14/3 Used For Two 15 amp Circuits?

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dafish

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I'm a bit concerned. I'm replacing some wall switches and outlets in my home and have noticed one location is delivering two circuits, breakers at 15A each, over one 14/3 bundle. I suspect this is both well outside any code (ever), as well as exceptionally unsafe. That said, I'm certainly not a certified electrician. What I see suggests that sharing a single neutral and ground in this way means:
  1. There is potential for 30 amps on the #14 neutral, well over its rated, or even safe, capability. Fire risk I assume.
    1. Side Story: My wife has always claimed her vacuum doesn't work right plugged into the affected outlets. Guess I know why now.
  2. The possibility that somebody could get shocked. Turn off one of those breakers and it would seem safe, but not be. The neutral would still be "hot" from the other breaker!
Question #1: Am I correct in thinking this is unsafe and needs corrected?

Question #2: if I read code correctly, I can't run a single #14 THHN through an attic or basement ceiling to deliver the additional neutral unless I add conduit. If true, the simplest answer seems to run a new 14/2 to support one of those 15 breakers. (Thankfully, wiring is independent from there on). Correct? Any better ideas?

Question #3: House was built in 2004 if it matters, by a well-respected GC who's fairly principled. Wired by an electrical company that I thought was using certified electricians, but this wasn't an accident. It was wired this way intentionally. Any idea what they were thinking?

Thanks for sharing guys!

Dan
 
You have a multiwire branch circuit. Very normal and code compliant. The neutral only carries the imbalance. Now working on them as a homeowner who doesn't know how they work may end up running 240 through them so I'd suggest you have an electrician do any work even replacing switches and outlets because of that.
 
Ah so. So you're telling me because they ran on breakers one right below another they are pulling Leg A and B and thus the current, when on, cancels at the common neutral save for imbalance. Pretty cute.

Would you share this with me? How is this not a situation where there is a shock risk still when one breaker only is off? Shouldn't those breakers have been connected so you had to turn them both off at once?
 
dafish said:
  1. There is potential for 30 amps on the #14 neutral, well over its rated, or even safe, capability. Fire risk I assume.
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Perfectly code-compliant and safe as long as the two hot wires are supplied by opposite hot legs of the service, so they would have 240v between them. That is the same as the power coming into your home.

If they were supplied by the same hot leg, they would have 0v between them, and the current could overheat the shared neutral as you surmised.

The two breakers should be placed together on the same side of the panel; better yet, a 2-pole breaker, or two breakers joined with a handle tie.

dafish said:
  1. Side Story: My wife has always claimed her vacuum doesn't work right plugged into the affected outlets. Guess I know why now.
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Possible, but not if the circuits are wired correctly.

dafish said:
  • The possibility that somebody could get shocked. Turn off one of those breakers and it would seem safe, but not be. The neutral would still be "hot" from the other breaker!
Click to expand...
Thus the 2-pole breaker or handle-tied breakers.

dafish said:
Question #1: Am I correct in thinking this is unsafe and needs corrected?
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Not if it's as I described above.

dafish said:
Question #2: if I read code correctly, I can't run a single #14 THHN through an attic or basement ceiling to deliver the additional neutral unless I add conduit. If true, the simplest answer seems to run a new 14/2 to support one of those 15 breakers. (Thankfully, wiring is independent from there on). Correct? Any better ideas?
Click to expand...
Again, not necessary.

dafish said:
Question #3: House was built in 2004 if it matters, by a well-respected GC who's fairly principled. Wired by an electrical company that I thought was using certified electricians, but this wasn't an accident. It was wired this way intentionally. Any idea what they were thinking?
Click to expand...
Yes, and I use 3-wire circuits when possible, and the two breakers should already be tied together.
 
dafish said:
Ah so. So you're telling me because they ran on breakers one right below another they are pulling Leg A and B and thus the current, when on, cancels at the common neutral save for imbalance. Pretty cute.
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Again, this works exactly like the shared neutral in your home's service.

dafish said:
Would you share this with me? How is this not a situation where there is a shock risk still when one breaker only is off? Shouldn't those breakers have been connected so you had to turn them both off at once?
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Again, thus the tied handles, which are for manual operation.


Added: See: https://forums.mikeholt.com/threads/understanding-the-neutral-conductor.140537/

I wrote that in response to a question a while back.
 
Thank you all. Looks like Larry answered my open question. The breakers should have been tied together. Easy enough to fix.

Thanks to you both!

Dan
 
dafish said:
.....Question #3: House was built in 2004 if it matters, by a well-respected GC who's fairly principled. Wired by an electrical company that I thought was using certified electricians, but this wasn't an accident. It was wired this way intentionally. Any idea what they were thinking?

Thanks for sharing guys!

Dan
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They were thinking that it is more efficient to use three wires to get the same work done instead of four. Sounds like the work of a very competent electrician.
 
dafish said:
Thank you all. Looks like Larry answered my open question. The breakers should have been tied together. Easy enough to fix.
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It's more important that they're placed for a handle tie, or use a 2p breaker.

Bus stabs in breaker panels alternate A-B-A-B-etc so adjacent spaces will have alternating supplies.
 
First, certified electrician and a licensed contractor means they took the test and passed it. That is it. It doesn't mean anything about integrity, self respect and respect for our trade.

Second, I am fairly certain that in 2004 NEC required breaker tie handles on MWBC that ended up on the same device.
 
It was in 2008. 210.4(B). I hate it so much it's one of the few code sections I know by heart. From the days of Thomas Edison until 2008 we ran multiwire branch circuits with no requirement for handle ties or common trip breakers. Now you can't find a set of plans that allows shared neutrals. Most unfortunate. Wasteful in material, wasteful in efficiency and voltage drop and now there is a whole generation of electricians that have no practical experience working with MWBCs. Rant over.

Edit to add. A common trip was always required for MWBC conductors that landed on a common device also known as a common yolk. That requirement makes sense.
 
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Opps. Sorry. I didn't intend to kick off debate about efficient vs safe, nor split hairs about when it was or wasn't legal.

I can say that in my simple mind I see MWBC as an efficient choice when breakers are tied so as to prevent an accidental shock/electrocution should only one be off. Since mine aren't I'm going to fix it be it code required or not.

Again, my sincere thanks for the coaching!
 
dafish said:
Thank you all. Looks like Larry answered my open question. The breakers should have been tied together. Easy enough to fix.

Thanks to you both!

Dan
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The main reason that the NEC requires a singe two-pole breaker or handle ties is for situations where you need to work on the neutral wire. If you turn off only one breaker (say L1) and there are loads connected to the other side of the MWBC, then there can still be current on the neutral wire. The voltage will still be close to zero and it will not be dangerous to touch the neutral.
BUT if you disconnect the neutral as part of your work, the downstream side of the neutral will immediately be at L2 voltage. And if you touch both sides of the break simultaneously you will be shocked.
Some members of the Forum, experienced electricians, feel that they are capable of working on an MWBC in a way that will not interrupt the neutral, and turning off the other side of the MWBC is not necessary and is unnecessarily disruptive to the customer. The NEC does not agree.
 
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