# 3 Phase Wattmeter Wiring



## Big John (May 23, 2010)

So I goofed up and went to put in a 3 phase wattmeter where I was sure I had two PTs, but it turns out I only have one.

This is the connection diagram for the meter I'm using:









I'm curious what would happen if I paralleled a single PT across terminals 2 and 6 with the grounded conductor being put on terminal 8?

Don't know how this meter works, so I don't know what it needs both voltages for.

-John


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## wildleg (Apr 12, 2009)

looks like ref to ground on the transformed side (don't know what the ration is from that diagram)

looks like it works by measuring the watts on 2 legs and calculating the 3rd ?

I found one article on this, didn't really read it but it looks like it describes the principle:

https://www.ohiosemitronics.com/pdf/tech_papers/Two-wattmeter-method(E).pdf


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## jmsmith (Sep 10, 2011)

Big John said:


> So I goofed up and went to put in a 3 phase wattmeter where I was sure I had two PTs, but it turns out I only have one.
> 
> This is the connection diagram for the meter I'm using:
> 
> ...


John, it appears that the pt in your diagram is a single MULTI-TAPPED pt. I have seen this before, in new substation. It used one voltage for the power calc, and the other was simply there to power the electronics that were internal to the meter. If I remember correctly, the two jumpered terminals are there for reference points for both circuits. Hope this helps.
- Jim


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## Big John (May 23, 2010)

Well, I tried it, and it didn't work. So now I gotta come up with a plan B.

-John


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## jmsmith (Sep 10, 2011)

Big John said:


> Well, I tried it, and it didn't work. So now I gotta come up with a plan B.
> 
> -John


Wildleg was steering you in the right direction. In this instance you are using one meter in the place of two. Unless the pt is built to monitor 2 phases with relation to the third phase, you will have to use two PTs as the diagram shows. The only way I can see of using one pt with that particular meter is using one that is center-tapped, both on the primary and secondary. Does this make sense? I hope so!


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## jmsmith (Sep 10, 2011)

jmsmith said:


> Wildleg was steering you in the right direction. In this instance you are using one meter in the place of two. Unless the pt is built to monitor 2 phases with relation to the third phase, you will have to use two PTs as the diagram shows. The only way I can see of using one pt with that particular meter is using one that is center-tapped, both on the primary and secondary. Does this make sense? I hope so!


BTW, I goofed-up in my 1st response to you. If electronic, there wouldn't be a need for a separate power source other than the control voltage needed for any relaying or monitoring devices. Sorry, my bad...


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## Big John (May 23, 2010)

I dunno, guys. To me that diagram appears to be two totally separate PTs with a common ground, hence the two different sets of primary fuses.

Regardless, I've gotta either find the CT circuit for another phase to try and make this work, or I've gotta come up with a way to measure watts with one PT and 3 CTS, and while it seems like it might be possible, that could also be wishful thinking.

-John


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## jmsmith (Sep 10, 2011)

Big John said:


> I dunno, guys. To me that diagram appears to be two totally separate PTs with a common ground, hence the two different sets of primary fuses.
> 
> Regardless, I've gotta either find the CT circuit for another phase to try and make this work, or I've gotta come up with a way to measure watts with one PT and 3 CTS, and while it seems like it might be possible, that could also be wishful thinking.
> 
> -John


Please see my last response to you. You are right about it being two separate PTs, and yes, you will need four fuses (2ea. In the primary). The secondaries are wired as per your diagram. The meter you have will act as a two-wattmeter system which allows for the use of only two CTs.. But either way, you still need two each: CTs and PTs. It was hard for me to get my head around that one, too! :laughing:
Hope this was some help! I just saw 3CTs? The two meter system uses only two, with the unmetered phase being common to the two meters.


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## wildleg (Apr 12, 2009)

If you don't have two pts as shown in the diagram, you won't be getting an accurate measurement. I don't see how you can wire it any way but the diagram and get an accurate reading (you are measuring the current on two phases, and the voltages from the same two phases relative to the phase for which you have no current transformer). without having all three phases present, I don't see how you can expect this meter to work (but I am by no means a metering expert, so feel free to correct me)


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## jmsmith (Sep 10, 2011)

wildleg said:


> If you don't have two pts as shown in the diagram, you won't be getting an accurate measurement. I don't see how you can wire it any way but the diagram and get an accurate reading (you are measuring the current on two phases, and the voltages from the same two phases relative to the phase for which you have no current transformer). without having all three phases present, I don't see how you can expect this meter to work (but I am by no means a metering expert, so feel free to correct me)


