Not bolt-on adapters on the bus fingers, rather at the main lug ends. Then the DIN rails breakers in another box feeding it. This was for the 225A QO breaker panel on output of Sunny Island fed by the battery inverters.
(On input side, I already had a 100A visible blade switch which previously fed the main lugs when this was grid-tie only. For OCP on input when installing the battery inverters, I mounted a DIN rail inside the switch box and put two dual breakers there. With house now powered through Sunny Island, I also added an interlocked "generator switch" breaker panel so Sunny Island could be bypassed to put house directly on grid.)
I was surprised too. I expected 60' of 6 awg to dominate resistance.
You would expect excess resistance to result in heating and tripping at low current. Those QO breakers work fine individually although I haven't tested their trip threshold. I do have a 100A breaker that started tripping well below 70A, and wouldn't reset until some days later. I wonder if it slowly cooked and changed its characteristics, perhaps due to poor connection with wire (no burned insulation, though.) Home Depo wouldn't exchange, so I plan to contact Schneider to take advantage of their lifetime warranty.
Sunny Boy or Sunny Island?
Sunny Boy wouldn't involve multiple parallel paths from same source to same load. Each is independent, with its own PV array, feeding common AC grid. No problem there; each just drives its power into whatever AC voltage is present.
Sunny Island can have 1 to 4 in parallel for 120V system (220V for European model.) It can have 2 wired for 120/240V split-phase, or 4 which puts 2 in parallel on each phase. Each monitors current to/from grid which is not to exceed 56 Arms. When internal relays are connected to grid, load draws current through two in parallel, or AC coupled PV Sunny Boys backfeed current in parallel. Master SI reports total power to/from grid, and the other three Slave SI each report the power passing through them.
QO breaker panels are available "main lug" or "main breaker". I replaced the single large lug with the following dual lug.
Commercial Electric's Mechanical Lugs are used to create a secure and reliable electrical connection between electrical conductors, such as wires or cables. Mechanical lugs can be either bolted or crimped
www.homedepot.com
I mounted a 6" x 6" x 2' wiring box above the breaker panel and put contactors in it for load-shed (house load disconnected at low battery so inverter can keep supplying AC to Sunny Boys while waiting for sun to come back up.) I added two dual 63A DIN rail breakers in the box so each SI has OCP.
I had thought SI could accept 56A from grid and add 50A (6kW/120V) for 106A load, but manual says 56A max output. Except surge of course.
Here's a big system, DC coupled with Sunny Island Charger rather than AC coupled with Sunny Boy. But it looks like multiple 3-phase clusters each with their own battery, which would then be connected together on AC bus with Multi-Cluster Box. No wires or relays in parallel.
This one is AC coupled to Sunny Boys.