[Shop-talk] unsafe wiring

[Chris Kantarjiev]

I need to do something that I know won't be quite right, and want to figure
out how to do it as safely as possible.

I have a sub-panel that feeds (fed) the heatpump - it's fed with three
conductors: both phases, and ground. The heatpump has been dead for at least
15 years; two owners back installed a swamp cooler right above it.
The previous owner left behind the extension cord that he used to
power the swamp cooler, and I've done the same thing for several
summers - but I've had enough. 

I know that I can get 125v from one phase to ground. I also know that this
isn't quite kosher, since there may be some voltage drop from
real ground to this phony neutral. I think that means I should use
a GFCI, and that will be reasonably safe - am I understanding this
correctly?

Or will the GFCI just keep tripping? :-)

Is using BX cable outdoors a reasonable thing? It appears to be what
was used to go to the heatpump, but I don't think of BX as being
weathertight. This is all under the roof eave, but still...

Thanks.