Current Transformer for 100A houses

#1

Noted in the blog post about the Sense CT, it handles current input of 200A and reads current output at 66.7mA (due to 3000 secondary coils). My house is 100A. So this means the current output is 33.3 mA. Does the halving in amplitude matter to current measurements? If no, why not?

Also - is it possible to buy additional CTs? Are these custom or available from a vendor?

Thank you.

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#2

A few thoughts on your question:

  • That means your max current output would be 33.3mA. The lower current from the CT shouldn’t make a very big difference to Sense. Sense does a calibration cycle when first setting up the monitor, and I’m sure the hardware chosen to measure the CT current (shunt resistor and ADC) is designed to get to squeeze sufficient accuracy out of whatever level of current is flowing from 200A down to nearly zero. The converters they use have 14 bits of resolution. That means that 12-13 bits would be used to represent 100A if calibrated correctly. Say it was 12 bits, that would mean that Sense could measure down to 100A/4096 or about 25mA.
    http://www.ti.com/lit/ds/sbas539b/sbas539b.pdf

  • The only place that the lower CT current might affect you more, is if there is some fixed level of electrical noise induced into the CT. You might experience a lower signal to noise ratio at your “rated” signal levels. But that affects all measurements taken closer to zero.

  • You can buy the solar option and get another set of CTs that plug into the monitor, but they are not much use unless you want to measuring solar, since the sampling and software analysis of the CTs are tuned for only that. What did you want to do with the additional CTs ?

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#3

Thanks so much for your answer. re: What did you want to do with the additional CTs? (now don’t laugh!) I’m building PCBs monitoring energy using the atm90e26. I’m doing this because I wanted to use the Sense monitors for my project, but folks @ Sense did not feel my project was worth a partnership effort. Does that stop me? Nah…I’ll just build my own. How hard can it be? (yah = ok, keep laughing…). Now us DIYers have the SCT-013-000 CTs for the 100A folks (actually this is in my home). But for the beefier 200A…well…i’m finding these more difficult to come by. Besides, the Sense ones are very cute. Stylish.

Anywho, if you what to laugh until you cry (or roll your eyes alot), here’s my github for one of the pcbs….

(BTW: here is a blog post I wrote where I go through my Sense install.

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#4

Interesting, but not rolling my eyes. I’m wondering if you would be better off just using a purpose built eval/development board from Microchip than building your own ? They use the same chip, but offer a software wrapper for calibration, pulling power data and adding your own secret sauce ??

https://www.microchip.com/wwwproducts/en/atm90e26

https://www.microchip.com/DevelopmentTools/ProductDetails/ATSAM4CMS32-DB

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#5

My “secret sauce” is a Flutter app (ios/android) that doesn’t do anything fancy…just builds awareness and mindfulness around energy waste in a home… I will check it out. And thank you for not rolling your eyes. Personally, I’d rather not do the hw at all. I realize it is dangerous. But I really want to see this project through. I very much appreciate your replies. THANK YOU.

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#6

ugh. re: microchip dev board. What is it with Zigbee? I am finding LoRa to be far easier, and much, much more friendly. I mean…pay to license Zigbee…pay more for chips…hmm…

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#7

To answer the original question, 100A panel should not pose a problem for Sense. We have some installs out there on 50A panels with no issues.

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#8

Thank you for the reply Ryan.

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