Sense says my dishwasher costs me negative $35 / year to run ... hmmm

Technical Questions
#1

I got the two addittional CTs for DCM. One of them is installed on the Furnace, the other on the dishwasher+garbage disposal (together). I’m not too interested in keeping track of the dishwasher+disposal, and I’ll move the CT to the AC when the weather warms up. Now, Sense is saying it’ll cost me about $ -35.00 per year to run the dishwasher. I thought one of the CTs was installed backwards, so I checked but it is ok.

The only small detail is the CT is mounted around TWO wires from neighboring breakers (one from each 120V legs), one feeding the dishwasher and the other feeding the garbage disposal. I’m not sure this affects anything because the CT should catch the sum of the two currents.

Any thoughts as to what I could possibly be doing wrong?

#2

@drjb, does the bubble come up when the dishwasher is running ? Does it come up when the disposal is running ? It should. If you don’t see a bubble, then it’s likely that DCM is registering as negative rather than positive which means something might have gone wrong in setup… I’m thinking that you could also have an issue if those two adjacent wires / breakers are on different legs, since the CT can only be associated with one leg voltage. I’m guessing that if they are on different legs, and Sense picks an L1 reference for the CTs, any power/energy used by the device on L2 will show up as negative since the polarities are reversed (voltage is positive when current is negative and voltage is negative when current is positive).

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#3

@kevin1. Thank you. I thought as well that the reason was because the CT sensor was wrapped around 2 separate circuits, that are fed from 2 separate legs. In fact, my other CT is also wrapping around 2 breakers that feed the furnace blower motor (also fed from 2 separate legs).

  1. CT1 - wrapped around 2 neighboring breakers (not-linked) that feed Dishwasher+disposal
  2. CT2 - wrapped around 2 neighboring breakers (linked) that feed furnace blower motor

What I think the difference is, is that possibly the breakers on CT1 are connected to a neutral, whereas those connected to CT2 are not. I have to re-open the panel and check as I overlooked that detail. If that is the case, then one of the wires must be wrapped around. I think I saw that somewhere on this site, but don’t recall where.

It kind of makes sense now (I understand it better) but I got to check.

This has been quite a learning experience … very fulfilling.

One question though. The two 120V legs feeding the panel, are ther exactly in phase or is there a relationship at all between the two? I’m fully understand the 3-phase circuits (Wye/Delta) and all the relationships, but do not fully understand the 2× 120V legs yet. Some reading is in order here.

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#4

Found it, it’s actually much simpler than I thought (and way simpler than 3-phase) … The linked PDF explains it very well (check the first figure).

The two legs (without the neutral) are at 240V. If a neutral is used, then we have two branches, at 120V each, with a 180 degrees phase difference. So, since I installed my CT on two neighboring breakers, the $-35 yearly estimate is the difference (not the sum) between the consumption of the two devices.

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#5

Most US home get split-phase power from the distribution transformer. That means the two legs are 180 degrees out of phase. Multi-unit buildings might have three-phase where an individual apartment, condo, etc. will operate on two phases separated by 120 degrees.

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#6

Correct. So then with the split phase, it’s either 120 or 240 V. Whereas if it’s two phases from a 3 phase system, it’s either 120 or 208 V, because of 120 degrees phase shift (and not 180). It all makes sense now. Thanks again. I just need to go back and loop one of the wires backwards on the CT.

#7

And possibly redo the setup for DCM, so it associates the right voltage leg with the corresponding CT leg.

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#8

Yes, good point. As a consequence of my mistake, I also now understand another side-effect with Sense. When the dishwasher is running, I see two large bubbles of ~500 W each, though the numbers are not exact (and NO gray/others bubble). I tried to merge those two bubbles in the past, but without success. Somehow Sense is smarter because it knows those devices are NOT the same. One of them (Heat 2, detected by Sense AI/ML), the other (on DCM) includes not only the dishwasher but also the disposal… Gotta love it when technology tells you you’re doing something wrong.

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