CT extension: distance to parallel AC conductors

[peppersass]

Ben

In this post and a number of others, you made these recommendations:

CTs can be extended, we've had customers extend up to 100FT. CAT5/6 is recommended for extensions as its twisted pair and shielded (2 wires per CT). Try not to run parallel with power lines as they'll introduce noise.

I need to extend several CTs from a new subpanel to my GEM, which is located at the main panel about 20 feet away. We have a number of AC circuit runs along that path, so it won't be possible to completely avoid running the twisted pair cable parallel to one or more AC power conductors. Presumably, the amount of noise pickup depends on the distance between the CAT 5/6 cable and an AC cable, so is there a distance beyond which noise won't be an issue? For example, will it work if I maintain at least 12" between the cables?

Also, should the Cat 5/6 cable be grounded at both ends (say, to the GEM ground at one end and the subpanel ground at the other end), or just at one end (and if so, which end?)

[ben]

Ben

In this post and a number of others, you made these recommendations:

CTs can be extended, we've had customers extend up to 100FT. CAT5/6 is recommended for extensions as its twisted pair and shielded (2 wires per CT). Try not to run parallel with power lines as they'll introduce noise.

I need to extend several CTs from a new subpanel to my GEM, which is located at the main panel about 20 feet away. We have a number of AC circuit runs along that path, so it won't be possible to completely avoid running the twisted pair cable parallel to one or more AC power conductors. Presumably, the amount of noise pickup depends on the distance between the CAT 5/6 cable and an AC cable, so is there a distance beyond which noise won't be an issue? For example, will it work if I maintain at least 12" between the cables?

Also, should the Cat 5/6 cable be grounded at both ends (say, to the GEM ground at one end and the subpanel ground at the other end), or just at one end (and if so, which end?)

Best bet if you can is to just try it out and see if there's any noise. The shielding and twisting may eliminate it.

[peppersass]

Best bet if you can is to just try it out and see if there's any noise. The shielding and twisting may eliminate it.

I have the extension working, with some interesting results along the way:

When I first hooked up the extension, I got a constant reading by the GEM of 19W, regardless of the power drawn by the circuits in the subpanel, and even when I switched off the breaker to the subpanel! And when I connected the cable shield drain lead to GEM, the reading dropped to zero.

I was baffled for a few minutes until I realized my mistake: I had run two lengths of CAT6 and failed to mark which was which, and connected the GEM to the wrong one -- i.e., not the CAT6 cable that was connected to the subpanel. So the unterminated CAT6 cable I did connect to the GEM acted as a giant antenna, maybe picking up EMF from other AC lines nearby.

The fact that the reading went to zero when I connected one end of the shield to the GEM suggests that this is sufficient for noise suppression. I think connecting the shield at both ends, with one side connected to the GEM ground and the other side connected to the AC ground, might be asking for ground loop trouble.

After connecting the subpanel to the correct cable, I got the power reading for the equipment connected to the subpanel. Connecting or disconnecting the shield drain didn't affect the reading, probably because the circuit was completed by the CTs in the subpanel and no longer acted as an antenna. Nonetheless, when I finish installing the rest of the CTs I'll probably connect the shield drains to the GEM ground.

[peppersass]

Following up, I finally finished a long-standing project to trim all CT leads to the shortest length and dress them neatly (will post photos later). Also completed the CT extensions from the new subpanel to the GEM via twisted pair cables.

By switching off the breakers in the subpanel, I found that there was noise on the two CTs monitoring the well pump and heat pump hot water heater (HPHWH) -- readings of up to 20W when they should have been zero. Grounding one end of the CAT cables to the GEM ground reduced the noise to 5W or less. Grounding the CAT cables at the subpanel reduced the noise to 0W.

Conclusion: in this particular case, grounding the CAT shield drain wire at both ends is best.

