The four 24 volt PV modules on the roof are grouped into two 48 volt pairs as described in The solar panels article. The + and - cables from each pair descend from the roof and enter the combiner box. Two 50' MC-3 cables arrived with the combiner box. The connectors on each end come marked with a + (male) or a - (female). I cut these two 50' cables into four 25' cables, two for each 48 volt module pair.

Before bringing the cables down I marked the cut ends with a + or a - using white fingernail polish, a fast drying enamel. I also marked the cut ends with East or West in case I ever need to know that. If the connector on a cable was marked with a -, I marked the opposite cut end with a +. That way I would know which was which down inside the combiner box.

The + cable from each pair fits into the bottom of a dedicated 15 Amp breaker. The top of the breakers fit onto a common bus bar at the top of the box. The white neutral system wire runs from that bar out of the box to the PV + connection in the charge controller. The - cables from all pairs connect to the PV Negative bar on the left and the black 'hot' system wire runs from there to the PV - connection in the charge controller. The Ground bar on the right receives only ground wire from the system and the DC lightning arrestor.