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I am about to purchase a house with knob and tube wiring.

I'd like to protect a circuit of outlets as best as possible (without rewiring with new 3-wire from outlets to breaker box). Same old story, right?

Before you mark this as duplicate let me explain what I will do but I also want to ask about what I think would be better. That's the more interesting question.

What I will do

I will add a GFCI on the outlet closest to the breaker with line and load connected properly. I won't mess grounding anything...just tighten the green screw and wrap the outlet with electrical tape. I will also change all the other outlets in the circuit to standard 3-prong outlets with nothing hooked to ground. I'll test my work and expect an "open ground" error, which is OK. I'll put the stickers on the outlets to indicate that these outlets do not go to ground.

So, first off can you confirm that what I plan to do is "the right thing to do" as being both permissible by the NEC and sensible in your judgement.

What I think would be better

ok, so here is the idea. I won't do it because I know it would be against code. Also please read it through as I don't believe it is anything like a "bootlegged ground" but may sound like one! I think its a great idea, but of course I am biased and mostly uneducated.

I want to ground my modern, 3-prong devices (i.e. I know the "neutral" is grounded at the box, I'm talking about the ground plug). So, on every outlet I would purchase two things

  • A dead-front GFCI
  • A standard outlet (likely tamper-proof, likely 20A)

Line would go into the GFCI and load would go from the GFCI into the standard outlet. I would have to have a large box to hold those two. I would then send the two (hot/neutral knob and tube) on to the next outlet pair.
Now, for the scheme:

  • I would take the ground of the outlet and run it to the neural which is located between the GFCI and breaker.
    • I agree that if I were to do this between the ground and neutral at the outlet, this would be a bootlegged ground.
  • The ground of the GFCI would remain unattached.
  • Rinse and repeat for the other outlets...they all get the pair.

What would happen

  • This setup is no worse then the "proper way" described above and I think better.
  • The device ground and the outlet ground would typically have little/no voltage and current. That's good. However, if hot happened to hit the ground of the device, current would flow through the bypass, and the GFCI would detect that delta in-out current and throw. This behavior is exactly what a 3-wire grounded system should do.
  • A big downside of this is that you need a double box, it might look a little strange in your 100 year old house
  • The other big downside is you need to buy and install this set at each outlet.

Would this work as I described and provide improved safety over the "proper way"?

Edit: I've included an illustration for clarification purposes. enter image description here

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    I'm very happy to hear that your theory is NOT what you're actually planning on doing but is just a "thought experiment". You seem to understand that if it were safe, it would probably be a code approved way of doing things, but since it's not code approved, it's likely not safe at all.
    – FreeMan
    Sep 30, 2021 at 13:12
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    GFCI/AFCI breakers seem like a better solution to this one until you get to replacing the wiring. Protect the whole circuit and the house. As for your alternate idea, plenty said already about what a bad idea that is.
    – Ecnerwal
    Sep 30, 2021 at 13:35

4 Answers 4

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The first method (GFCI in lieu of ground wire, subsequent 3-wire receptacles off Load all protected) is perfect. The only thing you need to do, which nobody does, is to label the GFCI to indicate that it has no actual ground and the other receptacles both to indicate that they have no actual ground and to indicate that they are GFCI protected.

The second method is actually dangerous. The problem is that if any hot gets shorted to ground, it will end up on the neutral on the GFCI. But the GFCI won't actually know about the problem because it will see the current going out on hot and back on neutral as it expects under normal usage. That's different from a properly grounded receptacle where that current going back on ground means less current going on neutral and an instant GFCI trip. But if you don't connect ground then either the current goes nowhere (in which case, no harm) or it goes someplace else (leaking into "whatever", yes including possibly people) and has a chance at tripping the GFCI.