The well pump, which definitely should be 0W when off now shows that. The brand new HPHWH also shows 0W in standby, which I believe means the electronics consume less than .5W when the WiFi connection isn't active. It'll show hundreds of watts to several KW when heating, depending on the mode, then drop down to 6W or so for a few minutes after heating stops before entering standby. Looks good.

[N.B. 1: After trimming the CT leads, I checked every circuit I could by turning off the breaker. No noise on any, so no noise jumpers needed. I had needed a few before trimming the leads.]

[N.B. 2: Not strictly relevant to this post, the HPHWH has been reporting at least 1KWh more consumption per day than the GEM. There are times when the HPHWH says it was drawing about 900W or so, while the GEM says it was about 450W or so. I'm sure the CT wiring is correct. Although the 240VAC line to the HPHWH is balanced, I'm using two CTs and not setting the channel to double the reading (doing the same for the balanced 240VAC line to the well pump.) I measured each leg of the balanced 240VAC leads at the breaker to the HPHWH with a snap-on current meter, and it showed about 1.8A on each leg, while the GEM showed about 430W -- which is correct. On most days, the HPHWH will record ~200W consumption for each of several hours throughout the day, while the GEM records none. I don't see these consumption periods in Total Consumption, either, which is monitored by two Split 200s on the Mains. I don't think they exist. I'm not seeing them in the Tesla Gateway records either. Very weird, but that's not the only strange thing about the HPHWH...]

[ben]

[N.B. 2: Not strictly relevant to this post, the HPHWH has been reporting at least 1KWh more consumption per day than the GEM. There are times when the HPHWH says it was drawing about 900W or so, while the GEM says it was about 450W or so. I'm sure the CT wiring is correct. Although the 240VAC line to the HPHWH is balanced, I'm using two CTs and not setting the channel to double the reading (doing the same for the balanced 240VAC line to the well pump.) I measured each leg of the balanced 240VAC leads at the breaker to the HPHWH with a snap-on current meter, and it showed about 1.8A on each leg, while the GEM showed about 430W -- which is correct. On most days, the HPHWH will record ~200W consumption for each of several hours throughout the day, while the GEM records none. I don't see these consumption periods in Total Consumption, either, which is monitored by two Split 200s on the Mains. I don't think they exist. I'm not seeing them in the Tesla Gateway records either. Very weird, but that's not the only strange thing about the HPHWH...]

Where are you getting the 900W from? If it's another monitor make sure it takes into account for power factor. Given it's a heat pump it's possible the power factor is very poor.

What you can do is compare VA on the GEM to Watts to see if there's a significant difference between the two.

[peppersass]

Where are you getting the 900W from? If it's another monitor make sure it takes into account for power factor. Given it's a heat pump it's possible the power factor is very poor.

I meant to say 900kWh and 450kWh, respectively. The 900kWh figure comes from the heat pump hot water heater's hourly consumption graph, while the 450kWh comes from the Dashbox hourly consumption graph. On any given day, the heat pump hot water heater (HPHWH) will say it has consumed roughly double what the Dashbox says it consumed.

So where do I find VA in the GEM? Do I just multiply the V and A readings shown on the Live Data page? When I do that, the V*A product is about 97% of the figure GEM shows for Watts. Does that mean Watts is taking PF into account?

And if so, how does GEM do that? I read your piece on Watts versus VA, and I fully understand the difference, but what I don't know is the difference between how Watts and VA are measured and calculated.

That said, I don't think the problem is the HPHWH's power factor. I'd be really surprised if it measures VA instead of Watts if it's possible the PF is 50% -- that would make no sense. Why tell the customer that consumption is double what it's supposed to be? That's poor marketing! What I'm seeing, at least on some days, is that the HPHWH consumption chart shows 4 or 5 hours where consumption is about 200W, while the GEM shows zero consumption for those periods. I'm inclined to believe the GEM because I don't hear the HPHWH coming on every hour. It's conceivable that it's briefly turning on the resistive element, but I don't see why that wouldn't be picked up by the GEM unless it's on for an *very* brief period and GEM is missing it. Could a resistive element draw 200kWh so fast that GEM would miss it? I don't think so. Say the resistive element draws 5000W immediately upon being activated. It would have to be on for 2.4 minutes to generate 200Wh, which the GEM would certainly see. Could the instantaneous power draw be many times that and the period so short the GEM would miss it?

