Jim, Some replies below.
By side by side, do you mean the breakers are installed directly across from one another? Connected to the same main bus breaker tie. (Both are fed from the same leg, Line.) Or, do you mean one is installed above the other, side by side? One fed from leg, Line 1 and the other fed from Leg, Line 2?
>>> Same leg/busbar. One 20A breaker above other.
Did you use Romex?
>>> Yes. 8/2 NM-B (not 10 as I said before) cable from Lowes.
Did you try to keep the 2 cables separated from one another at least 6" after getting them out of the electrical panel, as soon as practical? Especially a long parallel run?
>>>Yes. 12 feet apart.
Is it possible one of the 2 is running parallel next to some other branch circuit?
>>>It’s possible but not obvious.
>>>I also tried tying the ground wires to the same terminal, removing the ground (to test only) and moving other equipment to different runs to no avail. I’ll be honest, I abandoned messing around with it after an hour and removed one leg and used the cable for another project. The one 8/2 run on a 20A breaker into a 20A L5-20/L5-20P receptacle/plug sounds great.
By side by side, do you mean the breakers are installed directly across from one another? Connected to the same main bus breaker tie. (Both are fed from the same leg, Line.) Or, do you mean one is installed above the other, side by side? One fed from leg, Line 1 and the other fed from Leg, Line 2?
>>> Same leg/busbar. One 20A breaker above other.
Did you use Romex?
>>> Yes. 8/2 NM-B (not 10 as I said before) cable from Lowes.
Did you try to keep the 2 cables separated from one another at least 6" after getting them out of the electrical panel, as soon as practical? Especially a long parallel run?
>>>Yes. 12 feet apart.
Is it possible one of the 2 is running parallel next to some other branch circuit?
>>>It’s possible but not obvious.
>>>I also tried tying the ground wires to the same terminal, removing the ground (to test only) and moving other equipment to different runs to no avail. I’ll be honest, I abandoned messing around with it after an hour and removed one leg and used the cable for another project. The one 8/2 run on a 20A breaker into a 20A L5-20/L5-20P receptacle/plug sounds great.