Home wiring question

Factual Questions
1

I have your typical 15A 120V 2 plug outlet in my living room. The top plug can be controlled by either of two switches while the bottom one is always on.

There are three wires (really 4 if you count the Earth ground wire) coming into the outlet box: White, Black and Red. I took a meter to the wires and found the following:



SW1  SW2   White/Black   White/Red    Red/Black
Off  Off   120v          64v          36v
On   Off   120v          120v         0v
Off  On    120v          120v         0v
On   On    120v          64v          36v

My question is whether or not the White/Red pair should have 64 volts during the “off” stage. I plug a light into the top plug and it turns off when it should, but if I put my meter in the plug it still reads 64 volts.

2

Oops. Someone screwed up good. There should be zero volts present when the switch is off. Something (probably a lamp) is in series with one of the hot leads in line with those switches. If there is a lamp or other device plugged into the the circuit that these switches control, unplug it and I’ll bet the the 64 V goes away.

3

Not sure about the 64v question, but when dealing with 3-way switches, where a single outlet or light is controlled by 2 switches, it would be less confusing to talk about SW1 or SW2 being Up or Down, rather than On or Off.

When one of these switches is in the Up position, it could be either On or Off, depending on the position of the other switch.

4

How is the outlet wired?

Is the bare/green wire connected to the green screw on the outlet?

Is the white wire connected to one of the silver colored screws or pushed into a hole next to it?

Is the red wire connected to one of the brass colored screws or pushed into a hole near it?

Is the black wire connected to the other brass colored screw or pushed into a hole near it?

Can you check the switches?

Be careful. Electrical shock can cause serious injury or death.

5

Sparky here.

Typical open nuetral.
You have an open nuetral that is causing the voltage to float in that circuit. You will get different volt readings at different locations along that circuit, especially if you have lights or other things plugged into or turned on during the testing. Usually though, nothing will work if the nuetral is completely open. You could fry smaller appliances or computers, so be careful what you plug in till you figure out the problem.
If QED is right and there is a light in series then you have major probs and this would have been showing up earlier. Nothing should be wired in series in the home`s wiring. The red and the black are fed from the same breaker.

I just trouble shot one of these over the weekend. The light in the persons bedroom had been changed recently. The three wires that were spliced into the fixture`s nuetral were not making a good connection. When the light was turned on it would work fine but the rest of the circuit was still dead - until the light was on for about fifteen minutes. What would happen is the heat from the light would expand the wires in the splice just enough to make a good enough connection for the rest of the circuit to work. It took the light down and found the loose splice, tightened up the wire nut, and everything worked fine. I got readings on the circuit of 90v. to ground and 10v. from the nuetral to ground before the problem was found. The 10v. from the nuetral to ground told me right away that there was an open nuetral somewhere. Once fixed, I got 0v. from nuetral to ground and 115v. from the hot to ground.
Open nuetrals can be hard to find and can cause funny readings throughout the circuit.

Have you added/changed anything lately? Check out any recent splices or remodeled areas for loose connections of any white wires.

The other thing to check out is if the nuetral is switched (on purpose or accidentily) on one or both of the three way switches.

The other thing I see from time to time is a loose nuetral in the electrical panel. The white wires all terminate on a bar inside the panel.

6

I should have mentioned that the white wire is now referred to as the grounded wire and does carry current.

The grounding/green wire does not carry current during normal operation.

7

Correction: the white wire is the neutral. As stated it is a current-carrying conductor, but should not be confused with the safety ground.

8

whuckfistle, since you know your stuff, can you take a quick peek at my little problem ? It dropped like a stone the first time around, and I haven’t yet done anything with it.

9

whuckfistle, since you know your stuff, can you take a quick peek at my little problem ? It dropped like a stone the first time around, and I haven’t yet done anything with it.

10

Check the link.

11

Thin Ice, QED

NEC: article 100: definitions;

Grounded Conductor - A system or circuit conductor that is intentionally grounded. (nuetral)

Grounding Conductor - A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes. (safety ground)

12

I had it right. The white wire used to be called the neutral. It is now called the groundED wire. It is the return wire for the current back to the panel.

The bare/green wire is the groundING wire which is dead under normal circumstances. It becomes the return wire in case of a short- circuit.

13

Who’s brilliant idea was that? Oh yeah, that’s so much clearer than “neutral” and “ground”. :rolleyes:

14

I agree. It gets somewhat confusing. I think the reason for the change may be that neutral implies safe to touch. When you see hot you think," Uhoh, better not touch." When you see neutral you think, “Eh, must be ok, safe.”

15

Fat Bald Guy -
Sorry about calling them “on” and “off”. For clarity “on” refers to “up” and “off” refers to “down”.
Everyone else -
I’ve been in the house going on 5 years and we have never done any remodeling. I found this problem because I recently replaced the outlet and I was trying to figure out how to wire it up. For all I know it has been this way for over 5 years.

