Service Mains too big to put clamps on

Hi all,
This is my first post and I’m hoping I’m following all guidelines. :slight_smile:

I installed Sense in our main panel and ran into the situation that the mains coming from the outside have two cables each and together they are too large to fit the sense clamps completely around them. As I had no other idea and the wife demanded the power to be switched on again :wink:, I just put the clamps around one of the cables of each line. Sense is showing power usage and I have tried switching on and off various devices in the house and I saw the power draw change.

Is my setup really correct? Feels like this will have flaws.

Thanks for any input!

You are going to have to provide more information on your electrical setup.

  • Do you have solar?
  • Where do the main cables come from (or go to)? Do you have a diagram?
  • Can you access the main cutoff switch (probably located on the outside of your home, open the box) and take pictures?
  • Can you take pictures and post them of your meter setup.

You are using a Schneider Electric (Square D) breaker panel. It’s the QO series and most of your breakers are Arc Fault or dual function ‘GFCI and Arc fault’. The three breakers on the bottom left appear to be twisted (not square in the panel). You have room to add an inexpensive whole house surge protector that snaps into the panel. Square D QO 2175SB Whole house surge protector.
Available at Home Depot. NEC 2020 requirements for locations that have adopted the latest code.

From a quick review, it appears that you have connected the clamps around one set of wires properly. Black = Leg0 (phase A), Red = Leg1 (phase B)

The question is: Is that the correct wire coming from the meter? We need more data.

My recommendations is to keep the wire centered in the CT clamp. (I used a small piece of pipe foam to center the wire.) Make certain the CT is perpendicular to the wires and tape the CT end of the clamps closed with masking tape. (You don’t want them to open up when the panel is accessed in the future.)


It looks like you have a “double tap” which is illegal in the USA if those lugs are only rated for a single conductor (1 wire) and additionally, dangerous. There are cases where double taps are allowed, but the breaker or main will be labeled as such. If the second set goes to a service panel, there’s no protection between the service and the secondary panel which is dangerous. To further clarify the danger - if you clamp 2 wires in one terminal block, often times you only really clamp one wire well, this allows the other to get lose, when electrical wires get lose they arc. Arcs can quickly = fire. This can also happen when people clamp to insulation instead of the wire, which creates a similar effect.

It might be that you have correctly connected the clamps to the service feed which is what you would want to do. It’s impossible to tell by the pictures if you have the correct wires. The second set of wires “probably” goes to another panel.

The NEC has stated this for MANY years…here is a snippet:

“Terminals for more than one conductor and terminals used to connect aluminum shall be so identified.”

The main lug of a service is almost always rated for a single conductor. If rated for multiple conductors it MUST say so. The NEC code is Ch 1 ARTICLE 110.14(A)

When lugs are rated for more than one conductor it normally looks like this:

I would have a licensed electrician take a look at that and advise what the best course of action would be. I’m sure you could send that picture to a reputable local electrician and they could advise without charging you.


Hey guys,
Thanks a lot for your replies.

The house is a new construction townhome which was finished this year (and passed inspection). A licensed technician installed an outlet for EV charging a few weeks ago and the only thing he said was that it’s a 208 volt installation which is not super common but for some reason made sense at that location.

To answer your questions:

  • the house has no solar
  • unfortunately I have no diagrams
  • yes, I’ve used the main cutoff switch on the outside of the townhouses (it’s not on mine) but I think my neighbors won’t like if I start opening the panel
  • see pic of meter below

I’m unfortunately only back at the house in probably 3 weeks.

Thanks again for your help.

Was that inspected? What breaker is the EV charger on?

I haven’t been in construction in many years but I would love to hear from a licensed electrician on what they would do if they saw a double tap on a service main. I’m fairly certain it’s against code and dangerous. Maybe @JustinAtSense could ask one of their internal electrical engineers (the company that invested in Sense makes your panel, so I think someone should know).

Just trying to look out for you. Would hate to see something happen because of a shortcut someone took. If you Google “double tap electrical” you’ll see there’s nothing good that comes up. Also, passing inspection doesn’t mean much. Most inspectors are swiss army knives and miss the important things while flagging the nuisance issues.

