Is the only extra work wiring the vt to a breaker instead of plugging it in? How much more does the euro vt cost?
Reason Im thinking I shouldn’t use the 120v vt is due to having limiting 240v inverters supplying a Split-Phase panel…the inverters can slightly overfeed-backfeed-underfeed when legs are not well balanced…so I’m trying to figure out exactly how much unbalance makes the 240v inverters not supply power correctly. If I was using 120v vt during the unbalance I would not get the most accurate reading I could. During unbalance is when I really need things to be accurate…say the unbalance was 2000w from several extra loads on one leg. That’s around 20 amps and then also say the volts between legs are off by 2 volts then that’s 40w right there that would confuse the results on the graph during unbalance therefor not helping me see what the 240v inverters are doing during unbalance
To be precise, you would use two120V VTs and put 2 CTs on each inverter (total 4). Then specify a different VT for each of the two CTs on each inverter.
You would make that 240V circuit breaker connect to a duplex outlet with each socket a different leg and using the common neutral. That way each VT measures one of the legs.
Wait I thought I could just use one 240v vt for the two cts on my mains and connect that vt directly to a 240v breaker …owe I see u meant to have the most precise measurements. Can more than one vt be connected to one iotawatt unit? Doesn’t matter I guess cuz I’m not getting 4 vts…I could see me paying for 2 of them but not 4…u did say that using a 240v reference and halving that is better-more accurate for split phase Na power supply when loads are unbalanced correct? could I just use two 240v vts, one for both mains cts and one for both inverters cuz inverters share a common output coming out of the subpanel the inverters are connected to? Does the Iotawatt allow for voltage references from two different Vt devices?
You can.
Yes, up to three.
Slightly better.
I don’t see any advantage to that.
Yes, you can specify the VT associated with each CT. That’s how three phase works but you can just as well use it for the two legs of split-phase.
You will recall I recommended just using one 120V VT. I still think you should start with that. If it turns out you have a unique problem with imbalance and two inverters, you can always add another VT to get more data.
Therefore making me assume that if legs loads are more unbalanced (like having a 1000w micro on one leg) the individual legs voltages would be even more different from each other…
Legs could be unbalanced easily, so I’m glad I got two Vts to read more accurately.
I just want to point out that the entire range of this plot is 3V which is 2.5% top to bottom. The big swing at 22:25 is a 240V load as evidenced by both legs dropping the same amount. Although it may be caused by another user on the transformer, it would have no effect whatsoever on the accuracy of a single VT IoTaWatt.
The variation caused by the 400W pool pump looks to be about .25V. That’s 0.2%. So it would cause a very small variation in power. In the case of the pool pump, less than one watt.
I think the way these VTs are tracking your voltage right out of the box is phenomenal, but want to caution users not to get carried away overly complicating this. As much as this proves that the voltages vary, it demonstrates that modest imbalance will not cause your power measurements to be misleading.
I agree with everything u said…
Loads could also be unbalanced let’s say as much as 2000w instead of 400w which I believe makes my volts vary as much as 2v instead of .25 so the amount not read correctly could be as much as 34w in extreme cases…that would be 17amps (x) 2 volts. The 17 would be the amount of amps needed for 2000w and the 2 volts would be the difference in the legs voltage due to the unbalanced legs… I’ll get a pic tomorrow with very unbalanced loads…But again I agree that using just one voltage reference is very accurate…
Bob, is there an impact to the power factor calculation if you have a voltage imbalance on a split-phase service? I think that might compound the energy accuracy. A slight shift of the neutral point perpendicular to the 240V vector would mean the 120V reference voltage sees roughly double the phase-shift relative to the other leg.
I’m not sure what is going on with my system that is impacting accuracy, but I think the combination of solar and multi-tenant utility transformer is part of the issue.
I assume if the majority of (active) loads on a panel are 240V then you are better off with a single 240V reference, and if the majority is 120V you would be better off with two 120V VT’s; is this correct?
