240v from 120v ?

Yes. Assuming you wire the cord set correctly, and assuming the outlets you plug into are wired correct, and assuming nothing else is drawing power from the circuit (which will just reduce the max load) and assuming neither 15 amp circuit is GFCI protected. If it's a GFCI, it will trip when you pull the current from the hot leg but not the neutral leg on the same circuit.
This arrangement of course is not up to any code and is dangerous for multiple reasons. If the circuit breaker trips, most likely only one of the two will trip leaving the outlet and the equipment half-hot. There's the risk some equipment will malfunction (or be damaged) in a odd way if powered like that, but mostly, it's a safety issues because the equipment will seem dead most likely when in fact there is still power applied to it.
If you plug it into two outlets which are on the same leg, you will get a very low voltage across the hots (just the few volts that result from different amounts of voltage drop assuming something on one of the two circuits is drawing some power), but you will get 120 from either hot to ground. There's always the risk that that error could end up damaging your equipment. I wouldn't expect to to damage most things, but it's a possibility.
But other than those sorts of safety issues, it should work. Just don't blame me when your house burns down or someone gets electrocuted.

Yes, and also burn your ass if anything goes wrong. Gerry :-)} London, Canada

"dan" wrote: (clip) If I make up a cord set that has two 120v plugs feeding one 240v It ain't code, but I ran a 240v buzzbox welder in the back of my store this way for years without a problem, doing light wrought iron jobs. Since you intend to bring the welder home only occasionally, I hardly think it is worth installing dedicated wiring for it. Be aware that what you are doing is on the edge, so unplug completely when you're not using it. Be aware that only one breaker tripping could leave you with 120 v on what "should" be cold.
Of course, the rock bottom safe choice is not to do it. Do you, by any chance, have a stove plug or drier plug--that would be 240 v that you could use more safely, even if it involves temporarily unplugging the appliance.

What's that Lassie? You say that Curt Welch fell down the old rec.crafts.metalworking mine and will die if we don't mount a rescue by 25 Aug 2008 02:46:32 GMT:
I forgot about GFCI breakers and miss-wired outlets. Yah, I had thought about that. If the load was all 240v it should be OK, but many 240v things still have some 120v stuff in them. In that case it may be damaged, seem dead but still be live, or just not work properly.
I probably won't do this, but I thought of it and wanted to hear what others here would say.
Thanks,

On modern houses in the last 20-ish years, all garage, bathroom, wet-bar and laundry sink, and outdoor receptacles are on GFCI circuits.
If you try splitting the circuits, it will trip.
There's a reason they call that kind of a lash-up a "Killer Cord". Because you unplug one of the cords, and the male pins you are waving around are energized....
Two-year-old boys love to mess with things they don't understand, especially if those things can kill them. And since Daddy is over in the corner working, he can't supervise Junior every second.
Now then, where is your laundry center in the house, in the garage? Or a service porch right inside the garage door? Do you see that big 30A 240V receptacle there for an electric dryer?
They usually put that 30A 240V receptacle there, even if you use a gas dryer - and you should, it's rare when electricity is cheaper than natural gas as a heat source, and with Propane it all depends on market prices but usually gas has a slight edge.
And where is the main power service panel for the house - On the outside wall of the garage, perhaps?
That seems to be a very popular place to put it, and it's real easy to poke a hole out the back of the panel and mount a 30A or 50A 240V receptacle right there.
And if not... Run some heavy gauge AL service cable between the main and the garage, Place a small 100A 8/16 sub-panel on the inside wall of the garage - one circuit for the welder, one for the Hypertherm, one for the 5HP air compressor, one for the lathe and mill. Just don't try running them all at once, or the main breaker will let you know.
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Yes.

Bruce,
This 'buds' for you.
Bob Swinney
I am going to get a hypertherm powermax30 soon. It can run on 120v ac(reduced power) or 240v ac(full power). I will use it mostly at work where I can use 240v, but I might need to use it at full power at home.
If I make up a cord set that has two 120v plugs feeding one 240v receptacle, and plug each of the 120v plugs into outlets that are on different legs of the service could I then get 240v at up to 15 amps?

