Are there any good rules of thumb to go by when using multi wire circuits. For example residential 20 amp kitchen circuits, micro, fridge, dw, things like that. Neutral oversizing? Just any general knowledge would be informative. Thanks, Scott

It is best to avoid multiwire circuits where electronics are involved. The reason for this is the loose neutral, which is not usually not deadly to most old fashioned appliances if caught in a reasonable time, but can be an instant killer of electronic devices.

Of course, many of the old fashioned bullet resistant appliances are now equipped with electronics.

Neutral oversizing is not usually a concern in residential work or on single phase systems.

When running multiwire circuits, good connections are critical when splicing the neutral.

Tom

Tom,
There's not a darn thing wrong with multiwire circuits/ electronics if done properly. It's only lamebrain electricians that can cause a problem with one.
Maybe I'm missing something, but every piece of office furniture I've run into comes with a common neutral. You might as well trash all the offices that I've seen (which are probably fairly representative).

Multi-wire circuits can sometimes be tricky though. While re-wiring parts of an old house one time, the first thing I did was re-vamp the service and one of the circuits was a multi-wire circuit so I followed suit and continued it as it had been in the past, or so I thought. In the process of redoing some of the older circuits I was to fix all the 3-ways in the two upstairs landings. the first to second floor went well and started on the second floor to the attic. there was no bulb at the top of the stairs and the only one available was a small 40 watt so I put that into the outlet and proceeded to get the circuit working. And by the way the whole house was wired K & T and was not asked to replace it, and the 3-ways were the carter system this is important, after getting the light on I checked the switches from both positions on-off from the bottom and switched the top the light went on and when I tried to shut it off that little 40 watt bulb got bright like it was a 200 watt bulb. After pondering the problem I realized that that 3-wire circuit was never an original design in the original wireing scheme, by following what I had when I revamped the service I now had 240 volts on one of my lighting circuits. needless to say the multi-wire circuit had to come out.
Later I found out the house was originally wired 120 volts and somebody tried for a short-cut which ultimately didnt work.
So my thought on multi-wires is caution and know what you have ahead of time.
-Mark

The only consideration I have in using multi-wire circuits in a RESIDENTIAL kitchen installation is to not split the circuit in a 2gang receptacle outlet box. Something about the combination of 240volts, water, and the hapless homeowner doesn't sit well with me.
If you need to do it for economic reasons, you could run your feed to a j-box in the basement and split it there. It seems less likely to be tampered with there.

Electure,

I'm in agreement that there is nothing wrong with a multi-wire circuit, I use them all the time. You do have to pay attention to the workmanship, especially due to how easy the smoke can be let out of an electronic device.

However, if it won't break the piggy bank, don't share the neutral on certain cirucits. This adds an extra measure of safety, just the same as pulling in an equipment ground in a metal raceway, which many of us do, even though we never forget to tighten a fitting.

Ever since Hixson was called in as an expert witness in a lawsuit to explain the happenings of a failed "noodle" on a multiwire to the court, I've avoided them in resi work, I've used them in commercial though...

Redsy:
Could you elaborate a little, you lost me between Hixson and court.
Thanks
-Mark-

Motor-T, you mentioned: "the 3-ways were the
carter system..." What is the carter system?

CRW:
That was an early 3-way wiring system, where The common terminal of the two three-ways were fed to the light and the 120 volts was fed to the two traveler terminals, when exactly it was made illegal by the code I am not sure But I have an old wiring book dated 1936 and makes reference to this circuit as not being allowed anymore, primarily because it switches the neutral, but what I dont like about it is the fact that it used to be used for garage power for a light at the side door and at the garage and of course they used Keyless fixtures and in one switching sequence(of the four possible) the light are off but both the pin and the shell are hot, it was convenient but very lethal on a damp day.
-Mark-

Hixson was the fellow who trained me. He is a legend around here for many reasons other than being the best d@mn electrician in Greenbrier County. Caving was his true love, and took up the better part of his time. He is now aging, in bad health and lives with me now. My wife takes care of him, but it's the least we can do for the man.

I apologize for mentioning him as if eveyone here knows him, I'll try to be more contientious.

Someone was apparently in court with the PoCo because of burnt-out appliances and Hixson was called in to testify to the court on how this happens.

I figured I could avoid liability by avoiding the use of multiwires in residential work.

Sparky66
Thanks for that clarification, I even had the wrong name listed in my question.
-Mark-

I heard they may make you put multi wire circuits on a 2-pole breaker. So people don't get a false sense of security, when they flip 1 breaker off, and then get zapped by the neutral from the other device. It would also make sure the 2 circuits are on opp. phases.

I use em, because my company does for Disp/DW. But don't like em.

Nothing wrong with multiwires when installed correctly. Although I always use a double pole breaker with them even thought the Code doesnt require them. Maybe it should....

This is a very old argument and we won't settle it here. But that won't keep me from offering an opinion. I do not believe we can or should be building electrical systems to be worked on by unqualified persons. Failing to take advantage of the labor savings available in using multi wire branch circuits is either going to cost you work or income or both. People who will not take the time to understand multi wire branch circuits should keep their fingers out of electrical work.
--
Tom

In commercial It may be a different story.
But in residential a bag of 14-2 cost $15, and 14-3 cost $30. So not much money saved there except time. And that money is ussually burned up when you have a rookie sit there and stare at if for 30 minutes on the finish.

