Okay. So I wanna wire my garage. My old home has an old unused 3 prong 240v plug just chilling. My home is limited to 100amps. Old old home. So, the garage, 60ft away has no power. No power=no man zone. I would like to run a couple different options. Sub-panel or just an extension with a gfi outlet. I'm thinking about splitting the plug via a Junction box (new) prior to the old 3 prong plug *and remove said plug. My understanding of it is that it's running 2 hot and a neutral. Can I split the hots (2) to create more plugs and lights. Really just want 3 exterior led lights for motion detection and lights outside. Inside I would like modernized with a plug that can go 120/240 with 20 amps (so I can run my arc welder that requires a min of 20 amps on high or 15 amps on low- it needs a dedicated circuit so as not to be interfered when welding if I have a radio on and light. Help! Man in distress lacking a cave! Or what are some better options?
Yes, you can use existing wiring to feed a sub-panel. (you cannot put a plug on a sub-panel as far as I know.) If the wiring is not quite long enough to reach your sub-panel location, put a junction box there and splice to an additional cable.
Your sub-panel absolutely requires a ground wire to the main panel. Grounds can only be bare wire, green or green/yellow. If the inspector sees any insulation other than that color, it cannot be a ground wire.
If your wire also has neutral, then you can make your sub-panel a 120/240V sub-panel. Here's the trick: In the sub-panel, neutral must be isolated from ground. Most panels give you a way to do that, by removing bonding straps or removing green screws.
If your wire does not have neutral, your sub-panel will be 240V only. Use 2-pole breakers and NEMA 6 receptacles. If you power anything hard-wired, make sure it is 240V friendly - most new fluorescent ballasts are multi-voltage, as are many other products.
If you need to run 120V-only appliances out of a 240V-only panel, they sell step-down transformers for sane prices.
Jay welcome to stack exchange. You can create a sub panel from your house to the garage it will take a minimum of 3 wires and possibly 4 depending on where you live. 2 hot 1 ground, 1 neutral. The ground and neutral need to be isolated in a sub panel. The big question is the size wires. Wire size depends on the breaker you will be feeding it with and the type of load. a continuous load requires 125% where non continuous loads only require 100%. The size of the load 30A to provide an example. With a 30A load you need to make sure your voltage drop is less than 3% I use this handy voltage drop calculator. i put in 240v 30 amps 3% copper and it returned #10 (#10 is the minimum size for a 30 amp breaker) here in the U.S. standard breakers close to the size you need are 25,30,35. if you go to 35A you will need larger wire. I hope this helps.