I won't use them. Seen far too many bad events from such. I'm not condemning those who use them properly, but I will never recommend them. I understand that residential/light commercial construction is a very tight margin industry, I have been fortunate that most my time has been with higher end industrial work. I have seen many spec sheets that addressed this issue and required dedicated neutrals for general use branch circuits. Also harmonics and noise concerns are are growing issue due to changing load types. Another thing, I don't care for handle tied breakers on two single circuits, handle tied breakers are less reliable than one designed to open two circuits. As for space in panelboards, again I guess I am used to larger boxes, so thats not an issue from my view. I think you will see this issue readdressed by the IEEE and NFPA soon due to issues still evolving on fire prevention, consumer/worker safety, harmonics, line noise and limiting fault currents.
when a switch is wired...the black and white are run to the switch... naturally... the switch is in the hot line... black going to one side and white to the other... turn the switch on and the white wire coming from it will be at 120vac..and tie a bundle of such wire, all the lights or whatever fed by the switch... white wire you see.
Unless you tie white originally to a hot line, so that black runs out hot... its a problem you see of limited colors when you wire with two conductor romex and a ground.
if you dont want to have that problem...and I dont blame you...use three conductor romex.. then you can run a black into the switch and a red out... etc. and your white can remain thought the project as a neutral as God and the NEC intended.
would be back
is on a
I'l call circuit
indicated that hot
a home owner wiring job...such a mess can get dangerous.
to be safe, have her write he will so if she is electocuted you will not have all of your joint assets going through probate... :)
It would be good to call an electrician to estimate the scope of the errors..they could be minor or major. You need to know which.
Thats a fire waiting to happen since there is no breaker in the neutral...if you are feeding one neutral from two 20 amp breakers, the neutral could get a 40 amp load...if its size 12 as most are, or even worse you can very very easily burn yer house down (thats bad unless you have life insurance on your wife).
If you ran a neutral capable of carrying 40 amps (say #8 awg) then it would at least be safe.
Phil Scott Mech/ electrical contractor since 1830... (i'm OLD)
On Wed, 06 Oct 2004 04:02:28 GMT, Phil Scott put forth the notion that...
There is no such requirement for a shared neutral to be sized to carry the current from both circuits. The only requirement is that the two circuits come from opposite hots in the panel (in other words, 240 volts hot-to-hot), and that all taps from the neutral in boxes before the two circuits split off be made with a jumper on the neutral rather than passing the common neutral directly through the device. If the two circuits are balanced, the neutral will carry no current, and if they aren't balanced, the neutral will only carry the difference between the higher current circuit and the lower current circuit.
Although the description of the problem doesn't give 100% information, it seems as if the two cct's are wired using a 3-wire cable - shared neutral. If wired correctly, that neutral will never see 40A if fed from two out-of-phase 20A cct's. I haven't seen a cable with #12 hot, and #8 neutral, not yet anyway... :o)
Dumb wiring question: If you bring two separate circuits into the same box, are there NEC rules against tying the neutrals together? An electrician friend helped me do the wiring in my house's addition, and when adding switches to a box with two circuits feeding it, I noticed he connected all the neutrals in the box together, accidentally creating a shared neutral circuit of sorts. Except I don't know if the breakers feeding them are on different phases or not.
Each circuit is its own run of 12/2+G romex with the neutrals all properly connected at the panel, but they are also connected together in the switch box. This sounds vaguely wrong but I can't really think of a reason why.
In a second box fed with two circuits he kept the neutrals separate.
You are a very smart young man...I had not considered that at all, I had assumed that both lines feeding the same neutral would be on the same phase...then you would need a double capacity neutral... I had not considered the phase neutralization effects in the common at all.
What happens if a home owner then later moves one of the breakers so that both line feeds are served by the same neutral? In that case you could have a fire.... a situation that could sneak up on anyone but a fully qualified electrician that looked the entire system over before making a change... a common neutral wired somewhere in the attic could be easily missed.
I suppose all appliance and lights could turned on the neutral currents checked.
all Issues I had not considered.
what else can you say on this topic...risks ...hidden dangers etc.
I have always run a separate neutral with every circuit... so that for instance for a 40 breaker box, I'd have 40 neutrals going to the neutral buss in the breaker panel...
Why would anyone ever run a common neutral anyway? Maybe so they can run a 3 wire with a ground out somewhere and have 2
20 amp breakers on one run of #12 romex?
I guess that would be safe enough...it would be an obvious situation to someone seeing the 3 wire into the panel... they would know its a comon neutral. Ive never seen it though.
it was a shared neutral but with separate wire for each back to the panel so you would have the ampacity even if both hots came off of the same line.
it would be OK because while the neutrals were tied together you still had double neutral wire going back to the panel...it was not a shared neutral going back to the panel on one wire fed by two hots from the same phase.
neutrals all properly
in the switch
a reason why.
Its not wrong...see what checkmate has to say though.
You are correct...I had just never considered that option of the lines being on different phases, feeding one neutral and those net reductions in current in the neutral.... something new for me.
wiring that with both hots from one line and you could cook the neutral... and someone what didnt understand the issues could burn the house down.
I am surprised the NEC allows it... workable but leaves an opening for fatal error since there is no breaker in the neutral... a wire basically not protected against over load.
Id like to see more discussion on this...i was entirely unaware of this situation.
| What happens if a home owner then later moves one of the | breakers so that both line feeds are served by the same | neutral? In that case you could have a fire.... a situation | that could sneak up on anyone but a fully qualified | electrician that looked the entire system over before making a | change... a common neutral wired somewhere in the attic could | be easily missed.
