emonPi USA CTs

I'm in the US and want to monitor whole house usage. The incoming service is 200 amp split-phase. The service wires are 16mm (including the insulation) so the CTs available in the shop won't work. After some digging through the forums I found lots of discussion about the alternate CTs for the emontx. Seems the Magnelab SCT-1250 without a burden resistor are preferred. Using the Magnelab CTs requires the replacement of the standard  burden resistors that are installed in the emontx. 

Before I buy anything I want to confirm that the same Magnelab CTs would work with the emonPi in the same manner... are they the recommended CTs and would I need to remove the resistors on the CT inputs?



Robert Wall's picture

Re: emonPi USA CTs

As far as I'm aware, the input circuitry of the emonPi is a 2-CT version of the emonTx. So everything that's been written about this (e.g. EmonTx - Use in North America) applies to the emonPi as well.

The design files are here: https://github.com/openenergymonitor/Hardware/tree/master/emonPi/emonPi_...

Adam Byers's picture

Re: emonPi USA CTs

Ok, so I remove the resistors on R17 and R28 and replace them with 36Ohm resistors (assuming I'm using the Magnelab SCT-1250 without a burden resistor).

Just want to be sure before I drop a chuck of change on the emonPi and CTs. For a product ostensibly marketed to the US, and even kickstarted through a US company, sure is hard to get information on getting it working here... 

davekramkowski's picture

Re: emonPi USA CTs

I am in the same position as you Adam. I ordered a pair of these CTs for my 200A service. As was pointed out to me here by Bill Thomson, in most cases, the power company's transformers really can't supply you with that 200A capacity that your panel can handle. Most likely, about half that at best can be supplied. Also, unless I mis-understand, the 200A rating of the panel is 200A per leg. My main feed lines measured in at .59" (~15mm).

Based on all that, the 100A CTs *should* be fine for most applications, unless you plan on clamping the neutral (and even then, if you can't get much more than 75A from the power company...), although with the ground capable of supplying neutral, I'd think that would be a bad idea as it might not be able to provide accurate readings.

With the WC1's I ordered, based on a formula posted in my thread, I'm going to try either 12Ω or 15Ω burden resistors, and I'm going to build a pigtail into the CT's plug to short it for safe removal from the EmonTX I have coming.

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