Meter Socket

[SparkyEst.]

I installed a temporary service for job we are working on. The service requested was a 200A 120/240v single phase service, we typically install a 4-jaw meter for this application. Utility came back and said that the 4 jaw would not work for the configuration on the temp service since they would be providing 120/208v single phase service and we would have to install the a 5-jaw meter. attached is the diagram they sent me, my question is what exactly is the 5th jaw doing since it is tied directly to the neutral? Is something in the meter that is needed? Just trying to understand the metering difference from 120/240v single phase to 120/208v single phase, same amount of wires voltage still under 250v, just a little confused on this one.

 

[david luchini]

I installed a temporary service for job we are working on. The service requested was a 200A 120/240v single phase service, we typically install a 4-jaw meter for this application. Utility came back and said that the 4 jaw would not work for the configuration on the temp service since they would be providing 120/208v single phase service and we would have to install the a 5-jaw meter. attached is the diagram they sent me, my question is what exactly is the 5th jaw doing since it is tied directly to the neutral? Is something in the meter that is needed? Just trying to understand the metering difference from 120/240v single phase to 120/208v single phase, same amount of wires voltage still under 250v, just a little confused on this one.

Yes, the meter for the 120/208V system needs the 5th connection.

In the meter for the 4 jaw socket, the neutral just passes through and is not used by the meters sensing elements. It essentially measures the current in each leg and the voltage from phase to phase.

The 5 jaw meter measures the current in each leg and the voltage from each phase to neutral.

 

[david luchini]

See attached pictures for the 2 different meter types.

 

[SparkyEst.]

See attached pictures for the 2 different meter types.

Thanks for the clarification! The utility is asking to install a 4 jaw millbank "U7040RL-TG-KK-BLG" with an accessory 5th jaw Millbank "K5T" and to me it just seemed odd as I have not been required to do so. The engineer I'm working with couldn't really explain it, except that is what they are asking. Your clarification helped me understand better than a company who is requiring it. Sometimes they are just hard to understand! Thanks!

 

[mopowr steve]

A simple answer is that it has to do with the fact that a 120/208 service’s phases are 120* out of phase instead of 180* such as 120/240. So getting accurate measurements on 120volt loads needs the neutral to accomplish this.

 

[Hv&Lv]

The engineer I'm working with couldn't really explain it, except that is what they are asking.

That’s because he isn’t an engineer, he just happens to be working out of the engineering department.
we call them “stakers”. They are guys that go and lay out the jobs and drive the stakes in the ground for the construction crews, or in your case, parrot what an engineer has told them they wanted without an explanation as to why it is needed.

as the others have said, it is required in order to meter the consumption correctly.

 

[kwired]

A simple answer is that it has to do with the fact that a 120/208 service’s phases are 120* out of phase instead of 180* such as 120/240. So getting accurate measurements on 120volt loads needs the neutral to accomplish this.

120/240 can be metered with a single CT (and self contained meters likely do about the same thing internally) by passing each ungrounded conductor through the CT in opposite directions. The result on the CT output is additive when the phase angle is 180 degrees. Doesn't matter how unbalanced the 120 volt loads are it is still accurate. 240 volt loads are basically metered as two times current @ 120 volts.

But because of the 120 degree phase angle this method will not be accurate on a wye derived supply.

 

[meternerd]

Be aware that the fifth jaw can be installed at either the 9:00 (most common) horizontal position or the 6:00 vertical position in the socket. Ditto for the meter. Make sure your socket matches the meter.