My main panel is a Siemens G3040B1200. When the subpanel was installed (3" away from the main panel), the installer brought only the hots into the subpanel and left the neutrals in the main panel. Is this OK?
No, that is not OK. This was very shabbily done and you should bring in a competent electrician to correct it, or DIY.
- The supply breaker to the subpanel needs to be a Siemens QP250, not a GE which does not belong in this panel.
- The Square D HOM breaker also needs to go. What is wrong with people? Replace with Siemes QP230.
- Circuits entering the main panel that have had their hots extended into the sub, need to have their neutrals extended also. Grounds can stay where they are.
And even when you do all that stuff, you'll have panels that are completely full. No, this is not OK. You've been "living from one breaker space to the next" and look where it's gotten you.
If it were me
Look. I'm a CH super-fan. But just the same, I would tear the CH panel off the wall and throw it in the trash, and replace it with a 30-space Siemens panel. That way you can use the same breakers. 30-space because spaces are cheap, and obviously you go through breaker spaces really fast. 30 space not 30 circuits. With an accessory ground bar.
Same size as your existing panel, so all the ports will line up.
I would use a RMC metal conduit nipple for the feeder and several 3/4" metal conduit nipples linking side ports up and down the unit (just to make thru-wiring more convenient, often the wires can make it to a new breaker).
I have an older outdoor GFCI outlet on a dedicated circuit that trips periodically.
I have an older smoke detector that goes off everytime I burn toast.
The difference between my smoke detector and your GFCI is you can see smoke but you can't see ground faults, so I presume my smoke detector is doing its job, and you presume your GFCI is defective.
The GFCI is probably doing its job, and you're trying to shoot the messenger. Ground faults on outdoor wiring is really not a surprise. Try opening up all the boxes downline of the GFCI and cleaning out all the paper-wasp condominiums and fixing the water getting into the boxes.