Andrust:
I can argue that point, if you got two circuits to run 60 feet and 25 joist to drill through and can run a multi-wire instead sure sounds a whole lot quicker to run one multi-wire then having to run two single circuits.
I use them all the time its just when the DIYer gets hold of them that there seems to be problems with them, ie back fed breakers, because they added a circuit and didnt understand why a red wire was there instead of two blacks.
Its really a nice way to balance loads when done right.
-Mark-

For safety one should use a double pole breaker, but then ...

when one circuit trips the other one does also.

I believe a double pole {or handle tie} is code for a residential multiwire.
It won't matter here, as the AFCI's are all 1-pole, so multi-runs are out for now.

sparky,
I think it's (still) only required to be double-pole if you are supplying 2 devices on the same yoke with a multi-wire.

sparky-

If my memory serves me correctly from the discusion about afci's, at least one company (i forgot which one) is making an afci for multiwire circuits.

Rojosy is right.
The 2P is required only on a single yoke in a residential multiwire outlet.

Why is everybody so afraid of the neutral (noodle ) connection? You make it hard just like the rest, and forget it...or fix it!
Anybody got a burnt up residential neut to report? (from a Code installed system)

Electure, I've only seen pre-main CB noodles go south, aluminum with no antioxidant is almost always the culprit. Good point.

CTwireman, Cutler Hammer is the only manufacturer of 2P AFCI CBs that I know of...

hi,
my question is WHY would you want to use a multiwire circuit in the first place? except for a 240V outlet. first, GFCI will not work on a multiwire circuit.

there has been a time or two that i would like to use a 3/C #12 to deliver two circuits to one area but if you every need a gfi down there you will need a seperate neutral.

i just think that its easy and safer to use a single circuit.

-regards

frodo

Frodo,

Once the 3 conductor cable is divided into 2 circuits there is no problem using GFCI Receptacles on them.

Sparky,

I just saw a CH rep at a local trade show and was assured several times that 2 pole (independant trip) AFCI Breakers are available. She gave me a cd with product information on it. I wasn't able to locate anything at the website, but here is some info from the CD;





Bill

[This message has been edited by Bill Addiss (edited 04-13-2002).]

Frodo,

For me, reducing voltage drop on long runs is a compelling reason to use a multiwire homerun.

Al

I don't have strong feelings about multiwire circuits. Except when they are not obvious. My nephew is fond of wiring power to switches and then to devices. Because of this he happened to run the lights and fan of a ceiling fan on 2 different circuits. A strange way to share a neutral. Despite the fact that the devices were not on the same "strap" I put a 2 pole breaker on the 2 circuits.

Voltage drop I do have feelings about.

I run a 50amp "multiwire" circuit to a subpanel and then put 2 wire circuits in. This keeps the voltage drop low. And keeps the wiring simple. (I figure a 50amp panel with 8-12 circuits will serve 625-750 sq ft.)

Redsy;
my bad, a local inspector wishes to update by insisting on 2-Pole's for all multiwires.

Bill,
interesting data, i was aware of the AFIT, but not the AFGF
can I assume the dif is the 4-6ma limit?

[This message has been edited by sparky (edited 04-14-2002).]

The problem with multi-pole CBs is that if lights are fed from the (MW) ckt. and a problem occurs on one leg, you're in the dark everywhere. Obviously this is more common in commercial & industrial facilities, but it's probably the main reason for the absence of a multi-pole CB requirement.

hi,
anytime i have ever tried to use a GFI that shared a neutral with another circuit the GFI trips.

how do you wire a 3 wire circuit with gfi's so that it will not trip? it works fine until you turn on a device on the OPPOSITE circuit. it may work if everything slaves of the gfi..?

as far as voltage drop and multiwire circuits goes, it is easier to use a number 10 than a number 12 than to install a multiwire circuit to account for voltage drop.

less time, less material, less confusion.

-regards

frodo

frodo,

Once a 3 wire Multiwire circuit is divided up into individual circuits there is no problem using a GFCI Receptacle on 1, or both of them.

If you think about it, the service itself coming into the house is a multiwire circuit. There is no problem with GFCIs on individual circuits coming off that.

Bill

We install multiwire circuits in residential projects all the time. How can you say the running two 10-2's is less costly and less time consuming than running one 12-3? Its just the opposite. The material cost for the two 10-2's would cost about 3 times as much as the one 12-3 and the labor would be at least double. As Bill Addiss just said there is no problem installing GFI's on these circuits once there converted back to two 2-wire circuits. For the kitchen counter receptacles run a 12-3 to the first receptacle then from there run two 12-2's from this box to the other receptacles. Connect the black of one of the 12-2's to the red from the 12-3 and connect the white of this 12-2 to the white of the 12-3 with a pigtail. Install a GFI in this box using the black from the 12-3 and the white pigtail for the line and the white and black from the other 12-2 as the load. A second GFI will then be installed in the first box that the 12-2 connected to the red is run to.

Frodo,

Multiwire circuits are used every day. I have never seen a problem with multiwire circuits when installed properly. Of course, you must watch your Box fill, maintain proper grounding and most of all remember that the pigtail on the Grounded conductor is necessary because of 300.13(B)

I would not recommend using #10 wire on standard grade devices.

Bill

[This message has been edited by Bill Addiss (edited 04-15-2002).]

frodo-
I'm not trying to sound like a wiseguy, but I suggest you read the instructions that come with a GFI receptacle. They are very detailed and describe how to avoid the problem you had with using them on a multiwire circuit.