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USA Residential Supply - Single or Three Phase?

02/01/2012 8:44 AM

Hi all

I am conducting research in the UK on the Low Voltage Distribution Network and am using an American software package to model cables. In the UK it is normal for residential homes to have single phase supplies and contain two conductors; live and neutral.

I would appreciate some guidance on the following:

Is it standard for residential properties to have a single phase or three phase supply in the USA?

How many conductors do the cables contain that connect residential dwellings?

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#1

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 9:17 AM

The short story - typically US residential service will be single phase consisting of two hots (120V each) and a neutral.

The two hots fed to an appliance such as an air conditioner will provide 240V (120V + 120V).

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#2

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 9:25 AM

My house is wired as KJK/USA says.

Most circuits are 110V, two conductor. The stove/oven, AC and pool pump are 220V.

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#3

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 9:56 AM

Brilliant,thanks, just one quick question when you say 2 hots i assume this means 2 live conductors (?) from different phases? If so -is one phase dedicated to lighting for example?

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#4
In reply to #3

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 10:25 AM

No, not 2 phases. Two ends of the same phase, tapped in the center for a neutral point between them.

It is a single phase service drop in 99.999999% of residential installations. People can, in some areas, pay exorbitant amounts of money to their local utility to run 3 phase service, but it is rare because it is expensive. Utilities generally don't run 3 phase into residential areas so to get it, you have to be willing to pay for them to run the other wires from wherever the nearest viable connection point is.

I wrote this explanation for someone in another forum who asked why we don't call it "two phase", read it to understand how we do it here in the US.

http://forums.mikeholt.com/showthread.php?t=142765&page=3&p=1369541

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#5
In reply to #4

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 10:40 AM

Yup, just like JR said

EDIT - in reply to #3

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#26
In reply to #4

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 9:28 AM

A piece of terminology that I have heard and used is "split phase" for the US system. As you describe, one leg of a 240V three phase line is center-tapped to have two 120V hot lines with the center tap being "neutral".

I am a bit surprised no one used this term in this post (unless I overlooked it), as I think it makes the power supply simple to understand. At least it does for this old mechanical engineer!

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#27
In reply to #26

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 9:39 AM

I'm an electronic Engineer....and we use different terminology then electrical engineers do for the same things sometimes.

And have been involved in some discussions over that in the past...and every term has its own nuances of meaning.

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#19
In reply to #3

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 8:37 AM

Two live ends of the same phase actually with a center connection called Neutral.

You can also treat it (wrongly) as two phases 180° apart (not 120° as in 3 phase), that add together if used together to produce 220-240VAC.

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#6

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 11:10 AM

Excellent that is great, the thing that made me suspect 2 phases was the reference to 240 and 220v..... i take it for these type of appliances 2 conductors are connected to increase the ampacity of the cable and there must be a transformer in the plug or appliance to step up the voltage?

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#7
In reply to #6

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 11:18 AM

Sorry, I may have confused you. Listen to the experts.

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#8
In reply to #7

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 11:36 AM

no need to apologize, i appreciate the discussion, and your input, I am a researcher and sit in silence most of the day thinking! It makes it easier to talk

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#9
In reply to #6

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 12:30 PM

In the USA a typical insallation as described above cannot simply have the two live/hot wires connected, as the power in the one is 180deg out-of-phase with the other. Were one to do so, then a number of circuit protective devices must operate to disconnect the fault before it becomes an exciting irritation for the local fire brigade, and an embarrasment for the user towards fire insurance companies.

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#12
In reply to #9

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 2:54 PM

PWS - You are correct, there is a 180 degree (technically 120) separation on our 120v residential service but the circuit protective devices and wiring are actually quite simple.

When we wire a 240V single phase appliance such as an AC unit, we bring the two "hot" (120V) conductors to the appliance, we also bring a ground conductor per NEC.

On 240 volt single phase circuits we use a two pole CB like this one with the handles interconnected for a simultaneous trip in any event:

This is a the wiring practice for 240V home appliances in the US.

