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KWH Meter CT Placement and doubling?

Sean S

Titanium
Joined
Dec 20, 2000
Location
Coos Bay, OR
Hi all,
In my ongoing mission to get accurate Kwh readings from my 3 phase drop, I have purchased a Kwh meter.

The meter uses 3 current transformers and a line to neutral (and also a line to the legs I think.

All is good except I have an oddly wired drop and mains cabinet.

The service is 3ph 240 (Delta I've been told) with a hot leg.
The drop is 4 wire.
Now the weird part...
The mains cabinet has 2 identical weatherheads at the roof.
Each drop line splits into 2 lines (is tapped) at the end of the drop just before the weatherhead.
Basically each line becomes two parallel lines.
Now I would think this is because of an amperage issue with the wires that go through the weatherhead (doubling them up), but the actual lines going through the weatherhead are just about (if not) the same size as the drop lines.

I can only imagine that at some point the drop was of a wire gauge that wouldn't fit through the weatherhead and so two weatherheads (and two sets of wire) were used instead to get the amount of copper through there for the required service and since then the drops were replaced with a smaller gauge.
I'm guessing here since...as stated...it looks like the actual drops might fit through the weatherhead openings.

The end result however is that I end up with two sets of 4 lines inside the mains cabinet...which are simply paralleled back to the 3 bars and one to the ground (neutral?) bar.

This all works fine but here's my problem...
I want to install a Kwh meter that uses 3 CT's around the incoming wires (legs), but I actually have 6 if I only count the "hot" wires.
Obviously the reading would be incorrect if I only put the CT's around half of the paralleled wires.

The ID of the CT's is 2" which isn't enough to get 1 CT around 2 wires nor enough to get around the bar itself (if this would be OK or not....dunno?).

I actually have 6 of these CT's...just for reference.

So I'd like some advice on these options I've come up with....

1. Have the electric company come and re-wire the weatherheads for 4 wires instead of 8 (expensive).

2. Find a way to get 3 CT's around the bars (might be possible but I'm not sure bars have the same inductive qualities as wire...though it would seem they would).

3. Use 6 CT's on the 6 wires.... which I don't know if it can or should be done, nor if I would wire the 2/4 CT wires in series or parallel to get the correct amperage to the meter.
The CT's are 300/5a (60:1) if that matters.

Pictures coming...

Advice here would be greatly appreciated!
Many Thanks
Sean
 
What a mess. I think my count was wrong.
Two of my 4 drops are large, 2 are smaller.
The larger one's have 4 "taps", The 2 smaller have 2 "taps".
Here's the inside of the cabinet, the weatherhead, and the pole...

power410.jpg


power411.jpg


power414.jpg


power412.jpg


power418.jpg


Can anyone figure this out?

Sean
 
Well I am answering this for the second time. The formum went down for maintenance as I was posting but it does not tell you this until the post is lost.

Yes you have a paralleled service. If you could put the CTs on the bus bars that would be great but I think you would need a CT bigger than required to go around the two wires.

The solution I would try is use all six CTs, one on each of the three phases. Wire the CTs in parallel. Be sure that you phase them correctly. You can verify this by connecting one at a time to the meter and verifying direction of rotation. DO NOT LEAVE UNUSED CTs WITH POWER FLOWING THROUGH THEM with out shorting the outputs as they will self destruct.

Be sure that you have the correct style meter. Depending on the ratings of the meter and the ratio of the CTs you may not be able to direct read the meter. The reading may need to be scaled. This can be calculated using data on the meter frame and ratio of the CTs. A slight error could occurr if the CTs are not rated for the burden of the meter. I think utility actually compares to a known meter in veifying the correct scale factor for your installation.

I knew this stuff a little better a few months ago when I studied for a positon of metering technician. Unfortunately still looking for employment. But if you have more questions, I have books on the subject.

I was through Coos Bay last summer on motor cycle vacation. I noticed that most poles in town were covered in wires like yours in the picture. And worse yet, downtown they were stringing more wires on the poles as I drove through. I felt sorry for the poles.
 
