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#92270 03/11/05 11:10 PM
Joined: Mar 2004
Posts: 814
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BigB Offline OP
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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! [Linked Image]

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#92271 03/12/05 12:01 AM
Joined: Sep 2003
Posts: 650
W
Member
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

#92272 03/12/05 12:19 AM
Joined: May 2003
Posts: 2,876
E
e57 Offline
Member
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. [Linked Image] 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%.
Quote
Multiwire branch circuits may be derived from a 120/240-volt, single-phase; a 208Y/120-volt and 480Y/277-volt, 3-phase, 4-wire; or a 240/120-volt, 3-phase, 4-wire delta system. Section 210.11(B) requires multiwire branch circuits to be properly balanced. If two ungrounded conductors and a common neutral are used as a multiwire branch circuit supplied from a 208Y/120-volt, 3-phase, 4-wire system, the neutral carries the same current as the phase conductor with the highest current and, therefore, should be the same size. The neutral for a 2-phase, 3-wire or a 2-phase, 5-wire circuit must be sized to carry 140 percent of the ampere rating of the circuit as required by 220.22. See the commentary following 210.4(A), FPN, for further information on 3-phase, 4-wire system neutral conductors.
If loads are connected line to line (i.e., utilization equipment connected between 2 or 3 phases), 2-pole or 3-pole circuit breakers are required to disconnect all ungrounded conductors simultaneously. In testing 240-volt equipment, it is quite possible not to realize that the circuit is still energized with 120 volts if one pole of the overcurrent device is open. See 210.10 and 240.20(B) for further information on circuit breaker overcurrent protection of ungrounded conductors. Other precautions concerning device removal on multiwire branch circuits are found in the commentary following 300.13(B).

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


Mark Heller
"Well - I oughta....." -Jackie Gleason
#92273 03/12/05 08:44 AM
Joined: Jan 2002
Posts: 1,457
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Member
Quote
But in residential and in audio work, it is considered bad practice.
Looks like I am hack.

#92274 03/12/05 10:18 AM
Joined: Jan 2003
Posts: 4,391
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Moderator
Brian (BigB)

Quote
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.

Sorry for going off the original question, I will go back to that. [Linked Image]

The original installation;

Quote
8 120 V hots leaving in pipe along with just one #8 neutral

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. [Linked Image]

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;

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

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

Here is a image from the 2002 NEC Handbook,


[Linked Image]
Quote
Exhibit 225.3 A 120/240-volt, single-phase, 3-wire system (branch circuits rated at 20 amperes; maximum unbalanced current of 80 amperes).

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 [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#92275 03/12/05 12:22 PM
Joined: Sep 2003
Posts: 650
W
Member
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

#92276 03/12/05 07:07 PM
Joined: Dec 2000
Posts: 4,294
Member
Would anyone object to it if it was wired like this?

[Linked Image]


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

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.

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