This isn’t the best way to address your accuracy problem. IoTaWatt is always much more accurate than you are reporting and these things ALWAYS end up being a configuration issue. I’m willing to try to resolve this but as I said in another thread, I don’t have a context to understand your setup, so I asked for some simple information as a starting point.
https://community.iotawatt.com/t/checking-accuracy/4503/6?u=overeasy
overeasy, after configuring my unit, learning some interesting insights and reading as many posts as I could find around monitoring one leg, each leg, or 240v, I’ve drawn some conclusions.
My older home with multi-generation wiring, etc. has quite and unbalanced load between split-phases and this is in winter before we start seeing voltage dips on some circuits below 106 in summer with A/C etc. as reported by my APC UPS devices.
I have little appetite right now for much re-balancing activity and am just finishing up my lab/network closet which I repurposed an old dryer circuit to a sub-panel with a direct ground run and 2 x 15A breakers, one for each leg. I could re-configure to allow for a 240v VT but have little other purpose for 240v in the lab.
Since I’m already down the rabbit hole here and having fun, what would you do if you were me and were already committed to either adding another 120v VT or switching to a 240v VT? I am using the single CT with the U loop method on 3 240v circuits.
This screenshot is with little use right now, just to show the config (mains phase delta hits 15-20 amps often, periodically 25+ and really causes the circuit to mains load delta to grow to 5+amps):
This is typical winter usage:
Thanks.
What are you trying to achieve?
My house typically has a KW more on one leg vs the other. This is not a problem. Even 5KW more on one vs the other is not a problem. If you regularly have 20KW more on one leg with standard residential service, it might be worth considering moving some of your grow lights or Bitcoin miners to the other leg (or running them on 240V).
Unless all of your loads are 240V there will be an imbalance most of the time. It is perfectly normal.
If you are concerned about accuracy, the difference (between legs) is likely about 1V at most, most of the time. That is less than 1%, which is about the long term accuracy you can expect.
I have 2 Iotawatts and I have been measuring the two 120V circuits for a long time. The data show that the difference between the legs is HIGHLY correlated to the current difference, and that it is typically less than 1 volt different (this is with a 10A or 1.2KW difference between the legs). The mean difference is about 0.5V. You will probably make the same decisions with data that has this accuracy than you would with data that is 10X as accurate.
Lols, yeah, this is an older home that has 6 dedicated circuits for the kitchen and the rest of the home shares the rest. Not balanced well circuit-wise either.
I think asking the goal prompted me to realize that I’m mostly interested in monitoring voltage on each leg (would love each circuit but I have UPSes to do that where I have tech) as we have heavy A/C load here in summer with unstable power and the voltage really fluctuates). My home lab barely breaks 5amp in reality and pushes PoE all over our property.
Short answer is you should be fine with your 120V reference.
You probably read this post, but in case you missed it:
So IMO while I think that using a 240V reference calibrated down to half is technically more accurate, it’s not perfect either and in your case would be splitting hairs. You don’t really have a large imbalance.
Thanks, I may leave the single 120v VT on its own didicated circuit in the lab to provide clean data and use the other phase for network. If I puruit the 240v VT, which unit would you recomend in the US and will it work in the 9v port?
Thanks
Thanks, I’ve ordered the Ideal VT from Newark and an international to NEMA 6-15P adapter. I’ll wire in a NEMA 6-15P with a 10 amp 240v mini breaker in a small 2-space SE dedicated sub-panel for 240v without neutral and leave the other identical sub as is with a leg on each phase outputting 120v. It’s my basement lab, it was just a matter of time anyway and 240v loads are bigger issue in summer with A/C.
When you have time can you send any recommended calculations, calibration, etc. for this setup?
Thanks for all the support in the forums, I’m happy to have found your product.
Thanks, all up and running with the 230/240 VT, calibrated to 50% of the built-in value (9.53 in my case), super simple:
Not worth the hassle for most, but I was still working on my lab wiring anyways.
It looks like you found an adapter from the US 240V NEMA plug to the Euro 2 pin. Can you describe it and where you got it?
I found the pass-through NEMA 6-15 plug on Amazon: https://www.amazon.com/dp/B09TP1XDKF?psc=1&ref=ppx_yo2ov_dt_b_product_details. It has a nice nug fit but certainly can let someone connect a NEMA 5-15 plug into a 6-15 receptacle if they didn’t know what they were working with.