Can Flex Add On Sensor monitor 2 x 240 Volt circuits?

I’m a new user and I’m very impressed with Sense! But I am a little frustrated with how long it will take to “learn” my power usage and devices so I want to install a Flex Add on Sensor and a couple of smart plugs. I want the Flex Add On sensor to monitor my electric furnace with 1 lead and stove with the other. The furnace is running on a 240 volt circuit with 2 x 125 Amp breakers (dual pole) and the stove is on a 240 volt circuit with 2 x 40 Amp breakers (dual pole).

Can I use 1 of the Sense probes on one of the wires from the 240 volt, dual pole furnace breaker to monitor all power usage over the furnace circuit? And the other sense probe on one of the wires from the 240 volt dual pole stove breaker to monitor all power usage over the stove circuit?

What smart plug model is best for my washing machine? Thanks for any help you can offer.

New Excited User

Welcome to the club, @charliedearborn!

Hunker down and get ready to wait a while… We all are! :smiley:

Hi @charliedearborn
Based on your description, you would be referencing this section of the Installation Guide: Installation will vary depending on whether or not the loads are connected to the neutral bus.

Hey Mr. Justin - Thanks for the very quick response and link! I did manage to find that information late last night. I have a dedicated 240V circuit for my stove and it is using the neutral bus. There are 2 x 40A breakers wired like this: Red to 1 breaker, Black to the other, and white to Common. I hope to use 1 probe on this circuit. The installation instructions don’t deal with this directly. Can I connect the probe like in the attached image from your instructions. (Both Red and Black wires from both breakers going through the probe in different directions.) Or can I simply put the probe around either the red or black (one of the breakers)?


No problem @charliedearborn. Could you add a few photos of the breaker in question so we can see how everything is arranged?

Hi Justin - I want to connect to the bottom 2 breakers. I discovered that the furnace is not connected to the neutral bus so 1 probe on 1 wire will work for the furnace circuit. Correct? I want to connect the 2nd probe to the breaker that is 2nd from the bottom. This is connected to the neutral bus. I have the Sense unit powered from this breaker as well.

fyi, you might have an issue trying to monitor a dedicated circuit that’s also power the Sense unit…I changed my dedicated monitoring circuit yesterday, and wasn’t paying attention and tried to monitor the breaker that’s also powering my Sense. Well step 1 is turn off that breaker, so then how can you talk to the Sense unit?! :stuck_out_tongue_winking_eye:

The Sense unit doesn’t pull much, so maybe you’ll be able to turn the breaker on without the bigger device on that breaker turning on, which would not work with the setup process normally…I dunno. :man_shrugging:

Yes, this should work for the furnace.

You won’t be able to go through the standard setup flow if you want to monitor the circuit that sense is powered on. One option is to install the CTs on another load of the same type (240V/120V) and on the same leg of the panel and then swap the CTs over to the stove breaker after setup, but this has not been thoroughly tested.

Thanks Edison517 and Justin - I will install a 15A 240V dual pole breaker and move the Sense unit to it so it will be on it’s own breaker. With this done will I be able to use 1 probe on the stove circuit as described in the detailed training and my first pic? (both wires in one probe but different directions)
Can you also confirm if this logic is correct.
My Hydro bill in winter is around $450.00 so I need to accurately monitor the furnace. I thought that if I monitor my furnace on one probe and my stove on another and then my washing machine on a Smart Plug that I would be removing a lot of “noise” for Sense to sift through. (at least 14 different heat sources and 2 motors). This would get me very accurate measurements on these devices and hopefully make it easier for Sense to learn what the water heater and dryer look like. These are the main items I want to monitor closely.

You don’t need to use the wiring method from your picture above for a 240v-only load like a stove. You say your stove does have a connection to the neutral bus. Can you confirm this is actually a neutral connection that’s in use and not a grounding conductor attached to a shared ground/neutral bus? Some older stoves might have had a neutral to run a 120v clock motor, but that amount of power is negligible and I’d just use the conventional 240v single-CT monitoring method without the loop and dual conductors thru CT that the 120/240 wiring method requires.

Hi pswired - yes. In the picture above you’ll see a white insulated conductor terminating on the common bus for the stove.

I guess I’ll invest in the Flex Add On sensor and a smart plug. I hope this will enable Sense to learn a little more easily and meanwhile I monitor high use devices immediately. Is this wise?

There appears to be some ‘confusion’ about this topic (at least from my side). If I understand this correctly, then there are two options by which furnaces (and ACs) are wired, since they both use a double breaker:

  1. Both breakers are connected to the Neutral (common) - Parallel ?
  2. None of the breakers is connected to Neutral - Serial ?

In situation 1, the CT needs to wrap around the two straight wires coming out of the breakers
In situation 2 however, the CT needs to wrap around both wires, but one of the wires needs to have a loop, so as to change the current’s direction. If that is the case, how does one know which wire to loop? … because, depending on which one, the current will be in-phase or opposite the main incoming power wires.

I reckon I’m approaching this a bit with my 3-phase background … and do not fully (yet) understand the working details of the two parallel legs coming into the house.

Any brave soul care to shed some light?

I asked the electrician who installed my Sense few days ago about why there is no 3-phase option in the US, and he replied “The rest of the world got destroyed in WW2”. I think what he meant is, most of Europe rebuilt ‘correctly’ by using 3-phase after the war, whereas in NA, we had to make do with a legacy system that was not necessarily ideal, but was too expensive to replace. Is that really the consensus ? I know, a bit off-topic, but nonetheless, a valid ‘trivia’ question.