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gotpasswords
08-17-2006, 05:30 PM
One for the electricians... What would be the likely result of having two GFCIs (ground-fault interrupters) in series?

At the moment, there is some UF cable trenched around a garden. In an ideal world, there would be just one GFCI in the line. However...whoever set this up had a "thing" about GFCIs.

In this case, there's a small outdoor breaker panel, and the UF runs from it to a box 40' away. At this box is a GFCI receptacle to power a pond pump. More UF runs from the GFCI's "load" terminals to another box about 30' away. This box splits the line to a short run of UF to a GFCI outlet to power another fountain pump, and the other split goes inside a shed, where there is, yes, another GFCI.

Will the existence of a downstream GFCI cause problems to the first one? Are we likely to have nuisance trips? If there is a true ground fault, will it be a race between the two GFCIs to see which trips first, or can we expect both to trip simultaneously?

You're asking - "He knows the first GFCI feeds those two others from its load terminals - why didn't he just pigtail the wires so there's simply a stand-alone GFCI at each box?" Because the first GFCI is in an incredibly TIGHT PVC box. There's one wire nut in there already for the grounds, and it's squashed in so that its between the GFCI and the side of the box - there's just not room for the GFCI and two more nuts in there. And the box is sort of cemented into a wall, so changing it to a nice normal 4x4 would involve busting up some of the wall. :smack:

Tripler
08-17-2006, 06:06 PM
I don't have my NEC 2005 book handy, but IIRC, GFCIs are now required for all outdoor receptacles, kitchen counter areas, and bathrooms.

What's the effect? I don't think there would be an effect on the 'upstream' ones or the 'downstream' ones. Aren't they designed to simply isolate the two receptacles on the duplex without affecting the rest of the line?

The house I just bought has GFCIs mounted around the kitchen counter. I can't see overmixing margaritas in my blender tripping the GFCI and also cutting off power to my little stereo on the same branch circuit.

Tripler
I gotta ask, just how many fountains do you have?

Crafter_Man
08-17-2006, 07:40 PM
I've wired multiple GFCI receptacles in series and did not encounter any problems.

And have you noticed that some hairdryers incorporate GFCI circuitry into the plug? Yet they work fine when plugged into a GFCI receptacle.

Chefguy
08-17-2006, 08:21 PM
The code allows for multiple GFCI outlets in one line, although generally you only need to have the first in line protected. My entire kitchen has GFCI outlets, all on the same circuit.

rbroome
08-17-2006, 08:34 PM
One for the electricians... What would be the likely result of having two GFCIs (ground-fault interrupters) in series?

At the moment, there is some UF cable trenched around a garden. In an ideal world, there would be just one GFCI in the line. However...whoever set this up had a "thing" about GFCIs.

In this case, there's a small outdoor breaker panel, and the UF runs from it to a box 40' away. At this box is a GFCI receptacle to power a pond pump. More UF runs from the GFCI's "load" terminals to another box about 30' away. This box splits the line to a short run of UF to a GFCI outlet to power another fountain pump, and the other split goes inside a shed, where there is, yes, another GFCI.

Will the existence of a downstream GFCI cause problems to the first one? Are we likely to have nuisance trips? If there is a true ground fault, will it be a race between the two GFCIs to see which trips first, or can we expect both to trip simultaneously?

You're asking - "He knows the first GFCI feeds those two others from its load terminals - why didn't he just pigtail the wires so there's simply a stand-alone GFCI at each box?" Because the first GFCI is in an incredibly TIGHT PVC box. There's one wire nut in there already for the grounds, and it's squashed in so that its between the GFCI and the side of the box - there's just not room for the GFCI and two more nuts in there. And the box is sort of cemented into a wall, so changing it to a nice normal 4x4 would involve busting up some of the wall. :smack:

It has always been my understanding that it was a bad idea to have multiple GFIs. A GFI basicly measures the amount of current flowing in each direction. (not really, but that is one simple model of the method). All the electrons flowing out had better flow back. If not, some of them must have escaped to ground-perhaps through some hapless homeowner. When the current becomes unbalanced, the circuit opens. Part of the process is to bleed a tiny bit of current into the ground wire if I remember correctly. If you have too many GFIs in the circuit, the sum of al those tiny losses might trip the protection. And it doesn't do any good to have more than one. Once one opens the circuit is dead and the others won't see any current to be unbalanced. It is important to put it close to the head of the circuit so that all the outlets are killed when it trips. But other than allowing electricians to charge $8 for an outlet that should cost $4, there is no advantage.

