Let's hope it is not connected. Two 20 amp circuits ruining the same ground and neutral is not good.

I believe all your assumptions are correct,it is safe and leaving the "mystery"breaker off is a good way of finding what it controls. Garage door opener??? Landscape lighting???? Burglar alarm?? Fire protection equipment?? Humidifyer or electronic hvac filter?? Gas range light and timer?? Stubbed out near electric furnace to allow installing gas instead?? Terminated in a box with blank cover waiting for optional equipmet to be installed??

Thanks Klem1. I've tried everything you mention all working fine (or non-applicable), can't find any blank face plates, stubbed out capped wiring etc.. It appears that they changed out an electric furnace for gas back in the early 90's judging from the mfg date on the furnace (and I can see some "big boy" circuits in the panel labeled for electric furnace). I'll just leave that breaker off for now and see if I eventually notice something and given it's running down from the receptacle, next time I'm in the crawl (and that part of the crawl is for very small people with no fear of tight spaces, rats, spiders etc.) I'll see what I can see.

Thanks tim45z10. So would you recommend I discontinue the continuing ground/neutral part of the circuit by terminating it at the vac receptacle (which would be easy to do) or just leave the breaker off to the mysterious hot and accomplish the same thing. I definitely need to swap out the outlet for a 20amp GFCI particularly because this thing is about 3-4 ft from a utility sink in the garage it looks like the previous owners installed themselves given the hack job on the sheetrock they did.

"Let's hope it is not connected. Two 20 amp circuits ruining the same ground and neutral is not good."
Tim,
Just curious but what qualifications do you have that would lead you to make such a statement?

The OP is describing a multiwire branch circuit (MWBC) which is done every day, is perfectly safe and legal.

The main service coming in to your house works the same way.

This post was edited by joefixit2 on Tue, Jan 13, 15 at 8:47

The other half of that MWBC will most likely be a dedicated circuit like the dishwasher, kitchen receps, microwave, washing machine etc.

Joe is right on this one (in both his posts).

Joe surely you don't believe that it is ok to run two separate 20 amp breakers on a single neutral and ground? What if you put a15amp load on each breaker the neutral wire is being asked to run 30 amps. I know this is not safe. Leave the second breaker off. My guess it was originally a single 220 circuit. Somebody swapped out the dual breaker with two single ones. They also changed the outlet.

If the mystery circuit continues bugging you landscapers/irigation guys have a device that basicly makes a wire an antenna easily detected by a secound device. Electricians use them too but lawn guys work cheaper,especilly off season. Or rental yard might have one.

"Joe surely you don't believe that it is ok to run two separate 20 amp breakers on a single neutral and ground? What if you put a15amp load on each breaker the neutral wire is being asked to run 30 amps. I know this is not safe. Leave the second breaker off. My guess it was originally a single 220 circuit. Somebody swapped out the dual breaker with two single ones. They also changed the outlet".

This is why I questioned your qualifications. No offense but I am trying to tell you in a nice way that you are obviously not qualified to be telling others how to do electrical wiring. A multiwire circuit that is properly wired cannot overload the neutral. The neutral only carries the unbalanced current. In your example of 15 amps on each leg, the neutral will see no current at all. Ever notice that the neutral coming into your house is smaller than the line conductors? Same principle. Multiwire circuits have been around since the beginning of electricity and are perfectly legal. They are also known as "Edison circuits" and there are millions in use, probably a few in your house.

I am suggesting in a nice way that you do your due diligence and study up before giving out advice.

{{gwi:2137763}}

This post was edited by joefixit2 on Wed, Jan 14, 15 at 21:41

Not here in California. Our nuetral and hot wires are all the same size.
And we are not allowed to run run a common neutral off of two separate hot wires. Not all codes are the same.

Not here in California. Our nuetral and hot wires are all the same size from the drop through the weather head into the panel.
And we are not allowed to run a common neutral off of two separate hot wires. Not all codes are the same.

Overhead services have a Steel reinforced "messenger" wire that doubles as the grounded conductor, and it looks the same size as the line conductors. Underground services of say 200 amps are typically 4/0-4/0-1/0 with 1/0 being the grounded conductor. YOUR WHOLE HOUSE IS POWERED BY ONE BIG SHARED NEUTRAL CIRCUIT.

And yes the SERVICE ENTRANCE CONDUCTORS are the same size simply because the NEC requires engineering calculations to determine the neutral reduction allowance and it is cheaper & faster to just make all 3 the same. The POCO follows NESC and runs a reduced neutral.

Again, I ask your qualifications, your statements are not making sense. Please refer me to the code you speak of that does not allow shared neutrals. And for what possible reason would they not allow it? Do you have any idea what dedicated neutrals does to pipe fill??

This post was edited by joefixit2 on Wed, Jan 14, 15 at 22:39

Sorry Tim, but that is wrong for Califronia, at least on a state wide basis. 210.4 of the CEC mirrors the same wording form the NEC. Multiwire branch circuits are certainly permitted by the CEC. In fact, the only difference between the NEC and the CEC in article 210 is that the CEC has some restriction on receptacle height not present in the NEC.

You're also wrong on neutral sizing being different. It is identical between the CEC and the NEC as well. While a MWBC usually can't have a smaller neutral because the possibility of an unbalanced load prevents it (all the 120V load could be on one side of the MWBC), on things like feeders, etc.... the neutrals are very much allowed to be reduced to that needed for the maximum unbalanced load (and other restrictions).

There's no difference between the CEC and the NEC for anything we're talking about here.

This post was edited by ronnatalie on Wed, Jan 14, 15 at 23:26

The hot and neutral are the same size. With the same amount of insulation. I have retrofitted enough of them to know what I am talking about. The first one did not pass city inspection because my neutral was smaller. But since you seem to know all. I will shut my mouth and go away. That's what you wanted anyway.
Sorry OP that your post got hijacked.

The post got hijacked???? You're telling the OP to shut down a perfectly safe circuit because you don't understand it, I am trying to prevent that. How is that a hi-jack??? I also find it disturbing that you claim to be doing electrical work yet you don't have a clue about some very basic things that a first year apprentice would know, and you refuse to indicate your qualifications. Are you a house flipper? handyman? hobbyist? hack? I tried to reason with you but unfortunately your stubborn stupidity and refusal to accept facts is not allowing this discussion to go anywhere useful.
I am going to respectfully bow out, citing this quote:

"I refuse to engage in a battle of wits with an unarmed man"

This post was edited by joefixit2 on Thu, Jan 15, 15 at 8:55

To the OP: Put everything back together and plug in your new vacuum, then get on with your life.

"The first one did not pass city inspection because my neutral was smaller"

Further evidence you are in over your head. There is a difference between the service entrance conductors and the service drop or service lateral, re-read what was posted.