3-phase 3-wire delta subpanel monitoring setup in USA

I believe i have a 3 phase 3 wire sub-panel that is fed from a 3phase 4 wire main panel. I’m trying to measure the total power of the sub-panel and a single 3-phase 3-wire load on the main panel. I’m getting confused with the VT and CT setup for the sub-panel and calculating total power of the sub-panel and an additional 3-wire load from the main panel.

Background

The building has a new installation of central heat pump water heaters. 12 outdoor heat pump units (Sanco model GS5-45HPC-0), significant storage, and an electric resistance ‘swing tank’ (Bock model CE119-1B with 3x 4,500 W elements) to handle the recirculation load during periods of low hot water use.

Our goal is to help guide this new installation to operate efficiently, and effectively tell the story of how we got it there so that other new installations can build upon the keys to success that we develop. We are using ‘total system efficiency’ to evaluate the system and the changes implemented. This total system efficiency is being defined as the ‘ideal heating requirement of the hot water’ divided by the electrical energy input to the system.

  • The ideal heating requirement of the hot water is being monitored with flowmeters and temperature sensors. the total volume of hot water used by the building is logged, along with the entering cold water temperature and outgoing domestic hot water temperature.
  • Electrical energy input to the heat pumps and swing tank is being monitored with an iotawatt

Electrical Setup & Monitoring

Using an iotawatt (i dont have a picture of model number at the moment, but it has the 3-phase AC Ref ports on the bottom) one default reference voltage transformer, and 6 CTs.

  • 3 CTs are monitoring the incoming phases to the heatpump subpanel
  • 3 CTs are monitoring the triple breaker that serves the swing tank. this is in an older existing panel next to the new subpanel (The outlet for the reference voltage transformer comes off this older main panel)

My current understanding of the electrical system:

The subpanel just has the three phase wires entering (black, red & white), so my understanding is that this is a delta configuration

  • testing these shows ~208-211 volts between the phases.
  • each heatpump has its own two pole breaker, with 15 amps marked on each pole.

The swing tank (in the old panel) has a three pole breaker (50A)

  • Testing these three wires confirms to me that the black, red and white wires match the black red and white wires entering the heatpump subpanel

The outlet used for the iotawatt reference voltage transformer shows me its powered off the white phase. this outlet is off the main panel.

  • tested with voltmeter from the outlet to the three phases of both the old and the new panel. less than 1V difference from outlet to white phase, and ~208 to red and black phases

Iotawatt setup

On the original install day i was in a rush and it was my first time. i figured i’d get accurate amp readings and voltage even without setting up the three phase settings in the iotawatt at all, so thats how i’d left it and been thinking about it over the last month

This week i returned to test phases, determine my phase A, see if i could figure out B & C, and configure the iotawatt to match

Determined the VT outlet is on the white wire, so this will be phase A for this iotawatt.

  • When all phases were associated with the reference voltage (at 120v) (before checking off the ‘enable derived three phase’ box on the iotawatt), this was the only phase that had been reading a positive wattage, and had been showing a power factor very close to 1. the other phases had been showing negative wattages and power factors of .43 & .52

Adjusted the phases associated with the black and red wires based on which phase setting gave the highest power factor (understanding that this is not what the instructions say to do), settling on:

  • Black Wire as phase C (+240°), which is wired into the left bus in the HP panel
  • Red Wire as phase B (+120°), which is wired into the center bus in the HP panel
  • White wire as Phase A as described previously. this is wired into the right bus in the HP panel

Confirmed with voltmeter that the black, red & white of the swing tank corresponded to the black, red & white of the HP panel (it did)

  • adjusted the phase of those inputs

Adjusted the calibration factor for the voltage transformer from 11.43 to 19.72 to give me a reference voltage of 209.2v compared to my line-to-line measurements at the time of 209.6v, 208.8v, and 210.9v

  • I don’t see instructions to do this anywhere, so this feels like it was likely a wrong move

Troubleshooting with forums

1: Phase mix-up (please call me out if it sounds like i’m wrong, understanding that i chose phase B & C based on power factor readings from the iotawatt)

  • when i attributed the phases to the electric resistance swing tank, the resultant wattages didnt make sense to me, so this raises questions for me about my whole process: wire 1 has 0.24 amps set as phase B, shows 16 watts; wire 2 has 0.13 amps, set as phase C, shows -2 watts; wire 3 has 0.05 amps, set as phase A, shows 0 watts

2: Reference voltage, i calibrated it to 209v thinking that this seemed appropriate , but not sure if i should’ve (didn’t feel like this was addressed in instructions i saw)

  • my current thinking is that i should change this back to its original calibration to 120v line to neutral

3: Iotawatt amp readings are not matching the central Heatpump controller (iotawatt is about double), is there potential for mixup in the amp readings?

  • I think that the amp readings of the CTs are the raw data and not impacted by any of the voltage or phase adjustments that i’ve made. reviewing the logs of amp readings seems to support this as typical values have remained unchanged before/after these adjustments
  • my newest understanding here is that i should be averaging the amp reading of the three (or two?) wires, not adding them

4: I’d felt confused on wattage for each phase, and combined wattage. and my current understanding is that i just shouldnt worry about phase wattage, and should just do total wattage with the 2 wattmeter technique.

  • phase wattage = (line to neutral voltage) * (amps of this phase) * (powerfactor for this phase)
  • Total wattage = sqrt(3) * (line to line voltage) * (average of phase A, B & C currents) * power factor (i’ll say the average)

Sub-panel:

Older Panel:

Iotawatt showing phases, amps, and wattages. voltage calibated up to ~209

Thank you so much for your help!