I misread the original post. With the ground connected to the Line side of the GFCI - i.e., between the breaker and the GFCI receptacle - the ground will function more like a traditional ground and the GFCI should trip if any normal current goes over the ground wire (e.g., short to metal case). However, that is still no good as ground and neutral are only supposed to be connected in one location for the entire electrical service, normally in the main panel. If you install GFCI breakers instead of GFCI receptacles then that problem goes away, assuming there is only one breaker panel, as then the ground is being connected in the main panel. However, if you can run a fresh ground wire ("retrofit ground") all the way back to the panel, you might be able to run NM cable instead - and get rid of the knob and tube wiring at the same time.

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  • What you are describing, I believe, is what others call a bootlegged ground...wiring the ground directly to neutral. I understand and agree why that isn't a bad idea because it essentially provides no additional protection of ground. But what I'm suggesting is different...the outlet is on the load side of the GFCI as normal. However the ground of the outlet is connected to the line side of the GFCI. Now, if any current flows through ground, it bypasses the GFCI and the GFCI will detect this difference in current between the hot and neutral passing through it. Sep 30, 2021 at 16:39
  • Aha! That is slightly different. That would take care of the GFCI issue (i.e., GFCI would detect "current on ground" as a difference from normal and trip. However, it still risks creating island grounds which are normally not a good thing. If taken to the extreme of GFCI breaker instead of GFCI receptacle - in which case this is "ground wire back to the panel" then you're actually doing a retrofit ground, which is 100% legal (but not always so easy to do). Sep 30, 2021 at 16:47
  • The ground wire typically provides a "better path" than your body. In a 2-wire system, there is no such ground return path since there is no ground wire. So, I agree the breaker GFCI will detect human body shocking but it won't allow for that "better path".I'm suggesting does allow that better path.You mention only connecting neutral and ground at the box and I understand why that's a rule.However, in this case my belief is an exception should be made. I realize this has turned into a discussion. I'll try to update my question with an illustration and maybe improve my argument. Thanks. Sep 30, 2021 at 18:38
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Consider simply retrofitting ground. This was broadly legalized in the 2014 NEC. You simply run an appropriate sized ground wire from the outlet to any junction box or grounding electrode which has a sufficiently large ground back to the panel.

Your idea is "Since neutral and ground is bonded at the panel, therefore neutral is ground". So yeah, your question really is the "Same old".

But have you noticed neutral wires are insulated? Why would that be, if neutral is ground?

Because neutral is the normal current carrying conductor (unlike ground), and so it has current on it routinely. If neutral were interrupted from a simple loose connection, the current would be interrupted but the voltage would not be. Neutral would float at/near 120V!

You certainly understand that the "protected side" of the GFCI is safer than the "unprotected side". So why would you attach everything that is grounded - chassis of equipment, etc - to the unprotected neutral wire? Which due to a wire break is now floating at 120V. And since ground bypasses the GFCI, nothing will trip if you get between that "ground floating at 120V" and anything actually grounded.

A number of other ordinary malfunctions can cause similar conditions, e.g. the "Lost Neutral" when a utility power drop fails.

So yeah, terrible plan. This is what makes 3-prong dryers and ranges so dangerous, and why they banned that in 1996.

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    Good idea on the retrofitting ground. I look at the ground as a dead-end leg of neutral. Its connected to neutral at the box and runs "nowhere" electrically. However if hot every comes in contact with the metal of a device, since the ground is connected to the metal of the device, it will complete the circuit, allow current to go through ground to the box. A GFCI, from what I see doesn't directly sense the current through the ground. Instead it just monitors the current through the hot and neutral and then triggers if the currents don't match. Sep 30, 2021 at 16:49
  • I wanted to add, I don't completely understand your response, so I'll study it more carefully and try to learn a bit more. Thanks. Sep 30, 2021 at 16:50
  • @RobertLugg well, a wrong mental paradigm will definitely impede understanding. Ground isn't anything to neutral. Neutral runs from the utility transformer to your loads. Ground runs from your earthing spikes (grounding electrode system) to your device chassis. Parallel but entirely separate systems. In a European home they are also isolated from each other. In a North American home, neutral has an equipotential bond to ground. Due to that bond, grounds are allowed on the neutral bar, so a lot of homes have 1 bar for both. Confusing! Oct 1, 2021 at 0:30
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Ok, this is going to be hard to hear, but you need to hear it: you need to remove and replace all your knob and tube wiring. The main reason is simply because it's ancient wiring and you won't be able to get a decent homeowners insurance plan with it I place. You also can't insulate any cavities with K&T wiring unless you fancy a house fire.

