Burden Resistor Removal


I recently obtained an Iotawatt for purposes of measuring my panel and providing as secondary metering source of my PV installatiion.

I purchased two egauge revenue CTs and two ACTL-0750 Split-Core CT. Both of these use burden resistors. the ACTL ones provide the burden resistor value of 26.4, I have an email out to egauge to get the value for their 200A CTs.

From what I gather, iotawatt would prefer CTs without burden resistors. However, since I have already bought these I’m looking to figure out how to configure them. The spec sheet on the ACTL CTs says it uses a winding of 100:1 but that doesn’t give me proper readings on the status page.

It looks like the burden resistor on the channels I want to use these CTs needs to be removed from the Iotawatt unit itself? Is this detailed somewhere? I haven’t been able to locate it through the documents and forum. Thanks!

From what I can see, both of those CTs are 333mV output. That is to say they output 333mV at their rated primary current (50A, 100A, 200A etc).

While they can work with IoTaWatt, they will only use 33% of the ADC measurement range, so the granularity of the readings will be 1/3 of what an AccuCT or other 50mA CT will do natively. That said, there are plenty of users who have adapted to using one or more 333mV inputs.

To do so, you must first remove the burden resistor on that input. The burden is the resistor next to the corresponding input jack. Heat it up and remove. Then in the app under Setup->Burden Resistors, set the value for that input to 0.

The configuration for that input will change, specify model “generic” and set Cal to Rated Capacity / .333
So if you have a CT that outputs .333V at 200A, you would set Cal to 200/.333 or 600.6.

I will note that while these two CT offerings are sold as “revenue” and are very expensive, IMO the AccuCTs from the stuff store are just as high quality and using the full input should produce better results than those more expensive offerings.

Hi Overeasy,

Thanks for the quick response. I purchased the CTs from a friend so I got a good deal. If I need future CTs I will look at the store. I had success removing the resistors and dropping in the values you suggested. I have a follow up question.

  1. Through my searching of this forum it seems like the split core phase recommendation is 2.0, but I have seen it also suggested to be 2.5. I’m wondering if you can suggest which one would be best for me as I am using 2.0 currently. Thanks!

All I can do is give a WAG. For a 50A CT maybe 2 and for a 200A maybe 1. My procedure for developing those numbers uses a custom instrument and often yields unexpected values, so it’s a guess at best.