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#13
In reply to #12

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 4:34 PM

KJK... I am really surprised to have read this comment from you. There is no 180° phase shift involved in the North American 120/240 volt consumer service.

Also... where and how does the 120° figure into it?

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#20
In reply to #13

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 8:39 AM

Greetings North of 60,

Have a look at this regarding 3 Phase. Check out the item "Generation and Distribution".

And this regarding single phase.

Here is one more on the single phase system.

My simple reference to phase angle was 120 degree phase angle applying to 3 phase, 180 degree phase angle referring to 120/240V single or split phase although going back and re-reading my post, I see I was not very clear on that

I probably gave more information to malby than what his OP required.

Regards - KJK

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#24
In reply to #20

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 9:01 AM

Hi... KJK

It is indicative of the nature of this issue that... both of the sites you linked to about the 120/240 volt system are wrong.

By the laws of physics... it is impossible to have 2 sources of equal magnitude that are 180 degrees "out" from each other that are connected in series to produce a voltage other then 0 volts.

(120) + (-120) = 0 not 240.

But this is a long standing point of contention in the industry... lets agree that... if you consider the "neutral" as the reference point, then yes... there is a 180 deg difference, but... if you discount the neutral as a reference point, then there is no 180 deg difference.

My whole point of this, and why I bring it up when ever it rears its nasty little head...

It is wrong to analyze the nature of our 120/240 volt system using the neutral as the reference... the proper way to analyze this system is... from "L1 to the Neutral" and from the "Neutral to L2". That is indeed how the system exists.

We may "choose" to consider the neutral as the reference... and in doing so... we create the 180 deg shift. This does not make the shift real... as it fundamentally does not "jive" with the laws of physics. Which all systems must.

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#29
In reply to #24

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 11:15 AM

YUP....................sort of like either (long E) or either (long I) don't cha think

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#38
In reply to #24

Re: USA Residential Supply - Single or Three Phase?

02/03/2012 3:42 PM

"But this is a long standing point of contention in the industry... lets agree that... if you consider the "neutral" as the reference point, then yes... there is a 180 deg difference, but... if you discount the neutral as a reference point, then there is no 180 deg difference."

How can you *NOT* consider the neutral as a reference point? It is connected to ground, and "ground" is by definition, our reference point when measuring voltage - unless we are comparing the potential difference (either AC or DC) between two different ungrounded points (or lines).

I also have a problem with this:

(120) + (-120) = 0 not 240

There is no such thing as 'negative' (-120) AC voltage. AC voltage waveforms of identical frequency are either in phase (and there is no potential difference) or they are out of phase by >0 degrees to a maximum of 180 degrees. At 180 degrees of phase difference you have maximum potential difference between the two.

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#39
In reply to #38

Re: USA Residential Supply - Single or Three Phase?

02/03/2012 5:08 PM

Actually, with two legs 180 out of phase one potential is always negative with respect to the other. The problem with the math is that we do not look at potential sum; we look at potential difference.

The equation should be:

120 - (-120) = 240

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#21
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Re: USA Residential Supply - Single or Three Phase?

02/02/2012 8:44 AM

Already answered.

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#18
In reply to #6

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 7:02 AM

The standard nominal voltages delivered to residences are 120 and 240 single phase, but some residences are furnished power at 120 and 208, by way of a four-wire delta connection.

This is a less-popular arrangement in the U.S. than it was fifty years ago, but one can still purchase major appliances that are designed to work at 208 volts.

Our church uses a similar arrangement to obtain 3-phase 240 volt service for the 1912-vintage motor that powers the organ blower, and 120 volts for lighting. The three-core transformer has been replaced with an "Open Delta" arrangement that requires only two transformers, but the principle is the same.

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#23
In reply to #18

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 8:49 AM

Actually we are missing the way of distribution used in UK [and almost all contries following UK] and USA [Include North American countries also].

