In the “How dangerous is your home electrical system?” thread, Rick related a story of how, even after cutting the power to the circuit he was working on, he got a nasty shock from the neutral wire shared by other circuits in the house. I was wondering if it was still allowed practice (i.e code in the US) to not disconnect the power to a shared neutral, and a little searching produced several questions.
First, a few minor ones :
Some of what I found implied that it is allowed in the code, though the usual case (or recommended practice?) is that if you have a shared neutral on some lines, you use a multiple-pole breaker so that killing power to one kills power to the others, and you don’t end up with a live neutral. Can someone familiar with the NEC verify if this is true?
When someone mentioned that they wanted to install a shared neutral connection, someone else said that if it was new installation into bedrooms, an AFCI would be needed, which ruled out the shared neutral. What I didn’t get was whether it was simply ruled out by the code, or if the AFCI wouldn’t work if the neutral was shared (maybe due to the way it’s installed?).
Then I realized, first, I didn’t know what the A stood for. After finding out that it’s an Arc Fault Circuit Interruptor, my question became,
How exactly do AFCIs work?
I mean the theory, or what components are used. For instance, how does it discriminate normal load current from arc current? So far all I’ve found is a mention about ‘chaotic spectral density’ and I’ll probably keep looking but an explanation from someone here would likely be better than anything I’ll find.
Well first of all, I do not understand how anyone can get shocked on a properly-installed and properly-functioning neutral line, shared or unshared. Perhaps someone can enlighten me on this. I mean, there might be a little bit of a voltage drop on a neutral line that’s carrying a lot of current, but enough to zap you?! Hmmm. I don’t think so. Of course, if it’s not properly installed, e.g. someone decided to switch a light on the neutral (instead of the hot), then a person could indeed get zapped by touching the neutral line (between the light & switch) when the switch is off.
I haven’t found much new, although I did get an answer (sort of) to my second question. It seems that the AFCI is required to be installed parallel, and to disable the entire circuit if it trips, so the shared neutral is fairly useless. Somewhere I read that they’re also going to make 4-strand + ground romex to keep it all in one cable.
In the future series AFCIs will be available to install at an outlet – they would trip only if the load itself is arcing. The sensitivity is also apparently set high enough so things like power drills and other normally arcing things wouldn’t trip it.
Ding ding ding we have a winner!
After posting that I went back and pulled the plate off the outlet to take a peek and refresh my memory. Guess what? 2 white wires one to the box and the other from another circuit (probably the light fixture in the room).
On this thread (the last one we did)here I explained a little on page two how a shared nuetral worked.
With standard house wiring, you have a single phase service with two hot legs and a nuetral (120/240 volts). Here you have 120 between any of the two hot legs and the ground or the nuetral (they are the same electrically). You will have 240 between the two legs. Thats because the two legs are 180 degrees out of phase with each other. So, you may network two circuits together in the home. This means you can use the same nuetral for both circuits. ONLY if they are on seperate legs. You can not network two circuits on the A leg or two on the B leg. You must use one from A and one from B. What you get then is two circuits 180 degrees out of phase with each other using the same nuetral. We can do this because the return voltage on the nuetral does not add up when the two voltages are out of phase with each other. If you have five amps on the A leg and three amps on the B leg, the most youll have on the nuetral is 2 amps. Correct me if I`m wrong, and I will let CRAFTER MAN or D. SIMMONS expand on this (them being engineers, I believe).
The tricky part is this, if you were to open up that shared nuetral you would see a combined 8 amps trying to get to ground. That is why it is dangerous (again, correct me here) and probably why RICK got nailed by the nuetral in the other thread.
Nuetrals can be dangerous and you must be aware of this when you take apart a splice containing the nuetral. You may have some activity there that is looking for the ground potential (nuetral).
The NEC does not require shared (networked) nuetral circuits to be on a common breaker. There is an exception, and I think it is in gas station pump installations, and some wet installations.
AFCI???
I think you may be refering to GFCI (Ground Fault Circuit Interruptor). You should install them without sharing the nuetral on the load side of the GFCI. I dont think the code says this, its more common sense, if anything. You can share the nuetral on the line side of a GFCI. Happens all the time in homes. Two circuits are needed in the Kitchen, so you pull 12/3 (two legs one nuetral) romex to the first box where there may be a GFCI and you can branch off to the other box with the other leg and share the nuetral back to the panel. Care must be taken to keep the load and line sides of the GFCI seperate.
A GFCI works simply by sensing any current that “leaks” from the hot leg to the nuetral or the ground. It trips like a circuit breaker but it is way more sensitive.
These are required in numerous places in the home. But not bedrooms. Off the top of the head, - six feet from any sink, the bathroom, the panel outlet, the garage and outdoor outlets.
You may not put one on the refridgerator ciruit, the sump pump circuit, the furnace or any house citical circuit that must be maintained.
whuckfistle: The neutral will carry the current difference when two circuits from opposing legs (A and B) are sharing the same neutral. The current will add when two circuits from the same legs (A and A, or B and B) are sharing the same neutral. But I wasn’t aware the latter was illegal.
