string gets its own individually fused input, and the box's output busbar carries the combined current of all strings at once, a layout question the MPPT count vs array layout calculator helps plan out first, alongside confirming your actual string count with the string length vs panel count calculator.
Why each string needs its own fuse
Without individual fusing, a fault in one string could allow current to backfeed from the other parallel strings through the faulted one, potentially exceeding the wiring's safe current rating and creating a fire risk. NEC 690.9 addresses this by requiring overcurrent protection sized specifically for PV source circuits, using the 1.56 factor applied to each string's Isc rather than a simpler rule of thumb, with the underlying conductor ampacity limits coming from NEC Table 310.16.
Why the busbar calculation is separate from the fuse calculation
Each string's fuse only needs to protect that single string's wiring from an internal fault. The combiner's output busbar, by contrast, needs to safely carry the sum of every string's current flowing through it simultaneously under normal operation, which is a fundamentally different sizing question using the same 125% continuous current principle covered in the parallel strings current calculator.
What happens with mismatched string sizes
This calculator assumes every string uses the same panel model and Isc rating, which is the most common residential and light commercial scenario. If your design mixes different panel models or wattages across strings, each string's fuse needs to be sized individually against its own actual Isc, and the busbar calculation needs to sum each string's real current rather than multiplying one Isc figure by string count. The string mismatch loss calculator can help quantify how much output that mismatch actually costs you, separate from the fusing question here.
Choosing a combiner box product that fits
Once you know your required fuse size and busbar rating, check that against the specific combiner box product's specifications, since manufacturers offer models with varying numbers of fused inputs and different maximum busbar ratings. A box rated for exactly your calculated numbers leaves no room for future string additions, so many installers choose a model with some headroom above the immediate requirement, similar to the downstream sizing covered in the wire gauge sizing calculator. NREL's PVWatts calculator can help confirm the array-level production numbers behind whichever sizing you land on.