Rob
I'm fairly certain, like a lot of folks, that your culprit is the tandem breaker being installed incorrectly, just put in a standard 240 breaker. If you have the room in your box, and you don't violate the panel manufacturers loading instructions, then the 120/240 tandem breaker mentioned in a previous post could simplify things for you.
The "Kill" switch is actually a very good idea, but you should have a device called a "manual disconnect", "electrical disconnect", "service disconnect", "circuit disconnect, etc, or something with those sort of word descriptors in the title between the breaker and your tools. It sounds as if you have a standard switch.
Because someone has to bring it up, I will.
Tandem breakers cannot be indiscriminately loaded into a panel. Virtually every panel manufacturer has a loading diagram that details which spaces, if any, can be utilized for a tandem breaker. Some panel manufacturers don't allow for any tandems installed in their panel. I know that a person can buy tandems at the 'Borg that fit, and work, but it's wrong, unless they are spec'd and installed per the panel manufacturer.
The problem with indiscriminately installing tandems is that a condition can unknowingly be created whereas the neutral returns become unbalanced and or overloaded. It's especially easy to do it when sub panels are involved.
Last edited by Mike Cutler; 08-15-2014 at 7:43 AM.
"The first thing you need to know, will likely be the last thing you learn." (Unknown)