I replaced an outlet with Line & Load wires in Rm-A with a GFCI outlet. The new outlet won't RESET and a preexisting GFCI outlet with Line wires only in Rm-B quit working. Each outlet is on a different breaker. When I tried replacing the outlet in Rm-B with a standard outlet thinking its power might be coming from the outlet in Rm-A, neither outlet worked.
TLDR: If you didn't realize GFCIs could protect other outlets, don't use "Load". The "Line" terminals can take 2 screws - read the instructions.
You should not use "Load"
I get it, it's a common thing, you have 4 wires to attach and you see 4 screws, so there's a sense of symmetry. And the instructions seem to say to do that. And who reads warning tape? LOL, I'm sure that's merely decorative!
"Load" actually does something very special, and you're not supposed to use it for anything else. As you perfectly well know, there is a GFCI module inside this gadget which protects the two sockets. Well if it can protect 2 sockets, why can't it protect 3 or 4, or 10? The "Load" terminals are to allow you to do exactly that. The extra sockets can be attached to this GFCI (and receive its protection) by connecting their hot and neutral to the GFCI "Load" terminals.
I bet you didn't even know GFCIs had that feature :)
This is a nice money saver when you can use it - but with great power comes great responsibility. You must - this is a Code requirement - you MUST know each and every outlet that is being fed off those "Load" terminals. You must identify them, and you must label them "GFCI Protected" (and "No Equipment Ground" where applicable). NEC 110.3(B), instructions 8(C). While this identification rule sound pedantic, it actually solves many problems, but mostly makes you think about what you are doing.
This is the only proper use of "Load". Where you went off the rails is you used those terminals for no good reason - you just needed a place to land 2 more wires. Mistake. The right thing to do is to attach both wires to "Line" - the terminals take 2 wires, the instructions say how.
When you have no interest in having the GFCI protect other outlets, and no interest in mapping and labeling those outlets, just don't use Load. Most people don't even realize GFCIs can do that, and expect to put a GFCI receptacle at every location - that is fine but they should never use Load. If I were king, I'd have 4 screws, all Line, and the "Load" terminals would be concealed and only electricians would know they were there. As built, GFCIs are very DIY-hostile.
And I'll say one more thing. Never using Load and just fitting a GFCI receptacle everywhere you want protection, is fine in most cases IMO. Using downline protection can be big trouble (especially if done by accident) -- we hear many cases where someone had an outlet "go dead for no reason", and paid an electrician $150 only to be shown a tripped GFCI elsewhere. "Thanks a lot, LOAD terminals!" Maybe never using them is better.
At the very least, while you are marking the receptacles "GFCI Protected", use a P-touch label maker and also state where the reset is.
Reset in hallway
A Multi-Wire Branch Circuit requires special handling
You say that this breaker tripping breaks stuff on another circuit. That is one of two things. Either
- The other outlet is illegally crossing neutral with this circuit, taking hot from another circuit but stealing neutral from this circuit instead of its own circuit. That sounds super clever until you realize neutrals don't have circuit breakers. Nothing protects neutral from overload, except faithful matching of hot to neutral (so all neutral current comes through the same hot breaker).
- The 2 hot wires are part of a legal, approved way of sharing a neutral wire, called a Multi-Wire Branch Circuit or MWBC. However, MWBCs require carefully following several rules to avoid overloading the neutral or shocking maintainers, and those are not followed.
A MWBC involves a 3-wire cable (black white red) coming from the panel to outlets. Let's go through the rules of MWBC.
Rule 1: Neutral must be pigtailed (the way ground is).
You are not allowed to use the 2 screws on a receptacle as a way to splice the neutral wire onward to other parts of the MWBC. That is because if someone removed the device (or put a GFCI there LOL), that would cut neutral to the onward part of the circuit. This is very bad. Due to how power works, the neutral on the far side of the cut will now be HOT. For instance you can get shocked changing a bulb (the shell of an Edison bulb is hardwired to neutral).
So using the regular receptacle as a neutral splice was bad, and using the GFCI Line/Load neutral as a splice was of course a disaster.
Rule 2: Breakers must be handle-tied
Think about what I said about the neutral on the far side of the cut being hot. That means right now, in the box you are working on, the neutral on the Load terminal is hot (if any loads on the other half-circuit are switched on). Because when you hunted down the breaker for "this circuit", you never thought you'd need to shut off any other breakers to make the box safe!
And this is why Code requires that all breaker handles on a MWBC must be handle-tied. The breaker manufacturer makes UL Listed handle-ties for their breakers, or you can simply use a 240V/2-pole breaker that has a built-in handle-tie (or only one handle in the case of "QO").
So you should identify all your MWBCs and handle-tie the breakers or replace with 2-pole 240V breakers (not to be confused with independent-handle tandems, which would be Very Bad for MWBC).
Rule 3 (most important): Must be on opposite poles
The two hots must be 240V apart! (or 208V in 3-phase country). This is because of the electrical sleight-of-hand the MWBC uses to support 2 hots on 1 neutral without overloading it. (see link in previous paragraph for a long discussion on poles).
When the hot wires are on opposite/different poles, with 240/208V between them, the neutral wire carries only differential current (12A on one, 15A on the other, neutral is 3A).
When the hot wires are on the same pole (0V between hots), the neutral carries the sum of currents (12A on one, 15A on the other, neutral is 27A - whoops!)
Use of a 2-pole breaker will guarantee correct phasing. And so will using a handle-tie in most cases. (GE panels are an exception here, as on their "thins" the hendle tie is no help at all, and their 2-pole "thin" can be forced into the wrong position.)
Rule 4: You can't use GFCI "load" on a MWBC
Aside from violating rule #1 about pigtailing neutral, it simply won't work. The GFCI will see the neutral sharing as imbalance current, and will trip.