North America, CTs and Calibration

Hello, I got a raspberrypi for fathers day and have been slowly building my emonpi system. I have it working (monitoring one circuit and 4 temp sensors). I am looking to expand…just need to verify things are going to work as planned.

I plan on buying 2 Wattcore CTs
Wattcore WC3 SERIES Split-Core Current Transformer (100:1000mV)

Can someone confirm I need to remove the burden resistor and change the calibration for CT1, CT2 and CT1+CT2 (emonhub) from 1 to 100?

thanks in advance

Yes and no.
Yes, you need to remove the burden resistor (as explained - it sounds as if you’ve read the ‘Use in N.America’ page, but no, the calibration that page refers to is in the sketch inside the emonTx part of the emonPi, not the one in emonHub.conf. You will need to change that, but first, are you buying the correct c.t? The reason I ask (and my colleague @Bill.Thomson knows the details better than I do) is you might not get near 100 A normally, even on the main incomers, so it’s worth making sure before you spend money.
If you do get those c.t’s, then the current calibration inside the sketch is 90.9, whereas it needs to be 100. So to get the Pi to the correct calibration, you need to make the power constants in emonhub.conf 100/90.9 = 1.1 (but that may still need fine adjustment, depending on component tolerances).

Thanks for the response!!!

Follow on questions… from your response, am I buying the correct CT?..any suggestions are welcome. (and If so, what do I need to change/modify in the emonpi


Second, if I go ahead with the Wattcore (100:1000mV) all I need to do is remove the burden resistor and change the cal factor (emonhub) from 1 to 90.9…(and down the road, adjust from there using my power bill for comparison).

If it helps, I’m in a new subdivision (Only house on the transformer…for now). The number on the transformer is 75.

again, thanks in advance

Hi Buz,

As Robert pointed out, if your max load is considerably less than 100 Amps, you may not want the 100 Amp CT.

I see several posistions in your load center have 15 Amp breakers. However, I see one 50 A double pole breaker and three 30 Amp DP breakers. Which CT to choose, might depend on your house being all-electric, vice having some gas-fired appliances. If your house is all-electric, and it’s not uncommon for you to operate more than one of the large appliances (or many small ones) at the same time, then you’ll likely want the 100 Amp CT. If however, you rarely operate the large appliances at the same time, e.g. one is a heater and another an air conditioner, OR you have gas-fired space/water heating, stove/oven, etc, you’ll likely want a CT with a rating less than 100 Amps.

The next lower rating in the Wattcore WC-3 series is 50 Amps, so deciding on which one to use is going to require some simple “number crunching.”

Take a look at the electrical appliances you use on a daily basis, and read their labels to get an idea of how much power they draw. Add up the numbers on the labels of those you use at the same time, Add 15% to the total for some headroom, and you have your answer.

If it’s more than 50-55 Amps, you’ll want the 100 Amp CT. Less than 50 Amps, choose the 50 Amp CT

I see from the label on the inside of your load center door that one of the double pole breakers is for the A/C unit, another for the dryer, and a third for the water heater, but I can’t read the one at the bottom left position. Is it the word welder?

Thanks!! You guys are on point!!

The house has gas (water heater, furnace and stove) and electric (A/c, clothes dryer, lights, etc…the 50Amp (welder) is used for 30amp/110vac power outlets that are rarely used). After reading your comment and doing a quick calculation it seems 2, 50 amp CTs seem more reasonable.

That being said, what is the difference (besides price) from the WC1 and WC3 series CTs
W1C -
Wc3 -

And will the cal factor change?

thanks in advance!!

Yep. Now that I see the label in its entirety, I’d say you’ve nailed it.

Wire window size. The WC-1 has a smaller wire window than the WC-3.

Your load center Service Entrance Wires are size AWG 4/0, so a WC-1 may or may not fit them.
I had to trim some of the plastic housing from my WC-1s to get them to fit my SEWs.

You’ve got plenty room in your load center for WC-3s, so I’d recommend using them.

I went ahead and ordered the Wattcore WC3 SERIES Split-Core Current Transformer (50:1000mV)…I will update the thread once I receive them.

Thanks again for the help!!!

Remember what I wrote about calibration. Here’s how to work it out.

The calibration factor in the emon part of the emonPi is set at 90.9. That’s going to be wrong, but it’s hard to change it.

90.9 is the current that gives 1 V at the ADC input with the standard c.t. and burden resistor. The power calibration in emonhub.conf needs to be 1.0 with that combination.

Your c.t. - whichever one you choose provided it has a 1 V output, requires a calibration factor in the emon part of the emonPi that’s the same as the c.t. rating, 50 for a 50 A c.t. or 100 for the 100 A c.t. So to get emonHub to pass the correct number on to emonCMS, you’ll need a factor in emonhub.conf that is 90.9 / [your c.t rating], so 1.1 or 2.2 depending on which you choose.

I received my wattcore CT’s yesterday. I installed the 3.5mm TRS connectors per the instructions on the "learn EmonTx in North America"page (white to tip and black to sleeve). Removed the burden resistors by flicking them off with a soldering iron. Updated the scale in the emon.conf (0.55 using a 50A c.t. with a 1 V output). I backed up my SD card to my computer using DD (sudo dd if=/dev/rdisk2 bs=1m | gzip > /Users/buz/Documents/emonpi.gz). Turned everything on, updated the inputs and feeds and everything is working as advertised!!! Thanks for the help and advice along the way.

Thanks again,

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