Run a Neutral for each phase conductor?

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210.4, this issue has been enforced heavy in our area. two things you can do, separate neutrals or breaker handle ties with shared neutral circuits. the other dumb one is the modular disconnect at the fluorescent light to kill power to it prior to servicing, 410.130.
 

benaround

Senior Member
Location
Arizona
Under the 2008 code rule, as an owner, I would be very very upset with my electrical engineer if he permitted multiwire branch circuits in my new building. There is no way turning off 3 circuits to work on one would be acceptable to me as an owner.

But your rivail across the street doesn't seem upset with his EE, as your buildings are

exactly the same ( only he allowed mwbc) and his build was $100,000 less than yours.

This is the other side of the story.
 

bdktoys

Member
Location
York, Pa.
bdktoys

bdktoys

Yes, we specify the exact same thing on all of our projects.
It is to reduce the risk of overloading the neutral conductors. Plus, I've seen electricians that used share neutrals that don't have the phase conductors on separate phases.
 

kkscheid

Member
Individual neutrals

Individual neutrals

OK guys, I've got to weigh in on this issue. Have any of you worked in a facility that was over 60 years old? Things get moved around all the time. Individual neutrals do increase initial installation costs, but eliminate many problems down the road.

I've seen and had to help correct many issues related to MWBC wiring years after it was installed. When they were installed, there were not the non-linear loads of today's facilities (i.e. electronic ballasts and switching power supplies in computers). Ever had to troubleshoot a problem where a main breaker in a panel is tripping even though none of the circuits are over 10A and the total load on the panel is much less than the MCB? Ever heard of harmonics (created by the non-linear loads)?

The main reason for specifying dedicated neutrals is to eliminate these issues later. Yes, there is a cost, but it is very expensive to go back and figure out things later. The cost of downtime due to tripped breakers, burned wires, etc is very high, especially in a situation where the burden rate of electricians is approaching $80-$90/hr. I am talking burden rate, not pay rate. Also, what is the cost of lost data and work?

Another thing, just because an electrician is a journeyman, doesn't mean that he is qualified. I've worked with many (>100) "skilled trades" electricians that can't wire a simple 3-way switching circuit. Earlier in the discussion, a comment was made about things being an insult to an electrician. Well, let me tell you, if everyone did things properly and safe, do you think that the NEC would be so thick? NO!!! The NEC is so large to protect people.

Sorry for getting on my soapbox, but I hate it when people think that Engineers don't know anything. What I've seen is that many times, these same people are the ones that don't follow the print and specs on a job and then later wonder "why doesn't it work".

KAS...
 

kkscheid

Member
Individual Neutrals

Individual Neutrals

I forgot to say something in my earlier post.

I've also seen many guys who have an engineering degree and the title "Engineer" who are totally clueless - and dangerous.

KAS...
 

mivey

Senior Member
Engineers use their minds, electricians use their hands :grin: (I'm both).

The rock-throwing between engineers, linemen, electricians, etc is sometimes done in jest, but sometimes done when you run into a knuckle-head.

Good engineers, linemen, and electricians all respect each other when each is good in their field. It doesn't mean you can't pick at each other's disciplines.

I don't find it offensive when someone upbraids a dumb engineer because I'm not one. Neither do I it offensive when someone upbraids a dumb electrician because I'm not one of those either. I have often thrown rocks at both.

But I have done dumb things and have thrown rocks at myself.

I find it usually works better to criticize what was done rather than who did it.
 

paul

Senior Member
Location
Snohomish, WA
kkscheid,

Separate neutrals on branch circuits for non-linear loads would not get rid of the harmonics at the panel. It just moves the problem upstream on your neutral feeding the panel. So the main breaker tripping problem you describe would not be fixed by separate neutrals.

And as for harmonics, I do believe everyone on this board knows about them and how to put the band-aid on the problem. It's not rocket surgery.
 

mxslick

Senior Member
Location
SE Idaho
More voltage drop

More copper

Larger raceways

More derating


While I agree 100% with Bob on the above points, there are some reasons why (especially in my current primary trade) that MWBC's are not allowed by spec.

