Single Phase CT Metering

Yes.

This depends on your material, like the meter unit. There are systems with 2 wire, 3 wire and also with reversed inputs. We do not know what country you refer to.

Good old USA

Big business for the meter reader, if one CT is used? Is the 180 degrees shift clear?

Not sure what you mean by 180 degree shift clear. Our kids have bought this house and the power bill is very sporadic 700$ this month for a 2500 sq ft house. Way out of the norm for this area.

That seems high, but do they have electric heat? Where is this, and about what price per kwh?

Yes, if the purpose is to add the power delivered through both lines.

You say "single phase", then talk about "the other phase". Are you referring to phase and neutral? If so (unless there was a nasty earth fault), the CT would either "see" double the line current, or zero (depending what you mean by "the opposite direction").

This is a 120/240 volt residential signal phase service. there is one CT donut installed by the power company. one of the power conductors goes thru from top to bottom, the other power conductor goes thru from bottom to top . I have never seen this arrangement before.

Because of their opposite phasing, if the wires went through the same way, you would be measuring the difference in currents; i.e., earth leakage. That is also done, but for a different purpose from measuring energy consumption. See #3.

In alternating current, any phase goes one way half the time and the other way the other half of the time.

Is this what you mean?

Do they have estimated readings every so often? If so they could be using more electricity on months that the reading is estimated. (This is sometimes called "Taking the reading from the bus") When an actual month's readings are taken adjustments are made to catch up on the actual vs. estimated. For example If they use $200/month and are only being billed an estimated of $120/month the difference accumulates and is billed along with one months actual usage on months that an actual reading is done. 5 months estimated at $120 estimated with $200 actual and the actual of $200 for the month of the reading is 5 months actual - estimated = 5*($80 shortage) = $400 difference between actual and estimated. This plus reading month of $200 = $400 & $200 = $600 for a actual month when the meter is read and billing adjusted.

This many not be the reason but it quite often is why for several months being the same billing and then either a lower or higher on the month the meter is actually read. Check the old bills or discuss this with the electric company.

Good Luck, Old Salt

By ct, I assume you mean Current Transformer. Are you sure this doughnut is a Current Transformer, and not just a ferrite core to keep RF from entering/exiting the home via the power line?

I'm quite familiar with current transformers, and fairly familiar with residential kWh meters. I've never seen a residential kWh meter that had an external CT. Of course just because I've never seen one does NOT mean they don't exist...

I have a picture being emailed tomorrow so will try to attach it. To the best of my knowledge and from past experience it definitely is a ct. This is baffling at best:-)

It might be less baffling if you read and respond to the posts that explain it.

I do appreciate all the comments but am only able to check the site periodically. I have read each comment and am grateful for all of the input.

Here the "electrical inspection" made us install a 400 Amps service with CT can and 500 MCI connections. They come to the service with a 1/0 for 200 meter to the transformer. Our monthly bill is less than $100.00. It happens all the time, come and see.

Understand who can.

For a residential home, a CT could be used if the meter is digital electronic type.

I the meter is a mechanical type with a rotating disc, it would be unlikely to have a CT.

In any case, the CT will have 2 wires coming out of it and connected to the meter. These are from within the CT and not the ones passing through the center of the CT.

"...the meter is a mechanical type with a rotating disc, it would be unlikely to have a CT..." Digital or mechanical, a meter is a meter.

It is common practice to use CTs in rural areas where the utility company/coop only stocks one type of meter and uses CTs to adjust for the difference in loads between a farmhouse and say a milkhouse, or where there is a single drop from the line where the CT measures the current ahead of multiple overhead secondary taps distributing power to a cluster of buildings.

Thank you all for your input. The utility company has been notified and we are taking it from there. You have given me valuable options to present to them.

"...A CT is meant to have only one line through it only...", a common misconception. A CT is a vector summing device, it uses both magnitude and angle to net out what passes out through the secondary. We all have CTs in our houses, they're in our GFIs where both hot and neutral wires pass through the same CT in opposite directions.

Maybe I did not use the full definition here: In this OP context, we are discussing a metering device and not a differential current measuring for protection, device.

In this context, a CT is used to measure the amount of current going through, in view of metering the power being consumed. therefore, You would not have 2 phases or 3 phases or a phase and neutral wires going through the same CT. This will fail the objective of this device as a current transformer only and not a protective device, measuring a current difference.

Therefore, your response to my comment is misleading and not precise. It does not explain or improve the flow of info to help the OP or any one else.

I do not know why you received any GA for your comment since it does not answer any question in this thread but just makes a useless contribution.

Thank you

It is precisely the opposite directions of the conductors that makes the OP's CT additive rather than differential. I have explained this more than once already; RAM is right and you are wrong. So have another OT.

