How should ground wires be handled when updating switch loops in an older home?

Question

I have a house built in the 1950’s with fabric covered NMC wiring still present in most circuits. Those original wires are all 12/2 with no ground. I am currently in the process of replacing some of my old switch loops with new 12/3 romex from the light fixture to the switch box in order to bring in a neutral for “smart” switches. The existing switch box is metal.

I want to go from a typical switch loop (my old wiring only has line and neutral)

To one that supplies neutral from the fixture to the switch box

_{These wiring diagrams belong to do-it-yourself-help.com and can be found here: https://www.do-it-yourself-help.com/wiring_switches.html}

My question is, what is the proper way per NEC to terminate the ground wire inside the new 12/3 romex? The light fixture itself has a ground wire that is currently floating. Should I leave the ground wire disconnected and just cap it by itself? Should I attach the ground wire at the fixture to the ground at the new cable in the event that ground may someday be brought to that receptacle? (This seems more dangerous to me since it’s definitely not currently grounded.) Should I clip it up to the romex jacket and pretend it isn’t there?

I know the technically correct thing to do would be to pull new 12/2 wire from the existing fixture back to the panel (basically rewire the whole house), but that is not economically feasible. I am looking for the most code-compliant way to handle this ground wire, but I am not having any luck searching through the 2017 NEC. Any advice on the proper way to handle my scenario would be appreciated, especially if they can point to the relevant section(s) of the electrical code.

Answer 1

No fake grounds: Don't create "islands of grounding"

Grounding must start at earth, via your grounding electrode system, to the main panel and its equipotential bond. Then it must be carried onward from the panel sequentially up the branch circuits.

If you don't do that, you create an "island of grounding" where a few nodes are grounded to each other but nothing is able to return fault current on the ground wire. In that case, you have simply created a "Ground Fault Distribution Network". (Bad thing).

It means that if any device on that island has a hot-ground fault or leakage, then all the devices on that island have an uncontained ground fault, and their grounded parts are energized at mains voltage. Every switch plate cover screw, every machine chassis, all become lethal to touch. It's the worst possible outcome.

Isolate the ground wires from the boxes. This is not necessarily easy, they are bare and will tend to be pushed against the metal parts of boxes and devices.

Consider retrofitting grounds

NEC 2014 made it a lot easier to retrofit just a ground wire, and continue the original cables in service. Now if you have an island you'd like to ground, you can ground it any which way you can - within some generous limits.

• Not any random water pipe will do, but some will.

• Neutrals can never be promiscuous (shared across more than one circuit*). But retrofit grounds can. So circuit 7 can grab a ground from circuit 9, provided they source from the same panel.

• The pathway must be big enough for the circuit, continuously: so a 20A circuit needs at least a #12 ground (#14 won't do), and a 30, 40 or 50A circuit needs a #10 ground. Metal conduit of any size is plenty for any of these. Here's a strategy: first wire a "backbone" ground with #10 to range, water heater, A/C, dryer etc. so you can tap it for other circuits. likewise run #12 grounds where 15A and 20A circuits are in the same area.

GFCI is a good band-aid

A great many ground fault problems can also be avoided with a GFCI breaker (or GFCI deadfront or livefront strategically placed). You still get shocked, but only for a few milliseconds.

---

* a MWBC counts as a single circuit, and its breakers better be handle-tied.

Answer 2

I see two places the NEC would shed light.

404.9(B)Provisions for General-Use Snap Switches - Grounding has an exception for adding switches where there's no grounding wire available, depending on the NEC rev applicable in your jurisiction, it could be as simple as adding GFCI protection to the circuit or using a nonmetallic plate. However I don't think this covers you in this case, since you are not simply replacing a switch.

I believe 250.130(C) Equipment Grounding Conductor Connections - Nongrounding Receptacle Replacement or Branch Circuit Extensions would apply, and the code specifies a number of ways you can add the EGC to the circuit. Details in this question.

Answer 3

Sometimes in the olden days, there was a ground wire but people didn't know what to do with it, so it's folded back and wrapped around a clamp or other component of the metal box. You might check to see if it's hiding somewhere.

Is it more feasible to pull a new cable from the switch to the panel? That would be another method.

If you choose not to bond to another ground elsewhere in the building, you may be off the hook, as I did find some relevant segments in the NEC:

410.42 Luminaires with Exposed Conductive Parts. Exposed metal parts shall be connected to an equipment grounding conductor or insulated from the equipment grounding conductor and other conducting surfaces or be inaccessible to unqualified personnel [...]

You may be able to convince an inspector that your light fixture is "inaccessible."

Moreover, there is:

250.86 Other Conductor Enclosures and Raceways.

Exception No. 1: Metal enclosures and raceways for conductors added to existing installations of open wiring, knob-and-tube wiring, and nonmetallic-sheathed cable shall not be required to be connected to the equipment grounding conductor where these enclosures or wiring methods comply with (1) through (4) as follows:

1) Do not provide an equipment ground

2) Are in runs of less than 25 ft

3) Are free from probable contact with ground, grounded metal, metal lath, or other conductive material

4) Are guarded against contact by persons

Answer 4

Piggybacking off batsplatsterson's response, the same article of the NEC states you can connect an equipment ground to any point on the grounding electrode system. Some components of this system you may have easy access to would include: metal water pipes (if you are sure there are no insulating joints and it is actually bonded to the main panel correctly), grounding electrode conductor (wire that connects your main panel to the water pipe and/or ground rod), ground rod outside the home, etc.