Monitor monitor line to line voltage in US

Support
1

I live in the US and have a “standard” 120/240V system. I’m have an ongoing discussion with my utility about high voltage issues. Phase to ground voltages of 127.5V for several minutes at a time are not acceptable! I would like to configure one of the CT inputs as a VT input and use it to monitor line to line voltage. I do not plan to use4 it as a voltage reference. From what I have read, I need to acquire a 240V to 9V transformer, connect the the transformer output to the tip and barrel of a stereo plug and then configure the input (planning using input 11) as a VT and check the calibration. Since I’m not using this as a voltage reference, I’m not overly concerned about phase shift, but based on on Overeasy’s posts I did buy a rather heavy transformer! Comments/suggestions please. All I really need out of this is a graph and data set for discussions with the utility.

2

I’d recommend just getting a 230V to 9V Euro wall transformer and replacing your 120V transformer with that. It has a standard Euro two pin plug. That’s what I use on my own home IoTaWatt. I adapted the plug by installing a standard us 240V socket, and changed the plug on a simple Euro small power strip.

Calibrate the VT with a voltmeter as detailed in the docs, then divide the resulting calibration by 2. You will now have a 120V voltage reference that is exactly half of your 240V supply. Create an output that doubles it and you can plot your 240V supply.

IMO this is a superior voltage reference as explained here 120V/240V split phase voltage reference

3

Thanks for the suggestion Overeasy. I’ll do it your way only with a euro extension cord instead of a power strip. FYI, your link to the Euro wall transformer went to a transformer that is not in stock. Estimated ship date was in January of 2026. I’m not willing to wait quite that long. I ordered an Ideal 77DE-06-09-ND instead. it should be here in a week or so. Thanks for your help. A utility engineer is supp0osed to call me back Monday or Tuesday in response to my complaint. That will be an interesting conversation. I have lots of single phase data to send them.

1 Like