In uk: Star or Y with 4 wires [RST + N ]. and to houses or shops normally single phase with 3 wires L, N and E. 220V nominal

In USA: The distribution is Delta "Δ" 220V nominal but Center-tapped for 110V nominal.

2 hot [220V] for fridg etc. [but not in all houses] and one 1 from center-tap [ I cannot call it Earth or Ground] marked Nuetral in your scheme.

Your 1st drawing is the actual distribution scheme.

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#40
In reply to #6

Re: USA Residential Supply - Single or Three Phase?

02/03/2012 5:58 PM

240 / 220 is relative to the Utility Company suppling power to the individual homes.

On a good day it may be 240 lowering amperages and raising efficiency. On a bad day it can drop as low as 195 wreaking havoc on your electrical components.

At least they have for the most part gotten rid of the stinger legs.

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#10

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 1:06 PM

What happens with 180° out of phase

Sorry about the voltages shown, I did the drawing for a UK supply

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#11

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 1:08 PM

Be aware also that conductor sizing, which is a function of cable current and voltage drop, differs each side of the pond on account of the different voltage and frequency in use. A UK installation based on BS7671 bears no immediate comparison to a North American installation, as practices differ.

Cable sizing has been done to death in this forum of late; the response to queries is nearly always along the lines of "follow the installation code applicable to the country and location of the installation".

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#14

Re: USA Residential Supply - Single or Three Phase?

02/01/2012 11:01 PM

I, too, have the typical single phase power in my home.

I had bought a 3 phase milling machine, but 3 phase is not available where I live so I decided to generate my own by hooking up one set of windings of a 3 phase motor to the line, manually spinning it up and applying voltage to it so the single phase will make it run. Out of ALL the wires of that motor come (induced) voltage. By connecting all 3 sets of wires from this motor to the 3 wires of my milling machine, I now have a working 3 phase milling machine

However, your generator motor must be large enough to run the appliance/milling machine/lathe/whatever or you will burn up the generator motor. Your generator SHOULD be at least 50% larger than the motor you want to run, due to IR Losses.

I hooked up, physically, a "pony motor" to get the bigger motor spinning so it gets up to speed. Then I apply voltage to the big motor and turn off the pony. It continues to spin, powered by the big motor but it essentially, does nothing; it gets a free ride.

I built a mount so the 2 motors have their shafts in line. The frame is long enough that the shafts are several inches apart. I then connected the shafts with a piece of hydraulic hose going between the shafts, so when I lit up the small motor, it turned the big one. It takes clamps on both shafts or you will burn out the inside of the hose bcuz there is so much slippage. I built a set of saddle clamps to put on both ends of the hose and tightened them with BOLTS. There is no slippage now.

I thought about building up a phase shifting set of capacitors to start the big motor but the pony seemed like it was a bit easier. Besides, I like designing and welding up "RUBE GOLDBERG" stuff. It works and that's all I care about....

Oh, yeah. The pony motor was free. I got it from a neighbor who took it out of a scrapped out washing machine.

If anyone wants a picture of my generator, E me and I'll pass it on to you.

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#15

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 12:59 AM

I'm struggling to understand how this excellent thread on how US domestic electrical utility supplies are reticulated can help you in the UK.

What's the objective of your research?

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#16
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Re: USA Residential Supply - Single or Three Phase?

02/02/2012 3:20 AM

Thanks for the comments, I actually thought the answer would be much simpler so I am glad I asked now!


OK, so tell me if which of these I have right (assuming one of them is correct!):

1) Homes in the US are generally connected to the grid via a cable consisting of three conductors. One conductor is a neutral, the other two are:

  • Live (120v RMS).
  • Connected to the SAME PHASE.
  • IN PHASE (0 degrees between them).

2) Homes in the US are generally connected to the grid via a cable consisting of three conductors. One conductor is a neutral, the other two are:

  • Live (120v RMS).
  • Connected to the SAME PHASE.
  • OUT of PHASE (180 degrees between them).

