http://www.hobartwelders.com/om/0900/o949b_hob.pdfhttp://www.houwire.com/products/technical/article400_5.htmlAbove are links to the owner's manual for your welder and the National Electrical Code applying to SO cords.
Looking at the welder pdf (page 17, figure 3-8) you will notice that there are only 3 conductors required, in this case L1, L2, and ground. Neutral is NOT needed for a welder. The fan is designed to run on 220 volts--nothing in the welder requires 110V so you don't need a neutral. Remember that neutral and ground are NOT the same.
Okay, now go to the houwire link, which shows NEC 400.5. In the left column you will find the AWG of the wire. Look down the list and you will find 8 AWG wire. Following across you will see that 8 AWG SO cord will handle 40 amps. Even though your welder will draw 47.5 amps, the welder's duty cycle at that level is so short that the SO cord will not get hot enough to cause a problem.
Modern ROMEX wire is sold with 2 (or more) current carrying conductors PLUS a ground wire. Remember that electricity will always find the shortest path to ground. The grounding wire is NEVER supposed to have any current on it UNLESS there is a fault of some sort in a current carrying conductor. This grounding wire will allow the current to go to ground (away from people) and trip the overload device, which is a fuse or circuit breaker. The wire is bare so that is the ROMEX wire is cut with a pair of cutters or has a nail driven into it, chances are that a short circuit will occur at the point of the defect in the wire, tripping the overload device.
If you are planning to run permanent wire from the house to the shed, you WILL need 4 conductors (L1, L2, neutral, and ground)
