Using a single wire is almost always better than butting two wires end to end, but FWIW, I use the following procedure to connect two wires together in a way that looks nice:
1: Get some large gauge butt connectors from
RadioShack or electrical supply shop. The Yellow ones in the RS link are good for 12GA, but not sure about anything larger.
2: Cut the plastic sheathing off the inner metal tube.
3: Drill a ~1/16" hole on both ends. This provides a place to feed in the solder.
4: Tin both inside ends of the connector.
5: Strip/tin the wire ends. Make sure the ends are tightly twisted first because if the wires are tinned while too loose, they won't fit into the end of the connector. Although, I've had this happen and just used a dremel sanding wheel to do a little trimming.
6: Insert the wires to each end of the connector and solder. Heatshrink the whole connector.
You should have a clean looking wire connection without a large lump where the connection was made.
BTW: If your power wires are 10GA, 12GA motor wires is plenty. You gotta remember that the motor wires, being 3-phase, will see 66% of the total current. So, 100A on the power wires will be around 66A on the motor wires.