TWO AC Voltage Samples - Use in North America?

I’m interested in getting an emonPi, but would like to modify it to have TWO AC sample voltage inputs.

I’ve run across some instances where there have been significant imbalances in the voltages on the incoming legs of the PoCo’s transformer.

In one case, a nearby house for sale was the culprit. The seller’s real estate agent stabbed a sign post through the underground line and severed the wire…

In another case, tree roots had grown under the concrete pad that the transformer rested on…

In the last case I noticed that the current imbalance between the legs was so great that the main breaker in the property had overheated on that side and had damaged contact points where it attached to the bus bar of the breaker panel.

All three instances occurred in properties that I own or otherwise take care of.

Case #1 was detected when half the house’s 110 volt electrical facilities worked and the other half did not. Turning on 220 volt items such as the water heater or stove caused some voltage to bleed over on to the failed leg and the failed facilities operated, albeit at a lower voltage.

Case #2 was similar to Case #1. In both cases, temporarily moving all breakers to all Odd or all Even positions in the breaker panel restored 110 volt service to the entire house. This, of course puts all the load on one leg of the PoCo’s transformer. They don’t like that too much. Breaker position was restored after the PoCo repaired their damaged wiring.

Case #3 is the exact opposite of 1 and 2. This property had all of its 110 volt loads already on one leg of the transformer. This imbalance, coupled with corrosion and a loose connection on that leg’s connection to the main breaker caused problems which burned the breaker where it attached to the bus bar of the breaker panel. This resulted in the loss of one leg to the property… Being that this was after the PoCo’s meter, it was the property owner’s (mine) responsibility to repair the damage.

After the scorching was cleaned off the bus bar and the main breaker was replaced, current readings were taken between the two legs and breakers were shuffled around to balance the load between the two legs.

Being that I’ve struck the lottery in being associated with several cases of voltage imbalance between legs, I’d like to monitor both current AND voltage on BOTH legs.

Can I modify an emonPi to have to 9 VAC inputs?

Welcome, Theo, to the OEM forum.

The short answer is, it will not be easy. Although the analogue inputs exist on the processor, accessing the unused ones will be almost impossible in practice. The most sensible way would be to design your own Raspbery Pi shield the same size as ours so that you could still use the same case, and equip it with two voltage and 2 or even 4 current inputs - if you can get the processor. If that’s all you want to monitor, you don’t need the RJ45 socket for pulses & temperature, you don’t need the radio, and although the LCD is nice, it’s not necessary once it’s set up.

Along the same lines, it would be more practical to take the design for our emonTx Shield for the Arduino Uno, and modify it for 2 or 4 current and two voltage inputs.

Both the above imply designing a p.c.b and getting it made.

Probably a more realistic solution for you will be an emonTx V4 + emonVs, and use a USB connection to a Raspberry Pi.

If you really want an emonPi, I suggest you wait for the next version, which I understand will have more capabilities - though I don’t know exactly what. Nor do I know when it might be available.

Did you check that the copper bars had not softened? If they had, you’re likely to have trouble again because the bolts will loosen in time if the metal deforms under the contact pressure.

I will wait for the new version of the emonPi. I recently tried to purchase several R Pi 3 and 4 units for a project and noticed that they’re WAY out of stock everywhere and scalpers have gotten a hold of them and jacked to price up tenfold.

EDIT: I see that things have changed in the way the hardware is set up. It appears that the emonPi is now obsolete and the emonTx4 with an emonBase is the current hardware.

I stumbled upon this project, as I built a pair of OpenEVSE units for my electric car last year and there is a link to the OpenEnergyMonitor on the built-in web-based management tool for the OpenEVSE.

I will have to do more research into the newer hardware.

The bars were checked by a competent electrician due to the fire risk associated and the fact that tenants reported the issue. (Cases 1 and 2, I was living in the house and accepted the risk of doing my own work. Plus the fault was on exterior wiring.)

Every now and then you can find deals like this one:

A Revenue Grade device with a nice enclosure for 79 bucks.
($59 for the hardware and $20 for shipping. New, the meter itself sells for $400)

I can’t be sure, as the labels aren’t visible in the photo, but it looks like two Revenue Grade CTs
are included. (they normally sell for 73 bucks each.)

Even if the CTs are not Revenue Grade, the “package” is still a bargain considering the non-RG
CTs normally sell for 60 bucks each.

The WattNode is capable of reading three voltages as well as three currents.
It can monitor a single-phase or a 3-phase service. It’s bi-directional, so it can be used to
monitor a North American split-phase service with PV, as well.

More info here:

Athough the company stopped manufacturing them as of 1 Jan 22, they still support the device.


The CTs are indeed Revenue Grade:

In addition to the calibration sheets for the CTs, you get a cal sheet for the meter itself.
The manufacturer charges an additional 60 bucks for the meter cal sheet.

If you’re in the US, this is one hell of a deal!

600 bucks worth of hardware and calibration for $79 is mighty tough to beat.