> >
>

# Layman's explanation of 240v

#1
06-12-15, 11:10 PM
Member
Join Date: Oct 2014
Posts: 656
Layman's explanation of 240v

I tried googling it and the technical vocabulary went a little over my head. How can you not have a return path for the current? Here in non us land we get 3 hots and a neutral coming into the panel. Which means that if use the multimeter between any two contacts on 3 pole breaker I get 380 and between two contacts on a two pole breaker I get 220.

But if I stick probes to two different breakers on the same phase I get 0. I've seen Ray correct several posters who have called 240 two phase, that it's single split phase. If that's the case shouldn't the measurement between the phase be 0 as well not 240? How can a 240v work without a neutral?

Wikipedia seemed to mention something about center tapping as being key but I dont understand what that means.

Does anyone know a website with a good explanation for a curious but uneducated diyer?

Last edited by Gunguy45; 06-14-15 at 05:11 PM. Reason: I just had to change the title
#2
06-13-15, 03:36 AM
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 15,144
I will leave more of this to others, but the two legs of the 240 are 180 degrees out of synch with each other. The current returns on the other leg.

As far as the center tap, see if this helps.
http://www.otherpower.com/images/sci...40/Trans22.gif

Last edited by pcboss; 06-13-15 at 03:55 AM.
#3
06-13-15, 03:41 AM
Banned. Rule And/Or Policy Violation
Join Date: Dec 2005
Location: USA
Posts: 39,967
You have to keep in mind this is primarily a North American based forum, so we concentrate on what we have here. Using sine wave technology, our return path is achieved every 60th of a second by the crossing of the two supplies. If you measure breaker to ground, you will get 120 volts. If you measure across the double breaker (which must rest on opposite "phases" for lack of a better term), you will get 240 volts.

We do not have two phase commonly here. It is either single phase (residential) or three phase (more commonly commercial).

Last edited by ray2047; 06-13-15 at 07:32 AM. Reason: 360th>60th
#4
06-13-15, 08:40 AM
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 58,655
You have a three phase Wye service/system.

You measure 380v between any two hots.
You measure 220v between any hot and neutral.

#5
06-13-15, 10:53 AM
Member
Join Date: Oct 2014
Posts: 656
PJmax, I understand that. I was just trying to understand a 2 wire hot split phase system. It came to mind because in another thread it was mentioned that dryers etc don't need neutral as a return which I thought that was a basic requirement.

Maybe instead of layman I should have asked for the dummies version because that picture pcboss posted is greek to me.

Is it similar to wiring batteries in series to double the voltage where you go vs in parallel to double the amp hours?

This is just intellectual curiosity, I though someone might know of a youtube video or something that explained it in plain english.

#6
06-13-15, 11:06 AM
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 58,655
Ahhh..... I can see the confusion at your end. Your power is just like the UK. You have 220v-230v to neutral. Your basic appliances run on 220-230v 50hz power while here our appliances run off of 120v.

Our power is more like your 3 phase power where the low voltages are referenced to neutral.

Years ago dryers ran on 220v period. All the items inside were designed to be run on 220v. Therefore there was no neutral required. So they ran two hots and a equipment ground.

Then they started putting in 120v motors and keeping the element running on 240v. Many conversions were made where the required neutral for the dryer was obtained off of what is technically an equipment ground.

That is all fine and good until that neutral/ground opens and the case of the appliance becomes hot. That's the reason our appliances are now required to have a four wire connection.

May not be the exact answer you're looking for but is interesting anyway.

#7
06-13-15, 08:23 PM
Member
Join Date: Mar 2010
Location: USA
Posts: 4,074
If you measure 380 volts between any two terminals of a triple wide triple breaker in the panel then you will also get 380 volts between the two terminals of a double wide double breaker in the same panel.

You would get 220 volts between a breaker terminal and the neutral bus bar off to the side.

#8
06-13-15, 08:36 PM
Member
Join Date: Oct 2014
Posts: 656
2 pole, interrupts phase and neutral.

What I don't understand about chandler drawing is how it gets from a regular sine wave to that.

