multi-wire branch circuits / neutral Q

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malachi constant

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I am an engineering going through Owner's requirements while writing specs. This one doesn't sound reasonable to me: "The use of multi-wire branch circuits with a common neutral feeding loads is not permitted." This is a standard educational facility, nothing unique about it.

The way I read this, every circuit shall have its own neutral pulled. Typically the contractor would pull a neutral for every three circuits (in a 3ph system), correct? Am I reading this wrong?

Unless someone can explain to me why this is a reasonable installation I'm going to ask the Owner for a variance from their standards. Thanks.
 
Because it is an educational facility, it most likely has a lot of computers and other electronic equipment...which is suseptable to damage from MWBC type issues.
So, spend money up front and save potential issues for the future.
 
I am working on a project for which the owner has the same requirement. Their issue was one of safety. If you open only one breaker of the three that form a 3-phase MWBC, and work on a component powered by that breaker, you might get power back-fed to that point from one of the other two breakers. The 2008 NEC will require that every MWBC have breakers that open together (i.e., use a 3-phase breaker or use a handle tie that joins all three 1-phase breakers).

We did convince the owner to accept MWBCs, so long as we required (in our specs) the use of handle ties.
 
charlie b said:
Their issue was one of safety. If you open only one breaker of the three that form a 3-phase MWBC, and work on a component powered by that breaker, you might get power back-fed to that point from one of the other two breakers.
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Charlie, could you explain how this is possible. For the moment I disagree with your theory, unless you're referring to opening the shared neutral conductor.
 
infinity said:
Charlie, could you explain how this is possible. For the moment I disagree with your theory, unless you're referring to opening the shared neutral conductor.
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I was wondering if anyone was going to pick up on that. I regret not knowing how to draw a couple sketches and get them posted on this site. It may have saved a thousand words. But you?ll have to let me try with words alone. And I?ll have to ask you to remember that I am not an electrician, so I don?t know how wiring is actually installed inside outlet boxes. The following discussion (or something very near) is, I believe, the basis for the 2008 NEC revision requiring a common trip handle for MWBCs.

Yes, the issue has to do with the shared neutral. Consider a 3-phase MWBC, with 3 ?hots,? one neutral, and an EGC, supplying receptacle outlets in a large conference room. As you walk around the room, the receptacles are powered by, in order, circuit 1, circuit 3, circuit 5, circuit 1, circuit 3, circuit 5, circuit 1, circuit 3, circuit 5, etc. That is how I commonly lay out the design of receptacles.

I suspect that the way the wires run from outlet to outlet is that all five wires enter a box, and one of the hots and the neutral is then cut. You place the two ends of the cut hot wire, and a pigtail, under one wire cap, and run the pigtail to the receptacle. You do the same with the cut neutral wire. All five wires then run to the next outlet. You might be doing the cutting and pigtailing in a junction box in the ceiling, rather than in each outlet box, but the effect is the same.

You are given the job of replacing a receptacle. You discover it is powered by circuit 1, and you turn off circuit 1. You start to pull the receptacle out far enough to get to the wires behind it. The pigtail wires aren?t quite as long as you could wish (it?s hard to pull the receptacle out far enough), so you decide to replace them. You unscrew the pigtail wires from the old receptacle, and toss the receptacle in the trash. You can safely unscrew the wire cap for the hot conductor, since it gets its power from circuit 1. But as you unscrew the wire cap for the neutral, you have no way of knowing that it has current running through it (because of the loads on circuits 3 and 5). You think this wire is de-energized or is otherwise safe to touch. You unscrew the wire cap, and note that the wires are twisted together. But as you untwist them, you may very well have your hand on the neutral wire from a current-carrying circuit at the same time you have disconnected the neutral wire that runs from this point back to the source. You, then, become the wire that brings the current from the ?circuit 3 load? back to the source.

I am made to understand that this scenario has led to the electrocution of more than one electrician. The 2008 NEC will require that all three circuits be turned off by a single throw of a breaker, so that when you work on one circuit, there can be no current flowing via either of the other two.

Did that make sense?
 
charlie b said:
You think this wire is de-energized or is otherwise safe to touch. You unscrew the wire cap, and note that the wires are twisted together.
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Nine times out of ten, they're not twisted together, so simply removing the wire nut opens the neutral.
 
480sparky said:
Nine times out of ten, they're not twisted together, so simply removing the wire nut opens the neutral.
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And the tenth time?

Actually, even after the wires are no longer in contact, all it takes is for the worker to touch the wire that comes from one of the other two circuits. That would complete the circuit, with the worker being part of the current path.

