Randy wrote:I'll do some checking, but it is a 20amp GFCI circuit in the garage with a short run to the main panel.
I have an external voltmeter that I'll test with to verify.
Might be interesting to measure the voltage on a separate circuit (but the same leg of your 240V feed) to see if the voltage sag is detectable there, too. I wouldn't expect a 12GA / 20A circuit to sag under 12A of load... Have a rough estimate of the length of your run?
For example, a 20ft run of 12GA with 12A of load on it should only see a voltage drop of about 1V.