CT location with whole house generator - before or after transfer switch?

  • Mains CT’s on utility line (grid connection) before transfer switch
  • Mains CT’s on load side of transfer switch

0 voters

I’m interested in learning who has put their CT’s before the transfer switch and who has placed them after the switch.

My CT’s are before the switch (location 1 in attached diagram but new users please see followup question to ryan below and consider location 3 instead of 1 on a new install) so that they can measure all loads under normal operation including Generac’s oil filter heater, battery heater and battery charger which are all internally supplied from the switch itself. But when utility power fails and the generator switches on the CT’s no longer measure anything.

Alternatively place the CT’s after the switch (location 2 in attached diagram) and you’ll monitor your home’s entire consumption while on generator power and have the bonus of monitoring your generator’s frequency and voltage compliance. I suppose that some devices may not be recognized while on generator power as the less steady generator power may alter their signatures beyond recognition. But the all-winter long load of the oil filter (50 watts) and battery (40 watts) heaters will never be recorded.

And a follow-up for @RyanAtSense - I may want to move my mains CT’s from location 1 to location 3; I would need help from support as you’d need to make changes on the back-end. How is this support accomplished and do I lose any device detection if I do it? If I were to make the location 1 to location 3 move then once accomplished I could freely switch between locations 3 and 2 without messing anything up provided I maintained the identity and direction of the clamps. That could be the best of both worlds.

[edited to add wiring diagram]
[edited to add poll]


I’m not Ryan, but I might be able to substitute in.

Given the picture provided of the setup that you have, I would say that it would probably be best to definitely at least move the current sensors to position 3 if at all possible. So long as you are using the solar sensors from Sense, then the energy consumption readings will still be there for your devices when they are using solar power even at position 1, but position 3 is just better overall and wouldn’t require you to use Sense Solar unless you wanted to – which, I mean, it’s a pretty neat thing to have regardless.

At that position, you are right that you won’t be able to see any power provided by the generator unless it would be possible to also get the lines from the generator into the current sensors at the same time. I can’t physically see the setup, but I’d presume that’s likely not possible.

Unfortunately, there’s not going to a be a way to see both the generator energy and the energy used by the oil filter heater and battery heater given this setup. You would need to use position 2 in order to see the generator energy, but that would miss the energy from the devices on the transfer switch itself. Unfortunately, those are kind of the only options that are available right now.

If you do have Sense Solar and you don’t explicitly need/want for Sense to monitor your solar, you could put the main current sensors at position 3 and then attach the solar sensors to the lines that attach to the generator. You’d need to reach out to support for us to enable the solar for you since the normal application won’t work, but this way the solar energy recorded by Sense would be energy provided from your generator and you’d at least be able to view that. I do know of some other users that do use the solar sensors to monitor their generator energy, so it can work.

As for moving the sensors, going from 1 to 3 shouldn’t be an issue. 2 may be a little trickier but also fine. You just need to make sure that the same current sensor stays on the same main line other wise the phase and voltage association is going to be off and then you would likely have to do a reset. Otherwise, everything should be fine and remain the same, nothing that our team would really need to correct for.

Hopefully that answers your questions. Let me know if you need anything else!

Thanks @TylerAtSense.

Polls are no problem. See the last option.

@sol I don’t have solar…yet. But I do have a Generac and have had my CTs in position 1 and 2. I had to move from position 1, inside the house in my main panel, to position 2, outside the house in the meter can sub panel. This move was required when I added a circuit for my Tesla wall charger. It was easier to run the charger circuit this way. I figured it was more important to monitor the house and charger than the generator.

With this setup, the Sense is not powered when the grid power is off. A function of the meter can design. I do plan on re-routing some of the feeds so I can cover everything.

@afreedman - Perhaps you meant that you moved your mains CT’s from position 2 (panel with your branch circuits) to position 1 (meter can) rather than the reverse. My mains CT’s are currently at position 1 on the conductors between the meter can and the transfer switch. They and the solar CT’s are inside the transfer switch enclosure.

@sol - Apologies. Yes, 2 to 1.

Hi Tyler; nice to hear from you.

I think that I’d like to move the mains CT’s from “1” to “2”. Preserving specific line and direction is straightforward. My only concern is that with my current arrangement at location “1” sense has to add the raw measurement of both the solar and mains CT’s to get my usage. When my solar is producing and feeding back to the grid the mains CT’s are recording “negative” usage and this is indeed being successfully netted out of my solar to get my actual consumption. When I move them to “2” then my actual consumption will be measured strictly by the mains CT’s only and the solar production by the solar CT’s. The solar CT’s raw data should no longer be added to or netted with the mains CT’s raw data to get my consumption.

Will sense figure out this change without any intervention by me or customer service? And if the answer is yes how does it accomplish this wizardry? I suppose that it could kind of puzzle it out because as my loads vary on the current arrangement the raw data on both sets of CT’s will vary similarly and this will no longer happen after the move. I’m looking for assurance about how long this will take to be figured out and that I won’t lose device detection or much data (a day or so of loss is no problem) if I make this move.


Sense would not automatically correct for any configuration changes that it notices with the installation, but it would flag an error in our systems should the monitor not be reading correctly. That could take it a few days to do, however, so in this situation I would just reach out to our support team after making the change so that we can adjust the settings from the monitor in order to account for the change in how solar should be handled. (These same configurations are done when running the solar setup, so you may also be able to change it yourself by running through that again, but it’s best for our team to just make sure.)

If possible, I’d recommend either a Thursday or a Friday for that. Those are our lowest volume days most typically so we should be able to see it more quickly and make that adjustment faster for you so it shouldn’t lose much data. You don’t specifically need to address the e-mail to me, any one of our agents can do it. If it will be a Friday, then don’t specifically address it to me as I don’t handle e-mails on that day and am instead working on resolving bugs and fixing things, so it’s possible someone might see that you’re writing to me and you’ll just get a “Tyler isn’t in today but he’ll get back to you as soon as he can!” message and then I won’t see it until Monday.

If you need to make those changes on a weekend, that’s fine too. Just write in ahead of time and let me know when you’ll be making those changes. Our team isn’t in on the weekends, but if you let me know I can always set a reminder for myself and swing by the office to correct that for you.

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