Single DCM for both 120 & 240v

Thought I would have to use both my CT’s for my HVAC system, but it appears that’s not the case.

Am I correct in thinking that I can monitor both my 240v heat pump circuit (no neutral) and my 120v furnace circuit using a single CT.
Using one of the 240v circuit hots and the 120v circuit hot, with them entering from opposite sides of the CT. Allowing me to use the 2nd CT for something else.

Does it matter what phase they are on?

I assume Sense (essentially doubles the wattage from the single 120v hot of the 240v circuit) displaying everything as a single load/device?

I know I’m way over simplifying this and leaving out some extremely relevant physics. But if sense is able to differentiate between two loads based on which direction it flows through a single CT in order to determine one of the two 120v hot’s is 1/2 of the 240v circuit and the other is a single 120v circuit. With some additional programming wouldn’t Sense be able to separate these into 2 separate devices. Further allowing us to monitor 2 separate 120v circuits with a single CT and 4 with both CT’s.

You could run your dedicated sensors around your HVAC (L1/L2) and then add your 120v (L1) in the same phase ( L1 ) as your HVAC -240v circuit going to the outdoor unit of your heatpump. If they are too far apart in your panel… you could always re-wire your indoor air handler using a duplex outlet and put a Kasa plug there.

The single CT for a 240v is done by L1 going one way and L2 going the other way thru the CT… so the CT reads it as 1 phase.

I wanted go to go with the “Monitoring a 240V/120V load with a single sensor. This method should only be used if you are monitoring two loads, either both 240V or 240V and 120V.” method in the link above. If I understood right this should allow me to use a single CT for all my HVAC and keep using the 2nd ct. for my (120v) pool pump.
I think I may have understood it wrong the 1st time. Both hots for the 240v need to be in one ct facing opposite, while the 120v would be in the other CT which makes more sense. The instructions keep saying one flex sensor which I interpreted as the same sensor. If it has been worded Sensor 1, Sensor 2, or the other sensor it would be a lot less confusing.

Having the heat pump installed in the next 2 weeks. There is no existing circuit, so I have to install a new one. I’m going to have to run it back to my main panel (where Sense is) as the closer sub panel is near it’s limit. I was thinking of rewiring the Furnace circuit from the sub panel back to the main panel as well, simply so I can monitor it.
Per NEC a furnace has to be hardwired, meaning a plug is out of the question, though it would be nice.

I read that as you can defiantly monitor a 240v circuit with one flex and a 120v with the other. Shouldn’t have any problem getting 2 - 10awg wires in a CT or even 2-10awg and 12-awg like this. For indoor and outdoor hvac units

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Yeah, I knew about using a single CT for a 240v balanced load.
The images are just kind of thrown in there. Not really a figure 1, figure 2 thing to reference.

If your drawing would work that’s great. For some reason I was picturing something more along the lines of your drawing but without the top L1 in the CT. Just one line of the 240v & one 120v through a single CT. I confused myself wrapping my head around how Sense could accurately calculate it the way I pictured.

@obscuredtrip,
I don’t think it works that way. Sense isn’t able to differentiate current flows through a CT between two different devices - the physics of the CT just “adds” the two. And for them to be correctly added, they have to be phase and polarity aligned. That’s why you see some people contorting two different 240V lines with a neutral (neutral means the load is imbalanced), so one is reversed. That gives you a 120V x each individual current, phase/polarity aligned situation. I guess you could bunch a 120V furnace fan that is correctly phase / polarity aligned into that same CT to add one more 120V current to the mix.

But you can’t mix a single 120V and single 240V into the same CT.

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" in many cases you’ll still be able to monitor a single 240V/120V device via the [single 240V load ]method. 240V/120V loads should not be monitored with the 240V-only method, as both accuracy and broad device disaggregation issues can result."
-I’m confused. You can monitor a single 240v/120v device using the single 240v method but you shouldn’t do it?

Another section
"Sense can use a single CT, per load, to monitor up to two dedicated 240V circuits if the load is not connected to the neutral bus (balanced 240V-only). However, in some cases, it’s also possible to monitor a 240V circuit connected to neutral (240V/120V) with just a single CT. "
-Two balanced 240v circuits using 2 CT’s one for each circuit.
-One Unbalanced 240/120v circuit using a single CT.

“Monitoring a 240V/120V load with a single sensor
This method should only be used if you are monitoring two loads, either both 240V or 240V and 120V.”

