Home wiring: hot neutral & conjoined circuits

Welcome to Multiwire circuits. You are on a circuit that shares a neutral. Your life would be easier in the future if you bought a 2 pole breaker and put #1 and #2 on it or just put a handle tie between #1 and #2.

On Wed, 15 Sep 2004 20:58:45 -0700 Joe wrote: | I was replacing a receptacle on what I'll call circuit 1, which is a 20-amp | appliance circuit in the kitchen. The breaker was off, confirmed with outlet | tester. The wire nut came off of the neutral bundle, I went to put it back | on, touched two of the NEUTRAL (white) wires, and got a shock. I got my | multimeter and found 120 volts across two neutral wires. I found this VERY | strange. | | But the plot thickens: my wife then asked when the microwave would be back | on. She was right...the microwave was off, but the microwave is on a | different circuit (also a 20 amp appliance circuit), which I'l call circuit | 2. I tested an outlet on circuit 2, and the outlet tester indicated that hot | & ground were switched. | | So, I turned off circuit two, and the hot neutral in circuit one went away. | Turned both circuits back on, and everything works fine.

Turn circuit 2 back on, but unplug the microwave oven. See if the voltage is still present in the neutrals.

| This seems very strange to me...can anyone shed some light on this?

Sounds more like a case of neutral crossover. Someone could have wired the neutral of circuit 2 into circuit 1.

It is unlikely that ground was switched with hot (things would get really nasty if it were). The indication you are getting is probably because there is voltage between neutral and ground (you'd get this if ground was really hot), and no voltage between hot and neutral (you'd get this if hot was really ground. Hence the reading says hot and ground are switched. But that's not the only thing that can cause this. If hot is hot, and ground is ground, and neutral is hot (same phase as the other hot), you'd get the same reading. You may even get a false reading due to capacitive voltages when there is no load. If you were doing this reading with the neutral bundle all taken apart, I would not be surprised by it.

The big question is where does the wiring for circuit 2 go? If it passes through the box for the receptacle on circuit 1, that could have been why the neutrals were cross connected.

Are circuit 1 and circuit 2 on opposite phases? There would be 240 volts between hots of the 2 circuits, when live, if so. You can tell by checking their position in the breaker box. Phases alternate vertically, but are the same horizontally. If they are on opposite phase, and especially if they go through the same receptacle box, this could be a case of shared neutral. Such a circuit SHOULD be wired via a single 2-pole common trip breaker to avoid the shocking experience you had. But they are very often not wired that way.

So let us know more about the circuits you have.

Thangs Greg, a handle tie is now on my Home Depot list.

This answers a lot of questions for me, including why a 3-wire cable was coming into that box. However, I'm still having trouble figuring out why there was voltage across the two neutrals. Since the neutral is shared, there is only ONE neutral in this circuit--how can there be another neutral that is out of phase?

When you open up the neutral on a multiwire circuit things are upside down since you are now seeing the other phase reflected on the load.

Yes, apparently this is a shared neutral circuit. I was not aware of any such thing before I read Greg's post. A three-wire cable runs into the box, with red being tied to a 2-wire cable going back out of the box, so I presume that black is hot for my circuit (circuit 1) and red is hot for the microwave circuit (circuit 2), with a shared neutral.

They are on top of each other in breaker box, so out of phase as you suggested. Voltmeter confirms: 240 volts between hot 1 and hot 2.

Plann> | I was replacing a receptacle on what I'll call circuit 1, which is a > 20-amp

| This answers a lot of questions for me, including why a 3-wire cable was | coming into that box. However, I'm still having trouble figuring out why | there was voltage across the two neutrals. Since the neutral is shared, | there is only ONE neutral in this circuit--how can there be another neutral | that is out of phase?

When you opened the bundle, the neutral wire coming from the receptacle the microwave is plugged in to is no longer grounded. It is in fact connected to the hot wire through the microwave oven (or any other load on that circuit). Remove (unplug) all the loads, and then it is just an isolated piece of wire.

Any high impedance device (voltmeter, neon light tester, or your body) will get nearly a full 120 volts. A low impedance device (a light bulb with a high wattage) will get a lot fewer volts. Current is limited by the sum of all impedances, though that won't stop you from getting a shock since it doesn't take much current to do that (2 milliamps).

| Yes, apparently this is a shared neutral circuit. I was not aware of any | such thing before I read Greg's post. A three-wire cable runs into the box, | with red being tied to a 2-wire cable going back out of the box, so I | presume that black is hot for my circuit (circuit 1) and red is hot for the | microwave circuit (circuit 2), with a shared neutral. | | They are on top of each other in breaker box, so out of phase as you | suggested. Voltmeter confirms: 240 volts between hot 1 and hot 2. | | Planning on getting a handle tie for the breakers per Greg's recommendation.

Be sure your breakers are qualified (UL listing) for a handle tie. Not all are. Certain older breakers will not only not fully trip the other, but can also cause a mechanical jam that can prevent a trip even on a later fault current. If in doubt, a replacement

2-pole breaker (some come with integrated handle tie and some come with a single handle) is the safe way to go (leaving you with 2 spare 1-pole breakers for your next new circuit that will surely not be done as a shared neutral).

