Good catch, thank you – I had seen that the omega input wasn’t just for resistance but also a secondary voltage measurement, but I didn’t realize that the aux voltage was limited to 700 mV. Since the neutral-ground measurement reflects load on the circuit, and the reading was around 500 mV before the compressor kicked on, the inrush load of the compressor probably put it over that 700 mV threshold and perhaps measured with the primary reading. So it seems that I can’t measure voltage on the entire 3-prong outlet at once. But my main concern is really the hot voltage, so that should work fine.

Your idea for measuring current is cool. Is your extension cord setup effectively equivalent to a line splitter with a CT clamped into the hole? But with banana plugs to continue the connection from one side to the other, disconnecting them to slide on CTs and then reconnecting for the measurement?

I’ll have to come back to the air conditioner measurements – the outlet near it has reversed hot/neutral from the previous owners that I need to swap, and my earlier tests were done with the air conditioner hooked up to an extension cord, which even though it’s heavy gauge was another variable I just didn’t want in the mix.