GFI neutral

[RampyElectric]

Co worker of mine poses a question. On the same circuit.... if he takes a hot to feed a switch leg and the neutral from the load side of a GFI will it trip?
I wagered that the CT would sense the unbalanced current and trip but the light would stay on. OK GUYS LET ME HAVE IT

-Village Idiot

 

[iwire]

Is the hot and the neutral connected to the same GFCI?

 

[mdshunk]

If the hot is taken from the line side, and the neutral from the load side, it will trip. If both are taken from the same side, it will not trip. There are some obscure cases where switch loops and a neutral, both from the load side of a GFCI, will trip the GFCI.

 

[e57]

It will trip, (or should) and the light will turn off but remain energized if the light SW is not fed from the load side of that GFI.

 

[RampyElectric]

Now keep in mind this is on the same circuit, but only the neutral was tapped from the GFI. So no the hot isnt related to the GFI other than it was tapped from the same circuit down stream from the GFI outlet. The neutral was tapped from the LOAD side of the GFI. I think the light will operate but the GFI outlet will trip every time the light switch is thrown

 

[iwire]

There are some obscure cases where switch loops and a neutral, both from the load side of a GFCI, will trip the GFCI.

Please explain, inquiring minds want to know.

 

[e57]

Flouresant lighting and some types of dimmers used to pop them all the time, certain types of motors too. - I think both lighting and GFI's are made better today....

 

[mdshunk]

Please explain, inquiring minds want to know.

A difference in length of the hot wire and the neutral wire, on the load side of a GFCI, will create a difference (in mA) between the hot and the neutral. This will never approach the trip threashold of the GFCI, but it will be measureable with long switch loops and big sets of 3Ways and 4ways. Add this bit of difference from these bigger switch loops to the ordinary small leakages that exist in many appliances (which would also not trip a GFCI in and of themselves), and you have a GFCI that is tripping constantly or nuisance tripping. It has burned me exacltly enough times that I now do not do switch loops or "dead end 3ways" on the load side of a GFCI. I struggle to keep the load side hot and neutral conductors as close to the same length as I can.

 

[iwire]

Makes sense. :smile:

Now you have me asking the next question.

Why do you have long runs of 3 and 4 ways on GFCI protected circuits?

 

[jim dungar]

Now keep in mind this is on the same circuit, but only the neutral was tapped from the GFI. So no the hot isnt related to the GFI other than it was tapped from the same circuit down stream from the GFI outlet. The neutral was tapped from the LOAD side of the GFI. I think the light will operate but the GFI outlet will trip every time the light switch is thrown

True.
The GFCI breaker will see current flowing on the neutral that is not on the hot, therefore it will trip. However, because the GFCI breaker does not "open" the neutral conductor the light will stay on.

 

[mdshunk]

Makes sense. :smile:

Now you have me asking the next question.

Why do you have long runs of 3 and 4 ways on GFCI protected circuits?

3 prong retrofit in existing dwellings, with the "no equipment ground" sticker that lasts until me and the inspector leave.

 

[jim dungar]

A difference in length of the hot wire and the neutral wire, on the load side of a GFCI, will create a difference (in mA) between the hot and the neutral.

Neat trick.

What you are saying is: The current into a circuit is different than the current out of the circuit. Wow, Mr. Kirchkoff will be interested in hearing this.

 

[mdshunk]

Neat trick.

What you are saying is: The current into a circuit is different than the current out of the circuit. Wow, Mr. Kirchkoff will be interested in hearing this.

Okay, I'm wrong then on my reasons, but I have observed the phenomenon plenty enough times to know that it is real. My reasons must just be wrong then. Help me out, then. Why have I observed as I have explained; long switch loops and long dead end 3ways tripping GFCI's? Perhaps some sort of subtle phase shifting from the unequal lengths? I searched the memberlist for user 'Kirchoff', but I came up blank.

 

[Paul B]

Originally Posted by mdshunk
A difference in length of the hot wire and the neutral wire, on the load side of a GFCI, will create a difference (in mA) between the hot and the neutral.


Kirchoff is hard to find, he visits so infrequetly that his post are long gone before you know it. Anyhow I don't make much sense of your reasoning.

 

[George Stolz]

So, if we're done picking on each other...

That doesn't make sense to me either - the resistance of the conductors (or differences between the hot and neutral) shouldn't trip a GFCI, if the way I'm thinking is correct.

So that gets me wondering what's causing the trip - insulation leakage over the distance?

These educated fellers know who Kirchoff is, I'd have to google him.

 

[jim dungar]

The simple description of Kirchoff's Law is: the current entering a point must equal the current leaving that point - or what goes out must come back unless it finds a different way home.

Anytime you hae two parallel conductive surfaces separated by an insulator you have a capacitor. An NM cable effectively has two capacitors in it. What you are experiencing is the current "leaking" between neutral and ground or line and ground, if the conductors are different lengths then you will have different amounts of current leaking to ground. It is possible for a GFCI to see the difference and trip.

 

[Paul B]

Jim I respect your opinion, but how many hundreds of feet do you think it would take to cause a 4 mA leakage current?

 

[jim dungar]

Jim I respect your opinion, but how many hundreds of feet do you think it would take to cause a 4 mA leakage current?

Maybe I should have said "might be experiencing".
But, I did say possible, not likely/probable. However, if it is water filled conduit or water soaked NM there is a greater possibility.

 

[LarryFine]

However, because the GFCI breaker does not "open" the neutral conductor the light will stay on.

Are you sure? The last GFCI receptacle I demolished opened both circuit conductors when tripped.

 

[kbsparky]

... However, because the GFCI breaker does not "open" the neutral conductor the light will stay on.

I disagree here. GFCI breakers as well as stand-alone devices open all current carrying conductors when tripped.

The light will not work when it is tripped.