Watts look strange when clothes dryer on

Finally installed my unit after waiting 3 wks USPS delays! The daily energy consumption matches nbrs from my electric supplier for last 3 dys, so thats good news. Problem is, whenever my clothes dryer (240V red/black wires) comes on, the TOTMisc watts go up dramatically. The dryer is connected to a separate 100A subpanel and I have 2 CTs connected to it. Looking at the last photo, when dryer is ON, you can see that TOTMisc goes from 124.1W to 806.6W. And when Off, it goes back down to 120+W. Am I missing something here or is there something amiss with my circuits? Could the SCT013 CTs be defective and way off in its measurements?

My setup:


Comparison of Dryer Off and On:

I suspect that those are not SCT013-000 (100A:50mA). They may be voltage type SCT013-050, STC013-030 etc. If that were the case, they would read low configured as SCT013-000. They all look alike but the model should be printed right on them.

I’m sure these are SCT013-000. I got them from Amazon, (which doesnt mean a heck of a lot in terms of being the genuine stuffs!) but I did open it up and saw there are no burden resistors and it has the black diode on it.
Outside markings:

For comparison, I then removed both SCTs from the sub-panel and connected them to the same line as my HeatPump and below are the results (didnt select Double since its just to test). Amazing how much it differs from the H040-50 CT I got from you which is connected to the HP. If I then double the readings, SubA differs by 148W and SubB 232W. Is it normal to have such a big variance (>5%) for these CTs? I also tested with a clamp ampmeter which read 10.67A on the same line, so 10.67Ax123Vx2 = 2625W, which is pretty close to the H040-50 reading of 2604W.

I’ve asked AMZ for a replacement but assuming it comes back with the same variances, would you recommend setting these CTs as generic and adjusting the turns etc to get the measurements closer to the H040-50 or will it make things worse as the wattage changes?

I don’t recommend trying to use these. TBH I generally get decent results with SCT013-000. I can’t say what is going on here. I get a little nervous when you tell me that you took them apart to look for burden resistors. One of the most common causes of CTs reading low is an iron core is cracked or that they don’t mate perfectly.

Just looking at a status display doesn’t really tell the whole story. Without getting into my reasons, I’d like to see a Graph+ plot of SubA, SubB and HeatPump from the period when you did this last comparative experiment. A fairly high resolution of an hour or less, and if you could create an output that is HeatPump/2 and use that instead they would be at the same scale.

Here’s a 45 min auto (5s) resolution of the data:

Looks like just a couple of bad CTs. SubB is much worse than the other. I just put an AccuCT 100A and an SCT013-000 on the same cable with around 2,000 Watts and got a consistent difference of 9 Watts. That’s less than 0.5% and what I expected.

Doesn’t matter how good the monitor is if the CTs are the weakest link.

Bob… Thanks for looking into this. Hopefully the exchanged CTs will perform better. 2 bad out of 2 is not good … wonder if these are counterfeits!! Anyone come across any fake SCT013s?

I don’t know about counterfeit - but i have just gone through all 22 of mine from my 2 x IOTAWatt and there was a fair variance in them.

A number of them were marked as 100:50 and were not - they had the Burden resistors in them so i removed those and they calibrated much more closely to my 50/1v units (with the Burdens removed)

I have now gone through all of mine and find that for each Amp range their is a different number of windings that need to be specified (i went with 2.5 on the phase lead on all of them) - but within the amp range (such as 30amp) they all come out pretty close with the same winding numbers


Craig… thanks for sharing your experience with these CTs. I just realized mine could be clones. The brand logo on mine says “YHDO” rather than “YHDC” and its also missing the www.yhdc.com, white QC pass sticker as well as the patent markings on its side unlike the real YHDC CTs shown at the manuf’s website. Just my guess/observation but something to look out for anyone else looking at buying these CTs, even if its from Amazon as they all look the same at a glance.

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