Interesting side note… In my jurisdiction (Sacramento California) starting last year the permits actually say that even if you pass inspection and if you did the work yourself you’re liable for damages caused by your work now and even with a future owner. Speaks to the confidence in their inspections :roll_eyes:

If you truly have “a 208V” system, your townhouse is fed by 2 phases (and neutral) of a 3 phase supply which is typical for larger buildings, This is different than the typical residential split phase supply. Sense can work on a 2 of 3 phases supply, but you do need to be aware of differences.

ps: I’m wondering where both the conduits that contain mains thick black white and red wires go. One probably goes to the meter. The other ….

Hi @jc1 - welcome to the Community!

From my end, this looks like 3 phase service. I’ve included installation details for 3-phase below, but might recommend reaching out to our Support team here to ensure your installation is correct.

Below are some details from our portal regarding 3-phase configurations:

Sense is designed to monitor traditional homes in North America with split-phase 240V service, but it can also successfully monitor two out of the three legs of three-phase service. It is important that Sense is powered by a two-pole breaker across the two legs that will be monitored, and that the current sensors are clamped around the service mains for the same two legs. The power consumption of any devices on the third leg will not be monitored, but the monitoring capabilities of Sense will be fully operational on the other two legs.

Alternatively, if your home does not have solar feeding into it, you may consider using one of the Flex sensors (2nd set of sensors) to monitor that third leg, and leave one unattached. This is not an official solution though, and you would need to reach out to Support when you bring the monitor online so we can help set this up.

You can see more about compatibility with 3-phase configurations here:

1 Like

@JustinAtSense The panel shown in the original picture/first post is not a 3-phase panel. Check the Schneider Electric website and you will see that this panel is only designed for 2 phases. It is a standard residential panel. Have @jc1 post the breaker panel model number in this thread so that we can end the guessing.

@DevOpsTodd I agree with the comments made by Todd. You should not “double tap” the termination lugs in the panel. The inspector should have caught this.

@jc1 When you are back home and if you feel confident enough, “Cut-off” the main power switch on the outside of your house. Open the outside panel (box) and take pictures of the wiring inside the main cut-off switch box. You don’t need to open the meter box, just the main switch. The meter box is locked with a Power company tag. Don’t touch any wiring inside. One side of the switch will still be ‘hot’ - energized. We need a more complete picture of your setup. Take several pictures - far-out, right-angle, left-angle, etc. We need to see more if you still want support from the forum users.

If you don’t feel confident doing this job, then call an electrician. Have him trace your main wires and label them and provide a diagram. Have the electrician take pictures for you.

If you think you might be opening up the breaker panel in the future (or you have the electrician make changes to the inside panel), then consider using the Square D safety rubber protectors. They snap over the lugs. They are cheap compared to the alternative.

Square D 100 Amp - 225 Amp Service Entrance Barriers-PKSB1HACP - The Home Depot


@Dcdyer, @jc1, 208V is a very specific number - not likely a mistake. 208V shows up in 3 phase systems as either the phase-to-phase in a Y configuration or in phase-to-neutral in a Delta configuration (which I think is typical for a residential). Based on how Sense operates the Signals section is going to to see 120V on each phase leg with respect to neutral. But Sense won’t see that the two waveforms are 120 degrees out of phase vs. the regular 180 degrees (which ends up presenting 208V “leg-to-leg” vs 240V)

ps: I’m not a commercial electrician so I can’t give you more than that on practicals. My only knowledge comes from taking and teaching a couple of junior year college labs on 240V / 3 phase systems. It was taught in a lab that looked like it was out of Frankenstein - big exposed bus conductors and knife switches everywhere.

Yes, I used two phase breaker 20amp 240V breaker. (top right on the right side in the pic)

Interesting. I had also emailed support when opening this thread. Will let them know.

Thank you!

50amp - it’s the second from the top on the right side.

The builder’s spec sheet is saying “Service panel – 200 AMP”
Found a picture I took a while ago which seems to say 300A, 120-240V.