A few things I’d like to note:
In a properly wired and properly installed system (with no breaks in any shared neutral lines), the potential difference between neutral and earth ground is always around 0 V regardless anything else. It does not matter if neutrals are shared or not; it does not matter if they’re on the same leg or not; it does not matter how much current is on the lines; it does not matter if the circuits are live or dead. By definition, the potential difference between neutral and earth ground is always around 0 V. It is therefore just about impossible to get shocked by a neutral line. This assumes, of course, that there are no breaks in the shared neutral lines.
Let’s say you want to replace an outlet. You turn off the breaker and verify the outlet is dead using a meter. You remove the outlet. While doing so, you notice there are three white wires tied together with a wire nut, one of which goes to the outlet you’re replacing. You should be careful at this point, since this is probably an indication of shared neutrals. When you remove the outlet there will be two white wires going into the wall. One goes back to the breaker box; this line is safe. The other white wire goes to a load somewhere in the house (e.g. another light, outlet). If that load is on a separate breaker, and if the load is turned “on,” there will be 120 VAC on this wire! Furthermore, it does not matter if the load is on the same leg or not – it will still zap you.
If zapped by a shared neutral (as described in #2), the situation can be modeled as a voltage divider, with the load (e.g. light, outlet) and your body diving the 120 VAC. But because the resistance of your body is probably much higher than the load, you will likely receive almost 100% of the voltage.
Thanks for helping clear that up. What you said is what I meant, I worded it a little differently though.
"We can do this because the return voltage on the nuetral does not add up when the two voltages are out of phase with each other. "
My above quote should have said “return current”, not return voltage. In my haste to think faster than I type, I erred. Sorry for the confusion.
You can`t get shocked by the nuetral unless you “open” up the nuetral on a hot circuit. If you unsplice the nuetral without turning off the circuits using it you are in effect removing your ground potential. The electrons will look for an alternate ground path and in this case it may be your body.
Also, in relation to two circuits on the same leg (A+A) sharing the nuetral. Since the current adds when two circuits on the same leg share the same nuetral, you would have to oversize the nuetral to do this. Say you have 2 - 20 amp circuits sharing a nuetral. They are running at 16 amps each. You would have 32 amps on the nuetral. Now you would need to use 8 guage wire instead of 12 guage. (NEC chart 310-16, sect. 240-3(d)). Most devices won`t accept an 8 guage wire under the terminal screw (too big) and too expensive.
You are correct. One side of the load is connected to hot. After you cut the neutral, the other side of the load is floating. Even though there is no current, there is 120 VAC on this white wire. If you are grounded, and if you touch this wire, you will complete the path. As mentioned in an earlier post, you have created a voltage divider.
You are correct in that you must size the neutral wire accordingly when sharing neutrals, and it is especially important when the circuits are in phase.
No, it’s an AFCI; I had the same triple-question mark reaction myself, which is why I started this thread. A is for arcing, and they’re required for new installations in bedrooms. Here’s a brief description in pdf from the cpsc : http://www.cpsc.gov/cpscpub/pubs/afcifac8.pdf
Thanks for the help on shared neutrals & what is and isn’t allowed.
I’m just curious what the ‘unique’ current sensing for arcing is.
The NEC - article 210-12 (arc fault circuit interrupter).
Yes, January 1, 2002 was the start date for this rule.
“All 125 volt, single phase, 15 and 20 ampere receptacle outlets installed in dwelling unit bedrooms shall be protected by an arc-fault circuit interrupter.” Quote from NEC
I can`t add more info than the PDF offered.
Thanks for the heads-up, PANAMAJACK.
Note: not all jurisdictions adopt the code in its entirety. Some state and local codes may choose not to enforce this rule.
But it does sound like a neat device, one that could help prevent injury.
The Square D Arc-Fault Circuit Interrupter features a microprocessor for arc detection. It will detect and trip three types of arc faults:
· Series arcs (single conductor)
· Line-to-neutral arcs (short circuit)
· Line to ground arcs (grounded conductor)
According to the article, electrical arcing is responsible for a significant number of home fires. An arcing condition may not necessarily constitute an over-current condition, which means there’s a good chance a traditional circuit breaker will not open the circuit when arcing occurs. So manufacturers have come up with a circuit breaker that will break the circuit when it senses an arcing situation, in addition to an over-current situation. How does it sense an arc? I’m not sure, but I suspect engineers have defined the unique current and/or phase profile that occurs when an arc happens. The circuit breaker will break the circuit when it recognizes the profile. I’ll do some web searches to see if I can come up with technical info on this.
As far as the NEC is concerned, the article says the 1999 edition requires “AFCIs for receptacle outlets in bedrooms, effective 1-Jan-02… the requirement is limited to only certain circuits in new residential construction…”
‘Square D’ from whuckfistle was a good lead – I found this page of technical info on their website. They even have something on shared neutrals!
While reading some other stuff on it I realized the place I should be searching - the patent office. I found at least two (searching on ‘arc fault’ ) but don’t have the time to look them over right now.
It looks like it actually is doing some sort of signal analysis, given the uprocessor. And there seem to be patented algorithms, which is pretty fascinating. I was kind of wondering if it might be something simpler, the way a GFCI works.
Microprocessor? Hmmm. I was afraid of that. While the science may be sound, I’m skeptical about the long term reliability of incorporating a microprocessor into a circuit breaker. Is it fail-safe? Will it withstand voltage spikes over the long term? Let’s hope…