AC Efficiency: Fact or Fiction?

[Averagesupernova]

Ok russ, I will try to explain myself. Starting with 120 volts in either split phase system or the 208 volt system where there is 120 volts from each hot wire to ground. For the sake of my argument the voltage we stick with, for apples to apples comparisons, is 120 volts. As I explained, sharing the neutral in the split phase system eliminates current in one conductor. Actually it eliminates the current in 2 conductors if we split the neutral up and had 2 separate 120 volt feeds. You and I will certainly agree with this am I correct? Some folks will say screw the neutral and just use 240 volts for everything. Yes this is an option and as I have pointed out in another thread which is linked to in this thread doing that has its disadvantages. So, sticking with 120 volts, you cannot eliminate the neutral. In my water heater example I said you can and of course we both agree that is the case. But simply using the phrase split phase it implies 120 volt loads will exist. Otherwise there is no reason for the center tap on the transformer. So, that is my position and I think we can put that part of the discussion to bed.
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Sophie, I will try to address your points.

Split phase system (two extremes):
1. Totally balanced = 240V into a 1kW load (= 4.166A)
2. Totally unbalanced = 120V into 1kW load ( = 8.33A)
Doubling the volts uses half the current. I really can't see that's very relevant.

So it is your position that halving the current going to 240 volts is not relevant? I don't recall that being your position in other threads here on PF. 1000 watts may not be much going from one voltage to the other but what about a central air or heat pump unit? Last one I wired was a in small house for what is typically built nowadays and the breaker size spec'd was 25 amp 240 volt double pole breaker. Want to run that on 120 volts? Now you have a 50 amp breaker with much heavier wire.
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Concerning what you call 3 phase 240 volt wye system in the UK. I assume that it is 240 volts between neutral and each hot conductor with 415 volts between each hot conductor. We have the same thing in the US except the voltage is halved and we DON'T SPEC THE LOW VOLTAGE. Doing so seems insane to me from a safety perspective. We call it 208 volt wye. I don't think it is possible to get wire in this country rated for less than 600 volts that is used in construction so as jim hardy said it is a moot point.
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It boils down to this: 3 phase in any configuration and split phase take advantage of using currents that cancel and reduce the loss and wire size in some neutral wires. If you don't see this as an advantage over single phase then at this point I am not sure I can get the point across, although I am sure I will try. :)

 

[Averagesupernova]

This explains it very well.
http://www.allaboutcircuits.com/textbook/alternating-current/chpt-10/three-phase-y-delta-configurations/#02197.png
Want to take this opportunity to make a correction on one thing I have said. I implied, or outright said that
switching from 3 single phase feeders to one 3 phase set of feeders halves the loss in the conductors. This is not completely true. The link shows this pretty well. Probably the best link I have seen for explaining differences in various terminology. Jim, I'm not sure about the following:

One can have 240 or 120 3 phase let's take a look

three 240 volt 15 amp single phase circuits could move 240 X 15 X 3 = 10,800 VA over six conductors = 1800 VA per conductor

a single three phase circuit could move 240 X 15 X √3 = 6235 VA over three conductors = 2078 VA per conductor,

half as many conductors carrying 6235/10800 = 58% the power seems modest gain indeed

old jim

The single phase part is right on. 3600 watts per feeder pair. No argument there. Take a 240 volt delta system. It would be 240 X 15 / √3 since line current (current in the wire) is 1.73 times each load that is connected line to line. And then this would be multiplied by 3 since there are three loads. So, we have: 240 X 15 / √3 = 2078 watts per phase. Three phases gets you 6235 watts total just like you said. There is a reason I did the math that way. BUT take a 208 volt wye system. We will use a neutral conductor to easier illustrate current paths even though with a balanced load it is not needed. Line current is the SAME as load current in this configuration so we have gained here. 120 volts x 15 amps 3 times gets us 5400 watts on 4 wires. And if we have a situation where it is guaranteed to have a balanced load we can eliminate the neutral conductor. In that case we have cut the number of conductors in half as compared with 3 separate single phase circuits.

 

[sophiecentaur]

So it is your position that halving the current going to 240 volts is not relevant?