Last edited by Esand1; 06-13-15 at 09:09 PM.
#9
06-13-15, 09:13 PM
Member
Join Date: Oct 2013
Posts: 592
First off, I'm terrible at trying to explain this. You have been warned.

I'm going to steal Larry's picture for this one.

[ATTACH=CONFIG]51961[/ATTACH]

I'm not versed enough in your voltages to use them to explain, so I will stick with my 120V/240V system.
You need a difference in potential to get a reading which is why you got 0V reading on the same leg. As pcboss said, your legs are 180 degrees out of sync with one another. 60 times per second (60Hz) each leg switches from a positive charge to a negative charge back to postive etc. Referring to the drawing, when wire 1 peaks on its positive cycle, wire 2 has peaked on its negative cycle. So wire 1 is 120V, wire 2 is -120V. Therefore the potential difference between the two is 240V. When you measured the same leg, you were measure 120V and 120V, so the measurements cancelled each other out to read 0V. As far as not needing a neutral...
Your neutral ultimately connects to your ground, connected to the earth. Current flow as I was taught is argued whether or not it flows positive to negative or negative to positive but either way for current to flow you need that potential difference. The earth itself acts like a neutral body, accepting either a positive or negative charge. So back to the drawing, when wire 1 is positive and wire 2 is negative, and both are connected to a neutral, the earth is accepting both charges at 120V each, one positive with the earth negative, two negative with the earth positive.
Without the neutral, back to the drawing, while either leg is peaked positive the other is peaked negative, you have your positive to negative/negative to positive current flow at 240V.
3 phase works the same way, except when you pick any 2 of the 3 legs, at any peak the leg it is being measured with will be 208 degrees out of sync instead of 240.

Hope I didn't screw this up worse for you!

Attached Images
#10
06-14-15, 04:06 AM
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 15,144
My diagram was to show the center tap and how the 120 is derived from 240.

Electricity is trying to get back to its source. The earth has little to do as the resistance is too high.

#11
06-14-15, 04:47 AM
Member
Join Date: Mar 2010
Location: USA
Posts: 4,074
Let us consider a non-US single phase system with two 240 volt hots and one neutral. (This is not what Esan1 has.) Next consider Chandler's diagram but ignore the scale at the left. Finally consider a very brief moment in time when the black sine wave is at its maximum. Call that "plus 240." Directly below we see the red sine wave at its minimum. Call that "minus 240." Let's have the neutral stand for zero volts.

Now we have "black hot" to neutral of 240 volts. "Red hot" to neutral has a magnitude (absolute value to you mathematicians) of 240 volts. Hot to hot is 480 volts. Single phase ystems like this are used in many parts of the world where each home has just one hot and the neutral serving it giving 240 volts (or 220 or 230) for all lights and appliances.

A brief moment later (1/120'th second in the US) the red sine wave has risen to plus 120 and the black sine wave has dropped to minus 120 whcih is the definition of alternating current.

Next we move on to Esand1's situation, a 3 phase system. We have the same black sine wave but the red sine wave is staggered so its bottom is not directly under the top of the black sine wave. Instead the red sine wave is in the process of dropping down and its "minus 160" happens to be directly under the top of the black sine wave. Also there is a blue sine wave, also staggered. Its bottom is before the top of the black sine wave and it is on the way up, it has a "minus 160" directly under the top (plus 240) of the black sine wave.

To measure hot to hot we add actual values, not magnitudes. Start with the top or the bottom of one of the sine waves and look for where one of the other sine waves passes directly above or below. Then we use the magnitude of the sum. So the bottom of the red sine wave (minus 240) is directly below where the black sine wave passes through plus 160 so it adds up to 380. This is one way how we find out where the hot to hot 380 volts comes from.

There is also a gray sine wave that is the mirror image of the black sine wave and a pink sine wave that is the mirror image of the red sine wave but we don't use these. Three phase power needs just three hot lines, not six.

In the preceding paragraphs we chose zero to stand for the neutral. In some systems zero stands for something else, in some cases one of the three hot lines of a 3 phase system.

Last edited by AllanJ; 06-14-15 at 05:22 AM.
#12
06-14-15, 03:03 PM
Member
Join Date: Oct 2014
Posts: 656
Ok I think I severely confused things by mentioning my own electrical connection. I at least somewhat understand that because I understand how 3 off set windings on an alternator would induce power at an off set.

