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I see other questions on this topic and totally understand the dangers of bootleg grounds and bootleg grounds on the post-GFCI neutral. But my question is about the safety of a bootleg ground before the GFCI.

I have an old house with ungrounded circuits from the sub-panel. A relatively recent branch exists where the installer used modern NM-B 14/2 wiring for all outlets in the branch circuits, connected all the grounds to the outlets, but left it unconnected at the junction from old-ungrounded to modern 14/2.

Drawing:

drawing

I've added a GCFE at the first outlet in series to provide some amount of fault protection to the outlets but its not clear to me if the traditional dangers of a bootleg ground apply when a GFCI exists to prevent reverse-current?

Drawing:

drawing2

Whether or not this is against code, can someone explain to me why this is dangerous?

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  • Is the wiring in metal conduit or metal-jacketed cable?
    – Harper - Reinstate Monica
    Apr 8, 2022 at 22:09
  • The boxes in the extension circuit are all plastic.
    – Samuel Davidson
    Apr 9, 2022 at 17:51
  • I was asking about the original boxes, but I think you answered that. If they are metal boxes with metal-jacketed cable or metal conduit, fair chance that's a legal grounding path. You should check that. If so, the installation is grounded after all.
    – Harper - Reinstate Monica
    Apr 9, 2022 at 19:17

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Update: John Ward talks about this exact situation in a video. It struck me how similar John Ward's death diagram is to your diagram above, complete with GFCI (RCD) in that exact spot. It applies to "whole house GFCIs" in British houses with no ground rods, but it's the same concept exactly.

A relatively recent branch exists where the installer used modern NM-B 14/2 wiring for all outlets in the branch circuits, connected all the grounds to the outlets, but left it unconnected at the junction from old-ungrounded to modern 14/2.

NEC 250.110 prohibits extending a non-grounded circuit.

However you should check if the old work is metal conduit or metal jacketed cable, which is often a valid grounding method, which means the old boxes may be grounded after all.

Also most places it is illegal to do electrical work without an electrician's license, which is a 5 year apprenticeship, however it is OK if you are the owner-occupant of a single family home.

Also most places, a circuit extension requires pulling of a permit.

Sounds like the last guy left you a real mess.

An "island of grounds" is dangerous but GFCI helps

Because, when you have outlets with ground attached to each other but not to the panel, it does not arrest ESD or ground faults. It shares them with everything in the island.

So the laser printer throws a big spike. The power strip dumps that spike to ground. Except all that did was make sure all the grounded appliances received the full spike!

A PC power supply develops a bolted fault to ground. OK, now everything in the island that is supposed to be grounded is energized at 120V.

However, adding a GFCI in the first position at least renders it far less unsafe.

If you bootleg ground to neutral, it isn't ground anymore. It's neutral.

You say "but neutral and ground go to the same place!" Sure, when everything is working properly.

You may notice that neutral is insulated. Ever wonder why? Because there are very common conditions under which neutral will be energized.

enter image description here

So let's look at what your change really is. I've drawn the GFCI protected zone in green. Note how one wire color has been changed to reflect what it actually connects to, and here you can see the reckless and foolish thinking of bootlegging ground.

Note that while the hot and neutral are inside the GFCI protected zone, the, um second neutral is not inside the protected zone. So a neutral problem in the "rest of the house" department (or even off diagram) could energize neutral and light up all the "things that are supposed to be grounded" on the equipment. And what would the GFCI do? Even if it tripped, it does not disconnect the second neutral.

Neutral and ground must bond at exactly one place: the main disconnect, where Code says it must.

It's dangerous to take "a little bit of knowledge" and try to extrapolate that into a prioritization of safety risks. The fact is, humans are infamously terrible at that. That's why we have a science and data driven approach called the Electrical Code.

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  • Thanks for the information but theres no need to be so aggressive. So a couple followups: 1) I didn't do this extension, it was likely done in the 90s by the previous homeowner who didn't seem to be particularly concerned for safety. I just purchased this house and doing small safety improvements with GFCI outlets. 2) My home is located in Utah where I can do my own electrical work. 3) It wasn't clear from your answer if a GFCI protects against the traditional electrified grounds risk of bootleg grounds? Wouldn't the GFCI stop any current flowing through ground in my 2nd drawing?
    – Samuel Davidson
    Apr 9, 2022 at 17:53
  • @SamuelDavidson Sorry, I didn't mean to sound aggressive. Most homeowners can do their own electrical work, but permits and inspections are still required. I have edited to illustrate my point more impactfully, I suspect you are considering only certain problems inside the GFCI protected zone, and not thinking about problems with the supply to that GFCI.
    – Harper - Reinstate Monica
    Apr 9, 2022 at 19:43
  • Gotcha, thanks for the edited followup :) Its much nicer to read and I think answers my question better. "And what would the GFCI do? Even if it tripped, it does not disconnect the second neutral." The only way the ground/neutral is energized would be from the post-GFCI hot correct? If the GFCI is tripped the ground/neutral would stop being energized? The hot/neutral from the house is static ancient knob & tube unfortunately. No grounding.
    – Samuel Davidson
    Apr 9, 2022 at 21:00
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    No, there are lots of ways that neutral could be energized, see the "rest of house" part of your diagram. All sorts of problems in there could float the neutral wire up toward 120V prior to the GFCI. Now what?
    – Harper - Reinstate Monica
    Apr 9, 2022 at 22:34
  • So there would be no return current path through either GFCI hot/neutrals. And I would get electrocuted from the hot "ground" through my feet to earth or something?
    – Samuel Davidson
    Apr 9, 2022 at 23:03

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