Help needed with wiring gremlins!

[bandlaw]

Greetings all!

I'm working on my shop electrical, and successfully installed a 100A subpanel and wired 5 circuits. Circuits 1 and 2 are for the workbench circuits, Circuit 3 is for the table saw, Circuit 4 is for the Air Compressor and Circuit 5 is for the Shop Vac.

Circuit 1 and 2 are the ones giving me problems. Here's the general idea:
There are three two-gang boxes and the left side of each outlet is circuit 2, the right side being circuit 1. The first two-gang box is a GFCI on each circuit, with the goal to provide load-side protection to the second and third boxes. The boxes share a common neutral and ground, which after study, appeared to be both more efficient and permitted by code.

Problem is, I can use all of the first box, and the "right side" of the second and third boxes (i.e. circuit 1), but one or both GFCI's will pop if I utilize circuit 2 at all. Below is the wiring diagram, and I'm hoping the folks that are knowledgable about wiring can help me troubleshoot what I did wrong. I followed the Black & Decker Electrical Wiring Guide, which had examples of GFCI protecting standard outlets downstream and a wiring guide for a common neutral. I'd like to fix it as is, if possible. If not, I suppose there are two other options: switch to GFCI breakers and put in standard outlets on the first circuit, OR add a separate neutral wire for circuit 1 and circuit 2.

Thoughts, help, desperately needed. I am stuck in the house for the snow-pocalypse in North Carolina, so I would love to get this last kink worked out. Thanks!

 

[JimRB]

You shouldn't have common neutrals downstream of the GFI. The GFI is supposed to read the current flowing through the hot leg and the neutral. If there is a significant difference it is supposed to open the circuit. You can't measure a difference if you are using common neutrals. Actually you can and that is why things are popping. If you had no GFIs then you could use a common neutral. Just run neutral from the left GFI to the left receptacles and a neutral from the right GFI to the right receptacles and you should be good.
http://home.howstuffworks.com/question117.htm

 

[mm08822]

You shouldn't have common neutrals downstream of the GFI. The GFI is supposed to read the current flowing through the hot leg and the neutral. If there is a significant difference it is supposed to open the circuit. You can't measure a difference if you are using common neutrals. Actually you can and that is why things are popping. If you had no GFIs then you could use a common neutral. Just run neutral from the left GFI to the left receptacles and a neutral from the right GFI to the right receptacles and you should be good.
http://home.howstuffworks.com/question117.htm

This^^^^^. Replace the single 3conductor cables with two 2 conductor cables between boxes 1-2 and 2-3. Then each gfci circuit will be isolated from the other.

 

[bandlaw]

Thank you! I shall try this and report back!


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[LXCam]

You shouldn't have common neutrals downstream of the GFI. The GFI is supposed to read the current flowing through the hot leg and the neutral. If there is a significant difference it is supposed to open the circuit. You can't measure a difference if you are using common neutrals. Actually you can and that is why things are popping. If you had no GFIs then you could use a common neutral. Just run neutral from the left GFI to the left receptacles and a neutral from the right GFI to the right receptacles and you should be good.
http://home.howstuffworks.com/question117.htm

Yup like those two said. But the difference only needs to be miliamps to kick the GFI. A GFI looks at the current in both the hot and neutral and if they aren't equal that's how it knows there a potential ground fault, which in your case is not the issue.

 

[Milton Shaw]

I knew you were not supposed to feed gremlins after dark, but have never found any with terminal screws on them. ... LOL

 

[bandlaw]

Thanks, folks! That did the trick! We are 100% complete with this phase of the wiring project. Thanks so much for the help! I really appreciate it!


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[ard]

If OP doesnt want to rewire cables, he could put GFCIs at EACH location, correct?

So each GFCI is hooked to a common neutral and common lines and none of them provides 'downstream' protection.

Yes?

Not that it is better or cheaper...but.... if he was sheet rocked it would be the better solution.

 

[ard]

Whoops.

A minute too late

 

[bandlaw]

Ard, no worries! I think you are correct about the GFCIs, and I debated it at one point. The main reason I went this way was cost and the recommendation of the nice folks at City Electric Supply.


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[LXCam]

So each GFCI is hooked to a common neutral and common lines and none of them provides 'downstream' protection.

Yes?

Yes.

 

[pattenp]

Looks like originally you wired it as a multiwire branch circuit. Did you use a double pole breaker or two singles with a handle tie? With the changes you made how is the circuit(s) being fed at the panel? Even though it now works I hope you have a correctly wired circuit.

 

[bandlaw]

It is two singles but I am glad you asked because I need to invest in the handle tie. Thanks!!!


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