Quite a project. The good news is that I think this is a lot easier than you are making it.

First thing is to reset the VT to factory calibration. That is a the nominal 120V. Don’t try to compensate for what you perceive to be different phase voltages by adjusting it. The IoTaWatt is usually more accurate than most field voltmeters, and what you see as voltage variance now will change. It won’t make a lot of difference in the final measurements.

Once you reset the VT, the total of the three HP inputs should be the total load of that panel. Create an output:

TOTAL_HP = HP1 + HP2 + HP3

The problem with the Bock appears to be that the phases are not correct. The order of the phases for the HP feed, from top (black) to bottom (white), is CBA. Given that, the order of the phases for the Boch from top to bottom should be ACB. I can’t see any CTs in the pics, and all of the wires are black, so have no idea. You will need to figure it out.

All this depends on your phases having been determined properly.

As I read it you have determined:
HP1 is Phase B - Red Cable
HP2 is phase C - Black cable
HP3 is phase A - White Cable

Postscript:

You can easily monitor the HP panel using two CTs. Just select any two and set the reference to each and the third: EX

HP1 phase A-B
HP2 phase C-A

Add the two together with an output:

TOTAL_HP = HP1 + HP2

You can measure any individual 208V load in the HP panel using just one CT and referencing the corresponding phase-phase voltage. As you go down either side of the panel, the phases will repeat sequence If the first double breaker is say phase C and B, you will reference phase B-C. The next will reference C-A , the next B-C etc. In fact, it’s no different than if those breakers were in the main panel because they have no neutral.

Could it be 1.732x - the ratio of 208/120?

@NeilDonnelly, curious how this worked out.

I appreciate the help and the interest very much. I think we’re close. The heat pump Subpanel now has 5 CTs on the 3 wires to confirm the panel usage with two of the methods you mentioned.

Total_HPv1 = HP1 + HP2 + HP3

with HP1 on red wire and phase B

HP2 on black wire and phase C

HP3 on white wire and phase A

And reference voltage back at 120 on the white wire.

Second method is the ‘two wattmeter method’ Total_HPv2 = HP1v2 + HP3v2

With HP1v2 on red wire and phase B-C

And HP3v2 on white wire and phase C-A

If there was a phase A-C option for HP3v2 I would’ve used that, as currently it reads a negative wattage. Due to this negative, my output equation is: Total_HPv2 = HP1v2 abs + HP3v2 abs

These two methods give totals within 1% of each other.

I’m still questioning the Bock tank wattage. currently Total_Bock_Watts = Bock 1 + Bock 2 + Bock 3 with the same ref voltage and phases by wire color. CTs visible in the bottom right of the original photo. My current confusion on this one is that the wattage when its on is ~6595W, whereas the nameplate says 3 elements, 4500W each, 13,500W total, so we’re at about half of the rated total. This got me digging more last week into this tank, and it seems like its set up to be connected to 240v three phase, but is then wired from the panel that i believe was measuring at the same 208v line to line as the HP panel. My plan is to go back Tuesday to test the voltages at the tank and call the manufacturer if its looking like its set up for 240v but getting 208v and see what they say.


showing “H1 to H3” wiring so 240V to me… but breaker panel says 3phase 208Y/120

and just a pic of the current iotawatt setup with both the heat pumps and bock tank running:

It’s all relative. Just reverse the CT, either physically or with the reverse checkbox in setup.

As expected.

About the Bock. I have limited information here, but two things:

First, It looks syspiciously like the Voltage reference on Bock1 and Bock2 are incorrect. I would try swapping them. Bock3 is twice the value of Bock1 or Bock2 and they are supposed to have the same power. Out of phase will typically show half power. Also, power factor with the three CT Wye method should be 100 on all three, another indication of wrong phase reference.

The unit has elements that are 4,500Watts at 240V. That would be about 3,900 at 208V.

The transformer that you suggest may be the culprit I believe is a red-herring. From what I can see, I believe that is a relatively low power transformer to deliver single phase, probably lower voltage, to the control circuits that drive what appear to be two relays to switch on the heating elements. The transformer does seem to be wired for 240 instead of 208, but appears to be working OK. Your electrician should question whether the black wire on H3 should be connected to H2 instead.

I agree I thought the bock would have equal power draw, and I’ll try switching phases tomorrow, but it’s interesting to see that the amp are different on the different wires. I’ll report back what I see tomorrow.

That gave me pause as well. Another thing to check is that the CTs are all the same and configured as the correct model. Configuring a 100A CT as a 50A would cause this.

Another thing you might want to check is that all of the heating elements are being energized. The unit appears to be split into two banks, each with their own set of fuses. When I look at the set on the right, the leftmost wire feeding the fuseblock looks funky like it may have got pretty hot at one time. A couple of fuses might be blown or the relay may have burned contacts. I’d check for voltage where they exit the relays.

Tested voltages and that wire was loose & burned out. Electrician came and swapped out wires and the amps, wattages are now about equal and power factor is 1 for Bock wires. I feel like things are successfully installed and set up now, and i very much appreciate your help to get it there!

showing the amps of the two wires jump up to match the 3rd once the wire had been fixed.

total wattage is now showing at just over 10,000 watts vs the 14,500watts that the nameplate says for 240v, so seems to validate your estimate of 3900w per element for 208v

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BTW/ You really should be using graph+ rather than that legacy graph package. Try it out.