this post was submitted on 14 May 2025
6 points (100.0% liked)

Electricians

566 readers
2 users here now

founded 2 years ago
MODERATORS
[email protected]
6
Sanity Check: Smart Light Switch, No Neutral (lemm.ee)
submitted 1 week ago by [email protected] to c/[email protected]
9 comments fedilink hide all child comments
 

I was installing a TP-link HS210 3-way smart switch in my dinning room. On the side with the mains power I do have a neutral wire, but on the other switch I have no neutral wire from the wall (for that breaker). I do have a switch that's on the kitchen breaker right next to it, though, and that has a neutral and ground.

In my breaker box, both the neutral and grounds appear to be on the same row of lugs.

Running the neutral wire from the switch to the ground works, and I'm thinking it's because it's all going to the same place. This specific switch didn't explicitly say to do this, but other switches I've installed did.

Now, I could run the switch's neutral to the neutral on the kitchen circuit. I didn't at first because I had the other switch wired wrong, so I thought it was the no neutral switch causing issues.

top 9 comments
sorted by: hot top controversial new old
[–] pishadoot 6 points 1 week ago* (last edited 1 week ago) (2 children)

Kind of hard to understand what you're describing, but if what you're saying is that-

You're trying to install a switch that requires a neutral wire in a box without a neutral, and using a ground wire instead

-that is bad, and you should not do it.

Yes, it will function. However, you're electrifying your ground circuit, which is not supposed to happen. The ground and neutral busses are essentially the same thing at your main panel so yes you can do what you're doing, but it's not good. If there was a GFCI in this circuit it would trip, if that tells you anything.

If I understand your post correctly.

Edit: are you also saying you might use a different circuit's neutral? In theory you can do this, but it depends on where the breakers are in your panel. They need to be on different phases, and they need to have a mechanism to trip both at the same time. Again, if I understand you correctly.

You sound like you need to call and have an electrician come out, because you're just capable enough with what you're doing to be really dangerous.

[–] litchralee 3 points 1 week ago* (last edited 1 week ago) (2 children)

The ground and neutral busses are essentially the same thing at your main panel so yes you can do what you're doing, but it's not good.

I'll offer some more clarity on why swapping neutral and ground is bad. In North American electrical code parlance, the issue is known as "objectionable current" and is a result of electricity taking all available paths back to its source. Basically, while neutral and ground wires go back to the same place, putting current onto the ground wire means developing a voltage -- due to Ohm's Law -- and that presents a shock hazard for any upstream or downstream outlet's ground.

Imagine that downstream is a refrigerator or computer, where you get a minor shock just by touching the exterior metal case. This is very bad.

We do not want any voltage across the ground wire, so that requires that we must not intentionally put current through the ground wire. Ohm's Law is unrelenting here.

[–] pishadoot 2 points 1 week ago (1 children)

Correct on all counts. I didn't want to go into too much depth because it's hard for DIYers to understand some of this stuff but I think you nailed it.

OP, this is what I was alluding to when I said that what you're trying to do would trip a GFCI. You know, the things that stop you from getting shocked or electrocuted. And sharing that neutral in a different circuit, it can likely be done based on your written description, but I wouldn't trust you to do it safely/correctly.

Go with a different type of switch or call an electrician for your install if you are locked into your plan.

[–] [email protected] 1 points 1 week ago* (last edited 1 week ago)

I'm not a complete layman, it's just been a few years since my university courses on fundamentals of electronics (which covered AC fairly extensively).

The switch itself is basically just a relay and esp chip, so it likely is only drawing milliamps. But, I'll drop a neutral and ground line into the box and tie it into the junction box in the attic.

I was mostly just trying to avoid having to go into the attic.

[–] [email protected] 1 points 1 week ago (1 children)

Wouldn't that have an "objectionable current" on all grounds on that panel (what a great term, "objectionable current". Any idea when that was introduced to the jargon of electrical engineering? Sounds like it could've been 19th century, or from 20th century lawyer-ese).

I'm not an electrician - just know enough to be dangerous, so I try to follow the rules from the maximal safety perspective (and I'm always trying to learn more, I've always found electrical networks intriguing).

[–] litchralee 0 points 1 week ago* (last edited 1 week ago) (1 children)

Objectionable current propagates its hazards in two ways: 1) any part of the ground wire that is actively carrying the objectionable current back to the utility neutral, that's lava, and 2) any branches off of a part of the ground wire which is carrying objectionable current, that's also lava. In scenario 1, there must exist a voltage w.r.t. utility neutral, per Ohm's Law and due to copper still having a small resistance. Since utility neutral is connected to the site's ground rod, this means the generated voltage is a hazard to any person on site, because that person could accidentally conduct some of the objectionable current through their body, to the ground rod, and back to utility neutral.

