Because they are "in-phase". In order to get 240v, you need two 120v Alternating Current lines that are 180° out of phase, that is, opposite phases. Only when one line is +120v and the other -120v will you see 240v between the wires.
Your 2-wire system is actually connected to one "side" of the 3-wire system. In your utility's 3-wire system you have 2 hot wires that are attached to each end of a tranformer winding and the neutral is attached in the middle and grounded. Typically in the US this gives you 240v (or something similar) between hot leads and 120v from each hot lead to neutral or ground.
two hot one ground
There should be 2 or 3 wires. A hot, a neutral and sometimes a ground.
It is because the nec standard.
White wire is usually used as a neutral conductor and a 240v ac unit has no neutral. Each leg of the 240 volts is a "hot" leg However, very often a 12/2 or 14/2 NM cable is used to power small window ac units using 240v. In this case one of the conductors is white, but is not a neutral. It is good practice to mark the white wire with black tape so it is readily identified as a hot wire.
The dryer is a 240v dryer so two of the wires are your hot wires, or the ones with power on them. One is your ground wire. And the forth is your neutral.
In North America it takes two "hot" wires to obtain 240 volts.
No, it makes no difference. The selection of red and black "hot" wires on your 220 line is arbitrary.
3 prong the 2 black wires are your hot and the braided is your neutral.
Not a hard job or complicated but could be hazardous. Hire a professional.
There are no adptors to plug a 240v plug into a 120v receptacle. 240v requires two hot wores and a neutral and ground. 120v requires one hot wire, a neutral and a ground. If you have something that runs on 240/120 you need the cord and adaptor that came with the equipment as you willl need the wires to mate up accordingly.
On a 240 volt circuit both line wires are hot, so they may both be black, depending on the wire used. There is normally no neutral required unless you are also tapping off 120 volts between hot and neutral.
You should never trust any wiring without using a tester (get one at hardware or home improvement store. ) in USA, White/grey/bare wire is neutral, colored wire is hot (live). In a pure 240v connection, there are two hot wires and no neutral wire. Sometimes they still use black and white for this. If you are seeing black and red wires you might be looking at a split phase panel -- you'd best check that out
120v and 240v cords usually have different end configurations and will not plug into the different recepticles. However, if you changed the plug end, and the cord has the proper size rating, then yes, you could use the same cord. But, it also depends on the cord too. Most 120v cords only have three wires in them. One "hot one "neutral" and one "ground" wire. A 240v cord would have FOUR wires, two "hot" wires, one neutral wire, and one ground wire. Therefore, if you changed the voltage from 120v to 240 using a 3 wire cord, you'd not have a ground wire and that could be VERY dangerous. Note that occasionally a 240v device (e.g. some motors) will only need three wires (red,black,green, no neutral) and can be wired with a 120v cord if the cord is rated for 240v.
4 wire household wiring is black, red, (hot wires) white (neutral) and bare or green (ground wire). You say 3 wires. Is it 120v or 240v. If its 240v which is more common just use the two hots and the ground and cap off the neutral wire.
The wires are probably getting too hot.
Well, maybe. What is the current rating on the spa circuit? What is the current rating on the welder? If the welder is larger than the outlet, then no. It must be the other way around. If the current ratings are OK, then yes. Your welder doesn't need 110V so it doesn't connext to neutral. Connect the hot leads to hot and ground to ground.