Identifying AC Phase: Making 240 out of 2x 120.

[badwiz]

So I am staying with some family for the summer who live on the edge of a sound (body of water). A boat lift, wired for 240VAC, was recently installed but no wiring was laid for it (it is not powered). I want to use the boat lift to lift a small sailboat (my hobie 14) out of the water. Rewiring the lift for 120 is possible, and would certainly be enough power to lift my tiny boat, but I am forbidden from doing this. I am also forbidden from installing a new 240 circuit in the house.

to overthrow this totalitarian regime without breaking any rules, i must develop a source of 240 using only existing 120 circuits. This would be a simple enough affair if the circuit box for the house were well labeled, and accessible and I had a schematic for the entire house; lets assume this is not the case, and I don't feel like trying to get to a very well hidden circuit box.

Now then, I have all the required wiring and plugs to create 240 from 2 120 circuits, but my question is the following:
Who knows an easy way to identify the phase of two different household circuits without seeing or knowing anything about the circuit box?
Preferably, it would be an easy test I could perform at the wall socket. And through some process of elimination, go around to some exterior wall plugs until I find two of different phase.

I have the following tools:
1. A digital multimeter with AC settings.
2. Various 3 prong and 4 prong plugs, plenty of raw wiring, and other hardware.
3. A few simple circuit elements like capacitors, resistors, inductors, diodes. Maybe a few transistors laying around.

No i do not have an oscilloscope. I wouldn't be having this conversation if I did.

[moiraemachy]

Could you elaborate on what you intend to do after you identify the phases? Honestly, this should be simple (test AC between two wires until you find 220V), so I have the impression that you got something wrong about what to do.

Are you sure you need 240V from 120V phases, and not 220V from 127V ones?

[badwiz]

I actually found a 240vac outlet that wasn't being used around the house, and used that today. Simplest answers are often the best.

My original intention was to use two 120vac outlets to re-create the split-phase 240 coming into the house. I was working under the premise that either phase of ac was used throughout the wiring of a typical US home; some circuits are one phase, some are others.

And yea, I thought also of just using a multimeter and an extension chord to go around looking for 240 across two hot legs.

[Yakk]

Not an electrician, but yes, I think your impression is right, that different plugs use different phases. In fact, they alternate on my circuit breaker, so you can have a double-wide circuit breaker that uses both phases to generate a high voltage outlet.

[Goemon]

And yes, you're completely correct - the easiest way to do this is to plug an extension cord into an outlet, carry the other end over to another outlet, and measure the voltage between the two "hot" leads. If they're on the same phase, you'll get zero; if they're on opposite phases, you'll get 220.

Examining the fuse box won't usually tell you much, unless someone has been kind enough to add a diagram showing which fuse powers each outlet throughout the entire system, which NOBODY ever has attached to their fuse box.

[Carnildo]

I actually found a 240vac outlet that wasn't being used around the house, and used that today. Simplest answers are often the best.

My original intention was to use two 120vac outlets to re-create the split-phase 240 coming into the house. I was working under the premise that either phase of ac was used throughout the wiring of a typical US home; some circuits are one phase, some are others.

From a theoretical standpoint, that will work. The problem is the practical implementation: I don't know how the circuit breakers will deal with a hot-to-hot flow, and if there's a residual-current detector somewhere in the path, it's guaranteed to trip.

[Goemon]

The circuit breakers shouldn't really be an issue, since an overcurrent load would cause one of the breakers to trip and leave an open circuit - pretty much what is supposed to happen. Using the wires in a 110 or 220 config doesn't affect what size breakers would be needed.

But a residual current detector (aka GFCI) upstream on either circuit would definitely prevent you from using the pair as a 220.

[wumpus]

1. You don't have an oscilliscope, but do you have a voltmeter? Placing a sufficient load (incandecent light bulbs work well if they are still available) across two leads and then measuring the voltage.

My answer would be to ditch the idea of wiring up leads from unknown sources. It is completely unrealistic to assume that they could come from separate sources, but as an EE I am well trained by Murphy's law. The obvious solution is a 1:2 transformer that will simply step up 120 to 240. This should cover all issues with phases, ground faults (the solution presented above will trip any GFI breaker in the chain) and other hot/neutral issues.