I suspect that many electricians would try to twist the wires back together before installing the new wire cap. It is a real danger, though I admit I did not realize the fact until I had a client insist (as I mentioned in an earlier post) that no MWBCs be used at their facility. I had to draw a couple simple sketches, before I understood what it would take to shock the worker.
 
Mwbc

Mwbc

malachi, educational facilities in this area are known to use maintenance folks that are often not to knowledgable of electrical to perform electrical tasks. as charlie pointed out, not allowing MWBC could easily be viewed as a safety net.
 
Since you're speaking of opening a neutral I agree with you. But I have to say that any electrician that attempts to open a shared neutral of a MWBC is asking for trouble. Let's forget about the shock potential and just think about the unbalanced loads on the other two circuits. You could end up with well over high voltage on one circuit and a very low voltage on the other. This could destroy any equipment that is subject to the high voltage.
 
ONE loose wire nut can equal THOUSANDS of dollars in damage.

Been there, fixed that.

I simply removed a 4S cover plate one day, heard a slight arcing, and several of the machines nearby started to smoke. This particular building had previously suffered a $14K equipment loss due to a lost neutral on a MWBC and I happened to have one of the engineers right there with me so I was not held liable.
 
charlie b said:
As you walk around the room, the receptacles are powered by, in order, circuit 1, circuit 3, circuit 5, circuit 1, circuit 3, circuit 5, circuit 1, circuit 3, circuit 5, etc. That is how I commonly lay out the design of receptacles.
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You could spec the layout of a MWBC in a safer manner. "The first device of junction of all MWBC's, the 2 indiviual circuits shall segregate into 2 2-wire circuits, and never the twain shall meet."
 
whenever you deal with 3 phase service or "networks" as I've heard them called, it's usually a good idea to remove the panel cover and take a good look at the conduit your conductor is entering. Even with the new code requirement with the ties or 3 phase breakers it still scares me. I've seen too many 3 phase panels made up incorrectly. It can get confusing with red, black, blue everywhere especially when you have multiple networks in the same conduit. Just my two cents :grin:
 
I thought the problem was, computers produce harmonics. The harmonics can induce tons of extra current onto the neutral.
 
sparky_magoo said:
I thought the problem was, computers produce harmonics. The harmonics can induce tons of extra current onto the neutral.
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I THINK I have seen that firsthand also. I was installing new conduit/circuits to a panel in a 100,000 aq ft building full of computers. I went to terminate the ground wie and saw a little spark. I did a "what the..." and took it off/ touched it a few more times to verify what I was seeing. My CCC's were not even hooked up yet???

I guess that the neutral path was restricted and the power was trying very hard to get back to ground wherever it could. I investigated all the obvious things and couldn't find anything wrong, loose or disconnected.

I told them to call someone who went to college and do some load calcs.
 
i have a post in the works..if wire nuts are a Device see the first two words
of ART.100 Connector,Pressure(Solderless) 300-13,250-148 would only be violated by not twisting.Therefore would solve all our problems of continuity.
In 25 years i've never seen a Twisted wire fail.Other than an undersized conductor for the load.When my wife has time to type it up MAN,WILL IT BE A hot THREAD! LOL
 
Thanks everyone for your posts. Got pulled away from writing specs to work on another project and nearly forgot about this thread. All the responses were very helpful. I'll specify wiring to meet the Owner's requirements, no questions asked!
 
augie47 said:
malachi, educational facilities in this area are known to use maintenance folks that are often not to knowledgable of electrical to perform electrical tasks. as charlie pointed out, not allowing MWBC could easily be viewed as a safety net.
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I work in an educational facility and I hate MWBC they are always cannibalized because of the qualification of maintenance personnel at some places..So we spend money replacing MWBC on a regular basis..So I would vote no skip the up front cost savings unless you can ensure proper usage of the MWBC..
 
JohnConnolly said:
ONE loose wire nut can equal THOUSANDS of dollars in damage.

Been there, fixed that.

I simply removed a 4S cover plate one day, heard a slight arcing, and several of the machines nearby started to smoke. This particular building had previously suffered a $14K equipment loss due to a lost neutral on a MWBC and I happened to have one of the engineers right there with me so I was not held liable.
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That is not the 'fault' of MWBCs, that is a problem with the person who is making splices.

Make it all two wire circuits and they can still cause trouble when installed by the unqualified.

Keep in mind that same person also must have been splicing the EGCs.....
 
cschmid said:
I hate MWBC they are always cannibalized because of the qualification of maintenance personnel at some places..
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How about hiring some qualified people or sub the work out?

If they can not safely work on MWBC what else do they have trouble with?
 
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