OpenEnergyMonitor Community

Remote CT to control GTI export limit 250 feet (~75 meters) away

Hello, I am in need of a remote CT transmitter and receiver (I think). My split phase 240 supply is approximately 250 feet from my solar shack. The solar shack contains everything related to solar input back to the main panel, but I am not allowed to export to the grid, so I need to send the CT output to my grid tied inverters which are also 250 feet away in the solar shack - so that the GTI’s will limit export to 0, but allow me to self consume during solar production hours.

I’ve been looking for a solution far a few weeks now and I think (hope) the emonpi or TX would be able to send the data over my network to a emonpi at the other end of the network and then somehow interface with something (not sure what the something would be) that can read the real-time data from the receiving pi and convert it back to current for the grit tied limit input.

I’m not an EE and not a developer, so the solution to this problem will need to be relatively straightforward to implement or I’ll be unable to set it up and get it working properly.

Anyway, am I barking up the wrong tree with emponpi or is this something that is pretty common and easy to set up?

Thanks in advance for any guidance you are able to provide.



Welcome, David, to the OEM forum.

If I understand correctly, you want to monitor the power at your house, where the service entrance wires come in to your distribution panel, and then convey that power reading over your LAN to the solar shack, where the second part is to turn that number into a quantity that the GTI can accept in order to control its output.

The first part of the problem - measuring the power - is relatively easy; whether you use an emonTx and (say) a Pi Zero W to launch the data onto your LAN depends on what else you want to do with the information.

The second part isn’t something we deal with regularly. What will the GTI accept by way of a command? You suggest it’s current - what current, or can it be something else, like numbers over Modbus, a d.c. voltage, a 4-20 mA current, or what? If it’s not in published documentation, you might need to talk to the distributor or manufacturer to find out.

If the GTI can only accept a current from a c.t., the major difficulty with what you’re suggesting will be simulating that current from the numbers. That’s going to require development work, and somebody to manufacture a bespoke unit for you.

Trying to think of all the alternatives, I’m guessing that it’s impractical, given the distance, to run an “extension” cable for the c.t.(s) all the way to the house, and equally impractical to relocate the GTI next to the incoming supply and extend the d.c. cable from your solar panels.

hi I made CT extension cables that are about that length using cat 5/6 . since the CT has 2 wires (usually ) i made special adapters usually based normal cat 5 pattern so you can just use off the shelf pre made cat 5/6 wires … i just make all the striped wires connect to the ones side o the ct and all the solids to the other side of the CT… the wattage will be off slightly( a couple of percent depending on wire length on GTI but zeroing the grid works fine ). but if the limiting GTI you use on your split phase only has one ct you have to add another to get the second leg then it will zero out properly. …

From what David wrote, it would seem impractical to run a cable to extend the c.t. But if it is possible to do that, it is by far the simplest, cheapest and most reliable solution. It’s not cheap though if there isn’t an existing duct and you need to bury armoured Cat 5 cable.
[Edit] Checking the prices, ordinary 2-core 0.75 mm² armoured cable is a lot cheaper than armoured Cat 5, and better for this purpose.

Robert, Stephen, Thank you,

RW, the grid tied inverter has a two wire input that attaches directly to the CT, I do not know how the GTI senses the CT ‘input’ - I speculate the GTI senses current instead of voltage, but really it could be either or both, I don’t know how these things work.

SK, I thought the line would pick up too much noise on a run of 250 feet. I could run it for the first 90 feet within my attic from the far Southeast corner of the attic to the far Northwest of the patio to a bundle of 5 buried CAT5 UTP that I have running to the shack. If I can use the unshielded cat five, that would be great! BUT, I wanted to also measure solar production using emonpi, so I was hoping to kill two birds with one stone - I’ve only recently stumbled across this project (within the past few days) and find it to be interesting and useful. At least from a solar perspective, it’d be great to extend this remote sensing capability to handle the use case I am struggling with because it occurs more frequently than you might imagine, so having it as part of emonpi seems a natural extension of the emonpi solution.

RW, the GTI’s need to live in the solar shack, which is the name I’ve given to my temperature controlled shipping container at the far end of my property. The container has two six panel arrays (400W Trina panels) and another six joining the first 12 before the heat of summer sets in this June. I’ll be adding at least another 18 panels as time and money permits this year and hopefully another 18 next spring. I’m using several paralleled grid tied inverters to feed this power to my home for self consumption. The GTI’s live in the container for a few reasons, but the biggest is a space limitation in my home - I simply don’t have room to install them near the service inlet or anywhere else in the home, but because I can’t feed power back to the utility, I need to somehow extend the CT sense current from the service entrance all the way out to the container.

If using one or two of the buried Cat5 cables will work, I think my problem is solved, but I had assumed a 250 foot cat5 run was simply not an option because the “noise” I expected to pick up over the course of that run.

Anyway, thanks again to both of you, I appreciate your helpful guidance.



That confirms what I suspected - there isn’t really an alternative.

It’s going to be sensing both. It obviously needs voltage to synchronise to the grid (and to detect ‘islanding’ so that it can shut down) and it will be using the current to balance the power (or more strictly energy) to achieve the no-export condition.

