what wattage iron do I need for butting two 10 gauge stranded wires together about 3/8 inch of bare wire to be soldered and what solder and flux works

Rulon, thank you for your question. For the proper wire soldering we would suggest you contact the solder manufacturer. For electrical wire we would suggest 60/40 lead-free rosin core solder and a rosin flux. As with any soldering application in soldering large wires heat sink becomes the issue. The iron is 60 watt iron and operates approximately 880 degrees. However, the heat sink varies from the use of tip. With heat sink the largest possible tip should be used. In this set it does come with an ¼” chisel tip. A 60- watt iron should recover from, heat sink quick enough to perform the soldering.