Tips for North American installs

I’ve just installed my IotaWatt and would like to offer a few suggestions based on my experience. Although I tried to read the documentation carefully and plan out my install before making any connections, I still missed a few important points. Hopefully these suggestions will help others avoid the same mistakes.

Document Split-Phase Legs for the Panel
North American panels have split-phase power with two “legs”. These panels have breakers running down two columns. But the leg for each circuit in the panel is determined by the ROW in the panel and not the column. Until recently I misunderstood this important detail. This misunderstanding created problems with two situations in my initial install. For some of the CTs, I had wrapped the CT around the wire from more than circuit but didn’t realize that I had combined signals from Leg A and Leg B (resulting in subtracting the power of one circuit from the other). And I had a similar mistake by combining signals from Leg A and Leg B via headphone splitters (into one input of the IotaWatt). I cleared up this mess by creating an Excel spreadsheet showing the Leg for each circuit on my panel. This post on stackexchange provides a good explanation of the layout of the legs in a panel and I think it would be useful to include similar info in the IotaWatt documentation:

Allow Negative Power
I recommend that you enable “Allow Negative Power Values” for all inputs when doing your install. This setting helps avoid accidentally combining inputs from different legs.

CT aligned to VT
I recommend that you orient all CTs to be consistent with respect to the VT and oriented to generate positive power. If you have a solar input generating positive power and want your loads to appear as negative power, can enable the “reverse” option in the VTI input configuration to make all loads appear as negative power.

With “allow negative power” enabled for a CT input, the sign of the power reading for a CT (input) will be determined by the orientation of the CT with respect to the Load AND it will depend on which power Leg the VT is plugged into. To keep the CTs all aligned with the VT, all of the CTs on one Leg will be oriented toward the Load and all of the CTs on the other Leg will be oriented toward the Source. More specifically: If the circuit is on the same leg as the VT, orient the CT toward the SOURCE. If the circuit is on the opposite leg from the VT, orient the CT toward the LOAD. For “pure 240V” circuits, you will connect a single CT to one Leg of the circuit and it should follow the same protocol : if a CT on a 240V circuit is connected to the same Leg as the VT, orient it toward the source. For “120V/240V” circuits, you will connect CTs to both Legs and the CTs will have opposite orientation. The CT on the VT leg should be oriented toward the source. With “allow negative power” enabled on your inputs, you can test each CT to verify they oriented correctly before combining anything.


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It’s good to get feedback on installation. The documentation also covers the 240V cases with a slightly different approach.

I do want to discourage that folks use the “allow negative power” as a tool to validate the installation. IMO that’s an overly complicated approach that can also lead to other problems. The feature that automatically corrects reversed CTs is the basis for IoTaWatt’s ease of installation in split-phase load centers and really shouldn’t be disabled except for the mains in load-centers where the inverter feeds into a breaker inside the panel.

I get that there are times when it’s best to insure that several conductors passing through a CT, for whatever reason, are all going the same way. There is a simpler way to do that without disabling the bedrock auto reverse feature. Just look for the little ↺ symbol in the status display.

That said, it’s good to understand how the L1 and L2 are organized and to try to use a convention in orienting the CTs.

First… since I didn’t say this before… I am really enjoying my IotaWatt. It is a great product and the whole operation is very well done.

“Allow negative power”
I only needed this during the install to verify that I didn’t make mistakes in orienting the CTs. After installing the CTs and verifying that the orientation is correct, there is no need for this to be enabled.

Combining Circuits
In my install I used two approaches for “combining” circuits: passing multiple wires through one CT and adding two CTs with a headphone splitter. I actually have two panels (main and sub-panel) with lots of circuits. I combined circuits in a few places so I could track more info with the 14 inputs of the IotaWatt.

However, combining circuits only works if you pay attention to the power leg of the circuit. The “reverse” option for in the CT configuration cannot correct for mistakes when combining circuits via one of these methods. And I’m guessing that other people are likely to make the same mistake as I when orienting the CTs. If people are NOT combining circuits, then they don’t need to worry about the orientation of the CTs (auto reverse covers it). If they ARE combining circuits, they can easily make mistakes which can be hard to debug if they don’t understand this reliance on the legs. Since your documentation already mentions the option of combining circuits with headphone splitters, I suspect that some additional documentation regarding the “Legs” will lead to fewer support issues in the future. I understand your desire to keep the install simple and approachable. At the same time… if people are going to dig around in their electrical panel… they probably won’t be daunted by a bit more “process”.

Thanks again,

Right, my only point is that looking for the little ↺ symbol is the same as looking for a negative number. It means “This number would be negative if you had allow negative values checked”.

That makes sense. I probably saw the ↺ symbol occasionally in the Status page but got it confused with the ↺ in the Setup/Inputs page.


The reverse circle can be a little hard to see, but I found it to be very useful.

Be aware that some panels have half-size breakers. On panels with these, two half-size breakers will share the same leg, so 240V breakers must be positioned to use both legs. This can make it easy to get the CT wrong. A table makes it much easier to get correct.

I have three panels at the house, main panel where power comes in the house, generator transfer switch that also has some breakers, rest of house panel where most of the circuits are. So I had to make sure I understood which leg was which, since all three panels aren’t the same.