Combiner boxes combine the output of multiple solar electric (PV) source input circuits.
Combiner boxes are designed for installation near the PV array with each series string of solar modules connected to one of the fused/breaker circuits. The solar array input strings may be combined, yet only a single pair of wires is required between the Combiner Box and the PV controller location. This can result in a considerable savings in the cost of wire since fewer wires will have to be installed.
The smallest combiner box is offered by Midnite Solar MNPV3 which can accommodate up to 3 input circuits.
To determine the size of the combiner box calculate the number of series strings in your array, this may be determined by the maximum open circuit voltage of your charge controller.
Tip: Don't forget about expansion in the future its easier to add a few more breakers than to change out a combiner box!