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It's easy to verify which wire is the phase without relying on wire colors. But I'm not sure if I'm able to determine which wire is the ground and which is the neutral.

I have heard that using a multimeter it will show a higher voltage between the phase and the neutral than between the phase and the ground.

Is that method correct? Is there any other way to determine if the wire colors are correct and the installation was done properly?

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A voltage test will only tell you which wire you are using for neutral, not if it is the right one. And because of voltage drop from any load current on it, the voltage would be lower, not higher. –  Bobbi Bennett Jan 26 '13 at 16:06
For questions like this, it's worth mentioning a geographic location. Practices differ. Here, neutral is black in older installations (nowadays blue) but I believe the USA uses black for live! Earth/Neutral connection points vary too. –  Brian Drummond Jan 26 '13 at 16:30
Netherlands uses black for switched-live. So brown(live)--switch--swited(black)--load--neutral(blue) and green/yellow for earth. So yes geo info and colors is welcome. –  jippie Jan 26 '13 at 16:47
@BrianDrummond The colors don't matter for question. It's a question about the electrical differences between neutral and ground. –  Jader Dias Jan 26 '13 at 17:20
Please don't cross-post the same question to multiple sites; if it's a better fit for a different site, it will be migrated. –  Niall C. Jan 26 '13 at 18:04
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migrated from electronics.stackexchange.com Jan 26 '13 at 17:55

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4 Answers

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You've asked three related but not identical questions here which makes it hard to figure out what your real concern is and how to solve it.

  1. is the safety ground separate from neutral.
  2. how to identify and distinguish the safety ground from neutral.
  3. is the voltage between hot and neutral greater than hot and safety ground under load.

The first two questions have a common issue which is whether and where the safety ground is connected to neutral.

I will answer assuming a US installation.

For your first question:

Safety ground and neutral are supposed to be tied together in one and only one location, the main service panel.

Sometimes you see receptacles where the receptacle's neutral and safety ground are both connected to the circuit's neutral because the house was wired without a safety ground. If you are trying to detect this, a visual inspection of the receptacle's wiring is the most reliable method.

Sometimes you see houses with incorrectly wired subpanels. In a subpanel, safety ground and neutral are required to be separated. Visual inspection is the best way to verify this also.

For your 2nd question:

maple_shaft's answer is good with one exception. The problem is he uses the ideal wire model where there is no wire resistance. In fact, 14 gauge wire has a resistance of about 0.25 ohms per 100 feet. Assuming the circuit is wired correctly and safety ground is bonded to neutral only at the main panel, a heavy load like a hair dryer will generate enough voltage drop across neutral from the load to the main panel to be easily measured. I personally verified this ten minutes ago.

For example, 15 amps will generate a voltage drop of 3.75 volts over a 100 foot neutral. If you put your voltmeter between safety ground and neutral at the same receptacle as the load, you will see this 3.75 voltage drop.

For your 3rd question:

Under a heavy load, the voltage between hot and neutral will be LESS than the voltage between hot and the safety ground. In the above example, there will be 3.75 volts less.

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Usually your central heating and your water piping is grounded by regulation. Your ground wire shouldn't have any potential differerce with these installations (as there shouldn't be any current flowing through it), whereas neutral may carry a few volts (because of current elsewhere in the system lifting potential).

If your installation is behind a residual-current device and you have the live wire identified, another way to figure out neutral and earth is connecting a 25W lightbulb. If the bulb lights up you're using neutral, if the residual-current device trips, then you used earth.

Of course if you don't trust wiring or its colors, the only way to go is to pull the existing wires out of the pipes an do a proper re-wiring according to national regulations.

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There really is no way to tell the difference from the point of the load, because the ground and neutral are bonded together at the service entrance.

The only real way to be sure is to temporarily disconnect the neutral for the circuit in question at the distribution panel and verify that there is now no voltage between line and neutral at the load, while there's still voltage between line and ground.

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in my installation only the phase can be switched at the distribution panel, and the neutral and the ground aren't bonded together. –  Jader Dias Jan 26 '13 at 17:24
Also in the US only the phase is switched at the panel. You'd use a screwdriver to physically disconnect the neutral wire to do this test. –  The Photon Jan 26 '13 at 17:33
@JaderDias: If neutral isn't grounded somewhere in the system, then you wouldn't detect voltage between line and ground. –  Dave Tweed Jan 26 '13 at 17:58
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The neutral and ground wires should conduct the same voltage potential from the phase in a proper installation. Also in a proper installation the neutral and ground should show continuity on a multimeter because the neutral and ground blocks should be connected by a wire at the breaker box.

In a standard installation, the ground should either be green or bare. The neutral is the sheathed wire that has continuity with the ground wire (NOTE: when testing continuity, turn power off on that circuit just in case!)

If you are still unsure then locate the breaker for that circuit and find the wire that leads to it. You should see the sheathed wire from that line go to the neutral block, and the bare wire go to the ground block. Then inspect all of the receptacles and wired devices on that circuit to make sure that neutrals are connected to neutrals and bare wires are connected to bare wires.

In this way you can ensure that everything on that circuit was connected correctly and that they didn't do something foolish like wire a ground to a receptacles neutral or connect neutrals and grounds at the box.

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Let's not forget that for a good part of the world, grounding conductors are green and yellow stripes ;) –  bcworkz Jan 28 '13 at 21:11
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