You are right, it says 122 and 123 V in the Sense app.

Thanks, will do.

1 Like

The electrician mentioned 208V not only in a half-sentence but was basically telling me because of that setup, I’m getting a lower EV charge rate than what I expected and what I’ve seen with the same car at another house with the same EV setup (from breaker to car - not the panel/service mains/etc).

One last tidbit: I mentioned that this is one of 5 townhouses. Forgot to mention that they are on the side of a commercial building. (visitor center + offices)

Just because it isn’t a 3 phase panel doesn’t mean that it is being fed off of a 3 phase transformer. The building could be 3 phase, and they divvy up 2 phases across different residential units. You will still get 120v Hot to Neutral for standard outlets, and then your phase to phase high voltage appliances will be 208v rather than 220v.

I’m not sure how common this is, but I’ve seen it before in machine shops / farms etc… where the main service is 3 phase, but outbuildings or sub panels don’t need all 3 and are fed from 2 pole breakers in the main 3 phase panel.


@kevin1 and @ben are correct. You can have 208 volts measured across the two line feeds and still have 120 volts from the ‘line’ legs to the neutral. Looking at the SENSE voltage readings on the APP is not going to give you the data on whether you have 208 volt (2-phase) or 240 volt (split-phase). Since you stated that you are living near commercial property that uses 3-phases and you had a certified electrician state it is 208 volts, then you probably do have your townhouse fed by 2-phases (and a neutral) of a 3-phase supply. My apologies to @kevin1 and @ben for any mis-information. Most of my experience is with split-phase home residential. I will edit my earlier comment and remove the error.

@jc1 If you own a volt meter and feel comfortable enough, you can measure the voltage “line to line” (red main lug to black main lug). The information may be important to you in the future.


If the wires are indeed in parallel, i.e. they go to the same lug on both ends, Then the way you have it set up will work but will measure 1/2 the actual current draw. To measure full current, both wires have to be encircled by the clamp.

1 Like

Looking at the power consumption reported in the Sense app and what I see on the Utility’s website, it seems like that’s the case - the Utility shows exactly twice the consumption.

So, is it possible to get larger clamps? both wires together are definitely to thick to fit into the ootb clamps.

Or in this thread and also support mentioned that I could use cables from the solar package to measure another phase. Would it also work if I connect those cables and put them around the other red and black wire and have support somehow set that up in my account?

Ok, so there are no larger clamps for Sense that I’m aware of but here’s the bigger issue… you don’t know where those wires go. By the picture we can see it’s 4/0 aluminum which is rated for a 600v - 205amp service with just ONE wire/leg. There’s no need to have 2 x each leg for the panel size that you have.

If, for example, you have clamp #1 of the correct leg but clamp #2 is on the incorrect leg to then you will get 1/2 of the usage measurement (assuming there’s no/little load on the wire that goes who-knows-where). Both clamps have to be on the leg to the service side.

Since you don’t know what those two wires go to it’s a complete guessing game and no one here can tell you for sure what you have going on. You need to figure out where those go. The Sense app will read the correct voltage even if you have one or both CTs on the incorrect leg(s), but the usage should be imbalanced.

Another possible scenario is that you have both clamps on the non-service leg (assuming all 4 aren’t service) and you are simply reading the voltage for the dowstream panel, or wherever it goes. Point being that it’s all speculative at this point.

I’m still not comfortable with the double tap issue that you have either. I’m still going to recommend that you contact an electrician and figure out what’s going on. OR, if you have the blueprints from the building we would be able to tell what goes where.

This would be a question that I’m sure @JustinAtSense would have to ask the internal team about, since there’s two scenarios that would most likely work; 1 - It’s configured like a dedicated circuit or 2 - It’s configured like a 400amp panel. Neither is correct in your case if in deed they are in parallel. Again however, because all of this is speculative at this point it’s really mute until we know where those wires go/come from.

I’m going to hold off on making any official recommendations until we have more details on the schematics / wiring here, and would +1 @DevOpsTodd suggestion of bringing an electrician in or obtaining information about how your home is wired.

1 Like