This was more curiosity about how the a north american 240v connection works.

What I don't get is how you take a conductor which is measures 120v relative to ground at its peaks and get a 240v system that from chandler's graph looks like sin and -sin. From what I understand frequency conversion is complicated and expensive (my first post on this forum was about using a US dryer on my 50hz electric system - works fine btw).

Last edited by Esand1; 06-14-15 at 04:38 PM.
#13
06-14-15, 04:33 PM
Member
Join Date: Mar 2010
Location: USA
Posts: 4,074
Correction for example two posts above:

A brief moment later (1/100th second in many countries not including the US) the red sine wave has deviated from zero as far as it will to the peak voltage corresponding to Root Mean Square (technical term for average or square/rectangle waveform equivalent) plus 240 and the black sine wave has similarly dropped to the comparable negative peak for minus 240 RMS whcih is the definition of alternating current.

In a US single phase split phase 240 volt system system the red sine wave would be at the peak for -120 volts RMS at its maximum deviation from zero and the moment the black sine wave was similarly at the peak for +120 producing the 240 volts RMS hot to hot. Then both would start moving back towards zero.

Both the red sine wave and the black sine wave are at the same frequency so no frequency conversion is needed unless you import an appliance or device from a country that uses a different AC frequency.

Here is another example that may make things easier to understand. Yes, it is the two batteries in series example.

So we might take two 1-1/2 volt batteries and put them in a holder that holds them end to end. In addition to wires at the ends of the stack, put a third wire making contact between the two batteries. We have 1-1/2 volts (voltage of one battery) between the far positive terminal and the middle wire. We have 1-1/2 volts between the far negative terminal and the middle wire, referred to as minus 1-1/2 volts if the middle wire is called zero. And we get 3 volts (your double voltage) from end to end.

Subject to the amperes available from the batteries you can connect several things (for example LED lamps with appropriate resistors and supporting electronics) to the positive wire and middle wire, to the middle wire and negative wire, and to the positive wire and negative wire respectively.

Last edited by AllanJ; 06-14-15 at 05:07 PM.
#14
06-16-15, 05:22 AM
Member
Join Date: Oct 2005
Location: USA
Posts: 1,615
This MIGHT help. A voltmeter with two wires measures the voltage at the red wire and SUBTRACTS the voltage at the black wire (yes, even for AC). So, using the diagram above, you see our first leg of 240 goes up, while the other leg goes down. One is rising to 120+, the other falls to 120-. Now, the voltmeter sees 240 at the peak (120 - (-120)= 240).

If, instead you put the voltmeter black wire to earth, that wire sits at 0 volts forever. So, you end up with 120V minus zero, or 120v. That is our so-called "single phase" voltage.

I'm improperly using phase and avoiding the use of RMS vs peak voltage. But not pertinent for the explanation here.

#15
06-16-15, 09:00 AM
Member
Join Date: Oct 2014
Posts: 656
Ok I get the amplitude thing that voltage is a differential measure.

But power distribution is still 3 phases 2pi/3 apart right? So how do you take the 208/3 and turn it in to 240/2 or 240/1spilt.

Or is last mile distribution 415/3 with 240 L-N , if so that would explain why my Italian pressure washer has 380-415v on the plaque. Although that still wouldn't explain the graph.

From what little I think I remember from science class in high school is that w = v * a. What I thought I understood was that if you changed that V you were just going to change the amplitude of the wave, and that the period and offset would stay the same

Sorry if I'm being slow. I did amend that I'd need the for dummies explanation.

#16
06-16-15, 10:52 AM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,944
Each power company tends to do it a little differently on their primary voltages, but generally they will run a three-phase primary to a neighborhood. Up on the pole there will be three step-down transformers corresponding to each primary phase, with grounded center taps on the secondary windings. The secondary on these transformers will be set to 240V line-to-line, with 120V line to center-tap. Each transformer will then feed a block of houses to roughly balance the load of the neighborhood across the three phases.

