Caution: Flex Add On Sensor could catch neighboring circuit signal

I installed Flex Add On Sensor in my electrical panel on an AC device shut down for the season (should not read anything). Strangly enough, I was reading current on this dedicated circuit. After investigation, I found that I was picking up partially neighbour breaker current!

The reason seems to be that the neighbour circuit cable was too close (in contact) to Flex Add On. After pushing away the neighbour circuit breaker cable, my dedicated Flex circuit did not measure anything any more.

Bottom line, be caution that the Flex Add On Sensors could pick up electromagnetic fields from in contact cables located outside its loop.

Initial set up catching electromagnetic signal of neighbour cable:

Close up of initial setup catching electromagnetic signal of neighbour cable:

New set up with no cable toutching Flex Add On Sensor preventing it from measuring neighbour signal:

Could Sense experts tell if outside loop cables are detected by sensor per design?


Oops, that’s a serious problem, particularly in today’s crowded breaker panels.

This is indicative of a sensor that is not fully closed (and thus a gap is present in the CT loop, causing other signals to leak in). Do you mind if we take a look at your data?

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Please have a look at my data. Dedicated AC circuit was catching Coffee Maker signal on adjacent circuit.

It’s hard to say with 100% certainty, but it looks like your CT was slightly ajar initially, causing that interference to occur. An open CT can definitely manifest with this issue, as the loop is not fully closed around the conductor. It’s also possible for a CT to have a manufacturing error that prevents it from fully closing, but that does not seem to be the case here, as you solved the problem with some repositioning.

That’s rather surprising. I’m not challenging your observation but from my knowledge (school-acquired) in how those sensors work, the situation you describe (the sensor picking up current from outside wires) should ‘not’ happen.

I guess to understand this better is to show the current/watts while the specific device is on vs. off. Sometimes such devices draw a minimum current to keep their electronics working (on stand-by). I have a CT sensor around the wire feeding my Furnace’s blower, and even when the furnace is off, I still see a 15-17 W power draw. I attribute that to all the electronics and logic boards packaged with the furnace. Would that be the ‘same’ for your summer-only appliance? One can only speculate as we do not have all the details, but nonetheless, very interesting topic/find.

@drjb Electronic current could not be running in my dedicated summer-only appliance because the breaker is off in the main panel. No current could travel through this dedicated circuit, but still Sense measures some current when another cable, outside CT’s loop, is in contact in the location shown in the pictures.

@RyanAtSense To trouble shoot my CT to make sure it does not have a manufacturing error preventing from full closure, would you suggest me to put back an adjacent cable outside CT’s loop by the handle hinche?

If you’re concerned about possible inaccuracy due to a manufacturing defect, I’d recommend reaching out to the Support team and they can help run some diagnostics.

In any case, I still think the advice of trying to keep it away from other conductors applies. CTs in this style do not form a 100% perfect loop. Even with tight manufacturing tolerances, a split core CT can still present with air gaps. In most cases, it shouldn’t cause issues, but that depends on a variety of local factors. I can add a note to the installation guides about this.

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Clip on sensors are subject to some interference, particularly in tight wiring spaces. That said, they are generally “good enough” if you take care with the install.

That’s actually why the most precise current sensors are complete circles, where you have to disconnect the wire and pass it thru them. Clip on sensors are MUCH easier to install (particularly for mains…where Sense’s primary current detection is done), mostly not even needing an electrician, so that’s why they get used. Trading installation ease for a bit of accuracy is often a good trade-off.


I agree with @andy and @RyanAtSense comments.

If you could find a new location to place the CT clamps where the wiring is not so congested, that might solve the problem.

It also helps to tape the CT clamps closed. Use masking tape. When you re-install the metal panel cover, you can not see if the clamps are being accidently opened.

Last point, if possible, use some foam spacers to center the wire in the middle of the CT clamps. Without the spacers, you will get a reading, but when they are centered, you get a slightly better reading.


How big a difference could a foam spacer do? If the difference is significative, then I would suggest Sense to include foam spacers with the CT camps.

Review this old forum post. When I changed the position of my CT clamps - 90 degree angle to the wire and centered on the wiring, my results were better. Look at the charts. Review the complete forum thread/post to see other user’s comments.

As for the foam spacers, that would be difficult for the SENSE team to provide. There are just too many wire sizes and configurations. Also there is an issue with foam spacers being flammable. Flammable products are not allowed inside the breaker panel. I took a chance for myself since my spacers are small and I am familiar with the NEC codes and feel comfortable working inside my panel.

If you do make a change to your CT clamp positions, be aware that you might lose some of your current device detections. They will have to be re-discovered again. That might take more weeks or months.

These are decisions you will have to make for yourself. What are the goals / objectives you want from the SENSE product? I wanted to verify that my ‘total monthly wattage’ was being billed correctly by my energy provider. I am an engineer so I was looking for accuracy.

Extract your data from the WEB APP - before and after the change and chart it. That is the only way to know if a change was positive for you. Good Luck!