Anyway, if you add them up the 200kWh periods, they're roughly the difference between the daily totals reported by the heat pump hot water heater and the GEM. There are other periods where the HPHWH was clearly running, and the consumption it reports is considerably higher than what GEM reports -- often double. But not always. Also, the time periods don't line up perfectly. The HPHWH graph resolution isn't as good as the GEM's. Below are the yesterday's consumption graphs from the HPHWH and GEM:

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[ben]

Where are you getting the 900W from? If it's another monitor make sure it takes into account for power factor. Given it's a heat pump it's possible the power factor is very poor.

I meant to say 900kWh and 450kWh, respectively. The 900kWh figure comes from the heat pump hot water heater's hourly consumption graph, while the 450kWh comes from the Dashbox hourly consumption graph. On any given day, the heat pump hot water heater (HPHWH) will say it has consumed roughly double what the Dashbox says it consumed.

So where do I find VA in the GEM? Do I just multiply the V and A readings shown on the Live Data page? When I do that, the V*A product is about 97% of the figure GEM shows for Watts. Does that mean Watts is taking PF into account?

And if so, how does GEM do that? I read your piece on Watts versus VA, and I fully understand the difference, but what I don't know is the difference between how Watts and VA are measured and calculated.

That said, I don't think the problem is the HPHWH's power factor. I'd be really surprised if it measures VA instead of Watts if it's possible the PF is 50% -- that would make no sense. Why tell the customer that consumption is double what it's supposed to be? That's poor marketing! What I'm seeing, at least on some days, is that the HPHWH consumption chart shows 4 or 5 hours where consumption is about 200W, while the GEM shows zero consumption for those periods. I'm inclined to believe the GEM because I don't hear the HPHWH coming on every hour. It's conceivable that it's briefly turning on the resistive element, but I don't see why that wouldn't be picked up by the GEM unless it's on for an *very* brief period and GEM is missing it. Could a resistive element draw 200kWh so fast that GEM would miss it? I don't think so. Say the resistive element draws 5000W immediately upon being activated. It would have to be on for 2.4 minutes to generate 200Wh, which the GEM would certainly see. Could the instantaneous power draw be many times that and the period so short the GEM would miss it?

Anyway, if you add them up the 200kWh periods, they're roughly the difference between the daily totals reported by the heat pump hot water heater and the GEM. There are other periods where the HPHWH was clearly running, and the consumption it reports is considerably higher than what GEM reports -- often double. But not always. Also, the time periods don't line up perfectly. The HPHWH graph resolution isn't as good as the GEM's. Below are the yesterday's consumption graphs from the HPHWH and GEM:

Yeah, I would assume whatever the hot water heater reads should be accurate if it's a built-in display. Can you post the model? Maybe they have a minimum accuracy value.

The GEM should pick up those spikes, you can try looking at Peak Watts to see. Peak Watts is the highest value between packets (based on your send interval) logged on the minute.

The combination of 2 hour bars on the DashBox seem to reflect the one spiked bar on the water heater display pretty closely.

Are you 100% sure on the wiring? It's possible the CT polarities are cancelling from each other. You can remove one set of CT leads and see if the value jumps up.

Under the Advanced section if you choose Line Graph you should be able to do Watts and VA on the same graph.

[peppersass]

Yeah, I would assume whatever the hot water heater reads should be accurate if it's a built-in display. Can you post the model? Maybe they have a minimum accuracy value.

It's a Rheem Professional Prestige ProTerra model # PROPH80 T2 RH375-30. They don't provide a spec for power measurement accuracy.

The GEM should pick up those spikes, you can try looking at Peak Watts to see. Peak Watts is the highest value between packets (based on your send interval) logged on the minute.

