common (white) wire has voltage? is this normal?

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Old 07-26-10, 09:38 AM
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common (white) wire has voltage? is this normal?

I'm rewiring a bunch of 3 way switches back to 2 way home automation (HA) switches. These Insteon switches need a normal common (white) wire connected to the switch.

I've pulled out all the wires, found the hot from the breaker, wired through the red between the switches to the hot that goes to the light. Everything seems like it's working fine, lights work and everything. I then wired the white from an outlet from ANOTHER circuit to give the HA switches a common. I wired all 3 HA switches to that common. The common is getting voltage. I'm assuming it's fine because lights will pass voltage through the common and somehow it's comming to this common. Most outlets don't have voltage coming from the common right? Is this ok? Will this effect any appliance or anything?
 
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Old 07-26-10, 09:48 AM
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Originally Posted by justinm001 View Post
I then wired the white from an outlet from ANOTHER circuit to give the HA switches a common.
This is against code and unsafe; it should be removed ASAP.

The common is getting voltage.
Relative to what? Measured with what?
 
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Old 07-26-10, 09:49 AM
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Old 07-26-10, 09:58 AM
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Originally Posted by ibpooks View Post
This is against code and unsafe; it should be removed ASAP.



Relative to what? Measured with what?

Looking at a circuit panel all commons are blocked together. I can't just take a common thats from one circuit to connect use as common on another circuit? I'll definately remove it ASAP, just trying to understand the logic.


I used one of those voltage testers and it reads voltage similar to this one. I couldn't find my voltmeter yesterday.
 
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Old 07-26-10, 11:10 AM
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You could possibly overload the neutral from another circuit and cause a fire. The electrical code also requires that ALL conductors of a circuit be run in the same cable or conduit to avoid heating.

Your fix must be undone for safety reasons.
 
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Old 07-26-10, 11:26 AM
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Understood, I'll rewire everything correctly.

Originally there is 3 switches in the triple gang box, outer 2 on one circuit and the middle on another circuit. Is this allowed also?

Is it normal to have voltage (be able to be shocked) on the common/neutral/white wire?
 
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Old 07-26-10, 12:47 PM
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Originally Posted by justinm001 View Post
Originally there is 3 switches in the triple gang box, outer 2 on one circuit and the middle on another circuit. Is this allowed also?
Yes, you can have multiple circuits in one box as long as they are not interconnected; except for the grounds which must be interconnected.

Is it normal to have voltage (be able to be shocked) on the common/neutral/white wire?
Yes voltage on the neutral is normal, although high enough potential to be shocked is rare it is possible. That is one of the reasons that neutrals cannot be shared between different circuits. A serviceman may switch off one of the two breakers to work on the circuit, unaware that the neutral is also connected through another circuit and be shocked by wires which where supposed to be dead.

Also the type of tester you're using is prone to false positives (reading voltage when there is not actually a danger), so it is not surprising it would signal on a neutral wire.
 
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Old 07-26-10, 05:38 PM
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There shouldn't be voltage on the neutral wire with respect to ground (copper). Current - yes, voltage - no. Reason it is against code to connect two neutrals from two separate (in-phase) circuits is that you can overload the neutral and increase the risk of a fire as stated by pcboss (14 AWG wire can only handle so much current). It is ok, however, to connect two neutrals from two circuits only if they are on different phases.

I suppose you could receive a small shock from a neutral wire if enough leakage were to couple from the hot wire, and the ground/bare/copper was at a different potential, but this would be minor.
 
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Old 07-26-10, 10:42 PM
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Originally Posted by mossman View Post
There shouldn't be voltage on the neutral wire with respect to ground (copper). Current - yes, voltage - no.
There can be no current without a difference in potential (voltage); this means that a neutral must have a non-zero voltage with respect to the system ground.

V = IR

The current in the neutral wire (I) is greater than zero, and the resistance of the neutral wire (R) is greater than zero, therefore voltage (V) must be greater than zero.

Because the resistance of the copper neutral wire is usually very near zero, this also keeps the voltage low. However, if the neutral wire is damaged or has a high impedance fault like a corroded connection, the voltage in the neutral can increase to a dangerous level at some point out in the branch circuit.
 
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Old 07-26-10, 10:54 PM
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Originally Posted by mossman View Post
There shouldn't be voltage on the neutral wire with respect to ground (copper). Current - yes, voltage - no. Reason it is against code to connect two neutrals from two separate (in-phase) circuits is that you can overload the neutral and increase the risk of a fire as stated by pcboss (14 AWG wire can only handle so much current). It is ok, however, to connect two neutrals from two circuits only if they are on different phases.

I suppose you could receive a small shock from a neutral wire if enough leakage were to couple from the hot wire, and the ground/bare/copper was at a different potential, but this would be minor.
I type it in bold and no you can not do that as well you will have issue with crossed netrual especally if you have AFCI on it will trip real quick.

Merci.
Marc
 
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Old 07-27-10, 01:35 PM
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Yes, I understand that V = IR. My point is that if you have something plugged in (lamp, appliance, etc.) on the circuit, you can get shocked from the neutral if you are touching both the neutral and ground because you are providing the current an alternate path to ground. However, current wants to follow the path of least resistance, so the shock wouldn't be that severe.

And I need to correct myself regarding sharing neutrals. What I meant to say is that you can use one neutral for two separate circuits (e.g. 3-wire home run), as long as the circuits are on different phases (net current = 0).
 

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Old 07-27-10, 10:43 PM
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If they are MWBC { mulit wire branch circuit } it is not a issue however some states allready adopted new codes not too long ago { the states that adopted 2008 NEC code cycle } it must have real two pole breaker not a pair of single pole breakers.

Merci.
Marc
 
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Old 07-31-10, 07:13 AM
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Originally Posted by ibpooks View Post
There can be no current without a difference in potential (voltage); this means that a neutral must have a non-zero voltage with respect to the system ground.

V = IR

The current in the neutral wire (I) is greater than zero, and the resistance of the neutral wire (R) is greater than zero, therefore voltage (V) must be greater than zero.
Technically this is only true for DC circuits, for AC circuits it's only true as an average. Although that's good enough for most DIY projects.

A more important reason to not get hung up on the V=IR business is that many people don't understand that very small currents (and thus very small voltages) can be deadly. It only takes a tiny fraction of an amp to stop a human heart, if the current gets into the bloodstream somehow... yet I keep hearing people say things like "it's not a lot of amperage/voltage" like that made them safe. The only truly safe amperage is no amperage.
 
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