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

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.

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).]

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.


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

So can I assume that you only use two wire 120 volt services?
Don

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.

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)

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

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.

mxslick


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

I sure agree with you there.

Bob

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).]

Gosh guys I don't want to sound unappreciative (is that a word?) but I was hoping to get more feedback on the original topic of the one big neutral for multiple circuits. I was hoping not to start another multiwire debate. Right now on Mike Holt's site there is a multiwire thread 12 pages long with 169 posts so far!

The use of a 'common neutral' between _multiple_ sets of ungrounded conductors (as opposed to the ordinary use of a neutral in a multiwire branch circuit) is mentioned twice that I can quickly find in the 2002 NEC: 225.7(B) for outdoor lighting equipment and 215.4 for feeders.

In 225.7(B) the 'common neutral' must be sized fo be not less than the maximum net computed load current between the neutral and all ungrounded conductors connected to any one phase of the circuit.

In 215.4 it simply says that you can have no more than 2 or 3 three wire feeders or 2 four wire feeders sharing a neutral, and doesn't say anything about proper sizing.

The commentary in the McGraw-Hill NEC Handbook says 'A common neutral is a single neutral conductor used as the neutral for more than one set of feeder conductors....A common neutral may be used only with feeders. It may never be used with branch circuits. A single neutral of a multiwire branch circuit is not a "common neutral" '. I personally don't trust this particular Handbook, but it is what I have on hand.

I don't see anything explicitly prohibiting or allowing the use of common neutrals in article 210.

-Jon

Why not share neutrals.......... I don't get the 2-wire mentality either, but don't knock it.

As for the school of it, somewhere in the infamous "Ugly's" the is some refferance to the "Dangers of the multi-wire circuit". It must be those persons who don't twist wires before applying the wire-nut. Which is the only real danger in 3 and 4-wire circuits.

Back to the "Super-size neutral", I worked for a guy way back that used them a lot... #8 for 4 20A single phase, and #6 for 6 20A 3 phase. Use a split bolt for tap at the junctions. He only did this on circuits that were 120+' long for lighting in warehouses and shops, where the loads would be stable and even, which I thought was not cool. So one day, after me questioning it for the umpteeth time he loaded up the 6 circuits about 15A each, amped out the neutral pretty low, at under 10A. Said the only reason to make it larger than the hots was in case it got fed by only one phase for any reason. It's not a pratcice that I favor, but it does work and seems as safe as anything else to me. The only reason I don't favor it is that if the loads were not well balanced, say one or more banks of lighting were off, it would raise the amperage on the neutral. Seeing that these were usually off a contactor, it wouldn't happen unless the contactor or breakers were failing, or, some electrician didn't know the circuit was like that, and modified it for some reason..... The reason I don't do it, who knows who'll be there long after I'm gone.

Anyway I found some commentary on it an it appears that these circuits need to have the neutral sized at least 140%.

[This message has been edited by e57 (edited 03-12-2005).]

Looks like I am hack.

Brian (BigB)


Sorry for going off the original question, I will go back to that.

The original installation;


In my opinion is an NEC violation (240.4) had they used a 6 AWG 'common' neutral it could have been code compliant depending on breaker sizes and positions on the panel buss.

Eight 20 amp breakers all on the same bus could load the neutral 160 amps, the 8 AWG would certainly be toast if that happened.

Eight 15 amp breakers evenly distributed between the two panel busses could load the neutral to 60 amps, 6 AWG @ 75 C would be fine for that installation under any possible loading of the eight circuits.

As Jon (winnie) pointed out;


In my opinion, what is not explicitly prohibited is allowed.

Here is a image from the 2002 NEC Handbook,




They also show an example of a 208Y/120-volt, 3-phase, 4-wire system with eight branch circuits rated at 20 amperes, maximum unbalanced current of 80 amperes using a 4 AWG common neutral.

As far as proper sizing it seems that we just apply the same rules as always, use table 310.16 with the additional step of determining what the maximum unbalanced current could be.

Bob

I was a bit incomplete in my post. 225.7(B) applies to outside branch circuits for lighting, and 215.4 applies to feeders. But article 210 is the relevant article, and it doesn't mention common neutrals at all.

The idea of a common neutral just 'feels wrong', but thinking about it, if properly sized it would offer some interesting benefits.

1) Fewer current carrying conductors in a pipe for de-rating calculations.

2) Lower voltage drop, since as wires get thicker, the ampacity increases more slowly than the cross sectional area. A #2 wire has 4x the cross section (and thus 1/4 the resistance) of a #8 wire, but an allowed ampacity of 2.3x that of a #8 wire.

3) More robust shared neutral connections. This is just a guess, but I don't see you series wiring with a number #4 into a 20A receptacle. You would have to pigtail using some sort of wire connector or tap for large wires, and this would mean that you are unlikely to break the neutral circuit...breaking the neutral with one half of a multiwire live is IMHO the biggest danger of multiwire circuits.

The downsides are probably cost and confusion.

1) Confustion: While a competent electrician should know how to deal with a multiwire branch circuit, a common neutral circuit is a strange beastie, and I wouldn't expect most electricians to have seen one. As mentioned, the authors of the MCGraw Hill book believe them to be a code violation, even though I couldn't find the explicit violation.

2) Cost: You end up using quite a bit more copper for this installation. The neutral conductor has to be sized for the maximum possible unbalanced load...and as mentioned above, the cross section of the wire increases faster than the ampacity. In the example that Bob gave, which would be cheaper: 4 #12 wires (the 8 20A circuits done as normal 'multiwire' circuits) or a single #4? In the three phase example, it is 3 #12 conductors versus 1 #4...note that a #6 wire has the same cross section as 4#12 together.

-Jon

Would anyone object to it if it was wired like this?





How about a scenario of derating in a metal wireway or auxilliary gutter?
Up to 30 conductors in a cross section, derating isn't required. 31 wires in a cross section will require you to derate all of them to 40%, and quite possibly require a larger wireway.

I've never done this myself, but I can see where it might have some applications.