Bill, please could you elaborate why an open-circuit current transformer would self destruct if installed around a current carrying conductor?

My limited knowledge of electrical theory suggests there would be no current flow in an open-circuit coil of wire, so I'd be surprised how it could blow itself. Perhaps I'd entertain the idea that the ends of the wire would develop very high voltage and would maybe breaking down their insulation, but it does seem a bit of a stretch to me.

Graham
 
Graham, It may seem like a stretch but that is exactly what happens. These transfomers have a very high turns ratio, the primary only being passed through the middle one time (typically).

To quote from Handbook for electricity metering:
"... With an open secondary, the secondary impedance becomes infinite, the flux rises to saturation, and the voltage drop in the primary is increased due to reflected secondary impedance. ...... Voltages of several thousand volts are possible under open circuit conditions." It goes on to say this may damage the transformer and you.

I use some CTs to monitor currents in my RPC. But I only have one panel meter in the unit to monitor each of the phases. Since the loaded voltage out of the CT into my panel meter is only about .2 volts, I use back to back diodes across each of the CTs for protection.
 
Thanks Bill.
The CT's are 300/5a and the meter is designed for (3) 300/5a CT's.
Would that still be OK if running pair of CT's in parallel, or did I somehow change my ratio by doing that?

If I have an accuracy error of 5% per CT, could I possibly end up with a 10% error?

I really don't know what could be affected by paralleling 2 CT'S on one circuit, but it does seem on the surface to be a logical choice.

I would love to put the CT'S on the buss bar. I think the bar though is a tad wider than the 2" ID of the CT. If it were only 1/8...or maybe even 1/4" over, I'd be darned tempted to mill the bar down. The problem though is that I doubt I have room between the bar and the back of the cabinet to fit the half circle of the 1/2" thick CT.

I really don't know why it is like this at all? Why would a mains cabinet have two weatherheads?

For shorting the two leads of the CT's, please let me confirm that you mean shorting them to *each other" right?

Thanks
Sean
P.S. Bill, my experise runs more to the internet as yours runs to electricity so I'll give this little advice...
If you are typing *anything* that is long enough to not want to retype, copy your text (edit/copy) before hitting the "submit" button. That way if something goes wrong, you can open notepad and paste the text in for later posting.
I've been burned by just what got you soooo many times.
It only takes a few seconds to copy the text.
 
Sean, Yes shorting means shorting the two leads from the CT to each other.

I am hoping that the parallel will work well. It will work fine with perfect current sources but CTs are not quite perfect current sources.

Your measuerement error will not be double the CT tolerance. In fact, if some were + X% and some were -X% the errors could cancel assuming similar loads on all phases. However, there are several components of this error. The ones I can think of are if CT was calibrated with same burden as meter presents, meter accuracy (I think they are typically about .2% for low power settings).

I do not know why they ran the parallel service. Could be the electrician just used the wire and conduits he had on hand. I do not like short runs of parallel conductors as they are less likely to share current equally. Someone may have a better story. I notice that the panel has lugs for two wires (required by code if you are going to have two wires) so it looks like somebody planned this. I would think that the double lugs would typically be used to feed another panel as opposed to double service. What is the total amperage of this service? Where is your service currently metered or do you just get flat rate service? I do not see that anywhere in the pictures.

And I did use copy before I posted the second time. I have got burned a couple of times (slow learner) due to having faith in the power of the internet.
 
Sean S:

Is the panel as you have shown in the photo directly fed from the pole transformer without any disconnect, and where is the power company meter or meters?

How do you expect to install the current transformers if the panel is hot, or are they snap on?

A current transformer can be viewed as an approximation to a current source. In other words it has a high source impedance. This means for a given output current the output voltage is determined by the load impedance. Thus, theoretically you could get infinite voltage if the load impedance was infinite (open circuit). The sum of the currents at any point is zero. So if your current sources are of high impedance feeding a point, and these feed a low impedance, then there is little interaction of one current relative to the other to be summed at that point on an instantaneous basis. As mentioned your phasing must be correct.