Scenario 2 is a bit more subtle, but uses the same setup as scenario 1. If scenario 1's hazard was a dangerous voltage, then scenario 2 makes it worse by bringing that voltage -- unchanged -- to more destinations, amplifying the risk of accidentally touching that errant voltage.

For a typical USA domicile where all circuits and their ground wires are home-run back to a single main panel, the presence of objectionable current on Circuit A's ground wire will not create Scenario 1 on Circuit B's ground wire. This is because the flow of electricity is always back toward its source, in inverse proportion to resistance. From the perspective of Circuit A, no part of Circuit B's independent ground wire would help bring current back to the utility neutral. Of course, both circuits meet at the neutral/ground bar, but because of the ground rod, this point in the system is equal to earth. And so Scenario 2 will not appear either.

The equation drastically changes if -- unrequired but not prohibited by the NEC -- the separate circuit's ground wires are wired together in other parts of the house. For a common example, if two different circuits are ran into the same switch box -- such as by the front door for the porch light and the foyer light -- then it's quite common to tie both circuit's ground wires together.

Now we have a problem: if the porch light circuit develops objectionable current on its ground (eg rainwater in the lamp), then the foyer light circuit's ground is now a third path back towards the utility transformer, also carrying some objectionable current. And if the foyer circuit has an indoor receptacle with a TV attached, and that TV has a bunny-ear antenna attached via a coax connector, then there's a shock hazard when someone touches that antenna. This is totally unexpected, and that's precisely why it's so dangerous, irrespective of the actual amount of voltage on the ground wire. This is Scenario 2 in action, since although the antenna isn't actively carrying the objectionable current, the voltage from the original fault means a human could become an additional path for objectionable current to flow. Not good.

Because of this, it is absolutely verboten to intentionally put operating current to ground, and why GFCIs for outdoor circuits is basically mandatory in all regards now. That said, a lot of electronic appliances have "filtering capacitors" which block voltage transients from impacting their operation, but these work by shunting the transient to ground... This is not ideal, but transients by their nature are not part of normal operating current, so it's a reasonably acceptable practice. Whereas running a space heater 24/7 using live and ground would be a death wish.

More complexity occurs when subpanels are involved. Here, a fault causing objectionable current downstream of a sub panel can end up polluting all of its downstream ground wires, by way of Scenario 2. But not for other downstream ground wires in the main panel, due to it having the designated neutral/ground connection. The solution is still the same: GFCIs for active protection, and a vow to never abuse the ground wire. This is partially why NEC keeps adding GFCI requirements for circuits.

As for the term "objectionable current", I think it's solely a product of the NEC, which is published by NFPA, the fire insurance people. Yes, the electrical code is more about preventing fire and inherent safety, than it is about harnessing electricity. NEC is a floor for safety, but is not a design manual, so it mostly tells you what isn't allowed or what must be done. All else is fair game.

[–] [email protected] 1 points 1 week ago (1 children)

Duh, I wasn't thinking about the ground rod (see why I'm not an electrician?).

I appreciate your repitition of how voltage flows, always back to the source - this is the first I've heard this (again, not an electrician), I've always thought of it as flowing to the "easiest" path (I have more background in low voltage DC, , which is similar, but different. Learning DC first made learning AC harder, in my opinion). Seems this idea about voltage flow needs all your repitition - keep it up.

Yea, code is always about baseline safety - there's an electrician in my extended family (first licensed in the 80's), and we've occasionally talked about how much he's had to study over the years. I've seen how much new code has come in since about '85. Lessons learned from fires (or worse), I'm sure.

Thanks for your extensive explanations, I'm gonna have to read this a few times to fully grok the why's. Good stuff!

[–] litchralee 0 points 1 week ago

Phew! I thought I was beginning to sound like a broken record lol

I'm also not an electrician, but I did study some electrical engineering at uni. And for my job, I've spent a non-insignificant amount of time disabusing people of the awful stock phrase "electricity returns to ground"; it actively causes unbound confusion and the subsequent questions which somehow befall me.

[–] [email protected] 1 points 1 week ago* (last edited 1 week ago)

One weird thing about my breaker panel is that I have one breaker that will shut off all of the outlets in the house, and one that will shut off all the lights. The inspection didn't say anything about those being wrong, so I'm not sure how they wired it up.