My entire living room and bathroom are on this circuit, if that means anything. I know this because flipping the breaker for this outlet shuts off the lights and outlets in those rooms. While that doesn’t have to mean any thing else is in series, it could mean that. I’d have to trace the wires to find out and I haven’t a clue how to do that through walls. Although, I suppose I could turn off the breaker, short the outlet, and then measure the resistance at various appliances and other outlets.

Thanks for all the advice! I’ll keep you informed what I find out tonight.

16

Here is a wiring diagram of what you should have. Replace your outlet for the light.

17

Well, in a properly-wired system, it is safe to touch. As long as N-G voltage is ~0 V, there’s no danger in touching the neutral.

18

Are you using a digital voltmeter?

If so, then I think I might know what the “problem” is. I say “problem” because, if you’re using a digital meter, you probably don’t have a problem.

Most digital meters have a FET input stage, and typically have an input impedance of 10M ohms. This impedance, in conjunction with series impedances in a circuit, forms a voltage divider. This can cause erroneous readings. It has “bitten me” several times.

This explanation is best performed with an example. Let’s say you flip the switches so that the top outlet is off. This means that the red wire (which goes to the top plug’s hot terminal) is isolated from the rest of the world. Or at least you think it is. In reality, there is a finite amount of impedance between:

a) The red wire and “hot”.
b) The red wire and “neutral”.

In both cases, the impedance comes from two sources:

  1. Material resistivity (bulk and surface).
  2. Capacitance.

#1 is usually very high (over 50M ohms). In properly wired installations, most of the impedance is due to capacitive coupling.

Red-to-White Measurement
In this case, the circuit looks like this:

hot -> capacitor (wall switches) -> 10M ohms (digital meter) -> neutral.

Let’s say that, with the top outlet off, there exists 0.0003 uF of capacitance between the red wire and “hot”. (This is not much capacitance, and is easily created via the two switches and associated cabling.) At f = 60 Hz this capacitance has an impedance of 8.8M ohms. So what happens when you stick a digital meter (Z = 10M ohms) across 120 VAC with a source impedance of 8.8M ohms? You get a voltage divider. And the voltage across the digital meter is… 64 VAC.

Black-to-Red Measurement
In this case, the circuit looks like this:

hot -> 10M ohms (digital meter) -> capacitor (outlet) -> neutral.

Let’s say that, with the top outlet off, there exists 0.000114 uF of capacitance between the red wire and “neutral”. (This is not much capacitance, and is easily created via the outlet and associated cabling.) At f = 60 Hz this capacitance has an impedance of 23.3M ohms. So what happens when you stick a digital meter (Z = 10M ohms) across 120 VAC with a source impedance of 23.3M ohms? You get a voltage divider. And the voltage across the digital meter is… 36 VAC.

To prove the above stuff, do the following:

Plug in a lamp to the top outlet and turn it on. Verify the voltage across the lamp (while it is on) is 120 VAC. (To do this, partially pull out the plug to expose the bare terminals, then connect across them using your meter leads. Be careful!) Then toggle one of the wall switches to turn the lamp off. What is the voltage now? With the lamp still connected to the outlet, I’m confident you’ll find that it’s zero. If it is, you’ll know the 64 V and 36 V readings were due to finite source impedances as described above.

Keep in mind, however, that while the analysis above is correct, it is based on an assumption that there’s nothing wrong with your wiring and that you are using a digital meter with high input impedance. If either one of these is not true then I’ll have to come up with something else.

19

It depends what you call a neutral.

Let’s say I pull out an outlet and notice three “neutral” white wires tied together with a wire nut:

  1. A white wire going back to the breaker box.
  2. A short white wire going to the outlet I just pulled out.
  3. A white wire coming from God-knows-where (probably another outlet)

Now let’s unscrew the wire nut. Are all the wires safe to touch?

  1. Yes.
  2. Yes.
  3. ?

#3 could be hotter than a firecracker. If it’s “on”.

20

I spent a few hours working on this tonight, and I think I may have found the “problem”.

Crafter_Man - thanks for the detailed analysis and explanation. I hadn’t thought of the mutual capacitance causing the problems, although I was suspicious that my light wasn’t on dimly. You are right that I’m using a digital MM, though. As you said, I may not have a problem at all.

I was going to try and desribe what I found tonight, but I think typing it out would just make it confusing. I’ll just say that I was REALLY surprised to find one of the switches did not have a black lead to it. The wiring is okay, I think, but it was still surprising.

I may have a problem with one of the switches, but I’ll find out more tomorrow. Right now it is late and I’m off to bed.

Thanks again to all who responded. I sincerely appreciate all the info and helpful tips and advice.

One more thing…

For the last example Crafter_Man talked about, any wire is safe to touch as long as you isolate yourself (NOT GROUND!! ISOLATE).