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How to wire up a 240VAC receptacle
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Spork Schivago:

--- Quote from: james_s on May 19, 2018, 03:31:55 am ---
--- 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.

--- End quote ---

Yes, I realize it's changing so fast we can't see it, but that doesn't really mean it's not there.   I think I have a good understanding of it now, except  for the RMS.   I have a true RMS DMM, so when I measure the voltage from my receptacles, I see 120VAC.   I don't see 170VAC.   When the sine wave is at the peak, you're saying for 120VAC, it's really 170VAC +/- ?   And for 240VAC, it's really 340VAC +/-?   RMS = Root Mean Square.   The RMS value is 120/240, as you say...even my old analog volt meter that isn't true RMS measures it at 120VAC though.... so I'm still missing some pieces to this, but I'm getting there.   I don't understand why we use the RMS and not just the peak value when measuring or talking about the voltage sources.   Why use the RMS value?   Why not just call it 170VAC or 340VAC?

With the European 240VAC, don't they generally have 240VAC going down L1 and Neutral to Neutral, Ground to Ground (where Neutral would be connected to Ground, like in my breaker panel)?   If so, wouldn't they receive a short if they tried hooking my BGA rework station (which is wired with L1 = 120VAC, L2 = 120VAC, N = Neutral, E = Earth) directly into their outlets, just by changing the NEMA L6-30P I wired to the end of it?   We still have the sine wave, where it's going from positive to negative for one wave cycle right?   But now, because they're using 1-phase 240VAC, they don't have the L1 and L2, where the AC goes into L1, then back into L2, and so on and so forth...the heating elements would be wired up so 240VAC was feeding directly through the heating element and then to ground.   I guess that wouldn't cause a short though, would it?   The heating element would still be the load and would still have a voltage drop of 240VAC.   So nothing would change there I guess....hrmm.

I'm trying to think if there's any cases where just switching the plug on one of our devices that is made for our 240VAC to fit into their receptacles over there would cause damage.   I know we can do it here, most of the time, but does it usually work the same over there?
Spork Schivago:

--- Quote from: Gregg on May 19, 2018, 04:14:43 am ---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).

--- End quote ---

We gotta back up the train here a bit.   I understand the purpose of the neutral and ground in the house.   If we have a short, we want that current and voltage to go to the path of least resistance, hopefully back to the breaker, to trip it.

The transformer at the pole though....you said, "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."

Where are these loads coming from?    Some place in the transformer, or in the house?   Why would there ever be an excess of current?   A 60 watt bulb will always draw 60 watt, even if you have it hooked to a 200-amp breaker.   So how could you ever have an excess of current?   Even if you were to directly wire hot to neutral or hot to ground, if neutral and ground are at 0V, wouldn't there be an instant volt drop where they connected and a lot of heat generated?   I had an old camaro and the ignition went bad.   I had to hot wire it.   I'd use the fuse panel.   I'd hook to a 12V source, then use a wire to tie into the fuse for the Ignition.   Then I'd hook into the 12V source and tie into the starter, but just long enough to get running.   One day, I was a little careless and grounded that 12V source.   The wire evaporated in my hand and left a white indent.   At first it didn't hurt, then it hurt real bad.   But there wasn't no excess current that just traveled safely back to the cells in the battery.   Wouldn't an excess current be called a short?

Also, how come our neutrals and grounds in the house are tied together?   Why can we not have it like the transformers on the poles where neutral is seperate from ground?   Actually, I probably could, couldn't I?   Outside, there's a grounding wire that's buried in the earth that hooks to the panel.   If I separated the ground and neutral buss-bars in the breaker panel, and made sure my ground buss-bar went to the grounding wire outside, and the neutral went back to the pole, I'd essentially have the same setup as the pole.   Someone in this thread mentioned in their house they'd never have the ground tied to the neutral.   But in every home I've seen, this is how it's always setup.   Is there any dangers to doing what I just described?
IanB:

--- Quote from: Spork Schivago on May 19, 2018, 06:37:43 am ---With the European 240VAC, don't they generally have 240VAC going down L1 and Neutral to Neutral, Ground to Ground (where Neutral would be connected to Ground, like in my breaker panel)?   If so, wouldn't they receive a short if they tried hooking my BGA rework station (which is wired with L1 = 120VAC, L2 = 120VAC, N = Neutral, E = Earth) directly into their outlets, just by changing the NEMA L6-30P I wired to the end of it?
--- End quote ---

You have listed four wires (L1, L2, N, E), but in the L6-30P there are only supposed to be three wires (L1, L2, E). There is no neutral.

