Electrical Wiring in the Home/Separating an overloaded circuit
Our house was built in 1960, and the panel was upgraded to 150A service in 1971, which is problay when central AC was added. Old wiring is gray (Bufrmax?) 12-2G NM. One of the circuits includes plugs and lights for several rooms, incl. bathroom and family room. Laser printer is causing flickering of lights, which prompted my looking into this. I can see from the basement where the wires go up to each room. I would like to make this into 3 circuits: with family room and bathroom on the new circuits. Will the NEC allow me to run a new circuit(s) from the panel to a new junction box(es) in the basement (using same 12 gauge NM to match) where I tie into the existing runs where they go up into the individual rooms, or does ALL the wiring in the walls need to be replaced? If I don't need to replace wiring in walls, I think I can do this myself (with permit and inspection, of course) at a reasonable cost for wire in basement, new CAFI breakers, and a couple of junction boxes. (Bathroom plugs are already GFCI.). Thanks for your advice!
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That being said, it is absolutely allowable to split a circuit and re-feed a portion with properly sized and protected conductors on additional circuits. It might be a bit trickier than you think to separate sections out, as what you think goes up may come down in both places. It is not required to replace all the existing wiring.
It occurs to me that flickering lights can be a symptom of several conditions and a laser printer usually doesn't present a load that would cause that in a "healthy" wiring system. At 40-plus years I would consider that the "new" service may have issues. A failing service neutral connection can cause flickering lights when an applied load causes imbalances revealed by the defect.
Are the flickering lights and the printer on the same circuit or the same leg of the service? If not, I would examine the neutral throughout its path.