Neautral is ground in 120V system? - Page 1
Forum section: electronics / beginners

author: u666sa
author_rank: Frequent Contributor
timestamp: March 20, 2025, 10:44:31 am
content: Sorry for this newb question. I disassembled my outlet, 120V, Miami. Condo building with 208V triple phase. (two outlets combine to 208V).Hot and neutral connect to the ceramic socket. If I measure hot to metal box while the outlet is disassembled, it gives me 120V. Measuring resistance between neutral and metal box it's like 1 ohm.Measuring hot to ground in the ceramic socket has no results, I guess ground has to be connected to metal box with wire.Does this sound right, or do I have problem with ground?

author: jwet
author_rank: Frequent Contributor
timestamp: March 20, 2025, 02:57:31 pm
content: That's completely correct. Neutral and ground are connected together in the main panel. Any resistance you measure between them is the round trip resistance to your box.US power is delivered to you by the secondary of a center tapped transformer at the street. The outside of this secondary are the two big hot lines that enter your main box. The neutral is the center tap of this transformer and also enters your box. It is tied to a bus bar where all the white (neutrals) are tied. There is another bus bar where all the grounds (bare copper) landed. The ground and neutral are tied together and also runs to a 6 foot ground stake in the ground neare the power entrance.If a hot contacts the grounded metal chassis of piece of equipment, it will pop a breaker. For wet or below grade area, a GFCI is required. This look at hot and neutral and makes sure that the black and white wires have equal and opposite currents, (it uses mini transformer with a single turn primary of the white and black iin opposite direction- it produces no voltage if the signals are balanced (within 4-6 mA). This makes sure that small currents can't flow though ground either.There are other safeguards but what you are seeing is completely correct.

author: calzap
author_rank: Frequent Contributor
timestamp: March 20, 2025, 05:12:32 pm
content: There are some confusing things in the OP post. Both ceramic socket and outlet are mentioned. Typically, a ceramic socket is used for a light bulb or maybe a heating element. An outlet (aka receptacle) has sockets for cord plugs. Hot to metal box gave 120 V but “hot to ground” in the ceramic socket gave 0 V. I assume this means the center contact of socket to the box or maybe the ground wire. Assuming the ceramic socket is connected to hot, which is live, then there is a disconnect in the socket or the wire connecting it to hot or the connection itself. Also possible, due to miswiring, it’s the peripheral contact in the socket that’s connected to hot.Mike

author: TimFox
author_rank: Super Contributor
timestamp: March 20, 2025, 05:38:41 pm
content: A better statement is "Neutral is grounded".In US single-phase distribution, current should flow in the line and neutral wires, but the ground wire is there for safety purposes and should have negligible current flowing under normal conditions.

author: bostonman
author_rank: Super Contributor
timestamp: March 21, 2025, 01:59:18 am
content: QuoteI don't remove the panel cover, but I tend to forget that this is true.So neutral comes into the house and is tied directly to Earth ground along with all the breakers?My panel, much like many other modern ones, uses GFI and AFCI (?) breakers. I'm not quite sure how these are wired since they need to monitor HOT to NEUTRAL (?) to confirm nothing is leaking to Earth ground.

author: TimFox
author_rank: Super Contributor
timestamp: March 21, 2025, 02:55:01 am
content: Quote from: bostonman on March 21, 2025, 01:59:18 amIn my breaker box, two hot wires and one neutral wire enter, and the neutral bus is grounded to a rod near the entrance and to a cold water pipe near the water entrance.Normal GFI breakers use a current transformer to measure the difference between Hot and Neutral currents, not voltage from Hot to Neutral.Should some of the Hot current not return through the Neutral wire, the transformer senses the difference.

author: bostonman
author_rank: Super Contributor
timestamp: March 21, 2025, 03:16:46 am
content: QuoteYears back I had my service upgraded to 200A, so I got to witness the codes and installation methods.I can't visualize the inside of my box, however, I know it has many "pigtail" wires due to the GFI and AFCI breakers (or just the GFI); this causes difficulty trying to look at the connections.For some reason I've put in my head over the years that Earth ground isn't connected to neutral and kept separate, but this is apparently wrong.Unless I'm wrong, code states (or at least in Massachusetts): Earth ground goes to the cold water pipe on both into the water meter side and out of the meter side (this allows a permanent Earth ground should the meter be removed). Another wire goes outside and two rods go several feet into the ground; one not far from the house and another about five-feet from the house.I've had work done in the yard since the Earth ground wires were installed. Although I warned the workers about the wire, they still managed to putz around near it. Last summer, going by memory, I dug in order to make sure the wire wasn't cut. First I tried to find the rod, then resorted to locating the wire. The whole process was much harder than I thought, however, I found the wire, traced it to the rod, gave it a visual inspection, and measured it to make sure it wasn't cut.Upon getting irrigation installed, I did a 2D CAD drawing showing the approx. pipe layout and locations along with the Earth ground and rods.

author: calzap
author_rank: Frequent Contributor
timestamp: March 21, 2025, 03:59:27 am
content: At the service entrance panel, the ground and neutral buses should be bonded. If there are subpanels, separate hot, neutral and ground conductors should enter, and the neutral and ground buses should not be bonded. The section of the NEC (used at least in part in most states of the U.S.) that covers grounding electrodes is 250.52 and is fairly long and complex. There are all sorts of grounding electrodes that might be used … rods, pipes, plates, rebar in concrete, and others.Mike

author: Jwillis
author_rank: Super Contributor
timestamp: March 21, 2025, 07:02:03 am
content: Neutral or Common isReferencedto ground. It is not "Ground" and should never be used as a ground.This Reference to Earth prevents voltage drifting upward in your house wiring. Voltage potential difference between common and Earth can increase if the reference ground is to far away or not present. This can be several volts difference to even several Hundred volts difference, depending on service. The Earth ground line at the pole and Earth ground your house can have several giga-ohms resistance between the two points. So ground potentials can vary significantly from the pole to your home. Larger Structures may require several ground points.If Common was Not reference to Earth ground at the pole and your home, then the common can be just as hot as the live wires.

