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I have a mixture of knob-and-tube and NM-B. If I'm extending from knob and tube, but I have a (different) an NM-B circuit nearby, is it technically allowable to ground via ground wire in the other cable? If not, what is the right way to do it? (Please don't tell me to replace all the knob and tube -- it's not going to happen.)

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@NiallC. Thanks. So if I understand this, bottom line is, yes it is allowed, right? –  ThePopMachine Dec 9 '13 at 16:18
    
@NiallC. : Fortunately, I don't make a habit of adding rooms to my house. Oh, wait a second, does three rooms in 7 years count. I guess. But this is the first case where it is not practical to add completely new wiring to the panel. –  ThePopMachine Dec 9 '13 at 16:30
    
@ThePopMachine No. You cannot borrow the grounding conductor from another branch circuit. I guess I need to update the answer on the other question to make that clear. –  Tester101 Dec 9 '13 at 16:55
    
@Tester101: If you say no, then what does this part mean? For replacement of ... branch-circuit extensions only in existing installations that do not have an equipment grounding conductor in the branch circuit, connections shall be permitted as (C) Nongrounding Receptacle Replacement or Branch Circuit Extensions. The equipment grounding conductor of a grounding-type receptacle or a branch-circuit extension shall be permitted to be connected to any of the following: (1) Any accessible point on the grounding electrode system [...] (2) Any accessible point on the grounding electrode conductor –  ThePopMachine Dec 9 '13 at 17:16
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@ThePopMachine The grounding electrode system (of which the grounding electrode is a part), is a system of conductors used to connect the service equipment to the earth. Equipment grounding conductors (those found in branch circuits), are not part of the grounding electrode system. –  Tester101 Dec 9 '13 at 17:24
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marked as duplicate by BMitch Dec 11 '13 at 1:35

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1 Answer

Not to be snarky, but, if you mess up plumbing you get wet and stinky. If you mess up electricity, you get burned or dead. Is it really worth the risk?

As you touch the old unsafe and code fail stuff, replace it. No need to do everything at once.

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This doesn't really answer the question or give any references. It may be a bad thing to do, but without any references or explanation, it's just an opinion. –  Johnny Dec 11 '13 at 0:05
    
For the opposite view see diy.stackexchange.com/a/20279/5960 –  Bryce Mar 10 at 19:09
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