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I'm trying to tell what might be wired wrong with our dryer outlet. Using a Fluke 75 set to AC, I read 238V between the hot leads, 118V from one lead to ground, and 238V between the other lead to ground. Is this a simple breaker box problem or might I have bigger issues? FYI, the house was built in 1977 but I've been told the wiring is copper not aluminum.

I am comfortable doing the fix if I have good debugging tips...

Corrections now that I'm home: It's 120v between h1-h2, 120v between h2-neutral, and 240 between h1-neutral. Major user error in data collection but I feel better knowing it's just wired wrong. Please delete or edit at your whim. Also please ding the upvotes because it's a flawed question with bad data.

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Is there a metal water pipe within reach of your meter? If so, measure between L1 and the pipe (ground), between L2 and the pipe, and neural and the pipe. You should see ~120V L1-pipe, ~120V L2-pipe, and 0V N-pipe. Is this a 3 or 4 prong receptacle? Is it fed from a subpanel, or the main panel? What readings do you get back at the breaker? –  Tester101 Sep 18 '12 at 11:27
Did you check the batteries in your meter? –  Tester101 Sep 18 '12 at 11:40
Aluminum wiring is actually preferred for high load lines, providing that you increase the guage of the aluminum wire appropriately. The problem with 1970's aluminum wire is that it maintains the same guage that copper used, which was a big mistake. The high voltage power lines feeding your house along the power grid are aluminum, it can be safe if the design / installation is not botched. Anything (including copper) is unsafe it the design or installation is botched. –  Edwin Buck Sep 18 '12 at 14:46
This scenario seems impossible to me. The only possible way that the two hots have 240v between them is if they are (properly) connected to the different supply phases. Likewise, the 120v to ground from one leg demonstrates that the third prong is either neutral or ground (which should both be the same). It is simply not possible that the other leg meters 240v to ground. The numbers don't add up. Please take the meter reading again, and perhaps post pics of the configuration and reading. –  Matthew Sep 18 '12 at 15:53
@B0nk3r You're not taking measurements with the washer plugged in, are you? i.e. you're not pulling the plug partially out, and touching the probes to the blades on the plug? –  Tester101 Sep 18 '12 at 16:10
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5 Answers

up vote 4 down vote accepted

Based on your updated readings (120v between h1-h2, 120v between h2-neutral, and 240 between h1-neutral), it looks like Hot2 and Neutral have been reversed. Shutoff the breaker, verify there's no voltage, open the outlet, and correct the wiring so your wife and dry clothes without getting a zap. In the end, you should have this:

Proper voltage measurement diagram

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+1 based on the new readings, this answer is correct. I am quite surprised that this mis-wiring didn't have significant consequences.. and that the machine still worked! @BMitch, I would indicate in the answer what the readings are that you're referencing... this question is a bit hard to follow with the revisions. –  Matthew Sep 19 '12 at 15:02
Where does the earth pin connect to in that socket? –  Walker Dec 23 '13 at 16:32
On that specific socket, it doesn't appear to have a ground. There are different styles of 240v outlets for different amp levels, and like 2 vs 3 prong 120v receptacles, not all of them have a ground. Some 240v have a ground in place of a neutral, and others have 4 prongs for 2 hots, a ground, and a neutral. See diy.stackexchange.com/a/26964/2196. –  BMitch Dec 23 '13 at 17:11
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There does not seem to be a problem in this case. your breaker box does not regulate any voltage supply, (only cuts the supply in the case of a short)

Your power grid is supplied at a higher voltage (around 3300v), to your home, at the end of the utilities power grid will be an LV transformer which will transform your supply to the desired voltage. Due to the distances between the transformer and your home, the supply is slightly higher than the norm to allow for some loss within the span of cable.

This supply voltage is near to the 120v range, and it is normal to get these voltage levels. from Wikipedia:

national standards specify that the nominal voltage at the source should be 120 V and allow a range of 114 to 126 V ( RMS) (−5% to +5%). Historically 110, 115 and 117 volts have been used at different times and places in North America. Main power is sometimes spoken of as 110; however, 120 is the nominal voltage.

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The voltages are within tolerances, however, in the US it is not normal to measure 240V from L1 to ground/neutral. You should measure 240V from L1-L2, 120V from L1-ground, and 120V from L2-ground. –  Tester101 Sep 18 '12 at 11:36
Oops... did not read properly ... thanks –  Hightower Sep 18 '12 at 11:39
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This answer was based on a prior version of the question

The picture that Jeremy posted in his answer you is perfect. I do however disagree with the idea that there may be a short between ground and neutral. Ground and neutral should be at the same potential and intentionally connected/ bonded in the breaker box.

