Options for monitoring both 120V legs (PT-12V vs PT-480?)

[sarain]

I've been trying to pin down how the GEM uses the PT signal for its power and power factor calculations for each phase/leg.

The included PT-12V only measures the voltage on one 120V leg and appears to calculate power for circuits on both legs using that. I assume that means the system won't be able to see voltage differences between the two legs., Unless you are using a channel for net metering, it would just pick a 0deg or 180deg current/voltage phase relationship that results in positive power, and calculate power factor based on that.

The online store lists a PT-480 part (but no price) that it says works for 120V, 240V, and 480V and has two wires for the primary. If I wanted to include the voltage from both legs of my service in the power calculations, would this be an appropriate option? I realize it would only be able to average the voltage across the two legs for single 120V circuits (as described here) but are there any other caveats I'm missing?

There was a forum post a while back (can't remember where) about someone seeing an incorrectly reported power factor of 1 for a 240V circuit with 1 CT for a balanced load. For a reason I didn't totally understand, this was stated as expected behavior for a doubled channel (1CT for 240 load) because of some ambiguity without the second CT. Would using a PT-480 to average the two legs create any similar ambiguity that would affect power factor measurements for 120/240v circuits? I'm guessing not but want to confirm.

Lastly, are there any hardware/software provisions on the GEM that would allow each leg to be monitored individually with respect to neutral so that power could be calculated for each circuit based on the leg it is on? If so, could two PT-12Vs be used (one for each leg) or is there something you would recommend?

Thanks!

[Mustang65]

Here is a link to the same question I had regarding monitoring each phase.
Hope it helps

http://www.brultech.com/community/viewt ... f=3&t=1806

[sarain]

Thanks Don for that link. I hadn't come across your thread yet.

From Teken's response there, it sounds like the only way to monitor both legs individually is with 2 GEMs, with the PT of each on a separate leg. For the purpose of simply monitoring the voltage on each leg that seems adequate.

I was hoping there might be a supported way to monitor both legs individually from one GEM and use them to calculate loads on each leg with some way to assign a channel to one leg or the other. I guess that wouldn't be as meaningful for shared channels that have loads on different legs though.

The solution posted in the thread I linked to originally, mentioned using a 240 rated PT to measure voltage across both legs together and then just calibrating it to read half of the full measured 240 since the GEM expects the PT to only measure a single leg (120). That basically gave him an average of the two legs which the GEM used to calculate power. If the GEM can't monitor the voltage on both legs individually, the average of both legs would probably be my preferred option vs just monitoring one leg with each GEM, unless there are other caveats or downsides to that approach I'm not aware of.

[ben]

The only way to get the voltage for each phase would be to run a GEM on each phase. The PT can be used to get the average voltage between the 2-phases, but I don't think it'll give you much more accuracy.

[Teken]

I ran both 120 & 240 PT's and honestly didn't see any real difference and trust me I was looking. Purchasing the 240 PT for 1% accuracy was the main driver but in the end I found that the 120 PT was almost dead on with the 240 PT in terms of voltage capture and display accuracy.

This is why I wired each PT to capture both sides of the split single phase lines.

This in the end was a exercise in *Can it be done and what are the results* but the final outcome was the GEM is so accurate using the standard issue 120 PT that the only thing that gets better is that make belief in your head you tried everything to ring out that Nth degree of accuracy!

Ultimately, you will see more value in the returned metrics by investing the time in calibrating the PT referencing to a True RMS DMM. Coupled with taking the time to adjust each CT to a known and validated restive load for small, medium, and high.

Brultech only spec's the GEM down to 1 watt and by anyones standards that is truly great. But I am able to to see, measure, and display 0.1 ~ 0.9 watts in the Dash Box everyday with out issue.

YMMV . . .

[wci68]

I went with two GEMs, one for each leg of the split-phase service: http://www.brultech.com/community/viewt ... f=2&t=1509

In my case I knew ahead of time that the two legs did vary, sometimes by a bit. After my GEMs had been running for some time I had found that the voltage difference had actually reached almost 20V during periods of high imbalance in loads on the two legs but was less than 3V during periods of nearly-perfectly balanced loads.

Perhaps my case is unusual since the legs should not vary that widely around the neutral (ground reference). Most should find their legs fairly stable and balanced, and using the voltage of a single leg in power and energy calculations for both legs should be sufficient for good accuracy.

[sarain]

Thanks for clarifying my options for monitoring both legs. I ordered two GEMs and will put each of their PTs on a different leg.

One GEM will be about 50ft downstream on a sub-panel so I'll probably pick up some (hopefully minor) losses from that run on the leg I connect it to. I bought enough Micro-100 CT's to monitor both ends of the sub-panel branch circuit so hopefully I can identify any significant losses based on the difference between those and some manual measurements with a meter.

wci68: Thanks for sharing your experience with this. A 20V difference sounds pretty extreme. Any chance you have a loose connection somewhere on that low leg?