Sorry to disagree with you.

Ramconsult mentioned something else than the additive effect (which I did mention before as being additive and maybe required by the metering device as a multiplication factor (although this is unlikely since the meters do have settings for such...) Please re-read his comment instead of just siding as buddy-buddy...

Thank you for the OT it does not bother me. It is actually off topic since it does not fit in the answers to the op's. So should, also, be your comment! Maybe you should OT your's and I leave it to you.

You're wrong again, twice. But I can give only one OT, so you get a freebie.

Quote:>>>>>>

Explain, since you seem to be a specialist (or Dmsc...), why a CT will be having 2 wires from different phases ( or phase and neutral), going through it in a way to add the currents, for the purpose of metering the energy being consumed? <<<<<<

When I mentioned specialists, I meant other posters that studied enough and gathered experience with the US grid system. Not me. I am just checking posts and commenting to answers that are incorrect. (IMHO)

Our sponsors, who pay for this forum, deserve a no nonsense informative forum. I am happy to participate and see this as a minimum ethic, call it pride perhaps.

I have no clue where you are from and you might not be familiar with the split phase system.

In a regular single phase system as used in Europe and the UK there is no center tab in the 240 Volts (220 or 230) single phase, used as a neutral. One of the wires is connected to the neutral there. The other provides the hot line.

There I can understand, you do not put more than 1 wire through the CT because it makes no sense to put more, since the reference voltage is always the line voltage.

(say the same 220 or 230 Volts AC)

In the US, Canadian, Bahamas- system there is 120 Volts -N- 120 Volts, so that neutral is in the center of the low voltage part of the line transformer, making it possible to feed 120 volts instruments between the neutral and L1 or the neutral and L2. Suppose your girlfriend irons your cloths and uses a 120 VAC iRon between the neutral and leg 1 (the leg that goes through the CT)

The meter will register the power through the CT. (In this case 1 wire goes through the CT - the hot leg L1)

How can the same meter register the power used with my 120 Volts coffee machine that is connected between the neutral and L2?

Guess: by putting a second wire through the CT that runs the current, produced in L2 (the coffee machine)

Since in reference to the neutral the phase in L1 and L2 is reversed (180 degrees shift) the second wire has to go in reverse direction through that same CT.

Example: the iRon (the most practical tool of the "i" family) uses 750 watts and the coffee machine 1000 Watts.

With 1 wire, you use 750 Watts and the coffee machine has a free ride.

With 2 wires in the same direction, you will get 1000 - 750 = 250 registered (Look @ the minus sign, pointing to a differential as Tornado states in his posts)

With 2 wires, one in the opposite direction, you will get 1000 + 750 = 1,750 Watts. (adding up the values, what keeps the power company happy)

This being said, you could also put the neutral only through the CT, but that goes completely wrong: When the ironing is done and the coffee is ready it shows no power used. If I switch on the A/C that needs 240 VAC between L1 and L2, I go for a free ride on this one, since there is no current applied through the neutral.

As I wrote in earlier posts, the use of one CT or more CT's in residential power apps. has to do with the way the meters are able to receive info. Personally, I have seen systems with 2 wires through 1 CT as well as 2 CT's, each with one wire through it.

I get a happy feeling, to know that you will enter this new 2014 with a little more information that perhaps might find its way to enrich your knowledge. If this comes true, we both have learned something. Happy New Year. D.

Once more (and I hope the last), a GFI is a Ground Fault Interrupter and it measures the currents flowing in opposite directions from the Supply to the load, thus subtracting them to check that there are no leaks to ground from the side of the load. The cables are not run in opposite directions through the CT! It is the current that flows in opposite direction (maybe RamConsult can be informed by you? He will be reading this maybe).

This is different from CT intended to be used on Energy meters. Here there is no subtractions nor additions.

Happy new year to all

I find it a pity that no more "specialists" come up with their opinion. Just like in a few other cases where the right answer was been highlighted by only 2 individuals, answering the O.P. bluntly wrong.

I vote you a G.A. and Luke also gets another O.T. from me. I have been waiting for the evolution of this post to pick up the tread again. The function of the neutral has been misunderstood again.

If a few more specialists come forth, maybe we can have a real donnybrook.

There is only one phase.

In this system the reference voltage will be from L1 to Neutral for power and power factor calculations. Voltage imbalance between L1 and L2 will show power errors with a magnitude proportional to the voltage imbalance.

Thank you for the clarification details etc. GA

"If you have a single phase service, how can you have two phases?" - I asked (implicitly) for clarification on this in #4. Still waiting.

How can a single phase system have 3 wires with different potential(s)?