No spikes in the Peak Watts display. In fact, we were on vacation from 7/30-8/5, and I set the HPHWH to Vacation Mode, which I believe holds the water at 65F. The GEM reported no power whatsoever going to the HPHWH during that period, while the HPHWH showed regular spikes of about .2kW.

The combination of 2 hour bars on the DashBox seem to reflect the one spiked bar on the water heater display pretty closely.

Yes, I had noticed that. That's what makes me think the .2kW spikes are where the difference in daily consumption comes from. It's not clear whether each bar represents an hour or what hour it's supposed to be.

Are you 100% sure on the wiring? It's possible the CT polarities are cancelling from each other. You can remove one set of CT leads and see if the value jumps up.

The short answer is: removing one set of wires halves the power to about 225W when the HPHWH is running at max.

(When I say max, I mean compressor and fan on. The real max occurs when the compressor and fan are running and the resistive element is on. That's more like 4kW.)

I don't see how incorrect wiring could result in the GEM showing 0W for the .2kW spikes shown by the HPHWH, yet show about the same consumption when the HPHWH is running at max power (albeit with the hours not quite lining up.)

But I'm never 100% sure about anything, so I'll review what I did. Initially, I had a pair of Split 60s on L1 and L2 in the subpanel until the micro 40s I ordered arrived. That included the HPHWH and the well pump, but the well pump doesn't go on very often so most of the time the readings were just from the HPHWH when it was running. I always orient all CTs in the same direction: K facing source and L facing load for Type A split CTs and the wire side facing the load for Type B micro 40s. When using the Split 60s, I put one pair of wires on ports 1 and 2, and the other pair on ports 3 and 4. I switched the leads of one pair back and forth to determine which wiring gave the max power. Typically, it's about 450W (450 kWH per hour) when the HPHWH is running, even though the HPHWH might show a peak of 900 kWh at about the same time period. I couldn't find any combination of wiring that resulted in a higher reading. When I got the micro 40s, I put one one each lead to/from the well pump and hot water heater. I combined the black wire from one CT and the white wire from the other CT in port 2 and combined the remaining black and white wires in port 3. (And yes, I changed the CT type in the GEM as appropriate for the CT type being used.)

To make sure that there aren't significant losses in the ~30-foot twisted pair cable between the main and subpanel, I used a clamp ammeter on each lead between the subpanel breaker and the HPHWH while it was running, and got a reading of about 1.8A on each leg (it's an analog clamp meter on the 6A scale, so I had to interpolate between marks.) If I multiply 1.8A times 240V, I get 432W, which is about what the GEM says.

I should note that both the well pump and HPHWH are balanced loads, so I could have used just one CT and doubled the reading. I used to do it that way on the well pump when it was in the main panel because I ran out of micro40s, but now I have a bunch of micro 40s and decided to use two on each. In case you're wondering, I checked the wiring diagram for the HPHWH and it only has connections for L1, L2 and GND. No Neutral.

Under the Advanced section if you choose Line Graph you should be able to do Watts and VA on the same graph.

Ah. That's cool. Still no indication of the spikes. In the part of the graph where the HPHWH is shown drawing power, Watts looks to be about 95%+ if the VA value. That corresponds to the calculated Power Factor and the product of amps and volts in the GEM Live Data screen, which came out to about 97% of the watts value. So, I don't think power factor is the issue here.

I think the fact that the spiked bars in the Dashbox sum to about the value of the spiked bar in the HPHWH display, and the difference between the daily consumption readings is pretty close to the sum of the .2kW mystery spikes, suggests that the HPHWH thinks those spikes are real and the GEM doesn't. Which one is right?

