Wow, that brings back memories. I had to wire the same control panel on the same spa for a friend. I had the exact same problem.
I concluded that the controll panel was made to IEC standards (metric) and that's why the 3/4" non-metallic liquidtight connector would not fit into that "knockout" or cable clamp or whatever the heck that is.
I got so frustrated that I just ran the liquidtight right up to the box without a connector and threaded the conductors through the opening without tightening it.
Yes, it was hack to do that but I had no options. I didn't see a problem because the enclosure was all plastic.
That thing really needs to be redesigned. Those cheesy plastic lug connectors felt like they were going to snap right off when I was tightening down the #6.
Peter