3) Homes in the US are generally connected to the grid via a cable consisting of three conductors. One conductor is a neutral, the other two are:

  • Live (120v RMS).
  • Connected to the SAME PHASE.
  • OUT of PHASE (120 degrees between them).

If either 2) or 3) is correct, is it the local transformer (pole or surface mounted) in the distribution network that achieves the phase shift?

I have a suspicion what the potential benefit is from having 180 degree phase shift, I'll share that if it proved to be the configuration!

I am using software that has many preconfigured models for US cable configurations. To use the software to model UK cables I have to calculate the impedance matrix of each type of cable, if I can understand the US system it may help me find a shortcut….. Thanks!

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#22
In reply to #16

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 8:45 AM

Number 1 is the correct answer.

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#25
In reply to #16

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 9:12 AM

#2 is correct....a simple DVM measurement betweent the two hots is twice what you get between a single one and the return.

Put an oscilloscope on the two referencing the return (common) and you will see them being 180 degress out of phase with respect to each other, because they are opposite taps from a transformer at the pole. The center tap being the return...

If the were the same phase there would be no voltage differential. I am feeding my air compressor which is a single phase 208 volt Chinese mostrosity with a 60 gallon tank....from just the two hots....and grounding the unit for safety and code. True it results in a bit more than the 208v...and really should have a transformer to drop it to 208v but its been fine for two years now of intermittant use although ultimately it will reduce the life of the motor.

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#28
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Re: USA Residential Supply - Single or Three Phase?

02/02/2012 10:48 AM

http://electricaloutlet.org/ provides some information on worldwide power configurations.

My vote is for B.

and the terms 110,1150 and 120 are commonly used interchangeably for voltage in the U.S. as well ar the terms 220,230, and 240.

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#17

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 6:06 AM

I think number 2 is correct.

If number 1 would be correct, the voltage between 2 live lines would be 0 volt

If number 3 would be correct, the voltage between 2 live lines would be 208 volt

Only if the phase shift between the 2 live lines is 180°, the voltage between 2 live lines will be 240 volt

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#30

Re: USA Residential Supply - Single or Three Phase?

02/02/2012 3:10 PM

Hello just to give you an update which is valid for the US and Japn: 2 hots and one neutral - but beware there is no phase shft between the 2 hots so you get 120V for lamps and wall outlets, but 240V for airconditioners.

For Germany it is normal to get 3 Phase 440V and 240V especially if you use heatpumps in the household. Electric ranges are connectd to 3Phase. THe residential lighting and walloutoutlets will be split to the 3 Phases to equalize the system.

Greetings from Germany

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#31
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Re: USA Residential Supply - Single or Three Phase?

02/02/2012 3:59 PM

If you had no phase difference between the two hots you would have zero differential between them...when you in reality have a 220 volt nominal difference. (there is a 180 degree phase difference, when one it reaching max voltage the other is reaching min voltage)

I'm running a 208v single phase 3.5 HP air compressor using just the two hots....without the return. (but a ground for saftey reasons) If there was no difference between them, that wouldn't work.

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#32
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Re: USA Residential Supply - Single or Three Phase?

02/02/2012 4:35 PM

You said... "when one it reaching max voltage the other is reaching min voltage". That is impossible. The sum of such would be 0.

In your motor example... Your motor is in series with a 240 volt source. During one half of the cycle... One wire supplies current to the load while the other wires returns current from the load back to the source and then flips the other way on the other half of the cycle.

Given the system we are talking about... we have ONE secondary winding fed from ONE primary winding. The center tap cutting that secondary winding in half does not magically impose an inversion of one of the 120 volt wavefroms relative to the other. The voltage across the two 120 volt windings sums to 240 volts because it can be considered as ONE winding and not two of them.

So there is where your potential difference comes from. Both of the hots... are from the same source.

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#33
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Re: USA Residential Supply - Single or Three Phase?

02/02/2012 10:55 PM

No..you are viewing this wrong.

#1 referenced to the common reaches +110 v as #2 referenced to common reached -110V you have a difference of potential of 220v.

