# In US electricity monitoring scenarios, must both sides of voltage be measured?

My current electricity monitor has two CT clamps. Makes sense. It also monitors the voltage from both sides. I “get” how this makes sense. One clamp and one voltage - measure power. One for each side.

However-curious. Would the voltage reading be different on either side? I.e.: if one side read 112 volts, would the other side also read 112 volts? Or would there be a difference? It is entirely possible I don’t understand electrical concepts well enough. But I was thinking the two voltages would be the same since the are coming from the same source? Thank you for clarification (and being kind if this is truly a clueless question. I’m just trying to poke at my understanding).

Yes, there will be a difference. Although the emf induced in each half of the secondary winding of your local transformer is the same, if the current drawn - not just by you but including that taken by other customers fed in parallel with you - is not the same, then the voltage drop due to the winding impedance will be different. Add to that the different voltage drops due to cable resistance between the transformer and your house.

What your question really is, how big is the difference, and is it significant? There is no specification for the allowable difference, but reports say it will be less than 2 V. If the loads on the two legs are consistently out of balance by a lot in the same direction, then the answer is it possibly is significant. But if your loads are arranged so they are close to balance (as they should be, over time) then the differences will even out.

And it’s only the 120 V loads that will contribute to imbalance. By definition the current for the 240 V loads is always the same in both legs.

I’ve noticed measuring both V legs is pretty common in the pro US market. Sense have a fairly neat approach you can see in their install video:

It’s all a lot tidier when you don’t have to deal with external VTs.

IIRC, she has, or at least had, a Sense device. I say had, because of this post:

I don’t know if that means she sells them, or if she sold the one she bought, to forum user whatnick.

Hi Bill, I have a Sense monitor installed. I bought one for Whatnick (Tisham) in exchange for mentoring hours. Tisham has done an awesome project using the atm90e26. I have learned alot from not only his mentorship, but also the schematic and pcb layout. In fact, I got so “into it” I threw myself into a project that I’ve put on https://fithome.life

I’m not in sales. I’m better at spending money than making it these days. I jlove, love, love to learn. I am blessed to be able to do so.

Thank you VERY MUCH for all the knowledge you have shared.

It’s a great shame that there seems to be no interest in producing an emonTx that is more suited to use away from the UK domestic scene.

it is indeed common. I have a Sense monitor installed. This is what got me thinking. I had to use a double pole. i’m building a PCB and i have three channels for incoming 120V - L1, L0 and conversion to 3.3V to power the atm90e26 as well as a microcontroller (I just can’t make up my mind between wifi and lorawan…like a candy shop…). Then I started thinking… Can I just compress all these resistors, etc into one channel…now L0 = L1 and I can transform the line to 3.3V. I was thinking this way to save on parts but also the time for soldering (of which I have varying amounts of success ).

IT IS. The challenge I had when exploring it was the software was more complex than I wanted… the other challenge was the focus on UK. Then, I really liked the idea of a < \$1 chip (atm90e26) just puttering around monitoring…and I really liked “plug and play” which microcontroller and such i can use. Then I got into writing the iOS/android app… anywho - The IDEAL situation would be a US emonTX thingy. But then again, i don’t know if it will be easily adaptable to the scenario I want (https://fithome.life).

Thank you. As always, I learn quite a lot from you.

Echoing Margaret’s comment, indeed it is.

Part of that has to do with the fact that in the US, electrical service wiring and associated hardware are required to be in conduit and approved enclosures, (there are exceptions) and that electrical work is supposed to be done by a qualified electrician, which usually translates to “expensive.”

Because access to the US equivalent of UK meter tails requires opening said enclosure,
(the load center) the person performing the work is exposed to potentially lethal voltages with
no way to disconnect the load center from the mains. (again, there are exceptions)
That aspect alone is enough to scare the - insert your favorite term here - out of most homeowners
and keep them from doing the work themselves. And since hiring an electrician to do the job usually costs a tidy sum, they typically end up “scrapping their project.”

Another aspect is related to the insurance industry.
If a device that lacks UL certification is installed in a load center and is found to have caused a fire, there’s a very good chance the homowner’s insurance claim will be denied. Claim denial can also happen if the insurance carrier learns the work was performed by someone lacking the proper license / certification.

Hi Margaret,

I’m surprised I even remembered you had a Sense energy monitor.

I’ve seen Tisham’s work via his Tindie page. Nice work indeed!

Know what you mean about spending \$\$\$
My wife won’t let me forget.
Fortunately, my job as an Electronics Technician enables me to have a few hobbies. (electronics/computers/amateur radio)

Like you, I’ve had a life long desire to learn. I quit watching regular TV about 6, maybe 7, years ago.
Now, I watch YouTube channels like Smarter Every Day, Veritasium, Eddie Woo, From the Vault of MIT and Numberphile. (and others)

YVW!