Your current ratio is 300/5 or 60 to 1. Since you will feed a wire thru the transformer once this means the secondary has 60 turns.

You must protect any individual current transformer from having a high impedance load on its output (secondary). The easy way is to short the output when there is no other load. But you never want to open the output any time input current is flowing thru the current transformer. An Ammeter or Wattmeter on the current transformer output is a low impedance load. Many Ammeters are about 50 MV full scale. If your 5 Amp meter was of this rating, then the meter impedance would be Z = 0.050/5 = 0.01 Ohms.

Yes the current transformers must be run in parallel and with a low impedance load. A series connection of the outputs would be wrong.

Keep in mind you have a very dangerous circuit with which you are playing.

There are all sorts of factors that will affect your accuracy.

As I have said before I do not think that monitoring the power input to your building is the best way to get the information you desire. However, it is an interesting exercise.

.
 
Gar,

Sadly I don't have a disconnect. I my plan is to decide where the CT's will go and then have the utility company shut down the power for at least a few hours so I can get them in place. The CT's are solid of course.

I did this once on a 480v panel that was live... I wore a rain coat, pant, heavy rubber gloves and boots. I wrapped the heck out of my tools with tape and proceeded by standing intentionally off balance.
Not the smartest thing to admit, but hey... I don't skydive.

That box...in contrast...was quite clean with 3 nice straight lines to the bars. There is no way on earth I would attempt the same thing with the nest I have in this box.

The meter I purchased is here if anyone is curious...
Power Meter

It can trasmit a bunch of data via ethernet to software that can do all sorts of analasys. It should prove fun. There is a PDF link to the main features on the ebay page.

At this point I almost feel like the challenge and the learning experience is as important to me as the final readings and test results. Strange.

Sean
 
Bill, thanks for the clarification. Most of my electrical training comes from low voltage electronics theory, so sometimes I get caught out by the big stuff.

Graham
 
Jim...I wiiiiisssshh.

*I* don't have a disconnect (meaning some giant knife switch).

I'm sure the utility has a way to do it (quite sure).

The $$$ meter is at the bottom of the pole pictured above, at eye height. It is the glass bowl type...digital I think.


Sean
 
Ahhh...and upon looking closer at my own pic', I can see the loose mess of wires going to conduit that runs down the pole.
I guess the utility is using an external CT based meter as well.

Hmmmm.

Sean
 
This is all very interesting but WHY?

Are you trying to double check the electric use?

And is this to maybe disput what the electric meter says you use?

Dont they have you one way or the other, pay the bill or the lights go out?
 
Sean:

In your photo #5 showing the pole and the 3 50KVA transformers you can also see the Power CO. 3 CT's.

They are mounted at 90 degrees on the left side of the pole, under the transformer.

The wire comes out of the transformer, through the CT unit and to your weather head and on into your shop.

One CT unit for each leg/phase.

You'll also notice a small wire coming from each CT unit down to the meter loop then to the Power co.'s meter recording the power usage.

Hal
 
This could be much simpler...... Can you get a picture of the meter?

It may be in a box, behind another glass panel, so it could be hard to see. But a view of it might allow us to get a handle on simpler means of arriving at your desired result.
 
It looks like 2 large wires from the pole and two small ones. This might imply the primary load was expected to be single phase, and well balanced on the two sides of neutral.

.
 
Gary, I am trying to install a more sophisticated meter in the building that will give me real-time power calculations so that I can chart out my load for individual devices, groups of devices, sections of the building, and totals during certain states and time periods.
Hopefully this will let me determine what is inefficient and should be replaced, how to effectively use my electricity by avoiding low priority, high demand loads (that heater in the spare room...etc).
Things like calculating the difference between keeping a room heated 24/7 vs heating from outdoor temperature only when occupied...and what the cost difference would be...etc..etc.
The ultimate goal is to make informed decisions on what should be upgraded, where best to put money for upgrades and in what order...etc etc.
Upgrades may include:
Better (or better than zero) insulation.
Changing over to Propane where it is the most cost effective.
Automation of lights, heat...etc.

