What soladeck parts needed to combine 3 Enphase (17,17,11) trunks?
Hello, great question. The difference between the two Soladeck PNs in your question is the length of the DIN rail. The ...0783... has a 3-inch DIN rail whereas the ...0786... has a 6-inch DIN rail. For your application, choose the SolaDeck PN 0786-3R5 (CivicSolar PN cs-27149).
Prior to suggesting a part number for combining the three Enphase trunk cables, it is important to confirm that you are using Enphase M215 microinverters due to the two branch circuit lengths of 17 stated in your question. If you are using M250s, the branch circuit lengths are limited to a maximum of 16 inverters per circuit in a 120 / 240V electrical service.
Next, combining branch circuits in a SolaDeck is certainly possible, but one must consider accessibility for maintenance purposes. If the SolaDeck will be placed underneath modules, it is best to use the SolaDeck as a junction box to transition from Enphase trunk cable to homerun wiring. This alleviates future maintenance and troubleshooting that would require removing modules to access OCPDs. Additionally, if the roof is not easy to access (requires ladder and fall protection) it is generally good practice to not place OCPDs or other components that require manual operation.
If after considering access and it is determined that the SolaDeck is a good location to place a three-circuit combiner, the following PN would work great: 0760K3AC-PB20 (CivicSolar PN: cs-137329).