Help with whole house power monitoring (200amp/USA)

I have two of the below CTs and got them wired up to the emom-pi. I am getting reading but they are way off. I am getting -168 on one leg and 462 on the other. It is not possible for me to get more than 200amp per leg. Is there some config that I am missing?

That is not one of our “preferred” c.t’s; however you can use it.

Our standard current input, for all the Emon devices, is 50 mA. The calibration is set to show 100 A per 50 mA input current. Therefore, there is one of two things you need to do.

  1. If you feel confident, you can remove the SMT burden resistors inside the emonPi and substitute wire-ended resistors that will give the same voltage but with only 33 mA flowing. The value you need is 33 Ω. Then you change the scale factor to give the correct current reading. In emonhub.conf (accessible via your browser), for Node 5 change the ``scales = …` values for your 3 powers (Power1, Power2 & Power1plusPower2) to 2.0 ( = 200 [your current] ÷ 100 [our current] ).
    This will give you the best input sensitivity.
  2. If you don’t feel confident to do the hardware change, then just change the scale factors. In this case, the number you need is 3.03 ( = 50 [the original c.t. secondary current for 100 A] ÷ 33 [your c.t. secondary current for 200 A] × 200 [your current] ÷ 100 [the original current] ). If you don’t change the resistors, then at low currents, you will lose some sensitivity and hit inaccuracies sooner, but unless you are really going to be concerned with currents below about ½ A, it’s not likely to be significant.


You may want to have a look at this US 200 Amp Service explained or Do I need a 200 Amp CT?

Ok so I change the scaling and I am now getting close to what I get with a clamp on amp meter. I also swapped the cables for one of the CTs and no longer getting a neg number. I also did not notice at first that the input from the CT is in Watts when show in the interface. Well this is a problem, That means you are taking the volts x amps which is fine but the interface is showing the wrong volts. I have the calibration = 110V and the interface shows 110 but the voltage is really 116-117 meaning the watts number is wrong.
What is the fix to get the real watts usage?

Bill to your point I am the sole home on a 25kva transformer so I cant use 200 Amps available continuously but where I have to place the CTs the wire awg is 4/0 AL. Between wire and insulation it just under 3/4 in so the only CTs that I could find that would fit was the 200amp.

Hi Zach,

I’m in the US too, so have the same issue. i.e the AWG 4/0 Aluminum.
To get around that, I use Wattcore WC-1 and Magnelab SCT-0750-000 CTs.

Like yours, mine is the sole house on a 25kVA pot.

My main aim was to point out to other users reading the thread that 200 Amp service doesn’t
mean what it sounds like it means.

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Do you have an a.c. adapter measuring the voltage, or is your emonPi hoping that you have the nominal voltage?

seems to suggest that you do have the a.c. adapter, in which case, you are measuring the actual supply voltage, and using that in the power calculation.

Yes I am using the AC voltage sensor from the store. I connected a volt meter to the same outlet as the voltage sensor and got 117volts but the voltage sensor keeps saying 110volts. If I disconnect the voltage sensor will the input change to display amps?

That is probably down to manufacturing tolerances, which is ±5% for the adapter alone - add another 3% nearly for the parts inside the emonPi. You can safely adjust the power calibration again to compensate for that if you wish.

No. It will display zero power (or nearly zero) immediately, then if you power-down and power-up the emonPi without the adapter (or with it not energised), then it will assume 110 V (don’t ask me why that and not 120 V, we pointed out the error a while ago) and calculate apparent power based on that and the measured rms value of current, rather than real power based on the true voltage and current. It will continue then to ignore the a.c. input until it’s powered up again.

[Note: I mean powered up - rebooting the Pi doesn’t affect this, all the measurement and power calculation is done in the ATMega 328P front end.]

Well ok then sounds like I have gotten it as close as I can. Thanks your yall help.

I’m slightly concerned if you’re seeing exactly 110.0 V. That would suggest it hasn’t recognised the presence of the 9 V (nominal) a.c. from the adapter at power-up. An easy test is to temporarily unplug the a.c. input. If the power collapses to zero and then comes back, you are measuring the voltage.

If you compare the energy recorded against your supplier’s meter, and make small adjustments after reasonably long periods, then you should get quite good correlation between the two. Some users have claimed around ½ %.

@pb66 has clarified that for me. If you were looking at the EmonHubJeeInterfacer in emonhub.conf, then that “110V” is purely a “flag” (not an actual calibration value) that gets sent via a roundabout route into the ATMega 328P front end, where it selects the real calibration values. So the number being “110” there instead of 120 makes no difference, the actual calibration for the a.c adapter input has always been correct.

What would be wrong with leaving my North America volts at 220V in the config. while using a 120 volt AC-AC adapter? I have no use for any 120 volt measurements…all I want are 240V anyways.

Can I “fix” the calibration in ATMega 328P front end? I really do have 240V…not the EU 230V.

You can certainly leave it so that the factor for 230 V is used, it will indicate 230 V when it’s actually on 120 V.

You could, but…
In their wisdom, Glyn & Trystan decided that the software in the front end would be overwritten when you updated your emonCMS. So if you do adjust the calibration factor in the '328, you must forever remember to update your emonPi as as emonBase, not as an emonPi. That way, it doesn’t get overwritten.
So unless you need to adjust the phase error calibration, it’s probably safer to adjust the scale factor in emonhub.conf and let the '328 software get updated.
(I have put forward a suggestion to move all the calibration into emonhub, but it’s not met with approval.)

