Induced voltage on 3 wire circuits?

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Old 07-22-06, 07:54 PM
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Induced voltage on 3 wire circuits?

A question on wiring. I have recently had an additon to my house. I have 2 new rooms each with a switched plug and one a ceiling fan with separate switches for fan and light. All three circuits are on separate breakers and each uses a 12 gauge 3 wire cable. The interesting thing is that the black or the red wire in each of these 3 circuits has 50 or 70 volts on them when one of each respective legs is off. For example if the switched plug is off (red wire), there is still 50 volts at the plug (and 125 v on the non switched plug - black wire). If i kill the breaker it goes to zero, There is no continuity between the red and black wire either. Same with the fan situation. If the fan leg is off (black wire) there is still 70 volts at the fan location if the light switch is on (red wire at 125V). No continuity between red and black wire here either. My electician says it is not real and not to worry, however it does show up on my meters and is enough to set off the voltage detector. Is it ok to install the fan? Is it ok to install my lutron dimmer on the switched plugs. Is the wire defective. I could understand 1 but three independent circuits all exhibit the same type symptoms. What can be doen about it? Thanks.
 
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Old 07-22-06, 08:28 PM
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You must be useing a digital meter. Therefore your reading a "phantom" voltage. This would be the voltage on the nuetral and resistance on the cables, and yes perhaps a slight induction. What is the other voltage tester you've used? Proximity tester? If this is the case you're probably picking up voltage readings on the constant hot wire in the cable. Try a set of wiggies or a light bulb (2 wire) tester.

No you can not use a Dimmer on switched recepticals.
NEVER install an electronic dimmer with the power on.
That will turn a $20 switch into a $40 sw. in no time.
 
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Old 07-22-06, 08:29 PM
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This is the topic of a lot of discussions here on these forums. While many will tell you it is not real, they are wrong. It is induced voltage from the adjacent hot conductor. There is not enough current to be usable but the voltage is real, Many call it phantom voltage.

If the adjacent conductor is under a large current draw, the voltage will be higher. If the adjacent conductor current flow is very low, the voltage will be low.

This is actually the same principle a transformer uses to do what it does. There is no actual electrical connection between the hot conductors in your house and what the POCO sends down the pole wires. The voltages are induced into the wires that lead into your house.

I am guessing you are using a digital meter. The high impedance allows these voltages to be read while a tester (solenoid type) will not show any voltage present. A "tick tracer" will also pick up htese voltages.

There is no need to do anything about the situation. It is normal and acceptable.

Whoops, missed the dimmer situation. As lee posted, this is not allowed.
 
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Old 07-22-06, 08:40 PM
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Thanks. I am using a digital meter. The electrician put an analog meter on and it shows about 20V. This is with no current flowing. Sounds like it is OK? If a constant voltage (say 20 or 50) is on a fan motor or a light will that ruin the fan or cause my electric bill to increase? The dimmer to the switched plug has a special receptcle that will not accept a normal plug. There is a special plug to replace the lamp plug so that it will fit in the receptical. Thanks.
 
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Old 07-22-06, 08:47 PM
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The voltage has no current behind it so it will not cause a problem. It will not increase your bill since there is no current flowing. Not that I am advocating doing this but one can touch a conductor with induced voltages such as this and not be harmed. There is not enough current to actually flow against any resistance. That is why a solenoid type tester will not read these voltages.

If you are using the correct receptacle you are ok with that as well.
 
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Old 07-23-06, 07:08 AM
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The Phantom voltage can be a real pain when you are trying to trace a circuit. Ive known of people installing a resistor in paralell with there meter to get rid of it.
 
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Old 07-23-06, 07:28 AM
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Phantom voltage is not necessarily induced. You have to be aware of exactly how your meter connection relates to the cirtuit. If you measure across an open circuit, the internal resistance of the meter completes a circuit and allows current to flow, giving a voltage reading.
 
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Old 07-23-06, 07:48 AM
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Originally Posted by 594tough
Phantom voltage is not necessarily induced. You have to be aware of exactly how your meter connection relates to the cirtuit. If you measure across an open circuit, the internal resistance of the meter completes a circuit and allows current to flow, giving a voltage reading.
Well if you are going to toss this in here, explain where in an open circuit you are speaking of and exactly how and where the circuit leads to.

Since a single piece of wire laying on the floor is by definition an open circuit, I think you would be hard pressed to read from end to end of that wire and get a voltage reading.


For there to be current flow, there must be voltage to push it. If you have a totally isolated circuit and are using an analog meter that has absolutley no power source within, you are not going to get a reading no matter how you engage that circuit.

