Electronics > Beginners
How to wire up a 240VAC receptacle
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james_s:
L1 and L2 have 240V across between them, forget about ground, ground is a relative term. With AC the current reverses direction every half cycle, so half the cycle you have current flowing out of L1 and into L2, then the other half cycle it flows out of L2 into L1, there's your complete circuit. If you wanted you could call either L1 or L2 "ground", and you'd be able to connect it to earth ground *except* for the fact that the center tap is already connected to ground so doing this would create a direct short across one half of the transformer secondary.
james_s:

--- Quote from: Spork Schivago on May 19, 2018, 03:25:41 am ---
With those batteries though, that connection between them has to return back to either the bottom cell or the top cell to complete the circuit, otherwise, the electrons have no place to go.   That's what I don't understand with the AC, unless I was right about the L1 and L2 being opposite charges, one being +120VAC, the other being -120VAC, and the +120VAC passes through the heating element to the -120VAC, providing a voltage drop of 240VAC total over the heating element....is that correct?

--- End quote ---

There's no such thing as +AC and -AC, the polarity is changing 120 times a second. If you were to freeze time at the peak of one cycle then yes you'd see +170V on one of the wires and -170V on the other, but the next half cycle that will reverse. In reality the value is not suddenly flipping (square wave) but is a sine wave, so the RMS value is 120/240 for a peak of 170/340.
Gregg:
As previously drawn but maybe looking at the power from a different view may help.

The electric utility company has a transformer somewhere near your house; it is wired as shown in the diagram (in the USA).

As you can see the secondary winding is continuous and the output voltage of L1 and L2 are in proportion to the much higher voltage of the primary by the ratio of turns.  In the US it is 240 Volts nominal for most residential services.

The center tap of the secondary is brought out and called Neutral.  It is grounded to the earth near the transformer.  It is called Neutral because it is grounded and because the voltage between the neutral and either L1 or L2 is half the voltage between them, in this case 120 volts to either L1 or L2.  The neutral conductor from the utility carries any imbalance of the two other lines.  For instance, if you had 30 amps load on the 120volt side from L1 to neutral and 20 amps load on the other side from L2 to neutral, the neutral conductor would carry the 10 amp difference back to the transformer.

After the meter at the main disconnect panel (200 amp 2 pole 240v for this discussion) the neutral and ground are bonded together.  This is the only place they should be connected in your house.  The ground wires going to things in your house should not carry any current; they are there for safety reasons such as either L1 or L2 shorted to ground; then the ground wire should be able to carry the fault current long enough for the breaker to trip.

Ground and Neutral are NOT the same thing even though they are connected at the main panel.  Power doesn’t return to ground, it returns to the transformer. 

The potential between Neutral and Ground is very low, depending on resistance of the wiring, current in the neutral conductor and anything that may be leaking to ground (like Y capacitors, but we aren’t going there in this discussion).
Richard Crowley:

--- Quote from: Spork Schivago on May 19, 2018, 03:09:54 am ---So if L1 and L2 are going into the heating element, where is the completed path?
--- End quote ---
The path is:  From L1,  through the load (heating element) and returns to L2


--- Quote ---Where does it go back to Ground? 
--- End quote ---

Repeating: 240V exists between L1 and L2.  The current doesn't "go back go ground" (or neutral).


--- Quote ---So if I feed a heating element just two 120VAC sources, I wouldn't think it'd heat at all....that's where I'm getting confused here.
--- End quote ---
What you apparently don't understand is that 120-0-120 ("split-phase") is NOT "just two 120VAC sources."  You have 240VAC coming into your house between L1 and L2 (the two "phases"). It makes absolutely no difference whether L1 is connected to ground, or L2 is connected to ground, or if some point halfway between is connected to ground.  GROUND IS IRRELEVANT.   

If you insist on viewing the L1 and L2 voltages from the perspective of neutral/ground (at the center-tap) you should note that they are 180 degrees out of phase (opposite phases).  You are correct, if L1 and L2 were of similar phase, then there would be no voltage differential between L1 and L2, and no current would flow through your load.


--- Quote ---I was always taught you need a complete circuit, and without it, you don't get a working circuit.   Each load draws current and has a voltage drop.  By the time we get to ground, that voltage is 0VDC.   But without that ground, where do the electrons go? 
--- End quote ---

Current flow does NOT depend on "ground".  Else your cell phone (or flashlight or automobile) would be unable to operate with no connection to "ground".


--- Quote ---Just L1's electrons fly down L2's conducting material and L2's electrons fly down L1's conducting material, even though they're of the same charge?   Or is it because they're not in phase, this works?   If L1 is out of phase with L2, then L1 would be the opposite charge of L2?
--- End quote ---
You are beginning to get it.  L1 and L2 are of opposite "phases" when viewed from that center-tap ground/neutral.  The 240 volts coming into your house is split in half in order to provide 120V branch circuits to power your domestic appliances, lights, etc. 

But you have 240VAC coming into your house.  Your problem here is that it is center-tapped and grounded. But your PDUs want 240-0 (one side grounded, aka. "single-phase"), not 120-0-120 (center-tap grounded, aka "split-phase")  That is why you need a transformer to allow you to ground one side of the 240V to create "single-phase".

You do not have two identical 120V phases coming into your house.  You have 240V which happens to be center-tapped and grounded.  And electricians call it "split" or "two" phases.
Spork Schivago:

--- Quote from: james_s on May 19, 2018, 03:28:16 am ---L1 and L2 have 240V across between them, forget about ground, ground is a relative term. With AC the current reverses direction every half cycle, so half the cycle you have current flowing out of L1 and into L2, then the other half cycle it flows out of L2 into L1, there's your complete circuit. If you wanted you could call either L1 or L2 "ground", and you'd be able to connect it to earth ground *except* for the fact that the center tap is already connected to ground so doing this would create a direct short across one half of the transformer secondary.

--- End quote ---

That's what I've been missing.   I wasn't talking about the transformer here, I was talking about 120-0-120 split-phase that comes into the house, and trying to see how it's equivalent to 240VAC over in Europe.   Because over there, they'll have L1 and then Neutral hooked to the heating elements, which would still give the 240VAC.   I think I understand now fully how the AC works.

Now, why would connecting earth ground to a 240VAC source that comes from 120-0-120VAC USA split-phase or whatever you want to call it cause a short there?   It's because L1 and L2 are reciprocals of each other, right?  When one is 120VAC, the other is -120VAC.   When one is 75VAC, the other is -75VAC.   When one is zero, the other is zero.   If we added a ground, when either where anything other than 0VAC, we'd have our direct connection to ground, which would be horrible.   Am I finally understanding this?
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