Bill Door
08-17-2006, 08:37 PM
Here's something I found out the hard way. A ground fault circuit interrupter works by measuring the current flow in the hot leg, and comparing it to the current in the neutral leg, and if it detects that the currents don't match, it assumes the extra current has "gone to ground" and trips the breaker.

If you are not grounded, and grab both legs so that current can flow from one leg to the other through your body, the GFCI doesn't mind this a bit. It was the late seventies, and GFCI were fairly new, and I didn't know how they worked, and tested one this way. It was only 110V, but it kind of caught me by surprise. I have to admit, there were beverages involved.

danceswithcats
08-17-2006, 09:15 PM
You're asking - "He knows the first GFCI feeds those two others from its load terminals - why didn't he just pigtail the wires so there's simply a stand-alone GFCI at each box?" Because the first GFCI is in an incredibly TIGHT PVC box. There's one wire nut in there already for the grounds, and it's squashed in so that its between the GFCI and the side of the box - there's just not room for the GFCI and two more nuts in there. And the box is sort of cemented into a wall, so changing it to a nice normal 4x4 would involve busting up some of the wall.

I'd like to humbly offer that a device box that tight has already exceeded the allowable fill calculations for entering/exiting conductors and device(s).

Crafter_Man
08-17-2006, 09:54 PM
Part of the process is to bleed a tiny bit of current into the ground wire if I remember correctly. If you have too many GFIs in the circuit, the sum of al those tiny losses might trip the protection. And it doesn't do any good to have more than one. Once one opens the circuit is dead and the others won't see any current to be unbalanced. This is not correct. A GFCI doesn't even need the "third prong" connected to earth ground in order to work. If you want proof of this, consider that the NEC allows 2-prong, ungrounded receptacles to be replaced with 3-prong, GFCI receptacles (assuming it is properly labeled).

Crafter_Man
08-17-2006, 10:01 PM
Here's something I found out the hard way. A ground fault circuit interrupter works by measuring the current flow in the hot leg, and comparing it to the current in the neutral leg, and if it detects that the currents don't match, it assumes the extra current has "gone to ground" and trips the breaker.

If you are not grounded, and grab both legs so that current can flow from one leg to the other through your body, the GFCI doesn't mind this a bit.Yep.

FYI, a GFCI receptacle measures the difference in current between the hot and neutral lines using a differential transformer. If there is no outside leakage path, the currents will always be identical, and thus the difference would be exactly zero at all times. If you are completely isolated from earth ground, and you touch the hot and neutral lines on the "load" side of a GFCI receptacle, you will get zapped. And the GFCI will not trip.

IIRC, a GFCI receptacle trips when the difference in current is around 5 mA.

It was only 110V, but it kind of caught me by surprise.Minor nitpick: you actually got hit with a peak voltage of around 170 V, multiple times.

Bill Door
08-17-2006, 10:03 PM
I should have specified RMS volts.

Rhubarb
08-17-2006, 10:15 PM
I don't know what the NEC says about GFCIs in series, but my younger brother, a Master Electrician, advises against it. According to him, series GFCIs can lead to undesirable effects including nuisance trips (I have seen this myself) or failure to trip properly (I have not seen this nor pretend to understand how it could occur).

Regarding the overcrowded j-box, if your GFCI outlets are like the one I am looking at right now, it has pressure plate type terminals. On a pressure plate terminal, the wires do not go directly under the screw head, instead they slide between two metal plates. Tightening the terminal screw squeezes the wires between these plates. Per the NEC, it is permissible to have multiple (2) conductors on these types of terminals. So if your receptacle is like this, you can move the downstream wires from the Load terminals to the Line terminals and your GFCIs will no longer be in series.

Or you can do what I did in my last house and simply replace the downstream receptacles with standard ones. They are only a few dollars each for good quality receptacles.

Crafter_Man
08-17-2006, 10:33 PM
series GFCIs can lead to undesirable effects including nuisance trips (I have seen this myself) or failure to trip properly (I have not seen this nor pretend to understand how it could occur).I think I've got a pretty good understanding of how a GFCI operates, and I do not understand why this would happen. I'm not saying it doesn't; perhaps there's something I'm not considering. Can you ask your brother for more info on this?

Joey P
08-17-2006, 10:47 PM
I think I've got a pretty good understanding of how a GFCI operates, and I do not understand why this would happen. I'm not saying it doesn't; perhaps there's something I'm not considering. Can you ask your brother for more info on this?
I'd like to know as well. At my house, there are two outlets on the side of the house and one in the garage, all GFCI. (There are others in the garage, but I'm not sure if they're on the same branch or not off the top of my head.) Since I moved in about a year and a half ago. The garage GFCI has tripped about 4 or 5 times, knocking out the other two. To the best of my knowledge, this happened when nothing was plugged into any of them. I just went to use one of them one day and it didn't work. I spent a LONG time trying to figure it out before I thought to check that. At the time I didn't even know they were on the same branch. So for a while know, I've been wanting to rewire them. Seems to me that they should all be in parallel and all hooked to the line side. I'm guessing at least the other two are coming from the load side of the garage one, but I haven't opened them up yet.