The two go in opposite directions from each other being on opposing taps of the transformer.

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#34
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Re: USA Residential Supply - Single or Three Phase?

02/02/2012 11:04 PM

That's the whole point... using the neutral as a reference point distorts the nature of the system.

Anyways... I clearly will not change your opinion.

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#35
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Re: USA Residential Supply - Single or Three Phase?

02/03/2012 7:48 AM

I think its purely a case of you looking at it from the wrong perspective.

Being I got my degree in electronic Engineering 30 years ago, I do know what I'm talking about...and the fact I'm not wrong because we are using it exactly as I described both in my office and at my house...as well as others doing it in untold numbers across the USA.

If they were in phase with each other (as in ZERO degree difference on the sine wave)..you would only have ZERO volt differential between the two hots, you would literally be able to grab both in your hands and as long as you never grounded yourself nothing would happen...however thats not the case (being there is a 180 degree difference between them) and you would electricute yourself instantly due to the difference of potiential because they are not in phase with each other.

Both are AC....and your reference point is still important to get accurate readings as it always is.

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#37
In reply to #34

Re: USA Residential Supply - Single or Three Phase?

02/03/2012 3:26 PM

Here's what Wikipedia says about it:

A split-phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential applications...

Since there are two live conductors in the system, it is sometimes incorrectly referred to as "two-phase".

The two live or "hot" conductor's waveforms are offset by a half-cycle, or 180 degrees offset, when measured against the neutral wire. To avoid confusion with split-phase motor start applications, it is appropriate to call this power distribution system a 3-wire, single-phase, mid-point neutral system.

http://en.wikipedia.org/wiki/Split-phase_electric_power

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#36
In reply to #32

Re: USA Residential Supply - Single or Three Phase?

02/03/2012 3:15 PM

"You said... "when one it reaching max voltage the other is reaching min voltage". That is impossible.The sum of such would be 0" -- That would be true IF they were being summed at a summing junction, but they are not - we are looking at the potential *difference* between them, not their sum. If I place a resistor from one to the other, I will get current flow through the resistor from the top of the transformer to the bottom during one half cycle, and during the other half cycle, the current changes direction, flowing from the bottom of the transformer to the top. That gives us a 240VAC power source. When we are using only one of our split phase lines, the current flows from the top of the transformer to the center of the transformer through the neutral line, to the center of the transformer, and switching direction on the second half of the cycle. Because we are now using only one half of the transformer the voltage is halved to 120VAC. The voltage WAVEFORMS on the two lines (top and bottom of transformer) are *exactly* 180 degrees out of phase with each other when measured referenced to ground - which also happens to be connected to our neutral line and the center tap of the transformer. But they are not considered two different "phases" as the word is generally applied to electrical power generation. In power generation, "phases" refers to voltage wafeforms that peaks at different times regardless of polarity.

This difference of opinion really is simply a matter of semantics regarding the word 'phase' as it applies to waveforms on an oscilloscope, and as it applies to power phases in a generation system.

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#41

Re: USA Residential Supply - Single or Three Phase?

05/18/2015 3:43 AM

I realize that this post is a few years old and I am new to this forum - I was researching this same question when I found this website during a search for my answer -

AT FIRST, I thought this to be a SIMPLE ANSWER - and after hours of research I finally arrived at the ABSOLUTE Correct Answer for United States Most Common Residential and Light Commercial Electrical Service Entrance Panel Feeds - I will not address the rural 3 phase 208 Volt WYE service panel configuration because it is becoming more rare in the USA and mostly applies to rural / farm areas where there is a lot of distance between homes and limited supply lines. So sticking with the most common residential / small commercial type electrical service feed - it is defined as -

A split-phase or single-phase three-wire system is a type of single-phase electric power distribution. It is the AC equivalent of the original Edison three-wire direct current system. Its primary advantage is that it saves conductor material over a single-ended single-phase system while only requiring single phase on the supply side of the distribution transformer.

