danielA combiner box is used to combine multiple solar strings into a single connection. I plan to mount the box on the roof. Based on my solar panels and MPPT charge controller specifications I could wire them in a 1s4p (all four in parallel) or 2s2p (two serial strings of two combined in parallel). I chose to use a 1s4p configuration. If I had decided on a 24v battery bank I would have changed to a 2s2p configuration. With all four panels in parallel I will need to combine all the positive wires together and all the negative wires together.
At its simplest a combiner box can house a positive and negative bus bar that connects the wires from the panels and the larger wire that goes to your charge controller. Depending on your configuration you may need more stuff. For my system I need to fuse each wire coming from the solar panels. I am also going to add a breaker for the larger wire that then goes to my solar charge controller.
Why do I need fuses on the wires coming from each panel? The short circuit current (Isc 10.66a*4=42.64a) is higher then the fuse series rating (20a) of the panels. Explorist.life has a good write up on this topic. There is also a good guide here. You should also have a disconnect for each panel which the fuse holders I used will also enable.
Now that I know I need to add fuses/breakers what size do I need? NEC recommends sizing them 25% larger then Imax which is defined as 25% more then Isc. That means you should take the Isc of your panel or string and multiply by 1.56 (1.25*1.25=1.56).
My panel Isc = 10.66a * 1.56 = 16.63a. I will use 20a fuses for each.
For protection of the larger wire that connects to the charge controller I need at least 10.66a * 1.56 * 4 = 66.52a. I will use an 80a breaker which will work for the short run of 6awg wire I plan on using.
No box?
I had a change of plan. Since all my electrical panel would only be a few feet from the roof entry I decided to move all the PV combiner box components to the panel inside vs on the roof. The cost is negligible and now I can service all the components without getting on the roof.
I purchased a smaller enclosure and installed nine PG9 glands. A positive and negative for each of the four panels and a ground.
I then used butyl putty tape on the underside of the box and four screws to attach the enclosure to the roof. After the fun process of fishing a string through the roof I then pulled all the cables. Once the install is close to finalized I will tighten the cable glands and add a small zip-tie to “lock” the enclosure.
