I'm replacing the two-pronged outlets in my basement with 3-pronged grounded outlets in the United States (Illinois). All the wiring is in metal conduit with metal conduit boxes, which are grounded to the breaker box (I checked with a multimeter from the hot slot to the metal box). My goal is to wire all the 3-pronged outlets grounded to the metal conduit boxes, with a GFCI at the outlet closes to the breaker box (which I believe is proper code for a basement).
The metal conduit has three wires running through it: a red and a black, which are both hot, and a white, which is neutral - I tested all these with a multimeter at different outlets. These outlets are all daisy-chained together, sharing a neutral white wire, but powered by the different red or black wires.
To further clarify, the red and black wires are not in a series daisy chain (to my eyes and testing). When I opened the circuit breaker box, both the red and the black wires had their own breaker switches, but the wires ran through the same conduit.
At first I thought this was just weird, but chocked it up to odd 1960's construction, or my own lack of knowledge.
But, it is causing issues: when I plug something in downstream of the GFCI (powered by black) in an outlet powered by red, the GFCI trips, so I can't use those outlets.
I was using a used GFCI outlet (bought at a secondhand construction materials store) - I bought a new one in case it is the outlet's fault, but haven't yet tried to install it.
Also note that the breaker isn't set up for 240V supply; there are no appliances in the house running 240V (we have natural gas).
I drew a picture which I hope helps illustrate my description better. There is either a red or a black box around the outlets to show which color hot wire they are being powered by.
Thanks for looking! I am an electrical layman but could provide more info/investigate further if needed!
EDIT Thanks for the advice, everyone.
Checking the neutral wire at one of the outlets on this branch is 14 ga, which has a 15-amp rating - is that typical?
It is a Square-D brand breaker, type QO I believe.
Could I get some feedback on that assessment?
Finally, after reviewing the electrical code, it appears GFCI circuits are required in just unfinished basements, due to potential wet conditions. My basement is semi-finished (drywall , but exposed concrete & open ceilings), and not really wet. Could I get away with a non-GFCI 20-amp circuit breaker? It would save me about $70. I'd rather not die, though.