When our kitchen was overhauled I added two shared neutral circuits, 4 sets of outlets on 4 breakers, When you take the cover off of the outlets there is a label that says shared neutral cirxuit. Also labeled breaker box the same way. .

| When our kitchen was overhauled I added two shared neutral circuits, 4 sets | of outlets on 4 breakers, When you take the cover off of the outlets there | is a label that says shared neutral cirxuit. Also labeled breaker box the | same way. .

I would not do a shared neutral unless:

1. Every device that could interrupt the circuit (breaker, GFCI, etc) had a common switch/trip, factory made and UL listed that way.

1. In a common cable sheath or conduit.

2. Never split where one circuit goes to one box and a different circuit goes to another. This means each box must have all of the multiwire circuit so it is obvious.

1. Double neutral if three phase.

I would not recommend ever sharing a neutral even on single phase services. Handle ties for (2)120V circuits are not cool. Even on single phase services the phase 'legs' or hots alternate vertically in the panel. So any breakers that may be handle tied have 240V between them. If the neutral is lost between the panel and the shared point of connection you can easily have 240V across 120V equipment connected to these circuits. Plus since this is single phase you can easily overload the neutral ampacity wise. Three phase neutral sharing across individual circuits is even more hazardous since the the potential fault current is much higher than with single phase.

In short the proper fix for this is to run an additional neutral, which since you are dealing with cable means running a new cable for one of the circuits. I would not handle tie these breakers!

Matt

If there is 240 volts between the two breakers that supply a multiwire branch circuit then the current in the neutral will be the difference of the currents in the two legs rather than the sum. The use of a handle tie or a two pole breaker assures that when the circuit is opened manually in order to work on it that you will not end up opening a neutral that is carrying the current of the other leg of the circuit.

Multiwire branch circuits have been used safely and effectively for decades and there is nothing wrong with there continued use by qualified persons. If you don't understand multiwire branch circuits then call an electrician because you have no business doing any electric work.

-- Tom H

I use plain Jane Usenet to access the groups. This message was displayed in alt.engineering.electrical This is the only message that has been echoed to Usenet. Can someone tell me where the original thread originated from?

On Mon, 27 Sep 2004 17:53:08 GMT, Kilowatt put forth the notion that...

This was an old thread, and has probably been deleted from your server. You should be able to look up the original thread at g00gle.

On Mon, 27 Sep 2004 09:54:10 GMT, snipped-for-privacy@bellsouth.net put forth the notion that...

You'd be hard pressed to find an electrical contractor that doesn't use

3-wire feeds for 120 volt home runs. It costs too much to run separate neutrals, and it clutters up the panel and the space available getting into it. In 120/208 wye systems, contractors share one neutral with all three phases. If they're phased properly in the panel, there's no problem with doing it that way.

That explains it. Thanks

Shared neutrals have been quite common for some time, and as you point out usually aren't problematic when installed properly, and used with resistive loads.

However, where shared neutrals are used with harmonic rich loads (switching power supplies, electronic ballasts, some HID Lighting, etc.) problems with the neutral conductor can, and likely will occur. Triplen harmonics can be additive in the neutral with true RMS current levels significantly higher than any single phase conductor even where the current measured on each phase conductor appears to be well balanced, and the conductors are appropriately phased.

Louis

Isn't the triplen harmonic problem only applicable for 3-phase wye systems? iiuc These wouldn't add in a 120/240 centre tapped single phase system. Since the subject is Home wiring... I'm assuming this is what we have.

The subject was home wiring, but the post I responded to made reference to a

208Y120 3 Phase system as one where the neutral was commonly shared. A shared neutral in a 3 phase system regardless of the load type used to be the norm. Now that more has been learned in regards to additive harmonics on shared neutrals of such systems more attention has focused on wiring practices and components used, especially where a large portion of the known load is harmonic rich. A few examples of that are cabling systems with an oversized neutral available, and K rated transformers.

Louis--

********************************************* Remove the two fish in address to respond

I did not really state that correctly, you are right about neutral currents roughly, the current relationship you describe is accurate for straight up resistive loads only. Most loads on 120v branch circuits are more complex and can have harmonic issues. As far as being qualified, I am a licensed master electrician with I over 25 years experience, admittedly mostly industrial, not much residential. Many of the higher quality job specs I have worked with addressed this issue and required dedicated neutrals for all branch circuits where a neutral is required. In the last 15 years or so I have been more involved with service and less with installation and I have seen many examples of bad events from shared neutrals on 120v branch circuit, a few folks injured, and tens of thousands of dollars in equipment destroyed because of neutral failures on these circuits. I have not installed such a circuit in many years and never will again. If you insist on using these circuits at least install a single breaker that is designed to open two circuits rather than handle ties on two single circuit breakers, a weak or damaged spring in one and both may fail to open.

Matt