author: J-R
author_rank: Super Contributor
timestamp: March 21, 2025, 07:03:05 am
content: Some notes:Three phase 208V is common in commercial environments due to the ability to carry more total power compared to split phase for a given amount of wire.Also, to clarify, for US residential breakers, the coiled pigtail from the arc/ground-fault breaker connects directly to your neutral bus bar and the neutral coming back from the outlets/circuits connects to a second screw terminal on the breaker, next to the hot terminal. As mentioned, this is to be able to detect current leaving the circuit via another path, such as your wet feet on the floor.It's 100% true that there is only a single point where the neutral and ground should be bonded in a house (or at the service entrance). This is primarily to ensure that none of the ground conductors will have a voltage potential on them. If you connect the ground and neutral together in a sub-panel in addition to the main panel, then half of the return current from the sub-panel to the main panel will flow through the ground conductor.Plumbing and ground is a bit complex, but my understanding is that plumbing should be grounded, but you shouldn't use it a ground conductor. So if you had an outlet on the outside of the house, you couldn't use the water spigot as a ground point for the outlet. It needs a dedicated ground wire going back to the panel.Lastly, a neutral wire can have varying levels of voltage potential, depending on the conditions. When working normally, but at say 20A, you could observe up to 5V on the neutral compared to ground due to the resistance of the wire. If the neutral is disconnected, say at a different outlet in the circuit, you could have essentially the full 120V sitting there if a device is connected and switched on. So don't always assume the neutral is "neutral".

author: TimFox
author_rank: Super Contributor
timestamp: March 21, 2025, 02:18:23 pm
content: Quote from: J-R on March 21, 2025, 07:03:05 amBy Chicago electrical code for domestic service, the neutral bus in my house is wired to two ground connections:A rod driven into the earth immediately below the meter on an outside wall (north side of my house).A clamp on the copper cold water pipe near the water inlet pipe inside the south wall of my house.Both connections are done with a length of aluminum conduit with a large-gauge copper wire (8 AWG?) inside.Near each ground is a metal tag identifying the ground connection and forbidding removal.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 21, 2025, 03:05:26 pm
content: Quote from: u666sa on March 20, 2025, 10:44:31 amcatzap is correct. You have a problem with ground. I wrote a reply earlier but then saw that post and deleted.Voltage measured from the ground terminal in the ceramic socket to hot (black) should be very close to the voltage measured from neutral (white) to hot regardless of whether the box is connected to ground. Consider the case when plastic boxes are used. They are not connected to ground. The reason for running ground to both the socket and metal box is safety in case hot should touch the box.NEC-based handbooks at big box stores will show the correct way to wire a socket in a metal box. Generally, there should be a pigtail with 3 wires: incoming ground, wire to box, and wire to socket. An exiting ground wire (e.g., to the next outlet box) may also be present. Some variations may be allowed by code. In some cases, a "grounding" socket or switch may suffice for the connectiontoa metal box. That is, you will see a small metal strip that connects to the ground socket across the attachment ears so it presses against the box when installed. While that might be sufficient to provide grounding to the box, it is my understanding that it is not acceptable for providing ground to the socket for obvious reasons, e.g., when the outlet is out of the box, it is not grounded or the ground is not as robust as needed.What you are observing could be explained by that.It was/is common to twist all ground wires together and just leave them bare without a wire nut stuffed in the box. Now, you may see a wire nut used. I have read that while older electricians will still twist even with a wire nut, code allows depending on the wire nut alone. I have seen a problem in my own home with the latter, wire-nut only method. It is possible to have a wire inserted in a wire nut with other wires and not have good contact. In my home, it was a hot wire and a dead socket.Your problem may be the same but involving the ground wire instead.Those are two situations I would consider first. If neither applies, then dig deeper.I assume your wires are not in metal conduit from box to box. If they are, that changes the situation a little. It's best to check local code for that.

author: bostonman
author_rank: Super Contributor
timestamp: March 23, 2025, 02:23:25 pm
content: QuoteSince Earth and neutral are tied together at the panel, this makes sense. Prior to this thread, I MAY have connected it to the outside spigot.QuoteSounds similar to my wiring. As mentioned, I have a connection on both sides of my water meter so a connection exists to the pipe when the meter is removed (in MA they get replaced every five-years I believe - unless that has changed). Also, I have two rods outside, first is close to the outside wall of the panel, the second is 6 1/2' away from the house.I'll ask a crazy question. If Earth and neutral are tied together at the panel, does this mean that the return path (let's say a light bulb plugged into an outlet) is bleeding some of the current to Earth rather than through neutral and back to the power station?

author: bdunham7
author_rank: Super Contributor
timestamp: March 23, 2025, 02:36:47 pm
content: Quote from: bostonman on March 23, 2025, 02:23:25 pmYes, but only to the transformer serving your property. Such ground currents will be very small unless you have very high neutral currents or high resistance in your neutral line to the transformer.

author: MarkS
author_rank: Unknown
timestamp: March 23, 2025, 10:33:36 pm
content: Quote from: calzap on March 20, 2025, 05:12:32 pmIf I had to guess, he's looking at a very old panel with screw type fuses instead of breakers.