Harris Institute of Technical Training

Our line crew just recently changed out a couple of poles and changed the left what used to be the wild leg of a 4 wire service unhooked. My question is will the 4 wire delta meter accurately measure the power of the single phase service?

Yes, metering a single phase 240 Volt, 3 Wire service with a 4 Wire Delta meter is possible. There are however, a couple of items that should be addressed for clarification in our discussion of this.

First: If the meter you have is an electro-mechanical type, then I presume it is a Form 15S meter that we are talking about. If is this correct, then all of the potential coils in the Form 15S meter need to be energized for this meter to work properly, even if the customer is only using a single phase 240 Volt, 3 Wire service. This is based on the fact that when you test a single stator on a multi-stator meter (like the 15S, which is a two stator meter) all of the meter’s potential coils need be energized for this test.
A meter with all of the potential coils energized is what is necessary for the multi-stator meter to measure correctly with one stator being actively used. So, if the high-leg is disconnected from the meter socket’s line side, then there will be a metering error. The magnitude of the error I am not sure of, but there will be an error. To get an idea of the magnitude of this error, you could do a single element or single stator test on the meter, with only the single phase element energized. With this test you can see what the accuracy of this element is under test and this would give you a good idea what the metering accuracy would be when in service. And maybe this error will be close enough that you, and the company you work for, will feel it is correct metering.

Second: The other item that comes to mind for this discussion is the need for an on-site verification. It will ensure that the meter socket is (and has been) wired correctly. This needs to be done because a Form 15S electro-mechanical meter will measure energy, even in an incorrectly wired meter socket.
The customer will be in full power, but a metering error will exist, caused by the incorrect wiring of the meter socket (there is more to this but error, but it is not the focus here so I am not going to go in to that for now). The usage measured each month, for the company to send the customer a bill, won’t reflect the actual or correct usage caused by an incorrectly wired meter socket.
The socket wiring error I am talking about is based on the incorrect placement of the high-leg in the meter socket. With a Form 15S meter the high-leg phase to neutral voltage must be connected to the right hand side meter stator or element through the right hand side meter socket jaw (energized phase) and the bottom center-right socket jaw (neutral conductor). The socket and meter must be connected this way to measure the energy used by the customer on all of the phases correctly. The location of the disconnected high-leg will need to be verified to make sure it was removed from the right hand socket jaw and not from either of the other two socket connections (center or left). If the disconnected high-leg was or is in the center or left hand side of the meter line side socket jaws, then there has been (and will continue to be) a metering error that is not acceptable. If the socket was and is wired correctly then this will still leave you with the metering accuracy error described above so it will still need to be considered and addressed.

There is a fix for both of these problems that you can do to make it so that the Single Phase, 240 Volt, 3 Wire service, is wired and metering correctly – However, because these fixes are specific to each sites’ requirements, I encourage you to contact me directly to discuss these fixes and what is involved in making it right. My phone number is 503.901.6132. - Be Safe All Ways, Brad

Thanks for your response. My supervisor doesn't think there is enough of a drag on the stators to worry about, but I was just curious about the possible loss we were taking on it since they are always complaining about the bottom dollar here. I am attending the Mid South meter school this year for the first time hoping to learn more about these kinds of things. We do not have a actual meter tech here at the company I just get stuck trying to figure out what the problem is with anything we have wrong here, it is good to know that there is people like yourself out there that are willing to help those of us that are not that knowlegeable but are wanting to learn.