Yes. It certainly doesn't meet the electrical code, as the two breakers are not tied together, and if one of them trips, the other will still have power. You have to watch out for that. But, it will work.
Jon

What's that Lassie? You say that Leo Lichtman fell down the old rec.crafts.metalworking mine and will die if we don't mount a rescue by Mon, 25 Aug 2008 03:51:00 GMT:
Yes I do. And I think that's what I'm going to use. I'll have to get a long extension cord since the stove is on the second story of my apt.

What's that Lassie? You say that Bruce L. Bergman fell down the old rec.crafts.metalworking mine and will die if we don't mount a rescue by Mon, 25 Aug 2008 01:33:27 -0700:
Yah, I thought of that after my last post. Thankfully I don't have any kids, 2yrs old or otherwise.
Nope. Don't have any laundry center. Two room apt. over a two car garage(I get 1/2 of the garage). In my landlords basement. And the feed from the pole is on the other side of the house. But there is a sub panel in the garage that feeds my apt. But I will have to have an electrician run conduit to my side of the garage. Should be easy.

I didn't think of it at all when I posted my reply but it's probably the single most dangerous aspect of the system. That's probably the most important reason not to do it.
However, if you want to spend some money and add a few relays to your box, I think you could make it fairly safe in all respects. Use two double pole relays that can be powered by 120 V and carry the required current. Power one relay off of each power leg. Connect their contacts in two series circuits so each relay is switching both hot wires (one from each leg). Your 220 volt outlet won't go hot unless both relays are on, and if either leg is off, it won't be connected to the the load so you don't get the killer cord effect. If one side trips the circuit breaker, the relay for that side shuts off, and again, all power to the load is correctly disconnected.
By adding a third relay to this, one which requires 220 volts to close, you could also protect the load from mis-wired legs, or from plugging your box into two of the same power legs. The 220 volt relay would only activate if there was 220 volts across the two hots. When it activates, it would send that 220 power to the 220 outlet. If either leg was GFCI activated, the 240V relay would be enough load to cause the GFCI to trip as well indicating the circuit won't work for your 220V load.
Add a few indicator lights to the circuit so you could tell what was activated and what wasn't, and you would have a safe version of the system you could plug into just about anything you though might be separate power legs. It would only activate and send power to the load if the outlets were in fact powered by separate legs and were correctly wired.
Such a box wouldn't be cheap to make, but neither is a very long high amperage 240V extension cord! :) I doubt this solution would be cost effective, but it was just something fun to think about....

What's that Lassie? You say that Curt Welch fell down the old rec.crafts.metalworking mine and will die if we don't mount a rescue by 26 Aug 2008 01:07:42 GMT:
I think a 5kw generator would be cheaper. But I too had thought about some kind of control circuitry, but I couldn't think of a good way to do it.
I'm just going to use my oven outlet if I ever need to use full power. Thanks for the ideas anyway.

