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#92260 - 03/10/05 07:48 PM One big shared neutral
BigB Offline
Member

Registered: 03/31/04
Posts: 727
Loc: Tucson, AZ USA
I looked at a job today where they want an old Zinsco sub panel replaced, it is an interior panel located outdoors no less. When I removed the deadfront I noticed there were 8 120 V hots leaving in pipe along with just one #8 neutral. It all terminates somewhere in the very low attic crawlspace.
I have seen this scenario discussed before but I don't recall if it was ever determined a code violation, provided the neutral was sized properly for the load.
Anyone know?.........Thanks, Brian

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#92261 - 03/10/05 07:57 PM Re: One big shared neutral
electure Offline

Member

Registered: 12/24/00
Posts: 4226
Loc: Fullerton, CA USA
We discussed it a very long time ago, and everyone pretty much agreed (believe it or not ) that properly sized, there wouldn't be a violation.

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#92262 - 03/10/05 08:09 PM Re: One big shared neutral
mxslick Offline
Member

Registered: 10/24/04
Posts: 785
Loc: Atomic City, ID USA
Zinsco, yikes!! Almost as bad as FPE....

Not sure if Code allows it, but a few years back I did a refit on a screening room where the audio racks were serviced by 5 20amp breakers (on a 3-phase panel) with ONE #10 neutral for all 5 circuits!!! The client was wondering why the sound system had a massive hum and why he'd been replacing burned-out amplifiers every few months.
(edit: There were NO loose/bad connections, which was my first thought.)
To stir the pot again (see "Panel In Bedroom" thread) , I am of the school of NO SHARED NEUTRALS UNDER ANY CIRCUMSTANCES!! The cost of the extra wire is NEVER that great, compared to the potential problems.

After I had the EC pull in the proper number of neutrals and wire the rack recepts correctly, the system was quiet and no more burning amps.

[This message has been edited by mxslick (edited 03-10-2005).]
_________________________
Stupid should be painful.

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#92263 - 03/11/05 01:54 AM Re: One big shared neutral
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
 Quote:
I am of the school of NO SHARED NEUTRALS UNDER ANY CIRCUMSTANCES!!


I suggest you try some other schools.

What happens when you get to the panel?

You wanted to 'stir the pot' well you hit a nerve.

There is no reason whatsoever that multiwire branch circuits will in them selves cause a 'hum'.

Even if you do not want to believe it there was something else going on.

Electrically sharing a neutral on a branch circuit is not any different than what happens at the panel where all the branch circuit neutrals share a single conductor back to the source.

 Quote:
The cost of the extra wire is NEVER that great, compared to the potential problems.


Not true at all, it really depends on many factors. In a house I might agree that the savings is not that great, in a commercial pipe job the savings is tremendous.

1)Less total conductors result in smaller and / or fewer raceways.

2)Less 'current carrying conductors' results in less derating of the conductors.

3)Voltage drop is reduced on long circuits with the use of multiwire branch circuits.

It is really up to the customer or their engineer if the savings are worth the 'risks'.

Personnel IMO with competent electricians the risks our minimal.

We install predominately Multiwire branch circuits and we are not going back to fix things.

Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#92264 - 03/11/05 06:05 AM Re: One big shared neutral
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
 Quote:
I am of the school of NO SHARED NEUTRALS UNDER ANY CIRCUMSTANCES!!

So can I assume that you only use two wire 120 volt services?
Don
_________________________
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#92265 - 03/11/05 09:29 AM Re: One big shared neutral
mxslick Offline
Member

Registered: 10/24/04
Posts: 785
Loc: Atomic City, ID USA
I must say I stand corrected, Sometimes I type faster than I should. Of course in an industrial/commercial setting, it's not a problem. But in residential and in audio work, it is considered bad practice.

It has been proven many times in my line of work that a shared neutral causes noise issues. Many installs I've had to rework have been cured of noise by pulling dedicated neutrals for each circuit. I don't always understand why that's so, but it has proven itself many times.

 Quote:
Electrically sharing a neutral on a branch circuit is not any different than what happens at the panel where all the branch circuit neutrals share a single conductor back to the source.

