CT Basics for New Installation

Hi all, I’ve been considering adding an Iotawatt to my panel to help me track down some gremlins. Ultimately I will likely use it to monitor major hardware (HWH, A/C, appliances), but initially I will moving things around a bit. At any rate, I’ve been reading through the forum and I’ve figured quite a bit out, but I was hoping for some help understanding the CT’s. So a few specific questions if anyone can help:

  1. Service into the house is 200 amps (standard 120/240V US). Are the 200 amp CT’s sufficient for this (albeit barely) or do CT’s require some additional headroom above the circuit amperage? Same question regarding 100 amp circuits leading from the main panel to a sub-panel.

  2. I have two electric hot water heater working in tandem each on it’s own 2-pole 30 amp circuit. I’d like to be able to monitor these together. I have seen installations in the forum that route the two conductors for a two-pole circuit through a single CT in opposite directions. I have also seen where two single-pole circuits have been routed through a single CT with a high enough capacity to monitor them together. My question is if these two techniques can be combined to have all 4 conductors for the hot water heaters using a single CT? My guess is no, which leads me to ask if there is any other way to consolidate these two circuits?

  3. Are there safe ways to extend the cable on the CT’s sold in the Iotawatt store? I have a large panel inside a crowded cabinet that will likely require some routing longer than 5 feet.

  4. More reading led me to understand that the CT’s can develop very high voltages without a load or being shorted. I live in an area with frequent black/brown outs. Does the Iotawatt cycle back on after a power failure automatically? If not, am I correct in thinking that this could lead to a potentially dangerous situation if the supply cycles due to a blackout and Iotawatt stays off?

  1. The 200A CT is fine. The maximum full-time load is supposed to be no more than 80% typically. Most panels don’t typically come anywhere near full load. My all time peak is no more than 80A on a 200A panel. Think about why you want to monitor the sub-panel. I do that for the barnyard, since it is far away, but for a local sub-panel I would probably want to monitor some of the circuits in it individually. Make a list of all the circuits and prioritize what you want to see always and then what might be interesting for some time. Base this on what action you would take once you have some data, unless you are just curious. If you are just curious, looking at the mains and working to figure it out will keep you busy, no need for extra circuits. The extra circuits are for when you want easier math and automatic decision making.

  2. You only need one conductor of each water heater, since they are 240V devices. Most water heaters have 4KW to 5KW elements, so typically draw less than 25A, more like 20A. I keep mine on separate inputs so I can see if one is working harder than the other.

  3. People use standard headphone extension cables. Since they are current transformers, it isn’t too critical.

  4. The CTs in the shop come with TVS diodes to limit the voltage when they are not plugged in. When plugged into an Iotawatt, it doesn’t matter if it is on or off as the resistor is always to there. The Iotawatt does also restart automatically. I keep mine on a mini-UPS just because I like to see how many brown-outs we have. Also, these CTs are so small they are not a huge problem. Those warnings come from the CTs that are 5A (or even 1A) output ones. Those ones can be really dangerous if they are not handled well

Be aware that changing things around means that you will lose historical data or it will be much harder to understand. Depending on your willingness to delete what is mostly ephemeral data this might not matter to you. If on the other hand you are a data hoarder, it will be hard to make changes, unless you have a plan for how you are going to save all the data.

I have 2 Iotawatts, so 14 channels. It still isn’t enough to measure everything individually. I go back and forth on it. I was looking closely today and asking what decisions could I make with more channels. I already don’t look at most of the data most of the time, but some of the data is interesting some of the time. What I find is that short term monitoring is interesting if you have never seen the data before and you are willing to remove some base load and/or do something about the big power users. I have known that my bathroom heated floor uses a lot for over a decade, but other than a night setback I haven’t done anything yet. It is still on the list.

Similarly, my furnace blower uses a lot. I could replace the motor, but in a most heating climate and using propane for heat the waste heat from the motor is not really wasted.

On the other hand, the information on my water heater convinced me to get a heat pump one 5+ years ago. Continuing to keep an eye on it enabled me to figure out the heat pump was failing on it. Look at the numbers also convinced me to change the control system for my recirculating pump. The pump itself only uses 25W, but it pumps more water (for recirc) than I use for everything else. That means it was probably using at least 1/3 of the total energy for heating water.

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Thanks for the detailed post. How do you handle the data from 2 Iotawatts? Do you upload everything to a local instance of emoncms so that it can be combined or do you just look at each Iotawatt separately? Pvoutput only allows a few extended data points so that can’t really be used for the data from multiple Iotawatts. Or maybe you have a separate way (ie grafana) of looking at the combined data?

I do all of the above. I have used the cloud version of emoncms for a long time. It was great a decade ago, but doesn’t seem to have changed much. I am a “trained professional” and tried running emoncms at home. It was too much like work for me to stick with it.

I have been running the combination of InfluxDB and Grafana for years now. They are both evolving at a decent pace, but they are both commercial products so not really a fair comparison. The visualizations available in Grafana make it easy to generate beautiful dashboards and graphs. There is a significant learning curve, but at least both offer supported/official Docker images so installation is easy (if you know how to use Docker).

I run all of my home automation stuff on a small pre-owned mini PC (one of the 1 liter class). I find the price/power consumption/performance ratio to be far superior to the Raspberry Pi that many people use. I do have a RPi 2 that I use just for MQTT. It is incredibly slow to do anything on that machine, but it is more than powerful enough for MQTT. I have a RPi 3, but no 4. They were pretty much unobtainable for the last few years.

I currently have 3 different instances of HA running on 3 different computers, laptop, tablet and now tiny one. It was easier for me to just set up a new one than figure out how to upgrade the old instance. The new install with Docker should be easier to update regularly. Note that all the old ones are still working fine for what they can do. The newer ones are using newer versions of both InfluxDB and Grafana and they have improved significantly in the last 5 years.

But, I still look at and have some graphs on each Iotawatt for particular things. I collect a lot of data, but much of it really isn’t worth the time to look at unless something interesting happens. When that occurs, I can get a better picture of just how interested the event was.