True three phase is generally only available in commercial buildings or high density residential apartments. In the case of an apartment complex, each unit will usually get a 208V/120V service which is two phases and the "star" neutral from the building's central three phase service. This is close enough to the standard 240V residential service that most appliances will work just as if they are installed on 240V.

An example diagram with three phase primary at the top, transformer bank in the middle, and residential services along the bottom.
[ATTACH=CONFIG]52072[/ATTACH]
Image credit not mine.

Attached Images
#17
06-16-15, 01:15 PM
Member
Join Date: Mar 2010
Location: USA
Posts: 4,074
The 415/3 or 240 LN and 208/3 are all secondary or end user voltages and would not run more than a few hundred feet from the (usually pole mounted) transformer(s) that output those voltages. The pressure washer was intended for industrial or commercial use where voltages in excess of 240 were supplied by the utility in for use by other machines as well.

If 208/3 is supplied by the utility, either all three phases or just two of the three phases then 240 volts per se will not be available except via (private) transformers installed by the end user.

Most homes in the US use both 120 and 240 volts (or both 120 and 208 volts) so single 120/240 volt pole transfomers or three separate transformers that together provide 208/3 will be installed with each home getting two hots and neutral. Many countries using 240 (or 220 or 230) volts for household use will have 415/3 secondary lines on the utility poles in some neighborhoods where each home gets one hot and the neutral.

#18
06-16-15, 03:12 PM
Member
Join Date: Mar 2006
Location: USA
Posts: 33,597
Actually most homes are supplied with 240 volts single phase from a single phase transformer. The 120 is derived at the residence by using one leg of the 240 single phase power and a grounded center tap (AKA neutral) from the secondary of the service transformer. That is our main difference. No center tap in Europe just a straight 220 volts supply with one leg of that supply being the grounded conductor where as our 240 does not have either leg grounded.

Last edited by ray2047; 06-16-15 at 04:30 PM. Reason: Clarification.
#19
06-16-15, 04:17 PM
Member
Join Date: Oct 2013
Posts: 592
So how do you take the 208/3 and turn it in to 240/2 or 240/1spilt.
You don't.
Here, you have a residential setting where they connect a transformer to a single phase on the high voltage lines and give you 120V/240V.
Or a commercial setting where they connect a transformer to all 3 phases of the high voltage line and give you 120V/208V. Other voltages common in commercial settings are 347V/600V (in Canada at least).
Industrial settings see 3 phase 277V/480V as well, I would guess to accommodate machinery made in the US.
Commercially it is common to use multiple transformers for multiple voltages. The higher your voltage the more efficient your system is (I once read somewhere this is why Europe uses 220V. After everything was bombed out in WW2 a decision was made to rebuild with a more efficient system). So you might see High voltage line > 347V/600V step down xfrmr > 347V/600V Panel (run lighting, motors, and RTUs off this panel) > 347V/600V:120V/208V step down xfrmr > 120V/208V Panel (run receptacles off this panel)

#20
06-16-15, 09:30 PM
Member
Join Date: Oct 2014
Posts: 656
thanks for all the replys guys. It's been some very interesting reading.

Ray - you mentioned again the thing about the 240v legs coming out of a single phase transformer, I still don't get how that works so you get the 240v as two currents which are reflections of each other from a single sine wave.

I like my toys and there is no country that produces knick knacks like the US so I have my share of transformers thrown around the house like this one and the two unattractive paperweights from the other threa.

As you can see my coffee grinder is pluges Into it recieving 120v at 50hz. On the other end is 220v at 50hz.

But let's suppose that on the other end was something larger than 220 such as 380 (side note: is it happenstance that the local low voltage distribution to houses is 220/380 and medium voltage distribution to comercial is 380/660) or any other number larger than 240, how would I get 240v split phase instead of 120 on the same phase as the input.

#21
06-17-15, 05:28 AM
Member
Join Date: Mar 2010
Location: USA
Posts: 4,074
How to get 240 volt split phase if you really want it and you only have something else?

The transformer in question, whether a big one on the utility pole or a little one sitting on the table next to your coffee maker, needs a secondary winding (let's say it has red and black wires attached to the ends) delivering 240 volts when the primary is given the source voltage available (high four figures or low five figures up on the utility pole or perhaps 380 volts from the receptacle behind the coffee maker).

