What kind of panel does the wire go back to? Is it a breaker panel, or
fuse box? You may have trouble finding somewhere to attach a #6
as well as a #6 ground to, say, an old Wadsworth 60A fuse box.
Be aware that the innards of an old fuse box may have deteriorated over
years. Be very aware that, even if you pull out the main
fuse block or turn the main breaker to OFF, the service wires (the
that come into the house) and the lugs, screws, and whatever attaches
are all very much alive and capable of electrocuting you or explosively
the end of an insulated screwdriver, throwing bits of metal in your
It's very wise to wear goggles when working in a panel.
Also remember that the terminals must be properly tightened, neither
tight nor too loose.
Maybe you know all this stuff already. I just want to point out that
not something that should be learned from the people at Home Despot,
it's better to learn this some way that doesn't involve making a big
a blue flash.