Your BGA rework station is only supposed to have three wires too. In the European outlet there are two wires carrying 240 V between them, and a third wire for safety ground. In the USA the L6-30 receptacle has two wires carrying 240 V between them and a third wire for safety ground. So the situation is really the same in Europe as it is here from an electrical perspective. It is only the safety arrangements that differ.

We call a wire "hot" or "live" because it is effectively hot and will hurt you if you touch it. We call a wire neutral because--theoretically--it won't hurt you if you touch it (but don't rely on this). There is no difference between live and neutral as far as carrying electric current is concerned, there is only a difference with regard to safety and protection arrangements.
Spork Schivago:

--- Quote from: Richard Crowley on May 19, 2018, 04:56:02 am ---
--- 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.

--- End quote ---

I had temporarily moved away from the transformer for the PDU.   I understand completely why I need a transformer for the PDU and why I could never power it with just a double pole breaker hooked to my panel.   What I wasn't getting (but am now) is that the current for AC flows forwards and backwards, not just forwards like DC.   I had said early on I understood DC fairly well, it was the AC I struggled with, but now I feel I have a pretty good grasp of how the 240VAC works in the USA.   Still not sure about 1-phase 240VAC that is used in other parts of the world, or that would be used with my transformer.    Because we don't have an L1 and L2, because the 240VAC is on the same phase, where do the electrons go if they're not hooked to a neutral or ground to complete the circuit?    Somehow they have to make it back to the breaker, correct?   I use the word ground, but all I mean is a completed circuit.   I know and understand that a completed circuit doesn't need a ground or a voltage potential of 0V.   I could technically power a circuit where the supply is 24V and the return is 12V.   So long as the return is at a lower voltage potential than the supply, we can have the electrons moving.

Using the water analogy that everyone seems to like to use, we have our "pump" (the transformer I'd buy for the PDU, for example).   The "water" (electrons) flow out of L1 into the PDU.   But then what?   They need to make it back to the "pump" (transformer) somehow.    I know you say ground / neutral is irrelevant, but in this case, I cannot imagine how those pesky little buggers would make it back to complete the circuit without using one of those other wires that are at 0V potential.    The components inside the PDU would be the loads and by the time the current / electricity had passed through the entire circuit and was ready to leave the PDU, it surely wouldn't just be traveling up the L1 wire again, would it?   We'd have to use one of the additional neutral / ground wires in that instance.   Just like if we're dealing with 120VAC receptacles.   We cannot just hook up the hot wire.   That wouldn't do diddly squat.   We'd still have to hook up the neutral so the electrons could make it back to the panel.

With the 120-0-120, I understand how it works now.   I also understand why if I were to not use a double pole breaker but tried wiring up a NEMA L6-30R using a 30-amp single pole breaker in slot 1 and slot 2 (opposites sides of the panel), it wouldn't work.   I'd have 120VAC, not 240VAC.
IanB:

--- Quote from: Spork Schivago on May 19, 2018, 07:00:53 am ---Also, how come our neutrals and grounds in the house are tied together?   Why can we not have it like the transformers on the poles where neutral is seperate from ground?
--- End quote ---

Let me say something that will seem funny at first. If you take a transformer, any transformer, like the one at the top of the utility pole, or like the one earlier in this thread that might go in your basement, it doesn't matter. Anyway, if you take a transformer with two wires coming out of it carrying voltage, then at this point no wire is "neutral" and no wire is "hot". There is no neutral. Neutral is created by connecting one wire to a rod in the ground. Until we do that there are just two wires with voltage between them.

Think about an analogy with a 3 ft rule. First hold it up in your hand. The ruler is 3 ft long (240 V), but it is floating and not grounded. This is like the transformer with two wires 240 V (3 ft) apart.

Next stand it upright on the ground. Now the lower end (0 ft) is grounded, and the other end (3 ft) is 3 ft up in the air, but the ruler is still 3 ft long. You could call the lower end of the ruler "neutral".

Next, dig a three foot deep hole and drop the ruler down it. Now the top end (3 ft) is level with the ground and the bottom end (0 ft) is 3 ft below ground. The ruler is still 3 ft long, but now the top end is grounded. You could call the top end of the ruler "neutral" because there is no distance to fall between that end and ground level.

Lastly, dig a  1½ ft hole and stand the ruler in that. Now the ruler is half above ground and half below ground. It is still 3 ft (240 V) long, but now the middle is grounded. This is what split phase is like. The maximum distance to fall to ground level is 1½ ft (120 V), which makes it a bit safer.
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