As far as the readings you are getting L1 to Ground/neutral = 120vac and L2 to ground/neutral = 240vac, this leads me to believe there may be an interconnection of the two power leads. It would seem to me however, if they were somehow interconnected, you would see 240vac on either leg to ground.

I would suggest taking the cover off the plug and testing the hot leads to the neural lug and case ground again. Obviously be careful and skilled with working with open wiring! The next step would be to take the same measurements at the breaker. It should be a double pole 30 amp breaker. Look for any double taps off one leg of the breaker. The readings should be 120 to ground/neutral from each side of the breaker and 240 across the hot lugs of the breakers. If the same error exists, you may need to turn off the breaker, remove it and disconnect the wires and ohm out the individual paths looking for any shorts in the cable itself. This must be done with the wires disconnected from all power sources. Be sure to check for any voltage, even with wires off the breaker, before proceeding, just in case there is some other interconnection to a hot line.

This trouble shooting is dangerous and should only be attempted by someone qualified. If you are not sure of your abilities, call a pro.

I gotta think about this one a bit more. This situation and readings sound strange and improbable. Maybe one of my electrical guru buds on here can lend some better ideas.

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Thank you for the reply! It is a double pole 30 amp breaker at the main box. This is a strange problem and it's starting to look like it might be out of my skillset to deal with. I'm not totally deterred yet though. –  B0nk3r Sep 18 '12 at 15:10
IF there were a short between the two hot leads, his breaker would trip or house would burn down. –  Matthew Sep 18 '12 at 17:18
@ matthew: Sorry you are wrong. Breakers are current activated, not voltage tripped. Connecting the two legs will not trip the breaker. They are connected in every intentional 240vac appliance. Example; both legs used to power a 240volt heating element. –  shirlock homes Sep 18 '12 at 17:27
@shirlockhomes It should trip if you get a dead short between the legs. Consider Ohm's Law, I=E/R. In a dead short, resistance will be 0 (or close enough anyway) I=240V/0. The circuit will draw an infinite current (or some crap like that), and the breaker will trip (or start the wires on fire). A 240V heating element adds resistance to the circuit, and so prevents an infinite current draw. –  Tester101 Sep 18 '12 at 17:56
Here's some math. #8 copper has 0.6401 Ohms/1000', so... I=240V/0.6401 Ohms = 374.941 A. I'd say that's enough to trip a breaker. That's like a 500' long ~90,000W (300,000 BTU/hr) heating element, running though your walls. –  Tester101 Sep 18 '12 at 18:00
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This answer was based on a prior version of the question

I'm going to convert my comment to an answer and elaborate a bit on a comment left by @Tester101.

What you're describing here is not possible with residential electrical, unless you are testing with the washer plugged in (in which case all bets are off).

A 240V reading across your hot leads demonstrates that they are properly connected to the opposite phases of your supply. There is no other way reasonable way you could get such a reading.

The fact that one hot lead to neutral reads 120V likewise indicates it is correct. The voltage of a single supply phase to the neutral is 120V.

The wildcard (and user error, in my opinion) is that you claim the other hot lead reads 240V to neutral. The supply cannot be wired this way, and no erroneous circuity I can think of would result in such a reading.

For simplicity, you expect one hot leg to be -120V and the other to be +120V. Then, with neutral of zero you get 240V between the hot legs and 120V between each and the neutral.

Potential is usually considered from the RMS of the AC power wave. And this reading is compared additively.

The only answer applicable here is that your multi-meter is malfunctioning or you are using it improperly.

Here is the relevant math for why your description cannot be accurate (I did it on paper, easier than using the SE math writer, imho)
Section at the top-left are your readings.

voltage math

Jackie Chan was probably the more appropriate drawing, but I don't have those skills....

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Hopefully it's the meter. I will confirm tonight and get pictures. –  B0nk3r Sep 18 '12 at 17:02
I agree, the readings he is getting are not really possible. –  shirlock homes Sep 18 '12 at 18:58
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This answer was based on a prior version of the question

The readings don't make sense to me either. You might want to borrow another meter to verify it. What led to this investigation in the first place? Did the dryer not run? If you haven't plugged it in, its best to resolve this issue first before trying it. I would check voltages in the breaker box like Shirlock suggest. Make sure somebody hasn't tied onto the circuit somewhere.

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