The goal in the end is to reduce my electric bill, which can go over $1000/month at times.
My building is an old elementary school built in 1904, then added to in the early '50s, then upgraded and added to again in the '60's, and maintained and upgraded "as needed" until 2003.
Because of this, I have several different sizes of rooms, different types of lights, baseboard heat combined with 3ph fan driven heaters *in the same room*. Sodium lights, incandecent, flourencent...sometimes in the same room.
The list goes on.
My wife and I live and work here, so there are living areas, shop areas.
A good example of one of soooo many calculations I would like to make....
I have one room...about 2200sq feet. This is our "game room". It has 2 9' pool tables, an entertainment center...darts...whatever.
It also has baseboard heating and takes nearly two hours to warm up in the winter.
I would like to know just how badly I would like to shoot pool, and how much it costs me if I forget to turn the heat off when I exit the room.
I would like to know how long it would take to pay for running propane through the building only to the living area....or only to the heaters, or only to the 3 water heaters, or all at once, vs leaving them electric.

The building is 30,000sqft.

J Tiers,
The meter is easily accessible at the bottom of the pole. It is the "glass dome" type with a single hour usage digital display.
It is labelled "240 4 wire Delta" and "CT installation".
It has a straight conduit running up the pole from which the CT wires exit.
It's not very interesting but I could get a photo of it if it would be useful.

Gar, yes... it is two large gauge wires and two small. This is one of the things that throws me off. I am used to 3ph being 3 wires of the same gauge from the drop (and to the bars)
As we've seen, it gets even more confusing to the novice electrician (me) since the two large wires *each* are split to 4 smaller wires at the weatherhead. One of the two smaller wires gets split to 2 and the other only has one.
If I've observed this correctly, that's 4 wires to the weatherhead but 11 wires *through* the weatherhead (could be 12...I lose track)
At the box, the center bar only has 2 wires, but the 1st and 3rd have at least 4 each. I can see in this mess at least one of the wires coming through the weatherhead goes to neutral...maybe two.

What started as a mission to save some money on the electric bill is quickly evolving into me trying to learn what I'm talking about so I can speak intelligently to the electric company supervisor about getting some things changed at the drop, and potentially re-wiring from the weatherhead to the bars.

Ultimately they will argue that it's all "fine", in which case I think I'll need to see if they will let me get *my* CT's in at the pole somewhere since I cannot possibly squeeze 10 CT's in the top of that box....or I'll look closer at trying to get around the bars somehow.

Before anyone asks...it's just not my nature to give up in defeat, so one way or the other it'll get done even if ultimately it wasn't worth it. I bet a few here can sympathize with that nature


All the Best
Sean
 
Oh... just for reference...
The panel is rated @ 1200A It is larger than the photos would indicate.
The Power company CT's are labelled "400" so that would make sense although the one smaller wire out there that goes through one CT doesn't look like something I'd like to put 400 amps through (the other two are quite beefy).
My CT's are 300A. I'll risk that I never pull over 900 amps at once.

Cheers!
Sean
 
Well Sean, I hear ya... but I'm not a EE, just a ME that has had a big shop and my part was part of a bazillion sq ft facility, so I think I know what you mean. The facility owner installed a meter (480v/3ph) in my area and I assume he did the same to many of the other areas.

But I think your going about it the wrong way...
You already know by walking around the place what consumes energy, and you know that the building if not insulated, will continue to leak. Fix the leaks first, close off what you dont need, insulate where you do need. Upgrade what needs it first, next can wait till whenever.

Fancy computer reading of your electric meter is only gona tell you what you already know.

and Sean sez to...
Yo... you..told ya a million times...
TURN OFF THE FRIKIN LIGHTS WHEN YOU LEAVE A ROOM !!!!

Good luck
 








 
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