It is "not relevant" to the 3 Phase advantage argument because the angles are different. It merely shows that doubling the volts will halve the current.
Also, The "throwing away three conductors" argument is only a qualititive argument and it doesn't imply halving the resistive loss. But numerical examples will always lose details of relationships; each of the three remaining conductors will have current flowing through it that is different from from the three pairs situation.

we DON'T SPEC THE LOW VOLTAGE. Doing so seems insane to me from a safety perspective.

We spec it that way because it refers to the voltage of pretty well all domestic and industrial appliances, You measure 240V on a device and you know it's being supplied correctly. I can see that the US Mix n Match situation makes things much more complicated and that any installation you come across could have a range of system voltages. Actually, that seems much mre risky because there must be a greater chance of connecting equipment to the wrong supply volts.

i think the higher voltage question becomes moot

The specification for insulation is less easy than that for current, I guess. But even for current, the capacity of a cable is arrived at by considering an almost worst case thermal environment. It is more conservative than necessary if the installation is done well. (I am not recommending bad practice here!). Insulation is spec'd with loads of headroom but it isn't arbitrary. Your formula (twice plus a thousand) is based on statistics of fault incidents and also on a cost factor. Insulation is cheap, certainly for underground cables. How does it compare for HV overhead, though. I would imagine the cost of towers and insulating suspension must be more relevant. The choice of system voltage really is important though and I get the feeling that it's being dismissed in this thread. The fact that there's 415V between phases is negligible, perhaps, but it still accounts for a chunk of the increased power handling capacity of three phase, imo.

This thread about electrical supply is a good parallel with the recent thread about broadcast TV reception. The environments in UK and US are so so different that we often find ourselves at cross purposes. Looking in on the UK, it must appear that we are regulated to an absurd level whilst looking in on the US, it just looks like a free for all. We are separated by more than a 'common language', as the man said.
All the world's strange save thee and me and even thou are a little strange!

 

[Salvador]

Even though I found the link myself , it's a nice link about the phase explanations.but still maybe I'm getting it wrong but the three phase no neutral connection only goes for balanced loads , with unbalanced inductive/capacitive loads there should be unequal currents flowing so in all practical cases like residental areas there must be a neutral used.I mean it's not like all the neighbours down the street will come out to calculate their loads and then agree which one will plug in what.
So if you must use the neutral and it is being used , you gain what? a bit smaller neutral wire because the neutral currents are not so high as in single phase VS split phase?
but you then get larger live conductors because you have only 120V instead of 240V.Maybe if the split phase would have been made such that it has 240V on each side hot and neutral, maybe then the smaller neutral could benefit the same hot wire diameter beacuse of the higher voltage.

Also If I am getting this correctly , apart from the single split phase thinner neutral there are no other actual benefits of it?All the same things apply, like a certain amount of voltage can push a certain amount of current through a given resistance/impedance load, so who can say whether the thinner neutral actually outgunns the thicker wire needed because of lower voltage or vice versa?

Also to me the split phase seems like its just each of the three phase coils split in half and since current only flows through a closed loop , it kinda seems that the split phase is basically just a single phase with a lower voltage and the ability to double the voltage by connecting it to the full length of the coil of each phase but then it becomes essentially a european single phase except instead of having one hot from each phase and a common neutral it has both ends hot but since the voltage is the same it doesn't do anything in terms of wire diameters etc.

 

[jim hardy]

Insulation is cheap, certainly for underground cables. How does it compare for HV overhead, though. I would imagine the cost of towers and insulating suspension must be more relevant.

I don't know enough to give you a number
but from my friends in T&D i did learn that for those big overhead transmission lines the cost of oversize towers to keep separation when conductors are swaying in high wind, and the cost of the insulators themselves was a significant part of line cost. So when my utility built lines they insulated for twice intended voltage because it both gave better reliability and allowed future doubling of a line's capacity by just replacing the transformers at both ends.
Not bad ideas in a region with both hurricanes and rapid population growth.

old jim

 

[jim hardy]

all practical cases like residental areas there must be a neutral used.I mean it's not like all the neighbours down the street will come out to calculate their loads and then agree which one will plug in what.
So if you must use the neutral and it is being used , you gain what? a bit smaller neutral wire because the neutral currents are not so high as in single phase VS split phase?