I think you’re going to have to treat controlling the GTI and monitoring production as two separate tasks - certainly, it’s going to be simpler. Before going too far, I’d suggest you think about just what else you’re likely to want to monitor (and how) in the foreseeable future. You certainly want to see what data you can get out of the inverters, especially if you will have many. @Bill.Thomson has experience in this area. And many people also like to split the domestic consumption into areas so that they can separate their usage patterns.

Noise could be a problem, but the impedances involved are quite low, so it’s likely to be less of a problem than you might imagine. But it does depend on whether the cable runs adjacent to power cables, for example.

If you have unused Cat 5 cable in situ, then the obvious thing it to try it and see, and be prepared to stop if it does export power (which your meter should show anyway).

As his main objective is to prevent export, it sounds as if one of Robin Emley’s diverters would do the job. He’d have the energy available at an instant when he wants to “self consume” and when he doesn’t, it could be run to a water heater (provided of course that he has electric water heating) or other dump load. (I used a 55 gallon drum of water)

Hi dave,

I’m curious, Dave, what state are you in?
I’m in the US too. This is the first I’ve heard of “export not allowed.” (in the US)

But that depends on adequate dump load being available - and when it isn’t, David still needs to throttle the inverters.

So a diverter would be an addition, not a substitute.

I understand that’s the case in Spain (or parts of).

That’s not much of an issue. 5500 Watt immersion type water heater elements are widely available
and very affordable over here. Common ratings are 1.0, 1.5, 2.0, 3.3, 4.5 and 5.5kW. There are probably others as well. Those are the ones I’ve seen on shelves in several shops.

I ran one of Robin’s diverters into a 55 gallon drum of water dump load until I got my net meter.
It took about 9 months to get it. Robin’s diverter did the job very well. The only thing I had to do was top off the 55 gallon drum as needed.

I’ve heard about Spain, but I meant in the US. That was why I asked him which state he was in
as well as mentioning that I’m in the US too.

Hi Bill,

I should’ve provided more detail rather than say export was not allowed. It’s not allowed in my case because I do not have an agreement with the local utility. Those agreements end up costing many thousands that I’d rather spend on storage. My GTI’s are anti-islanding and I do not plan to export, so in effect I’m not tied to the local utility.

I’m in Arizona, our utilities are quite strict about feeding back to the grid and even with an agreement, if we do feed back to the grid, the utilities give us very little for our KWH’s.


I hear ya there, Brother. The town I live in (SW Oklahoma, pop ~900) gouges its resident 13 cents per kWh no matter what they pay for the energy. They give me 95% of their avoided cost (wholesale rate)
(gee, I wonder why they call it avoided? :grin:) which varies from 4.5 to 7 cents per kWh.

Any IEEE 1547/UL 1741 approved GTI will be anti-islanding.

If he can use 333mV CTs, Magnelab says of their CTs:

Q-What is the maximum length of black and white lead wires?

A-The current passing through thestandard lead wires of the Magnelab current sensors very low, therefore, there is insignificant voltage drop due to the length of lead wires. In practice, the units have been tested up to 7,500 feet and the output voltage and phase angle shift were withinthe listed specifications.

Frequently-Asked-Question-on-Magnelab-Standard-Series.pdf (54.4 KB)

Bill, that’s extremely helpful guidance/advice. I also reached out to Dent Instruments too:

Wish I would’ve found that link before posting here, but on the bright side, I’ve stumbled across what seems to be a great community here!

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That advice (Magnelab & Dent) will be true for any c.t. with its output at rated current in the milliamps (up to say 100 mA) region. It won’t hold true for 1 A & 5 A c.t’s - not that users here are likely to see those - unless you use the appropriate cable and the c.t. has a v.a. rating to overcome the voltage drop.

Correct Robert, a dump load would be in addition to, not in lieu of an extended CT run, but I do like the idea of a dump load too, as the weather here is perfect (at least by my standards) during our winter (33 degrees North) where it is currently cloudless and headed to a high of 68F (20C) so my panels, even with the small array I currently have connected, produces far more power than I use during the day and I don’t yet have enough battery storage to consume all that excess power, so a winter dump load sounds like a perfect solution.

During the summer, a dump load isn’t needed because I will consume 100% of my solar production and still consume quite a bit of power from my local utility provider. The weather here is great during the winter, but summer is oppressively hot - daytime temps routinely reach 115 (46C) and sometimes several degrees hotter with nighttime temps that drop to a wonderfully unpleasant 99 or 100 degrees F. Those oppressive temps require considerable cooling during the day (my office is here in my home - even pre-COVID) and almost as much during the evenings so that we can sleep comfortably. Our average July usage is 140KWh per 24 hours, so as you might imagine, I can consume everything my small 7,200 watt array will produce.

I’d need the dump load outside the home for (at least) two reasons:

  1. I already have a fairly high efficiency hybrid heat pump water heater

  2. even with panels set to 80 degrees to provide the greatest output during the summer months, I’m still producing at least 3,000 watts more than I consume during daylight hours, so I suspect a continuous 3,000 watts into my water heater for almost six hours each day would cause the water to boil and possibly launch it into LEO.

Anyway, thanks again Robert and Bill, I appreciate the guidance.



I used a 55 gallon drum with a 5500 Watt immersion heater along with one of Robin’s diverters.
At the time, my system was about the same size yours is now. I had no problems dumping all of the excess energy to the drum of water. Had to top ‘er off more when it was sunny, but other than that,
it worked like a champ. Robin’ diverters can work with more than one dump load.