I am starting this thread to expand on multiwire discussion which was brought up in another post.
I use multiwire circuits all the time and I think they are a Godsend in making our jobs easier and neater. I too mark each set of hots by taping them to their corresponding neutral in the panel.
Recently I have thought up the idea of having my printshop make me up a roll of adhesive labels saying something like: "Caution: This system utilizes shared neutral conductors. Re-arrangement of breakers by unqualified persons could result in a fire hazard"

Has anyone thought of or done this? Any opinions?

As far as Joe Homeowner screwing it up, I see far worse almost daily. Just today I found a bathroom receptacle wired with some single lengths of old #18 stranded, tapped from a 20 amp circuit. I wonder how hot they'd get from a 1500 watt heater?

Any opinions on the labels?

Considering the additional risks posed by multi-wire circuits, the construction of most romex used, and the likelihood of under-trained personnel working on residential circuits, I try to avoid them.

Even in other applications, I find myself getting away from them for power quality reasons.

I find that my love of saving wire is tempered by the effects of Murphy's law.

BigB: I think it is a great idea.

BigB, I also think they are a good idea when properly installed and marked.

Big b good idea. I use multi wire circuits all the time. As for some one screwing them up after I am gone, Stupid is as stupid does.

BigB...I always mark the neutral in the panel so other wiremen can identify it.We all know what happens when you loose the neutral on a multiwire circuit...right

shortcircuit

We multi wire anythig we can. I call it networking though. We never had a complaint or problem. We allways tape them together in the panel and some boxes depending on whats going on. Some panels we put the stick on numbers on the nutrals with the corisponding breaker. Other times we put the nutral near the corisponding breaker or in order. We put the networked breakers next to each other. On our homes the 1st network is always black, red, white. Then we save blue & gray for the other.

Ortganisation is the key. Some inspectors here require grooping the networks together in all boxes. I do hate going into a old panel with a pipe packed with wires going to breakers all over and nothing is marked.

I like the warning. I should put it on the panel schedules I print out.

I would add something to it like "The system utilizes shared nutral conductors TO PROVIDE BETTER ELECTRICAL EFFICIENTY." Something to make it sound more positive. Otherwise to a homeowner it might make them think you cut corners and something is not as safe as it could be.

Or maybe "Your electrical system has been ballanced to provide you with the best service. To keep the system ballanced only have a qualified person make chanegs inside service panel or branch wiering. Failure to follow this could result in overloading and a fire hazard."

Tom

I'm with John Steinke on this one, however I think the notice is a great idea.

"I run shared-neutral circuits to save on material costs, it's not a code violation, so I save where I can."

Sarcasm aside, what benefit is there in running shared-neutrals other than saving a little wire? You can sugar-coat it any way you want.

The signs are actually a very good idea. If it were possible to say something, ANYTHING, that would keep non-electricians out of distribution panels, I'm all for it. Anyone with a screwdriver can electrocute themselves, or create a fire hazard.

My personal feelings on the matter are that someone could have a fair understanding of home wiring and still not understand the importance of having breakers in the correct positions for shared-neutral circuits.

Dave

Dave I do not work in dwelling units so I do not know if there are benefits other than saving money.

However on large jobs there are real benefits.

First and foremost voltage drop is much lees of a problem.

Second raceway fill / derating is much less of a problem.

As an example a entire 42 circuit panel full of two wire circuits is 84 current carrying conductors. Make the same panel all multi wire branch circuits and you have 42 current carrying conductors.

by Dave55

I do not have to sugar coat it there are real benefits to the customer by utilizing multi wire branch circuits. There is nothing 'hack' about using multi wire branch circuits.

If you run all two wire circuits in a large commercial project all that you have gained is cost to the customer.


by John

Please explain the power quality issues brought on by multi wire branch circuits.

The panel feeding the branch circuits uses a common neutral, wouldn't this create the same power quality issues?

Bob

John, how are Multi-Wire branch circuits served from a single winding affected by harmonic distortion?

The harmonics would be the same on both sides of zero and would cancel the same as current cancels in any equal current flow.

Power quality would be a concern other than a shared netral when talking about single phase services.


Dave, there are many reasons to use Multi-Wire circuits and yet the biggest reason not to is the "unqualified person" making a mistake.

A qualified individual will look at the conduit or cable entry before lifting a neutral, and if there is more than one ungrounded conducted present not tied to a common trip breaker, they would know this is the case.

Roger

I have often thought that it would be a good idea if the manufacturers of panels were required to put labels on the cover of the electrical panel that said something like "CAUTION modificcation or maitenance of this system other than by a qualified electrician may result in electric shock or fire and could void your homeowners policy." In a lot of cases it wouldn't do any good but for some it might make them think twice.
As far as a shared neutral, as long as the circuits are identified in all locations accessable then it would be a okay but I think it can become a problem when several branch circuits are in the same junction box. In most installations it isn't a problem but I have seen it in larger custom homes.

I probably wouldn't object to multi-wire branch circuits if it were common to keep them together, and label, but all too often I'm doing a service upgrade and there is a conduit with five circuits and only three or four neutrals. I can either put the circuits on the same leg they came from in the new panel, or burn a few hours tracing wires.