One must distinguish between neutrals on high side and low side of the distribution transformer and keep his thinking straight

I walked around several blocks in the neighborhood where i grew up.
They carry three phases into the neighborhood at probably 7 or 14 kv

at each block they tap just one phase and feed it down the side street with a neutral wire

all the "Pole Pig" transformers for tha block come from that phase to neutral, which in this picture they called 'ground'

they divvy up the blocks among the three phases at the higher voltage level so the system will be fairly balanced by statistics.

In those photos the high voltage side neutral is low on the pole
where i grew up it's at top of pole to act as lightning rod

observe the high side neutral conductor uses a smaller insulator than the phase conductor , one that's there mainly for mechanical support.

those photos all from this link
http://www.science.smith.edu/~jcardell/Courses/EGR220/ElecPwr_HSW.html
which is a nice introduction to distributionOn the low voltage side the neutral is sized same as 'hot' conductors, in fact it's the bare cable that supports them. Lower left of bottom photo.
What do we get by using our split dual voltage 120/240 system ?
We evolved from Edison's DC system which was around 100 volts.
So - i'd answer your question "We got to keep the same light bulbs." Until EPA and CFL's came along.
I grew up with the split syatem. . I like it because no receptacle in my house has any voltage higher than 120 volts to earth. When i was about two, like so many kids, i stuck a bobby pin in an outlet and still remember how it burnt my fingers and glowed red before the fuse blew. I'm glad it was 110 not 230 volts.
So i will tease Sophie - his 230 volt system is 'unsafe at any speed.'
In my lifetime I've seen our nominal voltage grow from 110 to 115 to today's 120 volts.

Here's a tabulation of several distribution schemes.
http://www.ccontrolsys.com/w/Electrical_Service_Types_and_Voltages
note it includes that delta with "wild leg" .

And a more detailed introduction by a major manufacturer.
http://static.schneider-electric.us/docs/Circuit Protection/SD183.pdf

my two cents, hope it helps.

old jim

 

[anorlunda]

Jim Hardy is my all time favorite Internet surfer. Time and again, he digs up these golden nuggets of clarifying documents or diagrams. Thanks Jim.

But this thread is a hopeless mess. It started with the OP; I don't even remember what he asked. But then Sophie brought in low voltage DC distribution. Then it jumped to the bulk transmission system and HVDC where we deal with millions of volts and thousands of miles. Then it somehow shifted to household wiring. If I wanted to offer a reply comment now, I would have no idea what subject I'm commenting on.

 

[Salvador]

I don't know any HVDC system that would use " millions of volts" the highest one I have heard uses a bit over 800KV, if maybe you were referring to lightning even though I doubt one would call that HVDC system.

As for what you said Jim , a good reply , fair assumptions and opinion but I still would love to hear whether the overall single split phase supply wins with less copper or whether it's the other way round that it actually needs more copper due to the lower voltage, and the same need for the neutral wire in most of it's application cases.

as for safety I have zapped myself with 230V from a live conductor some times , and a few of them were when I was a kid and the electricity ran through my body because i was standing on conducting pavement, i still remember my legs being unable to get me up from ground for a few minutes.
but as for the safety I think simply using less voltage with all the drawbacks it causes simply because someone might stick some hairpins into the socket is not exactly a wise idea.rather make wall sockets with some safety features or locate them on a height that children can't reach etc etc.
otherwise if we would think like this we should stop using a lot of modern things simply because if they are used wrongly they cause death.like cars airbags won't save you if a drunk truck driver decides to crash into you and the steering wheel is being pushed through your body, or the good old example of someone stabbing a person with a kitchen knife , i mean should we now use our fingers to cut bread instead...

