Ok, when running 240v plugs, when do you need to run 3 wire cord (red-white-black & ground) vs 2 wire cord (black-white & ground)?  I noticed that my 240v welding plugs use the 3-wire setup, whereas my 240v compressors (20a) only need the 2-wire cord.  On the welding plugs, the black and red wires are my hot leads on the breaker, with the white going to the neutral.  On the compressor, my white and black wires are hot, with only a ground.  Why does one setup require a neutral, while the other doesn't??

Now I'm even more confused!

I may need to pick up a book and actually read up on this stuff.

The white wire in your 120 is nothing more than an "extra" ground wire with insulation on it.  They are connected to the same spot in the breaker panel.  





The ground wire is merely there for safety.

You need the 3-wire cord when the device needs 120V in addition to 240V.  There's 120V between the neutral and each hot wire.z

Jason280, you may have an actual neutral in your welder - it may be there for some small indicator lights, etc.

You'd have to actually tear the welder apart and see where that white wire is landed.

Many of the small suitcase welders are dual voltage, so without looking at it...



Not exactly.

The neutral carries the unbalanced load, and has the potential to knock you on your ass.

The neutral and ground are connected to the same spot at the service.  Anywhere else down stream they are on seperate bars.

This. If the range has a convenience receptacle or the appliance has logic boards it typically needsna 4-wire setup because it is illegal to use a ground as a current carrying conductor. 4-wire is now code, regardless of the receptacle used.

The white wire is the grounded conductor,  that gives potential. The green is the grounding conductor. Big difference that can kill you.

I installed the temp power at my last job, and it included 4 240v welding recepts in the Mech Bldg.

The Esso cords they had for them had 2 hots, N, and Grnd.

If you work temp power a lot, you see all kinds of whacko cords, and if you don't know, you gotta find out what the equip actually needs.

The welders had 4 suitcase welders, and all the labels, etc. were gone.  They had no idea about them, their cords had no caps on them, etc., so I had to tear one apart to see what was going on.

Anyway - got all the cord caps on, stuffed them all in a panel, and went my merry way.

Coupla months later the welding Super calls me - and he was pissed.

Guess a few days before someone hit one of the welding plugs with a lift and destroyed it.  Some other electrician replaced the recept, box, cover, etc.

And some how, some way, he fucked up.

When they turned the welder on, it smoked.  As in kaput for good.

The Foreman for the electrician that did it got in there and removed the new recept as (I was told) he knew I was on my way there and was pissed.  He never would tell me who did it.

I elevated it to the Shop and let them deal with the ass.

So we ended up paying for a new suitcase welder for them - as we should, actually.

IIRC it was about $2500.00.

That makes sense, as I would imagine the cooling fans and wire drives on the MIG welders would all run off 120v (unless the motors are 12v).

You actually have

2 hots, 1 neutral, & 1 ground

across the two hots is 240

1 hot to neutral is 120

some 240 only uses the 2 hots and the ground.

A lot of appliances that are traditionally 240v also use 120v.  Think of a stove with a digital display/timer.  The neutral is the grounded conductor, providing a return for 120v.  The green is the grounding conductor, intended to ground all the metal bits, and provide a path to ground.  Yes the white and green are parallel once out of the appliance, but I assure you, they are necessarily separate in the appliance.

Correct.
NEC only allows neutral and ground to bond at the first point of disconnect, be it the meter base or the panel depending on era it was installed.

Wait, I have confused myself once again. All of my 240v welders have standard NEMA style 3-prong plugs, and my extension cords all use NEMA 6-50 50a 240v male and female plugs.  My dedicated 240v plugs all use 6/3 wire from the breaker box:  red & black wires for each leg of the breaker, white to neutral in the box, and green to ground.  Now, in the actual plug mounted to the wall, the white and ground wires are hooked together, with the red-black wires for each female "prong" slot.  My extension cords are setup the same way, with white/ground connected together inside the respective male/female plugs.

Here's my question.  Since my white/ground wires are terminating together in the cords and at the wall plug, is there really any reason to even use this type of wire?  Wouldn't I accomplish the same thing using a 3-wire cord (black-white-ground), effectively negating any reason for 4-wire cord for my needs?

Yes. Better to just not connect the white and just use both phases or hots, and the ground. Do not bond the neutral and ground. Bad practice.

Humor me here, but why is it bad practice?  

Going by your advice, though, should I simply not hook up the white wire in the panel box and just use the red-black-ground?

As mentioned, never bond neutral and ground except in the actual system at the first point of disconnect. Always follow industry standard practices so the next guy will know what has been done or not smoke a piece of equipment as mentioned above. The colors really do mean something. Black, red, blue, white, green. Brown, orange, yellow, grey, green.

becuase you have the possibility to get current through a neutral, and if its hooked to a ground, you could energized a ground, which is connected to all the other grounds,

bad fucking ju ju.

Makes sense, thanks!

eta:  So, just leave the white wire unhooked in the panel box?