That was one of the things I couldn't get my head around, also. We tend to complicate things when that third phase enters the picture. A single-phase wattmeter is looking' at both HOTS with a reference to ground. Okay, now we are dealing with three phase. It all started out with using three single-phase watt hour meters. Then someone figured-out that you could save money (and be just as accurate) using two wattmeters to meter three phase. This roughly equivalent to using to transformers open-delta for three phase power. The meters would both be calibrated to account for the fact that they are both using one of the phases in common. The meter that John is referring to is actually TWO meters in the same unit.
Now that leaves us with the only other option, a true three phase meter, which would require three CTs and three PTs. You usually don't need that much accuracy unless you are a high-demand user, and want to part with the extra change for this equipment. What John has here is a very cost-effective way of metering... Two PTs, two CTs, and one meter. This is by far the least expensive of all the options! Hope you are getting' what I'm trying to say here! :blink: have a good one!


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## jmsmith (Sep 10, 2011)

BTW, Mr. Wildleg, I'm not a metering expert either! But majority of my 35 years in the field have been heavy industrial and utility, with the emphasis being switchgear, relaying, and controls. Loads of fun! There's still room to learn, tho! Y'all have a good one!
- Jim


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## jmsmith (Sep 10, 2011)

Hey, Big John--
Just was wondering...Did you ever get your metering deal settled? Enquiring minds want to know!
:blink::laughing:

Sent from my iPhone using ET Forum


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## ejbelectric (Nov 5, 2011)

what voltage was he working with and how was he reading the voltage?


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## recloserman (Nov 11, 2011)

*metering*



Big John said:


> So I goofed up and went to put in a 3 phase wattmeter where I was sure I had two PTs, but it turns out I only have one.
> 
> This is the connection diagram for the meter I'm using:
> 
> ...





Big John said:


> So I goofed up and went to put in a 3 phase wattmeter where I was sure I had two PTs, but it turns out I only have one.
> 
> This is the connection diagram for the meter I'm using:
> 
> ...


Couple of questions - what service do you have and do you have one PT or one CT? The diagram you posted is for a 3 wire delta service at either 240 or 480 on the secondary.

To answer your question at face value: the meter will not run if you wire it as you have asked. The nickel version of why it will not work: meters calculate usage based on the service and it vectors and phasors. They also account for power factor, which is not possible without the two seperate voltages. Any further explanation is well outside the scope of this post.

If you have one ct (you will not have one pt without a corresponding ct, but you can have one ct without a corresponding pt) then you most likely have a 3 wire, transformer rated single phase service with a double wrapped ct. However, you could also have a dual ratio, double tapped ct.

How did you get stuck trying to wire the metering? Electric metering is a complex specialty that requires the completion of a five year apprenticeship and an engineering degree? Beyond that, your local utility most likely requires the involvement of their distribution engineering department when you install any three phase services. Their engineering department will then forward the service specs to the meter department to size the metering requirements. The utility will have very specific requirements regarding the meter installation. Have you requested a copy of their service requirements?

Wish I could be of more help. I will check back tomorrow and see if you post any more details. I may be able to direct you to links and other info you will find helpful.


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## recloserman (Nov 11, 2011)

Big John said:


> So I goofed up and went to put in a 3 phase wattmeter where I was sure I had two PTs, but it turns out I only have one.
> 
> This is the connection diagram for the meter I'm using:
> 
> ...


Couple of questions - what service do you have and do you have one PT or one CT? The diagram you posted is for a 3 wire delta service at either 240 or 480 on the secondary.

To answer your question at face value: the meter will not run if you wire it as you have asked. The nickel version of why it will not work: meters calculate usage based on the service and it vectors and phasors. They also account for power factor, which is not possible without the two seperate voltages. Any further explanation is well outside the scope of this post.

If you have one ct (you will not have one pt without a corresponding ct, but you can have one ct without a corresponding pt) then you most likely have a 3 wire, transformer rated single phase service with a double wrapped ct. However, you could also have a dual ratio, double tapped ct.

How did you get stuck trying to wire the metering? Electric metering is a complex specialty that requires the completion of a five year apprenticeship and an engineering degree? Beyond that, your local utility most likely requires the involvement of their distribution engineering department when you install any three phase services. Their engineering department will then forward the service specs to the meter department to size the metering requirements. The utility will have very specific requirements regarding the meter installation. Have you requested a copy of their service requirements?

Wish I could be of more help. I will check back tomorrow and see if you post any more details. I may be able to direct you to links and other info you will find helpful.


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## SteveO. (Oct 17, 2011)

You need the two ct's and pt's because a wattmeter will always want to read the phase voltage, which is why you need at least two to make a star point between the pt's. It watches two of the lines in a balanced three phase system and that enough to make sure it's balanced and you're not single phasing.


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