P.S. I know in the former USSR and also in probably other parts of europe highrise appartment buildings all had mandatory wooden floors in the flats , like you finish the reinforced concrete floor which is the structural support with a wooden finish , typically pasteboard since its cheap.if the floor is dry you get isolation atleast at residental voltage levels aka 230V, so if some apparatus develops a chassis fault and has some voltage on chassis you won't be injured by that voltage (atleast in theory) beacuse your body doesn't complete a circuit, ofcourse unless you don't touch some conducting elements with your body at the same time, like the heating pipes or else.

 

[Averagesupernova]

Ok so is this thread ready to turn into split phase is better than 240 volt single phase used in the UK? If so I am totally on board!

 

[Jeff Rosenbury]

I think engineers usually try to keep voltage below the formation of positrons at about 1.2MeV. (They form as part of electron-positron pairs each with a rest mass of about 0.51 MeV.) Given phase and RMS calculations, 800KV is about the maximum.

However, under normal operations I don't think they would form since electrons would slow in the air long before they got the energy they needed. Under normal operations.

Russia had a long line (2000+ kM) at 1.1MV and China has several at 1MV. (According to Dr. Wikipedia anyway) I'm not sure how much radiation they would leak in an unexpected short though. Hopefully they have safety equipment to prevent that. Anti-matter is so cool though.

 

[sophiecentaur]

So i will tease Sophie - his 230 volt system is 'unsafe at any speed.'

You have a point there but the statistics of electric shock are pretty good in the UK because the spec for the equipment is very high and, for the past many years, there has been a requirement for RCD protection. You need to have a certificate for any domestic electrical installations if you want a prospective buyer to get a mortgage so there is very little that your average tinkerer can get away with. I couldn't get like for like comparison on relative number of fatal shocks in UK and US but http://rjqelectrical.co.uk/2013/04/electrical-safety-statistics/ suggests 28 fatalities per year (0.05 per 100,000 total) in UK and this link suggests about 0.2 per 100,000 (industrial) in the US. That doesn't lead one to think that the 240V issue is particularlyrelevant.

Ok so is this thread ready to turn into split phase is better than 240 volt single phase used in the UK? If so I am totally on board!

Better? 240V house wiring is cheaper and appliance leads are thinner. Afaik, the only reason for going away from a simple 120V (=100V) standard in the US was that it wasn't suitable for heating appliances (thick wire etc) and they thought quickly and came up with this dual voltage system which is compatible with both requirements. The split phase feature was a reasonable enough strategy which makes best use of the available copper. I would imagine that people in the UK are actually more scared of 240V and tend to treat it more carefully. 120V is just not scary enough; you can usually cope with a 120V contact without ruffling your feathers but 240V does make you jump. UK plugs and sockets are bulky and expensive but, at the same time, comforting. The fuse in every 13A plug is (should be) tailored to the lead / cord to each device to provide good protection against thin leads catching fire when abused.

 

[jim hardy]

The fuse in every 13A plug

your plugs have fuses in them ?

http://www.electricalsafetyfirst.org.uk/guides-and-advice/electrical-items/plugs/

 

[meBigGuy]

I wonder if the reduced current results in fewer fires from bad electrical connections. I hate it when I find hot plugs or sockets.
I've been hooked across 220 though, and that was a bad experience (I was pulled off).

 

[Averagesupernova]

Afaik, the only reason for going away from a simple 120V (=100V) standard in the US was that it wasn't suitable for heating appliances (thick wire etc) and they thought quickly and came up with this dual voltage system which is compatible with both requirements.

Seriously sophie, I am not sure I am going to put much effort into replying to you in the future. For someone who has such strong feelings about the differences between the split phase system and the single phase 240 volt system in the UK I would think you would remember how the split phase system evolved since I once explained it to you. https://www.physicsforums.com/threa...eutral-and-ground-needed.719066/#post-4554015

 

[sophiecentaur]

your plugs have fuses in them ?

http://www.electricalsafetyfirst.org.uk/guides-and-advice/electrical-items/plugs/