For non-power-sensitive loads, I agree. For audio gear, no. It's the same idea as using isolated grounds, IMHO. I do agree with all of your other points!
Don:
Haven't seen one in years! But 3w 240 is fine. I do recall some tweek audio magazine espousing the benefits of audio clarity by feeding all your gear from a 2w 120 service. (Back in the 70's)
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#92266 - 03/11/05 10:11 AM Re: One big shared neutral
tdhorne Offline
Member

Registered: 03/22/01
Posts: 344
Loc: Maryland, USA
Well I have to join in rejecting the idea that multi wire branch circuits are inherently problematic. I won't argue about the audio circuits because my experience does not include extensive audio system work. The housing industry is very competitive and every legitimate avenue to control cost must be explored. My present work is in home wiring repair and extension. I will pull a three wire plus ground cable if I even suspect that there will be a need for another circuit in the same part of the building. I undertook to wire a dedicated outlet for a sump pump as a "Christmas in April" project. I looked around the sump pumps location and there was the laundry equipment tied into a general lighting circuit. I pulled a three wire cable to the four square that contained the laundry equipment receptacle and ran the sump pump receptacle outlet from there. A very slight increase in cost and a marked improvement in the whole homes wiring plant. Since the laundry appliances are not left unattended for days at a time I used a GFCI receptacle for them but the sump pump is on a single 20 ampere receptacle.

I pull three wire cables to bathrooms now to solve the monster hair dryer problem. Not only does the basin outlet end up resupplied but so does the thousand watt plus unit heater that so many bath rooms have.

Multi wire branch circuits are also handy to supply existing out buildings. One car garages and small sheds often need just a little more but a feeder, grounding electrode system, and building panel is a budget buster.

[RANT MODE ON] It ties right into a sore subject. Anyone that cannot work safely with a multi wire branch circuit has no business doing electrical work. I am not opposed to home owner DIY work but if you can't take the time to understand enough about wiring to make a multi wire branch circuit just another useful technique then you haven't done enough studying yet. Invest the time to learn or hire someone who has done so. A person who gets hurt because they opened the neutral of a multi wire branch circuit is the definition of an UNQUALIFIED PERSON. [/RANT]
--
Ton H
_________________________
Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison

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#92267 - 03/11/05 11:08 AM Re: One big shared neutral
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
I suspect your audio problem had more to do with the size of the neutral than the fact that it was shared. With all of the harmonics you see in electronic power supplies I suspect the neutral was carrying more load than the phases at any given instant. In the example you gave it was clear the neutral was undersized, even if you were just serving light bulbs since you have a potential of 40a of neutral current. Super neutral cable (recomended for electronic loads)would have a #8 for 3 balanced #12 phases.
_________________________
Greg Fretwell

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#92268 - 03/11/05 02:38 PM Re: One big shared neutral
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
mxslick

 Quote:
Haven't seen one in years! But 3w 240 is fine.


Then you would be 'sharing' a neutral.

Your missing the point Don and I made, you share neutrals back to the source as soon as you reach the panel. The panel is not the source of the power.

There is nothing special in a panel to stop a hum.

Amplifiers can cause added harmonics on a shared neutral but that is a separate issue and not one that will cause a hum.

I would be looking for improper grounding and improper neutral to grounding connections.

tdhorne
 Quote:
It ties right into a sore subject. Anyone that cannot work safely with a multi wire branch circuit has no business doing electrical work.


I sure agree with you there.

Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#92269 - 03/11/05 03:28 PM Re: One big shared neutral
Fred Offline
Member

Registered: 04/30/01
Posts: 461
Loc: Straughn, IN 47387
I couldn't agree with Don and Bob more on the subject of MW branch circuits. I've used them for years in residential and saved thousands of feet of EMT and #12 THHN in comm/ind. But I do have to say I've witnessed a disturbing trend towards MW circuits in the training program. There are a couple of guys around here(my area) who have recently finished their 5 year program and passed their Masters and think MW circuits are irresponsible and dangerous. They got that from their instructors and I've seen the study materials they were given to back up that assumption. Which confirms my belief that you don't have to be new to the trade to have a poor understanding of certain areas of it. We've seen this in several threads on this and other forums concerning MW circuits. Makes me wonder if the pipe institute or copper consortium is sponsoring some of these training programs. You say multi-wire circuit to some and they automatically think "reckless hack".

[This message has been edited by Fred (edited 03-11-2005).]

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