I've tried to figure out whether the HPHWH is really consuming as much as it says by comparing my All Circuits group with Total Consumption and the figure for House consumption shown by the Tesla Gateway. However, before we got the HPHWH, and as I reported in another thread, there has been a significant discrepancy between these three values. Total Consumption generally ran about 1 kWh or so less than House consumption shown by the Tesla Gateway, and All Circuits generally ran about 1kWh less than Total Consumption. I still haven't gotten to the bottom of those discrepancies. So far, I've shortened all CT leads to the minimum required, eliminated noise on the subpanel CT leads, cleaned the PT plug and verified that the PT is putting out the correct voltage within .1-.2V of what my true RMS DMM says. Next step is to put some known loads on various circuits. I'm still unclear on whether CTs can be "tweaked". Changing Range doubles or halves, so that's way too much for tweaking. Does changing the CT type by small increments do that?

[EDIT: Possible explanation for the discrepancy: See my last post in this thread.]

[ben]

That's an interesting water heater.

I think there should be a constant load on it due to the WiFi/screen/etc. From one discussion I saw it could be as high as 40W.

Are you running your extensions parallel with any power lines? If so, this can cause signal interference, they should really be isolated from other sources.

Since it's balanced, you could try enabling the X2 setting and trying each separate CT in case one has a better signal then the other.

[peppersass]

That's an interesting water heater.

It's the fifth generation of this particular product. Unfortunately, Rheem made it worse than gen 4. Max noise is supposed to be 49dB. Ours is 70dB with a really annoying whine we can hear all over the house. Our installer didn't use PEX connection, which Rheem recommends to reduce vibration, but I suspect that's only making bad noise worse. Also, they dumbed down the UI. Used to be able to see all sorts of internal parameters related to water temp, but that's gone now. So it wouldn't surprise me if the .200kWh spikes in the HPHWH graph are bogus. Our installer promised to make good -- either fix it, replace it, get a gen 4 or get another brand. It'll be a long saga, I'm sure.

I think there should be a constant load on it due to the WiFi/screen/etc. From one discussion I saw it could be as high as 40W.

You're right. I hooked up my Fluke 189 in Amps mode to one of leads going to the HPHWH. As the HPHWH electronics power up, the draw is around 65mA (.065A), which translates to about 16W. At times it spikes up to 150-200ma for a few seconds, which would be in the 40W range. But within a minute it settles down and the standby current is about 23mA. That comes out to about 5W, which seems reasonable for an LCD display and WiFi module. But the GEM is reporting 0W. Hmmm...

When the HPHWH is running, the DMM shows 1.93A, similar to what I saw on the clamp-on meter. That comes out to about 463W. But the GEM shows about 435W. Hmmm....

So the GEM is reading low in standby and when the HPHWH is running. But not 1-1.5 kWh per day low.

Are you running your extensions parallel with any power lines? If so, this can cause signal interference, they should really be isolated from other sources.

In our basement it would be impossible to run the extensions without them being parallel to AC lines. All I can do is try to keep them as far away from parallel AC conductors as possible. The heaviest parallel conductor is the solar feed, which is maybe four feet away.

There's definitely some interference getting into the extensions because the GEM reads as much as 45W or 50W if I don't ground the shield drains of the extensions at both ends. But then the GEM reads 0W in standby, when it really should be reading about 5W. I thought maybe the shield drains are shorted to one more more of the twisted pair wires, but I checked and they're not.

Since it's balanced, you could try enabling the X2 setting and trying each separate CT in case one has a better signal then the other.

No significant difference between the two CTs.

I'm wondering if the problem is resistive losses in the extensions. I'm using somewhat old terminal blocks to make the connections at both ends. They're the type where you insert the wire and tighten a screw against a metal leaf that secures the wire. Could be some corrosion inside. Next I'll try disconnecting the leads from the terminal blocks and connecting them with snap-on splicing connectors. I can also try measuring current from the CTs at both ends of the extensions to see if there are significant losses.

Even if it's resistive losses, the missing standby current should only be about .120kWh per day and the error when running is only about 7%. That's nowhere near the 1-1.5 kWH difference between the GEM and the HPHWH daily consumption figures.