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 .