The fact that a DMM does NOT allow anything but an extremely small current flow (due to its' high impedance) is why a DMM will give you "phantom voltage" readings. An analog requires a greater current to initiate the physical movements of the meter. That is why a DMM reads phantom and the analog does not. Phnatom voltages are real but they lack the current to drive an analog meter.
 
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Old 07-23-06, 12:15 PM
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Originally Posted by nap
Well if you are going to toss this in here, explain where in an open circuit you are speaking of and exactly how and where the circuit leads to.
The antenna effect also induces voltage.
take 20 feet of romex wire and place your test leads on one white wire and the black wire, depending on the amount of stray magnetic fields in the area you can get a reading.

Since a single piece of wire laying on the floor is by definition an open circuit, I think you would be hard pressed to read from end to end of that wire and get a voltage reading.
Yes, hard pressed but not impossible.
it would no longer be an open circuit once you place your meter across the ends of the wire.

For there to be current flow, there must be voltage to push it. If you have a totally isolated circuit and are using an analog meter that has absolutley no power source within, you are not going to get a reading no matter how you engage that circuit.
I would add isolated and shielded.

Phnatom voltages are real but they lack the current to drive an analog meter.
Depends on the analog meter, and the strength of any stray magnetic fields.
 
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Old 07-23-06, 12:42 PM
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Originally Posted by GWIZ
The antenna effect also induces voltage.
take 20 feet of romex wire and place your test leads on one white wire and the black wire, depending on the amount of stray magnetic fields in the area you can get a reading.


Yes, hard pressed but not impossible.
it would no longer be an open circuit once you place your meter across the ends of the wire.



I would add isolated and shielded.



Depends on the analog meter, and the strength of any stray magnetic fields.
I do understand and agree but in the real world, your situations are generally of no justifiable concern.
I mean, if I am standing under a 738k power line, all bets are off but in normal situations what I described will be the norm.
 
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Old 07-24-06, 10:53 AM
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I love this one....been here before. The "phantom" voltage is "induced" in a in a generic sense. However, the techical reason for it is "capacitive" coupling. No load current is required for this to take place. This is noticed quite often in the circuits you are dealing with, when one conductor is energized and the other is not connected to a load. The only load is the meter itself.

As your electrician demonstrated, this can also be measured with an analog meter. It's more a matter of the impedance of the meter than the technilogy used to display the results. Don't get me wrong, I am not a digital meter advocate. In my opinion, they should be used where precision is much more critical than that needed in residual or commercial wiring, such as when dealing with electronic circuits. Besides usually being very high impedance, some of them can give weird results due to digitizing error. They can be tricked by distortions in the sine waves.
 
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Old 07-25-06, 03:00 PM
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Thumbs up Phantom voltage

This phenomenon has long ago taught me to never use a high impedance voltmeter for general electrical point-to-point testing. Once I was checking for an open line with one and the phantom readings gave me a fit. The very open line I thought was open gave me a reading I couldn't believe. Luckily I had the presence of mind to then use a simple lamp tester.
 
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Old 07-25-06, 04:14 PM
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I think the most reliable electrical tester for the
diy'er is a solenoid type.

http://www.homedepot.com/prel80/HDUS....jsp?pn=162357

Relatively cheap and almost bullet proof.
 
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Old 07-25-06, 07:46 PM
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I understand, and agree that a wiggy or such is usually adequate for many jobs but when I am installing a transformer and need to set the taps or I am having a motor overloading, I need to have an accurate voltage reading. Same thing for a control circuit troubleshoot.

A wiggy just doesn;t have the accuracy I need on a regular basis.

Just so you don;t think I think i am too good for a wiggy, I have an Ideal volt-con lite and a DMM in my bag at all times.

I have use for each of them very regularly.
 
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Old 07-25-06, 10:12 PM
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Nap, I wasn't suggesting that a professional only use a wiggy. My statement was for a home handyman/DIY type.

I own a TIF digital clamp-on ammeter (AC only), an Amprobe digital clamp-on multimeter (AC/DC), a Fluke 23 digital multimeter, a Fluke 787 process meter, at least three solenoid-type testers and probably a half-dozen proximity voltage detectors. I use them all and I would be lost without them. Then again, I have several decades of experience in using electrical/electronic measuring instruments.

A person needs to use the proper tool or instrument for the job they are doing. Most homeowner/handyman/DIY electrical jobs do not require the use of anything more than the solenoid-type tester. It is easy to learn to use and it rarely gives questionable information like "phantom" voltages.
 
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Old 07-25-06, 10:25 PM
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Down and dirty...... The right tool for the right job. Part of being good is knowing What one to use at what time.
Alot of times you don't know untill you use the wrong one at the right time. Now isn't that why we buy things we'll never need. When you do though.......There it is.
 
  #17  
Old 07-26-06, 05:19 AM
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right minds, right tools

furd and lectriclee: Well said, well put.
 
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