The single phase theory ends at the primary (e.g. 7kV) of the line transformer. And all that comes after implementing a split secondary is tradition, deception, misunderstanding and knowledge and hot stuff to write about to kill time.

Your question was answered in #2, #9, .

This is a single-phase, 120/240 volt, three wire residential service in the USA.

You may not be able to answer this from the UK perspective.

Dear Old Salt,

The 2 phase or 2 leg system mystery will not be solved in our lifetime. Perhaps after we all are metric (metricians?) It is similar to french(ed) fries that are called French fries.

I don't want to spoil your festivities but they, me, you the electrical code had a dilemma to deal with: 1. The long existing 2 phase system (that does not really comply with the multi- phase theory as newer technology implemented)

2. A split system, that is nursed as single phase system, but has all the specifics of a multi- phase system.

To name a few: the phase shift between phases in a 3 phase system is 120 degrees or in general 360 degrees (a complete circle rev.) divided by the number of phases.

A 4- phase system should have 90, a 5- phase 72 degrees, a 6- phase 60 degrees and so on. Some of these angles prove not being practical for circular fields.

So analog, the split phase has a phase shift of 360/2 = 180 degrees, what makes it a perfect candidate to be named 2 phase. My penny is that first all the old 2 phase systems need to disappear before this system gets the rights it deserves. AS long as this is not official, I will use L1 and L2 instead of phase 1 and phase 2.

Who is right and who is wrong? Just don't bother is the message.

A look at the distribution panel can be enlightening.

I am not a prophet or that clairvoyant but perhaps the 2 phase guys are progressive (renovators) and the L1, L2 guys the (disappearing fossil) conservatives?

I wish you a good year ending and a prosperous new year. D

To all who have presented an answer to #36:

I too have an answer to the question which I consider worthwhile, but at 45 answers posted so far and 9 GA points and 46 OT points is it very evident that it would be possibly be suicide to add to this discussion..

For those with the legitimate phase answers, thank you for your clarifications or reinforcement to what I already knew about legitimate phasing. With the 2 phases and being 90^o off of each other I even knew that. My EE father once explained to me about it and said "You will never need it". Then I had a prof who explained it in class. He also said "You will never need it and it won't be on any test". Gosh darn it, it wasn't on the test! They were then elevated to the level of intelligence hero's!

I am regretful though that no one answered my question about the flashlights! I guess I'll have to go to Menlo Park or West Orange, NJ, to find out. And here I thought you guys were the epitome of geniuses.

At 9 GA's and 46 OT's, I'll sit this hand out.

Good Luck, Old Salt

dvmdsc-

Thank you for putting time into the flashlight problem. I really don't have that type of phase problem because, as far as I know at least, DC has no phases or at least no more than 1/2 phase. Just watch now, someone of astute and self proclaimed superior intelligence will come down on me for being a dumb worthless creature wasting the air I breathe. NO, YOU ARE NOT TOO DUMB, at least as I see it from your answers to questions.

The mention of the batteries was said in jest and with a dose of sarcasm. It seemed to me that the original question was very short on information and that a few posters were getting quite venomous in their replies, some were very damning of others, some seemed clearly illogical and most of all forgetful of the purpose of the forum. Although I have been attacked personally a few times in the past, it has no place here and most members disapprove of it.

I am a human and make mistakes, the less the better. A few others, thankfully only a very small minority, look for an opportunity to attack, The high number of OT's and low GA's seem to show it from both sides on this OP. Some might forget that the purpose of this forum is to help others and not to attack them personally.

As with you, I am familiar with the concept of phases. Sometimes other's terminology takes a little time to adapt into my thinking but seldom stops it. My "question" about the phases greater than 3 was also intended as a ridiculous way of getting contributors back to earth and the original question. If I hurt anyone's feelings or caused them to lose precious time I am guilty and apologize profusely. EE is one of the most expansive areas of engineering and I will be the first to state that I, as hopefully others do, realize that I know only a tiny bit of all that it encompasses. First of greatest importance is the need to remember that we are humans and not perfect.

Thank you for your reply and I hope the NEW YEAR has been a good one for you so far and continues that way for another 364 days!

Good Luck, Old Salt

With regards to the real question "why this installation requires a current transformer ..." I can think of a few reasons that only the original poster can help clear up.

--Is the house the only facility being served throughout the meter?

--Were there ever additional facilities or structures served?

--What is the age of the installation?

--How big is the house?

There are (new) large homes we CT meter, and there are new large homes with three-phase service due to their size.

Yes, today you can do single-phase self-contained metering up to 380 amps, but there was a time when even 200 ampere self-contained metering was not common place, and central meter structures (aka central meter poles at a farm) were CT metered, and used a-base meters, good for 100 amps only.