If it's temporary get long enough wire and tap into a panel. Or if you could find a dedicated outlet you can make that into 220. You can even find a feed coming straight from a panel going into a general circuit and use the feed as 220 as long as you don't tie in the rest of the circuit. There's ways you can do it going off of two different circuits is not one of them
In a word, no. All of a given circuit's or feeder's conductors must be run together in a cable or conduit.
By being a wise guy (admit it!), you pointed out that the OP needs to see whether the L-L voltage suits the new load.
Combine two 15 amp 120 volt circuits to make a 30 amp 240 volt circuit?Leave it to Amazon…
Amazon does not care in the least if something a vendor sells on their platform is stupid or even illegal. Caveat emptor. I like Amazon and use them a lot, but I shudder at much of the junk electrical stuff they allow to be sold there. I've made reports to them and had them respond that it is not their responsibility to police the products. Some day someone is going to get hurt and find a lawyer good enough to take them on over that issue.
The source is a Single-Phase, Three Wire residential service. L-L voltage is nominally 240V. Load is single phase, approximately 3kW. Device would compare inputs to ensure separate phases are connected before energizing output. My biggest hurdle is how to prevent GFCI tripping.
To my knowledge, not possible, unless you do something like a VFD or double conversion UPS.
Well any 120 volt loads would still operate if it is supplied with both same "phase conductors" and a neutral.
Two 120V-120V 1.5kVA isolation transformers could be used with each driven by a 120V outlet. Then the 120V secondary windings of the two transformers could be placed in series to produce 240V. GFCI's on 120V circuits should not trip because there would be negligible common-mode current drawn from their outputs because each would be feeding its own isolated primary winding. A 2-pole breaker or handle tie between breakers would be needed as Larry mentioned above.
If the two 120V circuits were on opposite phases of a multiwire branch circuit supplied by a 2-pole GFCI breaker, then it should not trip with a L-L load across the two branch circuits. However, it would not meet the requirements of 300.3(B) which Larry mentioned in post #6 if the conductors of the L-L circuit are not contained within the same cable, raceway, etc.:
Nice.
Since you could do the same thing with a 120v primary, using a second 120v circuit would actually be providing twice the available power.
I haven’t read everything posted on this thread, but why not just use a 120 primary/120/240 transformer?
With that connection, unless the magnitudes of the L1-N1 and L-2-N2 voltages are equal, relatively high currents may flow between the circuits though the two primary windings. It's kind of like putting two low impedance sources of voltage in parallel.
I'm not sure if that's true if the transformers are plug and cord connected to receptacles.
A 3kVA load would exceed the capability of a 20A branch circuit, and so maybe that was part of the OP's motivation for using two different 120V circuits. The OP would need to clarify that.
Yeah, I didn’t read to see needed loads
But don't see a code compliant way to do it.