ECN Forum Forums Code Related Discussion NEC & other Code issues Multi wire circuits

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-