Do you have fuses installed? If so what amperage? In building wiring 14 ga copper is rated 15amp. 12ga 20 amp and 10 ga 30 amp copper. Around 90C rated insulation. The relay coil should only draw a few milliamps,To Save the light switch. I would buy a relay socket and solder wires to the socket. Soldering a heavier wire to a lighter wire just makes the lighter wire the fuse.