Also say there is a white wire connected to the midpoint of the 240 volt secondary winding of the transformer the subject of this discussion.

You get 240 volts between the red and black wires regardless of what is connected to the white wire.

From the white wire to the red wire there are half as many turns for that portion of the secondary winding so you get 120 volts from the red to the white regardless of what is connected to the black wire.

Ditto for the portion of the winding between the black and white; you get 120 volts regardless of what is connected to the red wire.

This is your 120/240 volt split phase.

Except for being inversely proportional to the respective voltages, the (here, 120/240 volt split phase) secondary current in amperes has nothing to do with the current drawn by the transformer primary whether we are talking input voltage of high four figures or 380 or 208. However up on the uitlity pole it is customary for the primary voltage to be hot to ground and the center tap (referred to as white above) is connected to the same system of ground wires. No current flows from the primary side to the secondary side because there is only one path, the shared ground conductors; in order to have a current flow you need two paths for outgoing and return respectively.

Note: If you feed the transformer (primary winding) a differnet voltage from what you fed it before, the output voltage will be different from (and proportional to) what it was before.

Last edited by AllanJ; 06-17-15 at 06:34 AM.
#22
06-17-15, 09:15 AM
Member
Join Date: Oct 2005
Location: USA
Posts: 1,615
Good thing granite is non-conductive

your last question: "how would I get 240v split phase instead of 120 on the same phase as the input."

Easy answer: "with a center tapped transformer of the correct turns ratio". And, although the above answers are correct, don't be misled. If you, academically speaking, have 380 three phase available, you can make 120/240 single phase supply. Just use ONE of the phases!

While we are on the topic of transformers. Do you understand what an "isolation" transformer is?

#23
06-17-15, 02:49 PM
Member
Join Date: Oct 2013
Posts: 592
I still don't get how that works so you get the 240v as two currents which are reflections of each other from a single sine wave.
I'll try throwing this out again...
You get two lines of 120V off your transformer. While Line 1 is peaked +120V, line 2 at the same moment is peaked at -120V. Therefore you have a difference of 240V between the two. And because one line has a positive charge and the other line has a negative charge, you have current flow from positive to negative.

I lied to you as well about getting 240V and 208V. We were talking about ******* legs in class today which I forgot about. If you want to confuse yourself even more, look up 3 phase 4 wire delta configurations. If your transformer is set up this way, you could see 120V, 208V, and 240V all in the same panel.

#24
06-18-15, 05:00 AM
Member
Join Date: Mar 2010
Location: USA
Posts: 4,074
"We are talking about ******* legs in class today."

What does the ******* stand for? (Repost, splitting up the word in question into two parts separated by a single asterisk or at sign or hashtag.)

The waveforms pictured earlier could have been viewed on an oscilloscope. For oscilloscopes, positive and negative are significant for alternating current as well as direct current. They make 'scopes that show two or even three waveforms at the same time.

Start again imagining a single transfomer secondary winding delivering 240 volts at the moment and with, say, red and black wires on the end terminals respectively. Also imagine a whire wire connected to the midpoint.

If you put the negative 'scope lead on the white wire and connect the beam 1 positive 'scope lead and beam 2 positive to red and black wires respectively you weil see the two waveforms that are reflections.

If you put the negative lead on the black, the beam 1 positive to the white (middle), and the beam 2 positive to the red, you will see two waveforms that are not mirror images but one is twice (for 240 volts) the height of the other (for 120 volts).

(If you have just a single beam 'scope, do the tests one at a time with the red transformer lead pigtailed* to the 'scope trigger input. Photograph (or sketch) the screen to get diagrams for each test to compare with. )

* Use of extra scrap piece of wire to reach an additional terminal or location.

Last edited by AllanJ; 06-18-15 at 05:17 AM.
#25
06-18-15, 07:01 AM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,944
There are lots of different versions of the asterisk'd words one might exclaim when they find a much higher voltage than they were expecting. I've found it depends on generation and area of the country as to which ones are preferred, but for the purposes the forum we can use the official name "high leg delta".

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Pingbacks are Off
Refbacks are Off