You’ll need to adjust the calibration to read the correct voltage and power (because of course power is calculated in the front end using the wrong voltage).

So do I! But I can’t tell you why 230 V rather than 240 V was chosen. Although the UK voltage is ostensibly 230 V ±10%, the real centre voltage remains at 240 V, as it was prior to “harmonisation”.

Thanks for the informative reply.

I thought the AC-AC adapter was to allow for the Hub and Pi math to get RMS accurate measurements.

Does the math make any difference if the AC-AC primary volts were selected for either 120V or 240V?

Is the AC-AC supplying Hz or volts or both for a reference input?

It would be a good thing to have accurate data here, maybe it is, just not easy to realize that.

And again, all my loads being used with my emonPi are N. America 240V.

Indeed it is - and accurate real power measurements too.

Of course. The a.c. adapters all give approx. 10 V out, for 120 V in with the US model, 230 V in for the EU model and 240 V in for the UK model. You want the reading to be 120, 230 & 240 V respectively, so the calibration must be different for each (notwithstanding component tolerances, which also affect the calibration).

Yes to both, but we don’t measure and use frequency in that sketch. (The phase-locked loop sketches for 3-phase do lock to line frequency.)

It is a matter of calibration. If you can read current and voltage accurately, then you can calibrate your emonPi accurately “on the test bench”. If you can’t, you get it as close as you can, then take readings against your supplier’s meter. Even if that is not 100% correct, by definition it is correct (because that’s the thing that determines the size of your bill). By making small adjustments over a period of time - weeks and months rather than hours and days - you should be able to get to around 1% agreement or better.

I’m missing something here how this all works.

If all the emonPi or hub sees only the 10V secondary of the AC-AC how does it know or even care what the primary voltage is?

I fail to understand why I need to edit the hub config file with the primary to be 120V since all related loads, in my case, are 240V…or said another way…how is my 240V different than your 240V?

Other than it is rather convenient to just plug it in to a wall socket here in N. America it doesn’t make sense why the emonPi or Hub needs to know the difference. What magic am I missing?

The magic you’re missing is the the transformer (AKA a.c. adapter) secondary voltage is, to all intents and purposes, a reduced-scale replica of your mains voltage. So by measuring the low voltage and then scaling the numbers up, we get an accurate idea of what the mains voltage is.
All this is because your mains voltage isn’t 240 V, constantly and consistently. It varies over time, according to how much load you, your neighbours and your town (and quite possibly, the whole country) puts on the system. Also, don’t forget your a.c. adapter is a 120 V one, so you cannot and must not connect that across 240 V. Therefore, we have to lie to emonCMS (via emonHub) to convince it that the 120 V that the adapter sees is really, as far as its sums are concerned, your 240 V mains, even though it was transformed down to about 10 V, then divided down to less than 1 V, for the '328P to be able to measure it. That’s really no different to what happens inside your multimeter (except that it won’t have the transformer/a.c. adapter).

Here’s what my mains looks like today - it varies between 232 & 246 V:

Where that scaling happens makes little difference. Historically, it was all done in the front end; and the emonPi front end tries to do that as a property that it inherited from the emonTx. However, it’s not nearly as easy to edit and keep the changes to the software of the emonPi front end as it is with the emonTx. However, the design of emonHub allows us to ‘fiddle’ the scales = ... parameter to achieve calibration, even though it was not put there for this purpose. The result is a rather messy, non-optimal, solution. But it works.

Thanks again Robert.

So my 120 V AC-AC secondary voltage (10 V) is not representative already to what a 240 V AC-AC secondary voltage (10 V) would be? Here comes that magic again…they are both the same (10 V)???

How does it know I have to edit hub.config file if it is the exact same value either way (10 V)?

No, the AC:AC adapters are a linear transformer, the output voltage is relative to the input voltage by ratio.

For example putting 120v into a 240v AC:AC adapter (of this type) would output half as much voltage (eg 5v not 10v).

In very approximate terms the 240v AC:AC adapters are 20:1 ie 240v in 12v out. Whilst a 120v AC:AC is around 10:1 ie for 120v in 12v out.

If I understand correctly you have a 120v AC:AC adapter on half of your 240v supply, there fore you must use the 120v settings (“110V”) to interpret the voltage level correctly out of the transformer, then Robert (I think) is saying you need to then double that to “represent” the 240v supply, essentially (I guess) because there is no “double US” setting.

[edit - I’ve just re-read your original comment and now understand what you are suggesting. Have you tried using the 120v AC:AC on one half and continue using the “230V” setting, ignore the number it’s misleading, just think of it as 2 modes “110V” and “230V” that just represent the scaling for a 120v AC-AC or a 240v AC:AC, it’s about the ratios and less about the numbers. In theory what you suggest should work I think, there is no harm in trying it. Just do not connect the 120v AC:AC to 240v, the voltage indicates what is safe as well as the ratio.]

[ hopefully Robert can confirm this, I wouldn’t be at all surprised if my math is back to front
my brain is overheating in this heatwave, we’re not used to this much sun in the UK]

I can’t confirm anything - for exactly the same reason! I’ll try to come back to this tonight when hopefully it’s a bit cooler.

Larry, I think you probably have a misconception or you’re making a wrong assumption somewhere, that I’ve failed to spot. I’ll re-read the thread from the top and then try to explain it from the ground up.