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Old 10-12-2008, 08:49 PM   #1
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How can both wires be hot?


I'M in my parents house trying to add on a switch to previously stand alone ceiling light with a pull switch on it. My dad had decided to take out the light after the roof/ceiling had leaked and left both wires just hanging there. Now i have to find the hot wire and the cold wire and both tested hot with 118v. Am i testing wrong or cam some one tell me what might be going on with this 100 plus yr old house.

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Old 10-12-2008, 09:09 PM   #2
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How can both wires be hot?


I have been an electrician for quite awhile and have never seen a "cold wire", hot and cold are for plumbers, in electrical you have a hot and a neutral and maybe a ground wire.

There is not enough info in your post to determine how you are testing to get 118v on both wires. Do you have a ground wire that you are testing each wire to individually?

If so you should not be getting voltage on both wires in reference to ground. If you do not have a ground in the box, you can take an extension cord and plug it into a receptacle, if your receptacles are polarized, and wired correctly, then the long slot of the receptacle is neutral, and the short slot is the hot.

Using this extension cord with a known hot and neutral you should be able to test the wires in the ceiling against the cord and determine which wire is hot and which is neutral.

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Old 10-12-2008, 09:17 PM   #3
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How can both wires be hot?


Well i have two wires coming out from the ceiling, neither are tagged as black and white. I tested both with a digital multimeter and touched one wire with the red and one with the black and then reversed and couldn't understand how both could be giving me a reading? I figured one would be positive and one neg and wouldn't get a reading if i reversed the test to find the charged wire. ( i guess Ive been working with electronics for to long, used to red pos. and black neg.)
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Old 10-12-2008, 09:20 PM   #4
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How can both wires be hot?


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Originally Posted by EBFD6 View Post
There is not enough info in your post to determine how you are testing to get 118v on both wires. Do you have a ground wire that you are testing each wire to individually?
Please describe what wires are in your box or provide a photo.

Are you in the US? Could this have been a 240V fixture?

240V would be 2 hot wires and a neutral.

Question to others here: Is there any reason not to suggest that he puts his meter, 1 lead on each "hot" wire, and if it reads 240, then he really has 2 hots in the box? Any reason this would not be safe to advise or any reason it would give inaccurate results? (it can be hard to know what is safe to tell people they can do when you can't see what they are talking about)

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Old 10-12-2008, 09:24 PM   #5
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How can both wires be hot?


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Originally Posted by zapagain View Post
Well i have two wires coming out from the ceiling, neither are tagged as black and white. I tested both with a digital multimeter and touched one wire with the red and one with the black and then reversed and couldn't understand how both could be giving me a reading? I figured one would be positive and one neg and wouldn't get a reading if i reversed the test to find the charged wire. ( i guess Ive been working with electronics for to long, used to red pos. and black neg.)
What color are the wires?

This sounds like a normal 120V line. Polarity isn't really an issue here the way your thinking of it in small electronics. On most testers, when either probe is energized and the other probe is run back to ground or to neutral, it is going to give you a reading.

You can just turn off the power and hook up a light fixture.

What kind of fixture are you planing on installing?

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Old 10-12-2008, 09:27 PM   #6
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How can both wires be hot?


Yes I'm in the U.S.. Do older houses usually not have a ground and two charged wires though? i thought all houses had a ground? I know the newer systems use a black, white, and a green wire( plain copper) with some using a red for a second circuit. So im assuming im testing wrong and should be doing it a different way?
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Old 10-12-2008, 09:30 PM   #7
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How can both wires be hot?


well..... I have two wires coming out of the fixture and neither is labeled in any way as to there nature of being the traditional black and white or having a tag as to which is the charged line. I'm just trying to find the right wire to use to install my wall switch.
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Old 10-12-2008, 09:34 PM   #8
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How can both wires be hot?


Whats the reading across the red and black? red to white? black to white?
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Old 10-12-2008, 09:36 PM   #9
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How can both wires be hot?


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Originally Posted by zapagain View Post
Yes I'm in the U.S.. Do older houses usually not have a ground and two charged wires though? i thought all houses had a ground? I know the newer systems use a black, white, and a green wire( plain copper) with some using a red for a second circuit. So im assuming im testing wrong and should be doing it a different way?
Yes, older houses usually dont have a ground, except when your house is wired with "bx" which is armored cable, and when it has a "bonding wire" a copper or aluminum wire inside the armor.
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Old 10-12-2008, 09:41 PM   #10
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How can both wires be hot?


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Originally Posted by EBFD6 View Post
I have been an electrician for quite awhile and have never seen a "cold wire", hot and cold are for plumbers, in electrical you have a hot and a neutral and maybe a ground wire.

There is not enough info in your post to determine how you are testing to get 118v on both wires. Do you have a ground wire that you are testing each wire to individually?

If so you should not be getting voltage on both wires in reference to ground. If you do not have a ground in the box, you can take an extension cord and plug it into a receptacle, if your receptacles are polarized, and wired correctly, then the long slot of the receptacle is neutral, and the short slot is the hot.

Using this extension cord with a known hot and neutral you should be able to test the wires in the ceiling against the cord and determine which wire is hot and which is neutral.
All i have is two wires comming out of the ceiling and neither has anything on them to lable them. So i don't know which is which. I tested them using a multimeter and then reversed the test and got the same reading at 118v. So what im i doing wrong to test for the charged wire and the ground. There isn't a green wire or a red wire just two black.
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Old 10-12-2008, 09:43 PM   #11
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How can both wires be hot?


Reversing the tester leads won't change the readings on AC circuits. One of your wires is "hot" and the other one is not. But the potential difference between them is 120 Volts (nominal - your reading of 118 is considered normal).

To determine which one of your wires is the actual "hot" one, you will have to measure to a different point, a known grounded point.

Someone else here mentioned using a grounded extension cord to establish a point of reference to ground. Start with that, and report your findings.
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Old 10-12-2008, 09:49 PM   #12
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How can both wires be hot?


Older homes are often not grounded outside of the service panel. But what you are asking is how both wires can be hot. They are not both hot. One is hot and one is the neutral, but reversing the meter leads won't tell the difference. There is a potential of 120 V between them, no matter how you measure it. Your meter can't tell unless you have a reference outside of the light box. An extension cord can be used to determine which is which, if the outlet the cord is plugged into is wired correctly.

The long slot on a properly wired receptacle is the neutral, so if you place one lead in the long slot, and test your wires, the wire that causes a reading of 120 is the hot. The other wire will read 0 or very near it.

P.S. KB beat me to it but... yeah
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Old 10-12-2008, 09:57 PM   #13
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How can both wires be hot?


Zap, you have to lose your DC thinking and try to comprehend AC.

Reversing the leads will NOT change your test at all. Just because you are getting a reading both ways does NOT mean both wires are "hot".

Listen to KB.
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Old 10-12-2008, 10:04 PM   #14
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How can both wires be hot?


we'll i'd like to thank all the electricians and other advice i got tonight and try it again tomorrow and i'll let every one know how things go. Thanks Alot.
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Old 10-14-2008, 04:35 PM   #15
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How can both wires be hot?


Yeah, you are using the wrong tool for the job. One wire is the hot, the other neutral unless there is a major major problem. To determine which is which you need to use a voltage detector. The cheaper ones are just a neon light bulb in an insulated handle. You can usually get them for less than $10. They look sort of like screw drivers and light up when touched to a hot wire.

Example:

http://www.harborfreight.com/cpi/cta...emnumber=96125

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Last edited by Bubbagump; 10-14-2008 at 04:52 PM.
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