In basic terms - this system mimics a 2 phase system but it is not as the primary of the Line transformer is SINGLE PHASE, but the secondary of that same transformer that feeds the residential service panel has a center tap - which is the Neutral (white wire), the L1 - Line 1 - is the Black Wire (hot) and measures 120 Volts to Neutral and the L2 - Line 2 - is the Red Wire (hot) and measures 120 Volts to Neutral and is 180 degrees out of phase with L1. The Voltage between L1 and L2 is 240 Volts (because they are 180 degrees out of phase with each other). The system is self balancing by design - so the load of L1 and L2 will balance through the Neutral back to the transformer secondary feed - this way, if the load devices (appliances, lights, etc..) are not balanced from a current draw perspective between L1 and L2, no voltage drop or increase would be passed on to the appliances as they would be if the neutral center tap design was not used. - Here is a link that defines this further - technically and historically.

http://en.wikipedia.org/wiki/Split-phase_electric_power

For larger commercial facilities that require more power & higher voltages - True 3 Phase is used to deliver power to the service panel (240V or 480 Volt 3 phase being the most common - higher voltages available).

In the service panel breaker box for the residential Single Phase 3 Wire system - the breakers are laid out so the odd number breakers are connected to the L1 phase and the even number breakers are connected to the L2 phase via 2 buss bars under the breakers. It is designed this way to somewhat distribute the load between the 2 phases and so if a 240Volt line was needed for a large window air conditioner (20Amp), or an electric Clothes dryer (30Amp) or an electric stove/oven (50A) - a double breaker of the correct amp rating would be used, both poles of the breaker switch are bussed together so they trip together and because one half would occupy an even slot and the other half an odd slot - they would use both the L1 and L2 buss bars, and because those 2 hot legs are 180 degrees out of phase - you would achieve your 240 volt potential between the 2 - 120 volt hot legs (if they were in phase with each-other you would read 0 volts). - Pretty cool when you think about it, a very efficient way to achieve both 120 volts and 240 Volts using a single phase transformer primary and only 3 wires (and ground).

There is a lot more information out there concerning how to wire this 3 wire (4 with required safety ground) system ("L1" - Black - Hot, "L2" - Red Hot, "N"- White -Neutral - and the "G" - Green - Ground - Safety Earth Ground required now for most building receptacles) - In summary - the 120 Volt receptacles are typically 3 wire, 2 pole, (black, white & ground) - range - 15 to 30A, and 240 (or 220/240) Volt receptacles typically 4 wire, 3 pole (black, red, white & Ground) for 30A to 50A and 220/240 Volt receptacles that are 3 wire/2 pole (Black, White & Ground) for 15-20A circuits that do not require a neutral - Black = L1, White (should be marked with red tape or marker) = L2 and Ground. 3 wire / 3 pole circuits have been phased out beginning 1996 in the USA as dual purposing the white neutral as a safety/chassis ground or dual purposing the safety ground conductor (bare wire) as both safety ground and neutral return has been deemed unsafe by National Electric Code.

Required AWG (wire gauge) is also another topic - some of the most common size requirements - 120/240 - 15A or less - 14 gauge, 120/240 - 20A - 12 gauge, 120/240 30A - 10 gauge, 240V 50A - 6 to 8 gauge.

HOPE THIS HELPS SOMEONE out there with future endevours - It has helped me in my project a lot.

Thanks

Chris C

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#42

Re: USA Residential Supply - Single or Three Phase?

12/15/2015 6:05 PM

There is only a single phase of which either half or all is connected to provide either 120/110 or 240/220. This is simple transformer functionality and has nothing to do with multiple phases. Whoever mistakenly said that the o-scope displays the two hot legs out of phase with each other is overlooking how they are connecting the scope probes. Connecting the same probe to the "center" neutral when measuring the two legs causes the measurement polarity to flip between the two legs which gives the false impression that there is a 180 degree phase shift. All that is at work here is simple transformer operation. There is no second phase, and the alleged 180 degree phase shift is because the o-scope probes are reversed.

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