• Two loads one 240v and one 120v using a single CT. Will this method work if the 240v load is balanced?

“If your second load is a 120V load: Find the load. Clamp one Flex sensor around the single ungrounded conductor attached to the breaker. The Sense house logo sticker (or blank side) of the sensor must face the circuit breaker.”
-Again will this work if the 240V load is balanced?

@JamesDrewAtSense Could you provide some clarification to all this?

Pretty common to have a 240v (balanced no neutral) heat pump or ac and a 120v furnace or air handler.
Maybe Sense would consider adding something detailing this specific scenario to the help section?

So if I had to clarify that you want to use DCM for one 120V and one 240 V device, I would use this method and use one CT to measure your 120 v device and a second CT to measure a 240 v device. As you already saw, please view here: Install Guide : Sense with Dedicated Circuit : Advanced – Sense.com

Generally, If you have a heat pump and want to have it detected and measured, it is best at this time to use DCM for the heat pump.

It appears you have found the info here, which is the page I recommend you follow: Install Guide : Sense with Dedicated Circuit : Advanced – Sense.com

If you found contrary information anywhere else, which you mentioned above, and can still easily access it, please give me the links so I can check up and see if it is still relevant or requires any changes.

Finally, @obscuredtrip, I’m excited to hear you are getting a heat pump and a little envious. I’ll be interested in hearing how it goes if you are interested in sharing.

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Hoping someone from deep programing of the Sense can confirm/ test this. …How I am understanding this… running the (120vac) conductor for L1 one direction in a CT of a 240v circuit and the L2 conductor (also 120vac) the opposite direction thru the same CT … you have tricked the CT in which phase L2 is in causing the CT to read this as 2 wires in the same leg/phase -“L1 (120vac)”

The CTs read amps… The sense meter used the voltage from the L1 and L2 connections to calculate wattage. Using a single CT on L1 and L2 (same direction) that was a balanced load such as a 240v heating element or a 240v AC compressor pulling 10 amps would cause the CT to read 0 amps/ 0 watts. Running L2 the opposite way as L1 will make it appear as you have 2 x L1 (120v) at 20amps then sense is calculating that out to 4800w depending on which L1/L2 is associated with which L1/L2 wire is connected to.

Figure 2 and Figure 1 are doing the same thing to the CT… Figure 1 has the wire physically going the opposite direction where as figure 2 is having the current going the opposite way by running the wire thru the back of the CT first.

More along the lines of what would happen if I used a single CT for a balanced 240v load and also put a 120v circuit in that CT.
I may just have to experiment with things once the HP is installed and find out.

Honestly our 25yr old furnace finally quit and needed to be replaced. We currently don’t have central AC, so what better time to upgrade. I would have never considered a heat pump but the rebates and forthcoming tax credits and refunds for HP’s will make the HP cheaper than a standard AC. My other half works for an HVAC, plumbing, electrical distributor, she gets everything at cost so we went with the top of the line system they stock.
Sense discovered my old single stage furnace blower motor, the new one is variable speed so I’ll need a CT on it. But I can already tell it uses substantially less electric.
I’m really looking forward to Sense showing me the heat pump’s consumption in various weather conditions. Our price of gas is quite low, the price of electric high. Knowing when it’s most cost effective to run one or the other will really help to minimize our utility bills. If only there was a Sense for gas… I will defiantly share my findings as I progress in this new journey.

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I can’t speak from a Sense perspective, but can from the physics perspective. A CT will “sum” the currents flowing through it. The problem with mixing a single hot of 240V balanced wire with a 120V wire is that the current will get summed but the voltage multiplier for power can only be 120V or 240V - you can’t have different multipliers for different wires through the CT.

Based on the physics, I would suggest the 3 wire through the CT approach treating that CT as a 120V device - the addition of the two 240V load legs will essentially do the 2x multiplication. But you’ll need to get all the phasing and polarities right or some of the wires running through the CT will do subtractions instead.

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Thank you for posting this information with links. I posted a thread in the wish list section and part of this info is what I was asking about but had no idea it was possible. Last time I used Sense this wasnt an option and now that I have Sense again in my new house I am learning about some of the new features availible. I already ordered a set of the extra CT clamps and plan on using them to monitor two different circuits that are for two of my split type air conditioners. I am using Sense in the Philippines and they use 230V single phase here so I am only using one of the CT clamps to monitor my main power coming in and it would be nice to use the second CT clamp to monitor another dedicated circuit so that i could monitor three dedicated circuits in my house.

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