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I'm rewiring an old house. It has a mix of knob and tube and romex wire. I'm planning to remove all the knob and tube. In the basement I found 12-4 Romex (Black, Red, White, White W/Red Stripe and ground) all in one junction box, neutrals all wired together and supplying different branch 120 v circuits. Inspecting it at the breaker box, it looks like both the black and red wires have been used as branch circuits on the same buss, but different breakers. From past experience I've learned 12-3 wire can share the same neutral but need to be on opposite buss. Is this the case with 12-4 wire?

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    FYI. What you're describing is called 12/2/2. x/4 cable is made up of three ungrounded conductors and one grounded conductor (Black, Red, Blue, White, and ground). – Tester101 Mar 15 '15 at 15:21
  • @Tester101 - This would be used for two separate circuits -- one black/white, one red/white&red? – TomG Mar 15 '15 at 18:33
  • @TomG That's correct. – Tester101 Mar 15 '15 at 20:06
  • @Tester101 that is not correct for 12-2-2 in this case.. it is made up of (2) ungrounded conductors (black + red),, (2) grounded conductors (white + white/red),, and one grounding conductor (bare copper) – CheffJosh Jun 29 at 21:00
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This is an unfortunately-all-too-common wiring error, even without a 12/2/2 cable to further confuse things. Most folks (even electricians) think "I'll just nut the neutrals together" since that's what works when everything in a box comes from one branch circuit, which is the case 99.9% of the time -- in the 0.1% case, such as yours, though -- this rule leads to a looped neutral, which violates 300.3(B):

(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

and 310.10(H)(1):

(1) General. Aluminum, copper-clad aluminum, or copper conductors, for each phase, polarity, neutral, or grounded circuit shall be permitted to be connected in parallel (electrically joined at both ends) only in sizes 1/0 AWG and larger where installed in accordance with 310.10(H)(2) through (H)(6).

This is easy to fix, though -- make sure that the neutrals in your 12/2/2 cable (white and white/red striped, or "candy cane wire" ;), are not nutted together anywhere.

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