The GFCI solution is really not a solution at all. It really does nothing at all. You have to ask yourself, why do we need a 3 prong receptacle? Is it because the device we're plugging in has a metal chassis that could potentially become energized by a hot wire? Or is it to facilitate the diversion of surge current from a lightning strike? If it's the former, then a GFCI will at least limit your electrocution to less than a second, but you're still going to go for a ride. If it's the latter, then a GFCI has nothing to offer. And any warranty claim or guarantee from a surge protector will be rendered null and void.

Consider replacing.

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    "GFCI in lieu of grounding" is a fairly common thing and 100% legitimate. I haven't done it myself - so far all the 2-wire receptacles have replaced (not a pro, so only work on my own house) turned out to have a ground wire available, but I was prepared to go that route if needed. It provides a very high level of life-safety protection at much lower cost (typically) than rewiring. Not just "metal chassis" but also vital in kitchen/bath/etc. As far as K&T safety - not necessarily that bad, as I understand it based on many other posts here. Sep 30, 2021 at 3:14
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    Also keep in mind that for a typical DIY, replacing old receptacles with new GFCI, etc. is a fairly reasonable task. Replacing a house full of knob & tube is, for most people, a big cost requiring a professional electrician. Likely a good idea, but the GFCI quick/inexpensive fix provides a lot of protection - i.e., far better than no updates at all. Sep 30, 2021 at 3:53
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    Funny. My house was originally wired with K&T. Still has some runs of it. Last I checked, my homeowners insurance was still in force. Also, OP knows he's got a house full of K&T, yet he's going to buy it. Probably won't pay cash, so there will be a mortgage. Mortgage (Latin for "death grip" - that's a freebee) lender won't loan money without insurance, ergo, will have a "decent homeowner's insurance plan".
    – FreeMan
    Sep 30, 2021 at 13:09
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    So I didn't tell the whole story. I contacted an electrician firm. I originally assumed that I did want to replace all the knob and tube and wanted to figure out what it would cost. They said not to bother and further said that none of their electricians would do it. I don't doubt that I could find another guy to replace the K&T, but these guys are union so generally know what they are talking about. That led me on this adventure. Sep 30, 2021 at 14:11
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    Yeah, we have that problem around here too. No one wants to do it. It's too much work! 😫 Just have to keep looking. And whoever told you you shouldn't bother is crazy. You do understand that K&T CANNOT be under insulation right? So if you've had I blown in to the attic or walls, you're a ticking time bomb for a fire.
    – DrSparks
    Sep 30, 2021 at 23:34
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I can only partly agree with previous answers and will offer a different perspective.

By attaching downstream ground terminal to a homerun neutral conductor, you might in theory be able to ground out just enough fault current to trip the GFCI in some specific fault conditions.

HOWEVER, you would have accomplished this by creating a ground terminal that is LESS SAFE in all circumstances. If your circuit has any other loads on it, your ground terminal is now part of the circuit for those other loads, and its voltage can only be zero when the resistance is zero. In fact, those "neutral" wires always have a small amount of resistance and a small voltage when under load, and anyone who touches that circuit path becomes a parallel load under the right circumstances.

Also consider that a GFCI DOES NOT protect the ground terminals. In case of a load fault, your outlets would always be connected to a circuit conductor, which is opposite the desired result of a circuit interrupt.

In summary, your plan would be IDENTICAL to jumping the line neutral terminal on the GFCI to the ground terminal. You are making the circuit less safe by doing that.

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