Oh yes. The Ring Main system is a very serviceable and cheap system for providing a lot of sockets in a house without a Star system. The ring is better than the Daisy Chain as there is no 'end of the line' voltage drop. Having the protection associated with each appliance by a fused plug is very good. A low powered lamp, supplied from a high current outlet is a fire hazard without something to protect the thin cord - I think you would agree. But those UK plugs are very chunky (good and bad thing) and are pretty expensive if you don't buy poor quality far-Eastern sourced ones.
There is one thing that amazes me about the fused plug system though and it's that all new plugs are supplied with a 13A FUSE IN THEM! Not a fail safe situation. I should have thought they should, by default, have a 3A fuse and then, when the user connected a new plug to a Kettle, the default fuse would blow - unless they had the sense to use the appropriate fuse value. (You can buy plug fuses in pretty much every shop in the high street except a fishmonger's.
I have read several objections (US origin) on PF to the ring main system but it is well tried and tested and those scenarios that the nay-sayers suggest, just don't seem to arise.

 

[sophiecentaur]

I wonder if the reduced current results in fewer fires from bad electrical connections. I hate it when I find hot plugs or sockets.
I've been hooked across 220 though, and that was a bad experience (I was pulled off).

The fire risk due to overcurrent is much reduced by the low level fuse protection and the risk of overheating connections is low because of the inherently high spec of the 13A plugs and sockets. Your comment about the lower current is also relevant.
The effects of a 240V shock are not nice but moderns systems all have RCD protection which limits the exposure to around 10mA for a very short time. That doesn't stop you falling off a ladder when you stick your fingers in a light socket, of course . . . . . Mains shocks can always be avoided when correct practice is followed in the workplace. I have also had mains shocks but it's always been my fault.

 

[Salvador]

sophies right about mains shocks , it's not like the electricity just magically decides to hurt you or your feelings it's almoust always someone who did something wrong.

As for the Uk plugs , well in the end everyone thinks their system is the best so it's more a mater of opinion than a matter of real life physics.For example here in the much laughed at (especially from UK by the way) eastern europe , we use a simple two pin plug, it has no fuse in it , but then again why would it have one if he fuse is in the apparatus and each device has its own current rating so a different type of fuse and also we now have cirucit breakers in like every appartment building , and given the quality of modern cirucit brakers I highly doubt one needs a separate fuse in the plug itself which makes the plug rather big and takes up unnecessary space.
i have had all kinds of deliberate test short cirucits and in all of them the cirucit breaker has done it's job so why would a fire brake out when the current is stopped in a matter of parts of a second.
and yes we use probably the simplest of all the AC distribution cirucits, the so called Y , were each building has a three phase cable coming into it's main utility box and from there each appartment or house or room has one of he phases together with the common neutral going in.so for single phase the phases can be balanced out and if one needs a three phase power he can simply tap into the utility box main cabl and have his three phases.
and even though I haven't done any maths on this, I highly doubt the single split phase has any real advantage over this simple three phase Y arrangement or the UK sysem too for that matter.

Just feels like history and tradition has a lot of importance even in physics , even though the differences are sometimes small if any.

 

[CWatters]

I've had shocks from 230V and higher. The most memorable ones were from a old CRT monitor and a cattle fence. I think the cattle fence shock was the worse experience. I was out walking and slipped over, I put one hand on the muddy ground and the other on a fence which turned out to be electrified. Every muscle in my body hurt for about 30 mins. I'd already walked about 12 miles and had to do another 12 afterwards to get back to the car which wasn't much fun.

 

[jim hardy]

There is one thing that amazes me about the fused plug system though and it's that all new plugs are supplied with a 13A FUSE IN THEM!

Hmmm fuses protect wire
i'd think the appliance itself would be fused internally, beyond where the power cord connects.

 

[jim hardy]

Aha ! It was those darn Europeans again...

In the United States, the Westinghouse Company chose to standardize the operating frequency to 60 Hz, as suggested by Tesla, eliminating nine other possibilities. In Germany, however, the standardization was much simpler because one company, BEW, had a monopoly on electricity. The outcome of BEW's standardization was 50 Hz, which was most likely chosen because it fit better with the 1, 2, 5 metric standard. This same company chose to supply its consumers with more power by raising their voltage from 110 volts to 220 volts in 1899. This trend, as well as the 50 Hz operating frequency, spread across Europe over the next few decades [10].
It has been suggested that the US switch to the 220-volt system. In the 1950's the US did consider switching but then decided against it since most consumers already had a number of 120-volt products. A compromise was reached when the US employed Edison's three-wire system: one wire supplied +120 volts, another supplied 0 volts, and a third supplied -120 volts, so that stoves, washers and dryers, and other large appliances could access 220 volts, while smaller appliances could still operate on the lower 120 volts [10].

http://illumin.usc.edu/122/a-powerful-history-the-modern-electrical-outlet/fullView/

 

[CWatters]

Hmmm fuses protect wire
i'd think the appliance itself would be fused internally, beyond where the power cord connects.

