Home wiring: hot neutral & conjoined circuits

which is a 20-amp

confirmed with outlet

to put it back

shock. I got my

found this VERY

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.

Phil Scott

one went away.

on this?

aware of any

runs into the box,

box, so I

is hot for the

that could be the case but its illegal unless the shared neutral is sized to carry the max current from each of the line 1 and line 2 sources .

The neutral should NEVER read 120v to ground.... if there is a read neutral to ground more than a few volts, 1 to 5 volts...you have problems.

phase as you

hot 2.

recommendation.

confirmed with

went to put it

shock. I got my

I found this

microwave would be

microwave is on a

which I'l call

tester indicated that

circuit one went

See if the

light on this?

could have wired

would get really

probably because

this if ground was

get this if

ground are

is hot, and

hot), you'd get the

capacitive voltages

with the neutral

go? If it passes

could have been why

would be 240 volts

tell by

vertically, but are

especially if

case of shared

they are very often

-----------------

-----------------

confirmed with

went to put it

shock. I got my

I found this

microwave would be

microwave is on a

which I'l call

tester indicated that

circuit one went

See if the

light on this?

could have wired

would get really

probably because

this if ground was

get this if

ground are

is hot, and

hot), you'd get the

capacitive voltages

with the neutral

go? If it passes

could have been why

would be 240 volts

tell by

vertically, but are

especially if

case of shared

they are very often

circuits, 4 sets

outlets there

breaker box the

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)

Romy

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.

Yes it is a violation of the "parallel conductor" rules.

notion

not

the

red

sized to carry

that the two

words, 240 volts

before the two

rather than

the two

and if they

between the

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

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?

Phil Scott

into the same

together? An

addition, and

I noticed he

creating a

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.

separate.

that would be right.

Phil Scott

message

circuit

off,

got a

wires.

fine.

(things

(you'd

hot

it.

can

and

neutral

the

labeled

in

amp

size 12

yer

your

awg)

information, it

shared neutral.

from two

and #8 neutral,

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.

Phil Scott

| 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...

Shared neutral circuits are quite common. Incorrectly wired shared neutral circuits are, unfortunately, not entirely uncommon...

Dan Lanciani ddl@danlan.*com

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

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.

---Snip---

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...

Romy

situation

making a

could

number of shared

Even as a

they

shared

uncommon...

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.

Phil Scott

situation

making a

could

ground) cable.

cable with the

routine that often

the service

it's HOT. Of

switching about the

someone screwing

would not lose sleep

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.

Phil Scott

cable -

fed

hot,

of

and

something

cook

issues

load.

used with three

"dangerous" when done

people who don't

danger as

their car, decides

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

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...

Phil Scott

On Tue, 12 Oct 2004 06:25:38 GMT, Phil Scott put forth the notion that...

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.