I am installing a outlet in a home for a electric stove. The existing j box is 18" up from the floor. I want to install a surface mount outlet. In order to do this I'll need to add about 12" of #6 wire. I want to know the best way to spice the new wire to the old.. Dave B.

Wire nuts are made large enough to connect 2 #6 wires... however, those connections must / need to be in a junction box.

And you can't just run individual conductors. You MUST run them as a cable or in conduit.

Also, if this circuit is an older non-grounding 3-wire circuit it cannot be extended. It must be re-run with the proper 4-wire circuit.