? about setting up CTs

I am excited to start participating. I am a novice. I hope someone can advise me on CTs. I live in the US (Seattle, WA). Included is an image of our wiring box. My goal is to monitor each individual circuit (the ones coming off the breakers) and total energy use (the larger wire … I assume…).

Q1: There are two circles on the image. One is around a thick wire. I assume this is where I would put a CT to measure total energy use? Is this a 4/0 Gauge ? (note: i ready the “Use in North America” section so I assume a 100A CT split core would work from any of the recommended vendors?).

Q2: The second circle is around a wire attached to the breaker that supplies current to a section of our house. Am I correct that to monitor each section (e.g.: washer, dryer…), I put a CT on each of these wires. These wires are much thinner than the other and also closely spaced. Can you please point me in the direction of which CTs would be best for this scenario?

Once I figure out how the CTs would work, I’ll figure out how to set stuff up with the firmware.

Thank you very much.

1 Like

Hi Margaret,

Welcome to OEM.

OEM is a whole-house monitoring system. Although it can be expanded / adapted to multi-circuit monitoring, there’s a newly developed device called IoTaWatt that would be well suited to your application. IoTaWatt is not yet available, but the OEM Shop hopes to have a small quantity of pre-production units in the next few months. See this post.

Just above, and to the right of the upper circle, is what appears to be 4 AWG. I can’t tell for sure because the photo is blurry. The number to the left of the letters AWG is the wire size. If the printing on the wire is 4/0 then the answer to your question is yes. If so, you’ll need one of the alternative CTs, as the SCT-013 sold in the OEM shop won’t fit wires of that size.


All of the breakers in your load center are rated at 20 Amps, so you’ll want a 25 or 30 Amp CT for each of the branch circuits you want to monitor.

When choosing CTs, keep in mind you don’t want CTs that produce an ouput of 0.333 Volts.
What you do want is a CT that produces either A) an output of 1 Volt, or B) a current output, sometimes referred to as a milliamp, type CT. Another name for that type of CT is unburdened.

1 Like

My moderator colleague Bill Thomson (from Oklahoma) is much more familiar than I am (mainly because I’m in the UK), but together we wrote the page in the Learn section about “Use in North America”. If you haven’t, you should read that.

Your assumptions are correct, to monitor the overall power you need two c.t’s on the thick cables (the incomers). (Beware - those nuts and bolts are live at 120 V to earth, with 240 V between them.)
Then to monitor individual circuits (the many thinner wires) you need one c.t. - but if you want, you can group wires fed from the same leg of the supply through one c.t. if you only want the total for the group.
To know which c.t’s to use, you need a realistic estimate of the maximum current you’re going to draw, overall and per circuit. You’ll need split-core c.t’s for the incomers, but for the individual circuits, you can use ring-core if you’re happy and competent to disconnect each wire. Otherwise, you need split-core ones for those too. If I can count, you’ve got 21 outgoing circuits, all at 120 V, as I can’t see any linked breakers?
There’s a big choice of large c.t’s on the N.America page, but smaller low-current ones suitable for individual circuits are harder to find, especially split-core (which tend to be physically larger too).

Aside from the spelling error, it appears she did…

Just a couple of comments on CTs if you think you may be using IotaWatt.

IotaWatt will monitor a 20A circuit just fine with a standard issue sct013-000.

The folks at OEM are also looking at some smaller split core CTs that are designed for these lower current ranges. I expect time frame is probably about the same as general availability of IotaWatt.

Your panel doesn’t have main circuit breakers, and judging by the way the ground and neutrals are segregated, I would guess it’s a branch panel. That’s not a problem in itself but could be if in the remote chance you are in an apartment or condo complex with three phase feeds and your box is 120/208. If you have a voltmeter, you can find out easily enough by measuring the voltage between the two feeds. If not, you might want to ask someone who knows where those feeds come from.

In a US split-phase panel, you have to be aware of which side each circuit is and either only group wires from one side, or pass the wires from one side “backwards”.

Thank you very much Bill. Your detailed answers are incredibly helpful. I like the design of the ioTaWatt. Well done! My husband has informed me I’m not supposed to electrocute myself so I’m going to buy a few CTs and see if I can get an electrician to walk me through…excellent work you have all done. I am excited to get started.

Thank you Robert. Your info is very helpful.

IoTaWatt was developed by OEM forum user Overeasy (Bob Lemaire), i.e. not me.
(Didn’t want you to get the wrong idea, as I had no part in its development) :wink: