Multiwire branch circuit tripping GFCI's

[iwirehouses]

I'm running two GFCI's, about 6 feet from each other, on two dedicated circuits in a shop. I ran 12/3 from the panel to a junction box right above the first gfci. From there I split the neutral to each gfci, and ran the red as hot to gfci 1 and the black to gfci 2. I figured this would work fine. And it did for days. Its progressivley been getting worse which I don't understand. The gfci's trip when tools are plugged into them. All kinds of tools, old ones and new ones, so its not bad tools. It's been happening on multiple gfi's, so i doubt its bad gfi's. It won't trip if a laptop, cell phone charger, etc is plugged in. I assume the power tools are making some kind of neutral imbalance and nuisance tripping the gfci's.

If I am understanding this correctly, I think have four options...

1 Take the gfi's out, which I assume would be a code violation (not gonna do).

2 Put in two pole gfci breakers on the three wireswhich is too expensive. (don't want to do)

3 If I just ran a 12/2 from the panel to the first junction box and gave each circuit there own neutral (abondoning the multiwire branch circuit). I think this would resolve the problem, right?

4 Run everything on the same, single 20 amp circuit. These outlets are bench top outlets. Will most likley be running sawzalls, jig saws, a shop radio, etc. It would be nice having seperate circuits.

Does any of this make sense? I like option 3 the best, but I hope I am understanding this scenario and it actually would resolve the problem rather then create a problem I didn't expect again (the gfi's tripping in the first place).

 

[480sparky]

Which wiring diagram shows how you did this?:

OR:

 

[iwirehouses]

I'm glad you asked...and understand. I first did it like the first diagram. I had never done it before but I thought people do it in kitchens sometimes. Well that didn't work at all. Any time you use the second gfci, it trips the first one. So I changed it to the second diagram, and thats where we are now. Initially it worked fine, but progressivly its getting worst and basically trips the gfi's everytime now, when a power tool is used. This is when I came up with the theory that its nuisance tripping over some weird kind of neutral imbalance...and could be resolved by running an additional 12/2 to the j-box and abondoning the red wire in the 12/3. I'd hate to do this tho, and have the same problems. Any ideas?

 

[480sparky]

When you say you changed it to the second wiring diagram, are you running 12/2 from the GFI to the downstream receps?

(I didn't think to change the red wire in the bottom diagram to black because there's no 12/2 with a red and white.)

 

[Jim W in Tampa]

What is triping the breaker or gfi.

 

[480sparky]

What is triping the breaker or gfi.

...................

I'm running two GFCI's, about 6 feet from each other, on two dedicated circuits in a shop. I ran 12/3 from the panel to a junction box right above the first gfci. From there I split the neutral to each gfci, and ran the red as hot to gfci 1 and the black to gfci 2. I figured this would work fine. And it did for days. Its progressivley been getting worse which I don't understand. The gfci's trip when tools are plugged into them. All kinds of tools, old ones and new ones, so its not bad tools. It's been happening on multiple gfi's, so i doubt its bad gfi's. It won't trip if a laptop, cell phone charger, etc is plugged in. I assume the power tools are making some kind of neutral imbalance and nuisance tripping the gfci's.

If I am understanding this correctly, I think have four options...

1 Take the gfi's out, which I assume would be a code violation (not gonna do).

2 Put in two pole gfci breakers on the three wireswhich is too expensive. (don't want to do)

3 If I just ran a 12/2 from the panel to the first junction box and gave each circuit there own neutral (abondoning the multiwire branch circuit). I think this would resolve the problem, right?

4 Run everything on the same, single 20 amp circuit. These outlets are bench top outlets. Will most likley be running sawzalls, jig saws, a shop radio, etc. It would be nice having seperate circuits.

Does any of this make sense? I like option 3 the best, but I hope I am understanding this scenario and it actually would resolve the problem rather then create a problem I didn't expect again (the gfi's tripping in the first place).

 

[sparky_magoo]

Have you checked for bond wires touching the nuetral in the receptacle boxes? I have found loose wire nuts and/or back-stab connections can cause gfi's to trip from high resistance connections. Are all of the connections tight?

Your original installation should have worked fine.

 

[winnie]

If the neutral is 'split' upstream of the GFCIs, then they should function correctly. Remember that the neutral is shared uptothe panel without causing difficulty.

I would suspect a resistive neutral to neutral fault downstream of the GFCIs. When the voltage drop on the neutral is high enough, sufficient current flows to the other neutral to trip the GFCIs.

-Jon

 

[Jim W in Tampa]

Being that it did work at first i would assume they are wired correctly. You might have bad connections and that could be anywhere from receptacle to panel. Also is chance that a nail,screw,etc has hit a wire. Be sure neutrals and grounds not accidently touching. Remove neutral and both hots from panel and ohm every thing out. Besure line and loads not miswired

 

[Howard Burger]

Have you checked for bond wires touching the nuetral in the receptacle boxes? ... Are all of the connections tight?

Might put a wrap of tape around the gif before putting it in the box.

Is the neutral tight in the panel bus?

What happens if the tools are plugged into a gfi on a 12-2 circuit (like in the bathroom)?

 

[K8MHZ]

I'm glad you asked...and understand. I first did it like the first diagram. I had never done it before but I thought people do it in kitchens sometimes. Well that didn't work at all. Any time you use the second gfci, it trips the first one. So I changed it to the second diagram, and thats where we are now. Initially it worked fine, but progressivly its getting worst and basically trips the gfi's everytime now, when a power tool is used. This is when I came up with the theory that its nuisance tripping over some weird kind of neutral imbalance...and could be resolved by running an additional 12/2 to the j-box and abondoning the red wire in the 12/3. I'd hate to do this tho, and have the same problems. Any ideas?

Take note that I have seen GFCIs get more sensitive after repeated tripping to the point of total failure.

Is the receptacle that the power tool is plugged into the one that is tripping or is it one upstream?

If it is the one the power tool is plugged into a quick test would be to swap it out with one you know will hold the power tool load. If that trips as well the wiring would be suspect. If not, the original receptacle.

Another question, when you say 'a power tool' do you mean any and all power tools or just a certain one?

 

[LarryFine]

The gfci's trip when tools are plugged into them.

Does the GFCI protecting one circuit ever trip when plugging something into the other circuit?

Is it always when plugging in, always when hitting the tool's trigger, or sometimes each one?

 

[iwirehouses]

I've got to go check out exactly which outlets are tripping. But for now, this is what I have. Shouldn't this be fine? Would it be suspect to nuisance tripping for any reason? 3 general benchtop circuits. All standard duplex receptacles are protected by an up stream gfci except the air compressor (17amp) circuit . There are four 20 amp breakers with 12/2 and 12/3. 4 inch square junction boxes 3 ft. above each device containing the splices.

 

[480sparky]

I've got to go check out exactly which outlets are tripping. But for now, this is what I have. Shouldn't this be fine? Would it be suspect to nuisance tripping for any reason? 3 general benchtop circuits. All standard duplex receptacles are protected by an up stream gfci except the air compressor (17amp) circuit . There are four 20 amp breakers with 12/2 and 12/3. 4 inch square junction boxes 3 ft. above each device containing the splices.

So, are the first receps in the line GFIs, or do you have GFI breakers?

 

[iwirehouses]

The rectangles are gfi's. All the non gfi receptacles are fed from upstream gfi's, except the air compressor outlet.

 

[cadpoint]

????'n @ #13 the neutral and the hot on any circuits is not on the same breakers ....

 

[480sparky]

The rectangles are gfi's. All the non gfi receptacles are fed from upstream gfi's, except the air compressor outlet.

That's why they're tripping. You must have a separate neutral for the two 'standard' receps on the top circuit. You cannot share a neutral with two GFI-protected circuits.

 

[iwirehouses]

Well this is the exact schematic I'm using. I split the neutral upstream of the gfis. The neutral is not coming out of the load side of the first gfi and going to the line of the second. This should work, shouldn't it?

 

[iwirehouses]

I guess the top circuit is the only one with problems. The bottom circuits seem to be running fine.

The 4" sq. box on the left (two "standard" duplex receptacles) is fed from the load on the gfi in the center box.

All my connections are extremley tight. You'd ned some linemen pliers and little time to get any connections apart. The only thing thats been brought up is the fact the I didn't wrap tape around the gfi's. There is a bare ground wire in each box and they are standard 4"sq boxes with raised metal covers. Its a good possibility a ground could be touching a neutral. I know this raises hell with arc fault breakers, but have never personally seen this cause a problem on a gfi.

When all is said and done, is my wiring correct? or should I run a seperate 12/2 to the middle box on the top circuit and abandon the 12/3 multiwire branch circuit idea?

 

[Dennis Alwon]

From what I see it appears your hookup is correct. The biggest mistake people make on this is using the load side of the neutral to feed the second circuit. As long as you have a splice on the main neutral conductor then this should not be an issue.