Our house--wired by a licensed electrician--included a number of shared neutral circuits with their hots connected to the same leg. Even as a mere home owner I was disturbed by this, so I fixed it...
[...] | I guess that would be safe enough...it would be an obvious | situation to someone seeing the 3 wire into the panel... they | would know its a comon neutral. Ive never seen it though. | | is it common?
Shared neutral circuits are quite common. Incorrectly wired shared neutral circuits are, unfortunately, not entirely uncommon...
When you have a shared neutral you run 3 conductor (plus ground) cable. The "extra" conductor is RED.
Even to a near clueless homeowner, the RED color is a CLUE!
Note that most 240 volt loads just use "normal" 2 conductor cable with the white being used for one of the "hots." This is so routine that often even electricians don't bother to mark the white wire inside the service panel: if a white wire is connected to a 2 pole breaker, it's HOT. Of course with GFCI breakers becoming more common ...
Clearly, it's possible to crease all kinds of dangers by switching about the wires in the service panel. I suppose the possibility of someone screwing up a shared neutral circuit is one of the dangers but I would not lose sleep over it.
Multiwire circuits have a lot of advantages. It is a way to get the utility of
2 circuits in a remote building without all of the requirements of disconnects and ground rods you have with a feeder. Multiwire circuits help with box fill, conduit fill (25% advantage) and derating (50%). If you are running lots of circuits it adds up fast.
The voltage drop is divided between 2 circuits, effectively cutting it in half if they are balanced loads. (the flip side of the derating issue)
Canada virtually requires it in their kitchen circuits.
The service conductor set to your house is a multiwire circuit.
It's pretty common to use shared neutrals, especially when used with three phase wiring. As for the dangers, it's not really "dangerous" when done properly by a qualified person. The only problem is when people who don't really know what they're doing, start "doing" things. Same danger as somebody who doesn't know how to change their brakes on their car, decides to do so...
Thanks for the info... I personally think its too risky for most applications...it shocks me at the NEC permits the practice...because you can end up with an overloaded neutral by various hard to notice errors.
I am really surprised to see a licensed electrician using common neurtrals with multiple single line source feeds.... I have done it for years on *control circuits though... 0.1 amp type circuits with #14 wire etc ...but not on power circuits.. in the controls business shared feeds and neutrals are pervasively common.
I hadnt considered any of this before... except of course using white commonly for a line on two conductor 240v circuits. I remain stunned that the NEC would permit even the possiblity of an unprotected conductor as with the potentially over loaded neutral. I suppose though some idiot could replace a 15 amp breaker with a 30 amp breaker easily enough on a 14 gage circuit and get the same results.
Three phase is at least a natural balance when single lines are run into the primary neutral..its sized to the feed lines. With these smaller shared neutral circuits there is much more risk as I think you agree. And I do understand your point. The NEC also obviously agrees.
its just that as I age my mind boggles more easily apparently.... I left an engineering project a few years age, a major industrial facilities engineering firm, main office in Detroit, branch in Phoenix...was designing a railroad locomotive repair 'round house' I had specified a 3" main compressed air line with 2" branches to each platform to feed
1" 125 psig compressed air to the huge impact air tools needed to remove the head bolts on these massive 10' tall 16 cyl diesel engines (driving generators that served electric tractor motors between the wheels).
These morons decided in unison that no air lines would be needed, and pulled the entire compressed air system, compressor and all from the mechanical set... since Black and Decker had just come out with thier NEW 18volt HIGH TORGUE impact screw driver ($99)... "and such pneumatic impacts were no longer needed"...
Finally I regained my ability to speak...by then however a team of 4 engineers were already dispatched to the City of Commerce calif to look at a locomotive repair facility and determine if they actually were using air impacts and why not the new 18v Black and Decker 'heavy impact' plastic screw drivers.... they seethed in unison on thier return .... and refused to discuss the issue.
None the less I understand the neutral issue and its in line I suppose with other risks, except that my mind has already shorted out ... and I am now beedy eyed and suspicious which idoocy, hose job, or cluster f*ck I will see from govt or industry next.
then our Commerce secretary, some twit broad named Chang... says not to worry about all those 150k offshored engineering jobs... and she says, and eagle beak, thas our pal GWB says that we will keep offshoring work because we are getting an equal number of jobs back in return....... in sales, dishwashing, lawn mowing...and toe nail clipping.
One staggers and weaves..and blinks but the tee vee keeps fluttering...
On Tue, 12 Oct 2004 06:25:38 GMT, Phil Scott put forth the notion that...
3-wire circuits are very common, and 4-wire circuits for three phase systems are also used extensively. I have seen cases where both circuits have been plugged into the same side of the line where the neutral could be overloaded. Another interesting situation comes up with Zinsco breakers, because they have two slots, enabling them to contact either side of the line, depending on which way you orient a removable bus clip on the breaker (the clips on the thin style breakers can't be switched, but the full size ones usually can). A homeowner could easily replace a breaker in the same panel location, and pick up the wrong bus bar if the clip in the breaker was orientated the wrong way. The advantages of using common neutrals are lower material and labor costs, as well as saving space in the can or the space available getting to the can. I've often wondered why the code allows it, given the possibility that someone will inadvertently overload the neutral after the initial installation, but apparently the code Gods haven't gotten around to bitching about that yet.
The NEC is written, based on the idea that work would be done by qualified people. If the concept of a multiwire circuit baffles you, calling a qualified person is your best option. You can't write codes based on what the dumbest person on the planet might do ... in spite of what the lawyers would have us believe.
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