No, that's dumb sending the power upstairs then right back down, just adds unwanted voltage drop. And SWMBO won't be amused tripping over the cord, and having her oven off when she wants to use it. And large sizes of SO Cord are Not Cheap, you don't waste it.
Get a breaker, a 4S box, a few feet of THHN copper wire, and an offset nipple, and put a 30A or 50A 240V receptacle right under your sub-panel; you should be able to do that yourself safely. I think the landlord will let you go on "his" side of the shared garage to plug and unplug the cord from the panel you pay for.
Then get a 25'-50' chunk of 10-3 or 8-3 SO Cord for 30A, or 6-3 SO Cord for 50A, and make a heavy gauge cord set to go to your side of the garage or driveway. You can take it with you when you move.
That long welder/plasma extension cord will also be useful at work - if you can't bring the work to the welder, you take the welder...
They don't make a cord connector body (female) in NEMA 6-30R or 6-50R - you have to build one up. Get a cord grip fitting, steel 4S Box, RACO 810 2.1" surface raised cover, and the receptacle.
NOTE: For abuse resistance put the 810 cover on "upside down" and the rim protects the receptacle, you may need to use a deep 4S box.
The 'Surface Mount' receptacles will not take the abuse they'll get mounted on the end of a cord - and they only come in 50A for ranges.
Make sure to permanently mark that receptacle as "Apartment B Panel, Circuit X/Y" so when you move and leave the receptacle the next renter knows what it is. And the landlord can't say "I didn't know!" and get 'free' power for his welder on the renter's dime - he has to put in his own welder receptacle.
If your sub-panel is full you might have to go get a "Quad" breaker to gain two more poles - (Examples given using Murray part numbers) take out two single pole MP120 and put in a MP2302020 or MP2502020. The two old circuits go to the single 20's on the outside, and your new outlet in the middle.
NOTE: The breaker manufacturer MUST match the panel, or be a sister or successor company - Or the UL Listing is void, and if it burns up you'll be neck deep. Ask here if you aren't sure.
They also make quadplex breakers with two 240V 2-poles in one 2" package, useful if this is one of those odd small sub-panels that only have 240V available on the two middle positions. The bussing goes A-A-B-B in some 8/16 panels instead of A-B-A-B, and you need A and B adjacent to get 240V - the center top and bottom positions only. Unplug the MP230 for the water heater, and plug in the MP230230 or MP250230
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What's that Lassie? You say that Bruce L. Bergman fell down the old rec.crafts.metalworking mine and will die if we don't mount a rescue by Tue, 26 Aug 2008 11:06:37 -0700:
I'm sure the landlord won't mind at all, he's good that way. The panel is flush mounted though, so a surface mount box will be needed. I just remembered, I've got an dedicated circuit feeding a single outlet for a window AC unit that I don't use. I think its a 25 amp breaker(120v). That and my 10 gauge extension cord will get me into the back yard just fine. Do they make them in NEMA L6-20R ? that's what the cutters cord is. They supply adaptors to convert to NEMA 6-50p and NEMA 5-15p.
I will label the outlet with the panel and breaker number. But the chances of the landlord having anything that would have a l6-20p plug on it is nil. Of course the next owner might. So a panel with full slots isn't really full.
I'll leave it up to an electrician.
And thanks for all the advice.

If you "color" the white wire black or red at both ends (outlet and panel) with tape, and you are sure it's a home run circuit, you can always change the AC receptacle over to a 240V circuit with a L6-20R or other suitable receptacle.
Oh, Youbetcha! Leviton 2323, and it's almost a given that Hubbell, P&S and the others make it too.
But if you are going to make an extension cord that can also be used for a welder, you'll want at least 30A 250V - which means changing it all over to L6-30 devices. Many of the welders have 6-50P plugs but never use over 24A, so you can size the breaker and cord at 30A safely.
If you want to go to a 50A cord system, I strongly suggest going all the way to the California Series (Non-NEMA but an industry standard) plugs and sockets for "Spider Boxes" used on construction sites - make it easy to mix and match.
CS63-69 Single locking receptacle - 50A 120/240V 1Ph (uses 7788CR or 7770 weatherproof cover) CS63-65C Locking Plug CS63-64C Locking Connector CS63-75 Locking Flanged Inlet
The rule is you can't go over 42 poles of disconnects if the loads in the panel are over 10% convenience lighting and general lighting circuits, and IIRC it has to be a bolt-on panelboard to go over. They build 80-breaker panels with 400A busses all the time.
But you can take a 20-position panel and use all Twin and Quad breakers and bump it to 40 poles if the buss stabs allow it (rejection notches that won't let twins plug in) and you don't overload the panel.
Some electricians play fast and loose with the rules, too. Call foul if you see an unsafe cheat like the cheap "Made in Bangladesh" breakers from outfits like "Unique Breakers Inc" - NOT U.L LISTED.
If they try to say the correct breakers are "Not Available", that's bullshit - but the OEM breakers for some panels are very expensive, so they shave corners with the knockoffs and pocket an extra $50.
Zinsco is, ITE Pushmatic is, Square D QO is. The only ones that aren't are Federal Pacific Electric, but IMNSHO they are dangerous a dozen ways and the whole panel needs to be changed.
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