Yes the fuse in a UK plug is to protect the wire from the plug to the appliance. Normally that wire is smaller gauge than the house wiring and smaller than the circuit breaker at the distribution board, so it's the first to fail if there is a short in the appliance.

 

[sophiecentaur]

Yes the fuse in a UK plug is to protect the wire from the plug to the appliance. Normally that wire is smaller gauge than the house wiring and smaller than the circuit breaker at the distribution board, so it's the first to fail if there is a short in the appliance.

My scenario is the bedside lamp, with a long length of minimal thickness flex. Someone moves the bed and one of the feet is bearing down on the flex. After a period (months) of people getting out of bed and other antics, the conductors (no earth) are squashed into contact. The flex is passing 8 or 10A and getting hot, sizzling through the PVC sleeving and charring the carpet. The mains fuse knows nothing of this; it's quite happy to pass much more current that the fire making machine is taking. In the UK, the plug fuse (it should be 3A, if the owner is being a good boy) will just blow. My example was of a lamp, which has no internal fuse.

Aha ! It was those darn Europeans again...

As ever. We have started most of the major wars, invented medieval torture instruments and are probably responsible for all the sexually transmitted infections. But don't you just love our culture and out use of irony?

BTW, Happy new year to you guys and all the other readers.

 

[jim hardy]

Metric Bolts, Pb-free solder, auto dimmer switch on the steering column not floor, 230 volt table lamps and Christmas lights
are some Euro things i dislike
but we should have stuck with England's right foot motorcycle shifter .

old jim

 

[jim hardy]

But don't you just love our culture and out use of irony?

Indisputably. I always wanted to be witty like Cary Grant.That line from Mary Poppins about the tea in Boston Harbor being rendered 'unfit even for Americans'
made me wonder a few years later --
Was James Bond's portrayal as a promiscuous gunslinger just for Americans ? We sure went for him.

And Terry Thomas's rant in Mad Mad World to effect "If American women stopped wearing brassieres your national economy would collapse overnight " ...

well they did and it did...

 

[sophiecentaur]

Metric Bolts, Pb-free solder, auto dimmer switch on the steering column not floor, 230 volt table lamps and Christmas lights
are some Euro things i dislike
but we should have stuck with England's right foot motorcycle shifter .

old jim

Remember Statler and Waldorf in the Muppets? That's you and me, that is.
I loved the T Thomas clip.

 

[dlgoff]

Indisputably. I always wanted to be witty like Cary Grant.That line from Mary Poppins about the tea in Boston Harbor being rendered 'unfit even for Americans'
made me wonder a few years later --
Was James Bond's portrayal as a promiscuous gunslinger just for Americans ? We sure went for him.

And Terry Thomas's rant in Mad Mad World to effect "If American women stopped wearing brassieres your national economy would collapse overnight " ...

well they did and it did...

You're killin' me.

 

[Averagesupernova]

Well to back step a bit, I happened to run across this:

http://forums.mikeholt.com/showthread.php?t=173330

Gets into a debate about DC power transmission on about page 5 or so. I would have to say some of the posters on each side of the discussion are a bit misinformed on some things.

 

[sophiecentaur]

New Year's Resolution: Avoid getting into discussions involving 'Neutral'.
I wonder how long I will manage that?

 

[dlgoff]

Avoid getting into discussions involving 'Neutral'.

But, but, ...
You are the engineer of the year.

BTW Congratulations.

 

[sophiecentaur]

Cheers digoff.
"But but" You have a point there.
I shall have to remain neutral in such arguments.