Multiple questions (double pole breaker monitoring, CT extensions)


Thanks again for the support you’ve provided to help me get where I am. I know in the past I asked about in which cases we can use a single clamp to monitor a double pole circuit (by checking the “double” option when configuring the inputs), and @overeasy you mentioned that in general if there’s no neutral on a double pole circuit, its a good indication that you can use the single clamp with the “double” option…

Recently in another thread I saw @overeasy mention that solar circuits can use a single CT with the “double” option checked and realized I can make that change in my setup (I’m currently using two CTs). I guess in my mind since I ran a neutral to the sub panel where I aggregate several solar related circuits (multiple string inverters, micro inverter strings), I was thinking for some reason that this circuit isn’t applicable to the “double” option/single CT. But I guess the actual 100A breaker itself which sources the sub panel doesn’t have a neutral on it. I guess I’m confused about how to identify which circuits “have a neutral” in this sense.

  1. Besides the main distribution panel - are there other common double pole circuits which usually need two separate CT’s to measure? Right now, I’ve got a single CT measuring the following double pole circuits successfully: Water heater, AC (and soon to be my Solar sub panel). The other double pole breakers in my main distribution panel are: Furnace (50A), Range (40A), Dryer (30A). I know those are common usage-circuits, do all of those sound like single-CT/“double” option circuits?

  2. I show my measurements and graphs to anyone who’s interested and one of my family members had a question for their situation. They like the individual breaker monitoring so they can isolate power consumers in their house, but they have a solar circuit in a not-so-nearby location to their main distribution panel. Is it possible to use a 3.5mm extension cord (e.g. headphone jack extension cord) with the IoTaWatt CTs of any amperage (to be able to use a single IoTaWatt device to measure their main distribution panel mains and their solar circuit which is far apart)? Would a special type of extension cord be required? Would the signal degrade over the length of the extension to make it unreadable/unusable by the IoTaWatt?


First, let me clear up that there are techniques to use one or two CTs on any 240V circuit. Some involve the [voltage] “double” checkbox, and some involve passing two conductors through the CT. I prefer to distinguish 240V circuits by whether all of the current is carried in two conductors (true 240V) or whether some can also be carried in a third neutral conductor (120V/240V). This is discussed in the docs:

There are exceptions, but generally the common true (two-wire) 240V appliances are:

  • Hot Water Heaters (not some heat-pump types)
  • Well Pumps
  • Heat-pumps
  • Electric baseboard heaters
  • Some air-handlers
  • EV chargers

Appliances that are usually 120V/240V

  • Clothes Dryers
  • Ranges and stove tops
  • Ovens
  • Some air handlers
  • Sub-panels

They are probably 120V/240V requiring two CTs or both conductors passing through one CT in opposite directions. In the later case you would need a 100A CT for the Furnace, and probably the Range. The Dryer should be OK with a 50A.

Yes it is. See this post:

As explained, the CT is a current source, so the resistance of the extension should have no effect on the CT output unless it is excessive and causes the CT to be unable to drive the current. The other signal on the line is the CT DC bias, which is effected by resistance, but is constant and compensated by the IoTaWatt firmware.

Thanks for the replies! I definitely have to do some further reading to solidify my concepts. It seems that each time I read through the same material I realize something that I didn’t before, so hopefully I’m on my way there.