Crafter_Man
08-17-2006, 11:17 PM
I'd like to know as well. At my house, there are two outlets on the side of the house and one in the garage, all GFCI. (There are others in the garage, but I'm not sure if they're on the same branch or not off the top of my head.) Since I moved in about a year and a half ago. The garage GFCI has tripped about 4 or 5 times, knocking out the other two. To the best of my knowledge, this happened when nothing was plugged into any of them. I just went to use one of them one day and it didn't work. I spent a LONG time trying to figure it out before I thought to check that. At the time I didn't even know they were on the same branch. So for a while know, I've been wanting to rewire them. Seems to me that they should all be in parallel and all hooked to the line side. I'm guessing at least the other two are coming from the load side of the garage one, but I haven't opened them up yet.One of the problems might be something called a "shared neutral" or "shared hot."

Here's an example of how a "shared neutral" might happen:

Let's say a bunch of receptacles, lights, and whatnot are connected to the load side of the GFCI. We say these things are "downstream" from the GFCI, and thus they are all GFCI-protected. Afterwards, someone decides to install a ceiling light. They connect the ceiling light's "hot" to a circuit that is not GFCI protected (or they run the ceiling light's "hot" wire directly back to a circuit breaker in the panel). They connect the ceiling light's "neutral" wire to a neutral that is downstream from the GFCI. This creates an automatic imbalance on the downstream side of the GFCI when the ceiling light is used, and the GFCI will trip.

Here's an example of how a "shared hot" might happen:

Let's say a bunch of receptacles, lights, and whatnot are connected to the load side of the GFCI. We say these things are "downstream" from the GFCI, and they are all GFCI-protected. Afterwards, someone decides to install a ceiling light. They connect the ceiling light's "hot" to a circuit that is downstream from a GFCI. They connect the ceiling light's "neutral" wire to a circuit that is not GFCI-protected (or they run the ceiling light's "neutral" wire directly back to the CB panel). This creates an automatic imbalance on the downstream side of the GFCI when the ceiling light is used, and the GFCI will trip.

danceswithcats
08-17-2006, 11:44 PM
Additionally, GFCIs don't like poor connections in downstream circuits. Granted, it shouldn't make a difference, but when I installed GFCIs in my late 70's home to upgrade electrical compliance, nuisance tripping was common. Only after I opened each device box, removed backstab connections and remade them to the screw terminals did the tripping stop. I can't explain it, but making good connections on the downstream receptacles alleviated the problem.

gotpasswords
08-18-2006, 12:02 AM
Trying too address the questions as they came...

Tripler - one pond and one fountain, in an area roughly 80 x 200 feet.
danceswithcats - agreed, the box is packed. But, I wasn't the person who installed a frippen-fracken switch box into the dad-blanged stone wall. I'd much rather it be a 4x4 junction box and mud ring, but it's not my wall to bust up.
Rhubarb - I'm pretty sure the thing has the screw-plate compression terminals. I've just gotten so used to pigtailing wires, rather than using receptacles for "wire-through" - that would alleviate the problem entirely. :)
danceswithcats - I firmly believe backstab terminals need to be outlawed. I've seen too many burned wires from those things.

casdave
08-18-2006, 08:07 AM
Your main problem is likely to be discrimination.

We in the UK call these things Residual Current Devices(RCDs), they work particularly well for us because our entire system is neutral grounded using a 3 wire system.

The problem with multiple RCDs is that they not only will have a tripping current, they also have tripping times, so that a surge will not cause unwanted interrupts.

Ideally you put RCDs with longer trip times further toward the incoming supply connection, so that those further down the line trip first.

Your problem comes when you install units with ostensibly the same tripping time, because the reality is that there will always be some small differance.
This can then lead to unwanted upstream tripping, and leave the downstream device still on and can then lead to problems working out which is the faulty circuit.

This is where you pay for a person who knows what to look for in manufacturers specifications instead of doing the job yourself.

This can happen with fuses as well, but fuses tend not to be used in the same way so the safety issues are differant.

