emonVs - Getting Voltage on Multiple Lines

oriol · 2023-11-18T07:55:48.612Z

Hello all. I’m considering getting an emonTx V4 with an emonVs AC supply, a couple of 200A CT’s and an optical monitor (if I can figure how to route the cable from the meter to the inside). I’d appreciate clarification about the emonVs voltage sensing functionality.

For single phase service (US, 200A @ 120V per line), is it relevant which line is the emonVs connected to? I ask because other in-the-panel energy monitoring solutions need to have the voltage reference on a specific leg matched to a CT.

What is the impact on accuracy by the emonVs not measuring voltage on both lines? Some say that for truly precise measurements it is needed to do it on both lines; although most power monitoring devices only do it on one.

Bill.Thomson · 2023-11-18T10:00:03.505Z

Hello Oriol, welcome to OEM!

WRT to your choice of CT, you may want to have a look at this article:

US 200 Amp Service explained or Do I need a 200 Amp CT? Hardware

A common question is “I have 200 Amp service, do I need a 200 Amp CT?” The answer is most likely no. Unless you have a very large house, a 100 Amp CT would be a better choice. (Large houses, typically have 320 or 400 Amp service and may actually need a 200 Amp CT, but this article addresses 200 Amp service installations) So what does the term 200 Amp service refer to? Simply put, it means the weakest link in your Service Entrance Wire, Meter Base, Meter, Main Breaker, Load Center (circuit b…

Robert.Wall · 2023-11-18T12:20:19.977Z

oriol:

For single phase service (US, 200A @ 120V per line), is it relevant which line is the emonVs connected to?

Ignoring for the moment the fact that you’re guessing a voltage, the worst will be you have the wrong sign to a power - meaning it looks like an export when it’s actually an import - if you don’t watch the orientation of the c.t’s.

You are aware that the 3-phase emonVs can be used to monitor both legs of your split-phase supply, and the software enables you to pair each c.t. with the appropriate voltage (or for a 240 V load with the sum of the voltages)? I think this might answer

oriol:

What is the impact on accuracy by the emonVs not measuring voltage on both lines?

Obviously, the answer to this question assumes a single-phase emonVs and depends entirely on how well balanced the two voltages are, and this in turn depends on the fault level (i.e. the source impedance) of your supply and what you’re using at any given moment.

oriol · 2023-11-18T16:54:36.274Z

I saw that post while researching earlier and had made a mental note of parsing it again. Thanks!

oriol · 2023-11-18T16:55:59.702Z

Robert.Wall:

the 3-phase emonVs can be used to monitor both legs of your split-phase supply

That’s good news. I had thought that that was maybe the case when going over the specs of the 3 phase emonVs but concluded that I was assuming too much. Thanks!

Robert.Wall · 2023-11-18T17:17:59.542Z

It is 3 isolation transformers etc with a common neutral, so if you wire the Black leg to L1, the Red leg to L2 and your White neutral to N, (and E to the protective Ground), you’ll have the two line voltages available and you can treat each leg and 120 V circuits on both legs identically.
(I believe at the moment, each 3-phase one is final-assembled by hand, so if you ask The Shop nicely, you might get a N.America 2-inputs only version.)

For a 240 V load (if you go that far) you use a single c.t. and tell it to use both voltages. The line in the sketch would look like:
EmonLibDB_set_pInput(4, 1, 2); which says c.t. input 4 will use voltages 1 & 2.
For a 120 V load:
EmonLibDB_set_pInput(5, 1); which says c.t. input 5 will use voltage 1.
All this is fully documented, along with example sketches, in the emonLibDB library zip download. EmonLibDB - Version 1.0.0

oriol · 2023-11-19T15:50:14.074Z

Can a voltage be assigned to multiple CT’s? So that for a 120V circuit I could for example use voltage 1:
EmonLibDB_set_pInput(5, 1);

And then for a 240V circuit reuse the same voltage 1:
EmonLibDB_set_pInput(4, 1, 2);

Robert.Wall · 2023-11-19T15:51:59.086Z

Of course.