Custom 200A CT's


I’m looking to buy a pair of IotaWatts and some CT’s for my panel. I’ve got 7 240V circuits, and 15 120V circuits.

My big problem is getting something on the mains. It’s really tight - mainly because there is a pair of doughnut CT’s already there, from a system installed back in the 90’s. I’ve got no idea of the specs on them, but they’re obviously current-mode (burden resistors are on the 90’s electronics) and likely totally unprotected. Removing them requires disconnecting wires that I’m uncomfortable handling live, or calling the utility to have them pull the meter so I can disconnect / reconnect safely.

The question is, what’s the likelihood that I could reuse these CT’s? I can measure the voltage across the existing burden resistor to get an idea of whether they’re in the right range, but turns, phase, etc. aren’t likely to be easily determined. Any thoughts?


A picture is worth a thousand words.

Hopefully it won’t come to that. For obvious reasons I can’t recommend that you do anything with your mains. Even if you have the utility pull the meter, they probably won’t put it back without an inspected permit or a licensed electrician to vouch for the work. Disconnecting your mains and reconnecting can be tricky, especially if you have aluminum wires.

Slim. Would probably involve changing the burden resistors in the IoTaWatt and/or adding external resistors to effectively do the same. Do you know if these CTs are working now?

You would need a good true RMS voltmeter and a good clamp on RMS current meter, and know how to use them.

So post a picture so I can see the layout. I still have a few ECS25200 clamp on CTs that you might be able to get in there.


I’ll get a picture tomorrow. The CT’s do appear to work - they’re driving a 1990 era LED display in the kitchen that I can make tell me the current power consumption of the house, which appears to be reasonably accurate. At one point, I considered just reverse-engineering the comm link (running on 4-wire telephone cable) between the display panel and the controller at the panel, but that would only get me overall power, not individual circuit power.

I was actually considering calling the utility to remove the meter as an option; but I’m guessing that you’re precisely right about them wanting a sign-off before they’ll reinstall. I’m not ready to go down that route.

I can borrow a good true RMS voltmeter and a good clamp on RMS current meter - I’ve got a friend who’s an engineer for Fluke and has lots of good toys available to him. Of course, I run into the same problem - no place to get a clamp-on ammeter into the panel, and no desire to take things apart to get the CT’s out of the panel.

One completely different question, if you have a moment - why use a 120V transformer to get the voltage waveform rather than a 240V one? You’re only seeing the voltage on one leg to neutral; is the assumption that second leg voltage will be (within system accuracies) the same as the first leg?


That’s the next step.

The short and practical answer is that most folks have a 120V plug available near their panel and a decent UL approved 120V wall-wart is readily available. The voltage on the individual legs does move around with load, depending on the resistance of the service cables. But the voltage across the neutral is a function of current, and if the panel is reasonably balanced, the current on the neutral is small, so the voltage across it is small. The hot leads carry all of the current, but then both the 240V and 120V transformers will pick up the voltage drop there.

It’s six-of-one half-dozen-of-the-other. I’ve monitored my split phase panel with two 120V transformers, and for the most part, they go up and down in sync. By far the greatest variations are caused by the collective use of the 4 houses connected to my transformer, and it affects both legs pretty much the same.


A few days late, but I got a couple of pictures if you’ve got a moment. Panel_Small

The first picture shows the external view of the box, note the small attached box in the upper right that holds the ancient monitor, and where I plan on installing the Iotawatt. The second picture is the inside of the panel, and the last is the wiring inside. Note the two donut CT’s installed on the mains, and that there’s about a 1.5" space there - and the current CTs are tightly wedged in there (it’s difficult to move them).

That’s why I was asking about using what’s already there…


Is the small box on the right made of metal? If so, the IoTaWatt May have a very hard time receiving a WiFi signal. You may want to consider replacing it with a plastic box. Also, where are you planning on getting power for the two IoTaWatt transformers?


Interesting… it looks as if there might be enough room for some clamp type CTs if those doughnuts weren’t there, but they are. Just curious what country you are in. I’ve never seen a panel like that in the US.

So let’s investigate those existing CTs. The first thing I’d try to do is identify the equipment they are connected to and try to find some documentation on it. That might reveal the specs of the CT.

Another approach, as I mentioned above, would be to observe or measure the burden value, then measure the voltage across it with a known current on primary. You say you can get a good voltmeter and clamp on ammeter. That you can’t clamp onto the main isn’t a problem, you can turn everything off except that EV charger and then plug in your car and measure the cable on the EV breaker.

If the CT turns is viable, I wouldn’t worry too much about the phase shift. Solid core CTs characteristically have very low shift, and it would be possible to measure it with the IoTaWatt if you have a good high power factor load. Maybe the EV charger.


Location is Phoenix, AZ, which generally qualifies as the USA.

ogiewon -
Good point - I was originally planning on using a Brultech and planned on just running Ethernet into the box. Switching over to the Iotawatt, I forgot that WiFi doesn’t penetrate Faraday cages very well. I’ll have to mull over getting WiFi into the box, because I’m a big fan of air gaps between my network and 240V, especially during thunderstorm season.

OverEasy -
I can get 120V on the backside of the panel inside the garage - there’s a convenient 4-position outlet right there. As far as the equipment in the box, it’s a Demand Controller built by Dencor, and appears to be a solid, high-quality design. The best documentation I can find on it is this patent:
It’s intention was to monitor maximum demand (maximum kW averaged over an hour), and shut down some loads (Hot water heater, A/C, etc) if the house was taking too much power. This was used in conjunction with a demand-based electric rate to reduce cost. Unfortunately, due to its age, there’s not much information on line. I can read the part number on the CT’s - but it’s a Dencor part number, and Google pulls a blank on it.
The next time I get a chance, I’ll pull the guts out of the box and figure out the burden resistor, and see if I can correlate voltage across it with kW. I’ll try with a couple of loads - the water heater or Jacuzzi heater are always good ones (continuous, resistive load), the EV charger is probably a good one at higher power, maybe I’ll try to max out the current (turn on everything) and see what I get.


Just wondering if you have access to the cavity below the meter and if the mains cables are exposed there.


Well, that’s a remarkably insightful question. I can’t get into the small panel that the meter is on without breaking the seal, but there’s a big panel below it that I CAN get into. It’ll be before the meter and have the feeders coming from the transformer - but they’d be perfect for a clamp-on meter to characterize the existing CTs. I’m pretty sure the electric company wouldn’t be happy about me putting permanent CT’s there, but it might be a case of “what they don’t know won’t hurt them”. Thinking about it, I’m also not sure what’s in the lower panel on the right - I’m guessing some of the circuits exit the panel there, but it’s likely mostly empty.

I guess I know what I’ll be doing tomorrow…


My favorite is “it’s easier to beg forgiveness than to ask for permission”. If you can get into that lower box and it has the mains exposed individually, I’d just clamp on a couple of CTs and button it up.