According to Richard J Kinch :
Everything seems fine right now, until I throw the disconnect on, at
which point it arcs at the main GFI in the house and trips the breaker
and disconnect hard.

Something to check: a half-high double-pole breaker on the wrong spot in
the panel creates a dead short.

Actually, what it produces is a 120V circuit with two hot wire, and two breakers
in parallel. Which won't do you a bit of good with a 240V load, because both
legs of the load are at the same voltage - applying _zero_ volts across the load.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.