We recently bought a 1978 home in Florida.
The kitchen wiring leaves me scratching my head. I think I have solved 2 of 3 major problems, but need advice on the last.
(see pic)
4 different 20A tandem breakers in play. One of them was split into 2 INSIDE the breaker panel, one was split in a gang box so I am dealing with 6 "hot" wires & only 4 neutral wires.
1) 12/3 Romex. Black/neutral on one breaker [16 L] leads to kitchen GFCI outlet –> regular outlet –> kitchen light switch –> kitchen light. Red "hot" [14 L] leads to light (former fan) in living room with light fixture neutral connected to neutral wire from the kitchen breaker.
To solve the shared neutral problem, we disconnected the living light and connected it inline with the living room outlets using 12/2.
(The kitchen light is downline from a GFCI which isn't ideal but I don't want to mess with what works.)
2) Breaker 16 L splits inside the breaker box with a hot single black wire through conduit to the other side of the kitchen. That wire powered the microwave, dishwasher + 2 outlets and had NO neutral. (Neutral for this breaker is on other side of room in the 12/3.)
In that SAME conduit are a hot/neutral from breaker 14 R. This was used to power the garbage disposal through an under-sink outlet. The neutral from 14 R was then wired up from the disposal outlet & connected to ALL of the other appliances/outlets on 16 L.
To solve this shared neutral, we 1st removed the microwave and put it on it's own 20A circuit.
Next thing was to disconnect everything & wire the 14 R breaker from dishwasher outlet —> disposal switch —> GFCI under-sink outlet.
3) That leaves us with 2 outlets adjacent to the sink with power but no neutral.
QUESTION:
Would it be better to splice in a neutral wire from one side of the kitchen to the other. There is a junction box directly above, so it should be fairly easy.
OR
Would it be better to remove the splice on 16L inside the breaker box and run a whole new 12/2 to the sink outlets?
My concern splicing a neutral is with using GFCI outlets and having the same neutral run through GFCI outlets on both sides of the room. Would that cause problems?
My concern with running new wire is using up 4 breakers just for the kitchen.
Thanks for any advice!
Best Answer
Ok. The fundamental rule is that currents must be equal in every cable or conduit.
Yes, current travels in a loop. But in actual practice, we squeeze that into a "tree" topology. Imagine you draw a tree on a piece of paper, and send an army of 2-dimensional ants (current) to explore the tree. They are not allowed to hop across branches, but they can wander from branch to branch without going back to the bottom of the trunk. At any point on the tree, if you count the total ant passages, exactly the same number of ants (current) go up any branch as come back down.
This equal flow assures the current does not induce magnetic fields and eddy-current heating into nails, sinks, piping and other metal parts of your house.
Now it's OK to have 3 wires, 10A going out on one, 2A going out on the other, and 12A returning on the third. That's still equal. That's how a sometimes-on switch wire can travel with an always-hot.
Multi-wire branch circuits
That is also how a multi-wire branch circuit works, which is what you have on 14L and 16L. It's not an accident they are 2 numbers apart; they must be on opposite poles or they will overload the neutral.
And it's perfectly fine for part of a multi-wire branch circuit to have one side branch off in another direction, as in 16L. However its neutral must run with the partner hot. So whatever is going on in 16L, you must make sure that wherever the hot runs, the neutral is right next to it.
If you're working in conduit, as I suspect you are, your local actual electrical supply house sells THHN wire in 11 colors and tape in 10. Use different colors. Gray and white are both legal for neutrals, so when you have two circuits in close proximity, ask them to sell you some gray neutral wire.
You see where there are points where the MWBC splits off to just one hot with just one neutral. That is a prerequisite for working with GFCI+receptacle devices. That is fine, again, keep the hot with the neutral at all times. Now, a few new Code requirements with MWBC:
Anywhere both sides rely on the same neutral, the neutral must be pigtailed. You must be able to remove the device without interrupting the neutral wire for either side.
Both sides of the MWBC must have a common maintenance shutoff, which in practice means thet must be on a 2-pole breaker. When dealing with duplex, you get a "quadplex" that is a 2-pole in the middle and two singles on the outside, like this one.