Recently, while performing a jobsite safety audit, I found several temporary power boxes (commonly referred to as a turtle or spider box) which come factory wired with GFCI protection, I noticed these having an additional GFCI protected pigtail plugged into the already protected receptacle. Per conversations with various co-workers and hear-say, I learned through my apprenticeship, and currently understand, that it is not necessary, and actually wrong, to plug in a GFCI into a GFCI. Do the modules, which detect current fluctuations on the neutral, counteract eachother? How does "back to back" GFCI protection affect the circuit? Any ideas would be great as I have a meeting with several risk consultants and superintendents, and I need credible evidence with mathematically calculated or scientifically plausible explanation regarding this matter. Thanks!
GFCI Receptacle with GFCI pigtail
- Thread starter juanoito
- Start date
- Status
- Location
- Massachusetts
It is not right or wrong, it simply does not matter.
As long as at least one GFCI is protecting the personal the codes and standards have been satisfied.
No harm whatsoever comes from multiple GFCIs other then maybe the inconvenience of find which one has tripped.
As long as at least one GFCI is protecting the personal the codes and standards have been satisfied.
No harm whatsoever comes from multiple GFCIs other then maybe the inconvenience of find which one has tripped.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
juanoito said:
You mis-understand how GFCI devices sense a ground fault.
Very very simply: a GFCI looks at the amount of current flowing out of the device on the line conductor and compares it to the amount of current coming back on the neutral conductor. If there is more than 6mA of difference, not enough or too much, the GFCI trips. GFCI devices do not actually look at the ground conductor at all.
There is no technical reason that GFCIs can not be connected in series.
wptski
Senior Member
- Location
- Warren, MI
Well, sort-of! It also monitors the N-PE connection too. If N-PE is shorted, it'll trip.jim dungar said:
This also makes me wonder if a GFCI installed too close to the service entrance will nuisance trip??
Last edited:
- Location
- Massachusetts
wptski said:
No, not really, not directly.
If the grounded and grounding conductors are connected down line from a GFCI that will cause an imbalance between the grounded and ungrounded conductors which is all the GFCI 'looks' for.
Thanks for your help. That is my main question, can GFCI's be connected in series. It seems the answer is yes.jim dungar said:
Additionally, with our temporary power supply boxes (turtles) what would be the main cause for GFCI's to "burn out." I believe if you have overcurrent situations, e.g. threeway pigtails with multiple power tools plugged into this single 20A circuit, the heat created by this situation works against the sensitive electrical components of a GFCI, but not enough to trip a 20A overcurrent protection device with a typical Interrupting Rating.
Thanks Again!
- Location
- Massachusetts
juanoito said:
The device is rated 20 amps regardless if that is twenty 1 amp loads or one 20 amp load.
GFCIs require testing as they often stop working.
GFCI modules on Turtle boxes
GFCI modules on Turtle boxes
Another question: With GFCI's installed on temporary power boxes, what would be the main cause for these to "burn out?" I am thinking that a typical 20A overcurrent protection device with typical interrupting rating is not as sensetive as the GFCI electrical component, therefore, when you have multiple power cords with drill motors, circular saws, tile saws, "chop boxes" and other high inrush current devices/appliances, these stress the GFCI but not enough to trip breaker?
GFCI modules on Turtle boxes
Another question: With GFCI's installed on temporary power boxes, what would be the main cause for these to "burn out?" I am thinking that a typical 20A overcurrent protection device with typical interrupting rating is not as sensetive as the GFCI electrical component, therefore, when you have multiple power cords with drill motors, circular saws, tile saws, "chop boxes" and other high inrush current devices/appliances, these stress the GFCI but not enough to trip breaker?
wptski
Senior Member
- Location
- Warren, MI
Nope! First GFCI receptacle, short N-PE in the receptacle itself and it trips. Short the load side N to PE and it trips. Try it as I have several times.iwire said:
brantmacga
Señor Member
- Location
- Georgia
- Occupation
- Former Child
some of the older gfi's will not reset but still have power available for use. lots of power tools running at once will destroy a gfi. i've got a particular contractor i work for who's framing crew has burned up two in the last month. i think i'm going to replace them w/ gfi breakers next time.
- Location
- Illinois
- Occupation
- retired electrician
That is because you have created a parallel path for grounded conductor current from other equipment that is served by the same transformer. Some of that current is flowing through the CT in the GFCI without being balanced by current on the ungrounded conductor and the device trips.
Don
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
wptski said:
No, a GFCI does not look at the N-PE connection at all. This is why a GFCI receptacle can be installed in place of an existing 2-wire receptacle without a PE connection.
The GFCI looks only to see if "what goes out comes back in", no more and no less resulting in a 6mA net difference. A GFCI device can only look downstream, it does not care what happens before it.
If there is a downstream N-PE short, the parallel path that is created will mean that not all of the current that went out comes back in resulting in a trip. Also if there is a downstream connection between multiple circuit neutrals (i.e. a multi- wire circuit) then it is possible to have more current coming back in then went out, again resulting in a trip.
There are other ground fault protective devices and leakage devices that involve a PE connection but not a GFCI.
wptski
Senior Member
- Location
- Warren, MI
There are two transformers(coils) or dual functions in the GFCI, one for N-L and another for N-G if either is unbalanced, it trips. It operates like mentioned with a single GFCI with nothing downstream.don_resqcapt19 said:
- Location
- Massachusetts
If we are going to continue this discussion we should all be talking the same language.
Ungrounded conductor (AKA the hot, the black etc.)
Grounded conductor (AKA the neutral, the white etc.)
Grounding conductor (AKA the ground, the green, bare etc.
"PE" has no meaning in the NEC
Ungrounded conductor (AKA the hot, the black etc.)
Grounded conductor (AKA the neutral, the white etc.)
Grounding conductor (AKA the ground, the green, bare etc.
"PE" has no meaning in the NEC
wptski
Senior Member
- Location
- Warren, MI
Is it okay to use N-L-G and then Line/Load if it comes to that?iwire said:
- Location
- Massachusetts
wptski said:
Sure, but technically there is no neutral at a most GFCIs
I think I may have found what you are talking about Bill and it is a fairly recent addition.
The illustrator is also not familiar with NEC terminology.
wptski
Senior Member
- Location
- Warren, MI
Bob:
What do you call "recent"? Pass&Seymour GFCIs installed in 1998 included that addition.
What do you call "recent"? Pass&Seymour GFCIs installed in 1998 included that addition.
- Location
- Massachusetts
wptski said:
I would be surprised if that is so.
It was fairly recently that the standards changed.
Even an 'old style' GFCI without that specific part of the circuit would trip when a load was applied and it had a grounded to grounding conductor fault.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
wptski said:
GFCI do not sense current on the grounded conductor. They sense a difference between the line and the neutral conductors. They do not care if the stray current is going to ground or if it is going to a different neutral conductor. If the net current in the GFCI is >6mA it is supposed to trip. This is why multi-wire circuits need to have their common neutral connection made upstream of a GFCI.
Look at the diagram that Bob posted. There is no difference in the conductors that are sensed by the two coils. And the sensing portion of the GFCI has no relation to ground.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
iwire said:
There are a lot of standard engineering and electrical terms that do not have a "meaning" in the NEC. It would be almost impossible to do my job if I was limited to using only NEC terms, after all the NEC is not a "design" manual.
- Status