Rhubarb
08-19-2006, 01:54 AM
I think I've got a pretty good understanding of how a GFCI operates, and I do not understand why this would happen. I'm not saying it doesn't; perhaps there's something I'm not considering. Can you ask your brother for more info on this?
My brother's coming over this weekend. I'll ask him about it and post the answers here.

danceswithcats
08-19-2006, 01:01 PM
danceswithcats - I firmly believe backstab terminals need to be outlawed. I've seen too many burned wires from those things. Heh. I've seen where Harry Homeowner stuffed wires into the release slots-hey-it's another hole-poke a wire in it and repeated arcing had burned the duplex to the point that it fell apart as I pulled on the device mounting yoke. :eek: A four-gang box whose cover was almost intolerably hot to the touch-way over fill rule-had I not found it, they would have had a visit from station 44 with BRTs when the kitchen burned.

nmchick52
09-11-2016, 01:39 PM
We do not have a GFCI outlet on our covered (not enclosed) patio but understand that if we were to replace the outlet, it should be with a GFCI one. However, our spa manual has a big red warning NOT to plug the 110 v GFCI cord into a GFCI outlet. Why would this be when the NEC says the outdoor outlet should be a GFCI outlet? Thanks.

Tim@T-Bonham.net
09-11-2016, 04:07 PM
One for the electricians... What would be the likely result of having two GFCIs (ground-fault interrupters) in series? GFCI's in series can cause frequent nuisance trips.
If everything is wired perfectly, this should not happen -- but in the real world, it's not all wired perfectly. (Especially if done by someone who overloaded a switch box.)

Because the first GFCI is in an incredibly TIGHT PVC box. There's one wire nut in there already for the grounds, and it's squashed in so that its between the GFCI and the side of the box - there's just not room for the GFCI and two more nuts in there. And the box is sort of cemented into a wall, so changing it to a nice normal 4x4 would involve busting up some of the wall. :smack:They make 'extenders' (http://i.stack.imgur.com/hGwTK.jpg) for nearly every size of box. Those make the box deeper, thus nearly doubling the room inside it for wiring. Of course, that will make the box stick out a bit from the wall. Also, in exterior locations, you will have to worry about making this weatherproof.

Quartz
09-11-2016, 04:55 PM
I'm no electrician, but shouldn't exterior wiring have lightning protection at both ends?

mixdenny
09-11-2016, 06:28 PM
I'll just throw in that I have several outdoor products that come with GFCIs built in to the cord, and the outdoor receptacles are GFCI also. So every time you plug one in you create this situation, and they are all UL approved. They include blowers for bounce houses, contractor's extension cords, a pool pump, the charger for a kid's electric car, and a sewer cleaner. Also window A/Cs which plug into an arc fault breaker in the bedroom.

Now, I am sure they are there for safety reasons in case someone plugs them into an unprotected circuit, but there is nothing in the instruction manual that mentions a problem if plugged into a GFCI. In fact if I plugged one into the extension cord it would be triple GFCI protected.

Dennis

Crafter_Man
09-11-2016, 08:05 PM
We do not have a GFCI outlet on our covered (not enclosed) patio but understand that if we were to replace the outlet, it should be with a GFCI one. However, our spa manual has a big red warning NOT to plug the 110 v GFCI cord into a GFCI outlet. Why would this be when the NEC says the outdoor outlet should be a GFCI outlet? Thanks.
Just a guess: your spa has some normal leakage to ground. And it exceeds 5 mA. So if you plug it into a GFCI receptacle the receptacle will trip. The GFCI that is built into the spa has a higher set point (e.g. 9 mA) or the GFCI that is built into spa senses the differential current downstream from the leaky components.

septimus
09-12-2016, 07:08 AM
May I piggy-back to ask my own question about GFCI?

Two wires from the street divide into three parallel pairs, which go to an upstairs Safe-T-Cut brand GFCI, a downstairs GFCI (same brand) and an air conditioner. (I don't know if bypassing the GFCI for air-con is a good idea, but that's not my question.)

The question is: Can misbehavior on the from-street lines cause GFCI to trip?

One day, the GFCI tripped repeatedly; I thought we might need a new GFCI or something, but my wife insisted that everyone was having trouble with their electricity that day and the problem was external. I didn't believe her until I noticed the upstairs GFCI, with very little attached to it, had also tripped. And indeed after the municipal electricity was more normal, we no longer had GFCI trippings.

I do not know exactly what was wrong with the municipal power, other than that we often have significant low voltage. I do know that once several years, for a few minutes, the two wires to our house, which are supposed to be neutral and a phase, were instead two different phases! :eek: (Two of our appliances were damaged that day; neighbors also had problems.)

excavating (for a mind)
09-12-2016, 07:28 AM
All this talk about wiring GFCIs in series has my head spinning.

Shouldn't these always be wired in parallel? Would they even work if they were wired in series?

Or, am I thinking wrong? (I'm not finished with my first cup, yet, so that is a real possibility).

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