I've got a 4 wire 240 volt receptacle set up to power an electric brewing controller.
There is a 240 30amp breaker in my main panel that then supplies a sun panel in the garage that has a 30amp gfci breaker, that then goes to the outlet.
The 240 is for a heating element, but the control box is set up to split off one of the hot legs, and connect it up with the neutral to power some 120 volt items, lights, pumps, CPU
So the receptacle will read 240 on the hot legs and powers the element light just fine. And will read between 110 and 120 connecting each hot leg to the neutral, but as soon as something is trying to get any power from the 120 circuit the voltage drops and there is no power.
Also went as far as to bypass the gfci breaker connecting it directly to the main panel. And no change.
Could the new 30 amp breaker in the panel just be bad?
One note is that we couldn't get the gfci breaker installed in the main panel without it constantly tripping, putting the gfci breaker in a sub panel fixed that, but I'm thinking there is still some issue in the main panel .
Best Answer
Time for a nightlight test
Hit a dollar store and get two 2-prong extension cords and two incandescent nightlights (incandescent, without light sensor, straight switch-only).
On the extension cords, lop off the plugs (pointy end) and throw the plugs away, we are not making a suicide cord today*.
On the cut ends, split the two wires for about 1 foot (300mm). Then strip about 1/2" (12mm) of insulation off each wire end. The ribbed side of the cord is neutral, the smooth side is hot, if possible mark them white and black respectively.
Test the nightlights, then plug them into an extension cord socket. Now you have a test cable.
With the main breaker off, land both extension-cord neutrals on the GFCI-breaker neutral (or the neutral bar if testing with a plain breaker). Then land one extension cord "hot" on hot L1, and the other cord "hot" on pole L2.
Power up and see what happens to your voltages
Make sure both nightlights are on (or should be on). I have a feeling one might not light up, that's why you test them first.
Now go back and do your voltage tests again. I bet whichever 120V supply was "slightly lesser" in voltage now goes completely away.
That would be phantom voltage, which is picked up by capacitive coupling from adjacent 'hot' wires to a 'disconnected' wire. There's no appreciable current behind it, and it can't do any useful work**. It is almost "static electricity" and is very easily neutralized by the slightest load (our nightlights).***
So now, I think the symptoms will be much clearer and easier to troubleshoot.
The trouble may be in the main panel.
* If you have an actual use for a cord-with-plug, then buy an extension cord long enough for both that and this, and cut at the appropriate point.
** unless tuning AM radio frequencies into a crystal earpiece is of interest to you.
*** I prefer nightlights to 22k ohm resistors because Radio Shacks are getting hard to find; plus nightlight cords have dozens of other uses in electrical troubleshooting.