In other cases it is a three wire romex or BX and it's obvious. I twist the wires together right up to the breakers where a non-professional will probably notice something is meant to be that way.

Dave

Considering that I don't do residential either, but a multiwire circuit would cause a harmonic issue in a residence with a 120/240 single phase service.
On the other hand, every project I work on in a commercial building (high tech type facilities), does not permit multiwire circuits due to harmonic loading.

Three phase circuits have problems with triplen harmonics (the third harmonic and its odd multiples) because they do not balance on the neutral; instead they add up. All other harmonics will balance on the neutral if they are from a balanced source (eg. the 5th harmonic from a three phase rectifier).

The neutral in a residential single phase multiwire circuit does not have a problem with triplen harmonics; they balance just like any odd order harmonic. However in this case the neutral will have problems with even order harmonics.

With rectifier power supplies, even order harmonics are much lower intensity than odd order harmonics. You only see even order harmonics from the hysteresis of the load.

-Jon

Ron I have always had a hard time understanding why moving the point the grounded conductor becomes also the common conductor would make a difference to power quality.

I understand the issue of overloading the common conductor by harmonic currents. That can be avoided by either not loading each leg of the circuit heavily or using a larger neutral conductor.

Assuming we have done one of the above, would there be any other power quality issues?

Bob

Bob,
Yes, if you reduce the load on the phase conductors, or enlarge the neutral it will suffice. It doesn't remove or mitigate the harmonics, it just gives the circuit a fighting chance from the heat.
Most folks do not do either, so that is why I generally don't permit it.
By the time you get upto the larger switchboards/switchgear, it is not usually needed to enlarge the neutral, as the other loads (of different types) on that larger piece of the distribution system will have "diluted" the harmonic load into a larger capacity equipment.

I too hate when the nutral is not marked with what hot it goes to. By adding a nutral for every circuit are you really making it simpler? It would seem to me that you are adding much more nutrals to get confused.

In the simplest form a residence if you had 2 hots and a nutral in a pipe there would be nothing to keep track of. Add another nutral and now you need to keep track of each one in each location. You want to worry about the homeowner? What's to say he will use the right one of those nutrals when he adds something. I don't think you can protect the HO from messing up the nutrals unless you keep it to only 2 wires per pipe.

It would be even worse of a mess when you have more than 2 circuits in a pipe. You can argue how many of what wire should go in a pipe. The reality is most HR in pipe have more than 4 circuits.

It is not just the cost of wire. Pipe costs a lot now. If you need to go to the next size up or more HR that costs $$. It seems to take a bit more labor for every size of pipe you go up. More labor in pulling, splicing, and keeping track of those extra nutrals. More wires you need to cram in a box or get bigger boxes.

I like what others have said about not tring to HO proof the instulation. If they wanted to do that they should sell tamper proof screws for any electrical cover or trim plate. Then only sell the removal tool to licenced EC.

Tom

I think that a label is an easy, inexpensive way to provide an extra margin of safety.

Last week I hooked some machinery up. I connected the transformers as 240V 3Ø corner grounded deltas, which to some could be quite confusing. (the vendor's technician had never heard of such a thing, although it was his company that requested a grounded supply system).

Labels to identify this cost about 40 cents apiece, and took me less than 5 minutes to make.
Well worth the effort, I say.

As to a homeowner making modifications to his system, if we were to label all the possible misteaks they could make, there would be no room left for anything but labels.
I love residential multiwire branch circuits. 2 wire branch circuits just waste wire and create more heat. As Roger said above, there isn't any issue with power quality (harmonics) on a 1Ø 3Wire system...S


[This message has been edited by electure (edited 10-04-2004).]

If your running multiwire circuits for everything. Then how are you wiring your ARC-FAULT BREAKERS in your bedrooms?

Doug, Arc Fault breakers are available for multiwire circuits.

Are we talking about non linear loads in the 3 phase multiwire? That isn't good for the nuetral, but a purely resistive load shouldn't have the harmonic problems. I don't see this being an issue on a single phase service,which is a multiwire circuit by its very nature. I use multiwire circuits in rewires. Pulling 3-wire into places too small for two 12-2's in quite a labor saver.

I wish I knew more about the harmonics on a 3 phase system. However, there shouldn't be any problem with installing three duplex receps on there own dedicated 20A ciruits (phases a, b, and c) using a single neutral is there? Oh, this is on a 208Y/120 panel.

I have noticed two distinctly different approaches to residential multiwires at the DB. In south Denver, CO aluminum breaker ties are used to couple both poles and the ties are commonly available at the orange box. In northeast Nevada, no ties are used. The use of a tie-bar certainly identifies the circuits as multiwire – what happens in your area?

I appreciate everyone's input.

Roger, Iwire, under 'power quality' I was including the voltage surges that can occur in a multi-wire circuit should a shared neutral connection be disturbed, corroded, etc. I agree that harmonics are not an issue here.

The use of a shared neutral may make it a lot more expensive to later GFI or AFCI protect one of the circuits; such a two-pole breaker costs a lot more than a pair of single-poles.

I have commented often on the need to ID neutrals- something that I have almost never found. I'm not even sure that the parts houses have ever heard of white wire with a colored tracer in it!

Ever notice that whenever you make something "idiot proof." they come out with a better idiot?