Harmonics; (Usually with dimmers by folks like AMX and Vantage (in fact both mfgr's prohibit MWBC feeds to their equipment), very rarely an issue with some audio gear);

Other odd noise issues that can't be readily explained but go away when MWBC's are replaced with dedicated neutrals, with NO other changes made to wiring or equipment;

And in one case I ran into some years back, there were 5 (!) circuits fed from a 3-phase subpanel that had only ONE #10 neutral ran with them. That client suffered from severe noise issues in his audio and repeated failures of several audio amps. After we ran dedicated neutrals all the problems went away.

Is it a placebo effect? Voodoo science? Who knows, and Bob and I have had some animated discussions in this regard. All I can say is that I forbid them in my audio rack circuits (MWBC's work fine and I have no problem with them for the general receptacles, projector motors and non-dimmable lighting circuits.)

And for at least the last 15+ years, I have NEVER had any issues with circuits that have dedicated neutrals as used in my audio racks.

Interestingly enough, the EC on my current install job actually added (on their own initiative) a 200% neutral for the feeder to my booth subpanel.
 
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iwire

Moderator
Staff member
Location
Massachusetts
While I agree 100% with Bob on the above points, there are some reasons why (especially in my current primary trade) that MWBC's are not allowed by spec.

You really want to go here? :grin:

OK you said your piece now I will say mine. :cool:


Harmonics; (Usually with dimmers by folks like AMX and Vantage (in fact both mfgr's prohibit MWBC feeds to their equipment), very rarely an issue with some audio gear);

Your combining two issues, manufacturers requirements and harmonics.

Manufactures requirements are often stupid but we must follow them.

As far as harmonics show me some documented damage in an otherwise well designed circuit.

Other odd noise issues that can't be readily explained but go away when MWBC's are replaced with dedicated neutrals, with NO other changes made to wiring or equipment;

No such thing, any electrical problem has a cause and an explanation.

And in one case I ran into some years back, there were 5 (!) circuits fed from a 3-phase subpanel that had only ONE #10 neutral ran with them. That client suffered from severe noise issues in his audio and repeated failures of several audio amps. After we ran dedicated neutrals all the problems went away.

And I am stating this as a fact.

There was something else wrong with the set up* , it was not the use of a common neutral. If it was the common neutral then the hum would still be there as the feeder uses a common neutral.

There is no 'magic' that happens at the panel that changes how your audio system reacts to a common neutral.

Is it a placebo effect? Voodoo science? Who knows, and Bob and I have had some animated discussions in this regard. All I can say is that I forbid them in my audio rack circuits (MWBC's work fine and I have no problem with them for the general receptacles, projector motors and non-dimmable lighting circuits.)

Every audio system you have ever installed has been supplied by a common neutral.

It's to bad that a guy as sharp as you is willing to believe in ghosts and just accept something as fact when you have no science to prove it.

And for at least the last 15+ years, I have NEVER had any issues with circuits that have dedicated neutrals as used in my audio racks.

Which means what in realition to MWBCs?

Again the feeder from source to panel uses a common neutral.

* For instance, perhaps the original 'common' 10 AWG was shorted to ground and when the wiring was re-done that extra bonding was cleared. Point being is you have ruled out any other explanation and decided MWBCs are evil.
 
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TOOL_5150

Senior Member
Location
bay area, ca
Under the 2008 code rule, as an owner, I would be very very upset with my electrical engineer if he permitted multiwire branch circuits in my new building. There is no way turning off 3 circuits to work on one would be acceptable to me as an owner.

Hold on a sec. Lets take the new 2008 rule out of the equation for a moment.

You have a 3 phase MWBC. The 3 conductors are NOT handle tied. Your job is to install a new receptacle, and you have turned off the circuit you want to add on to. HOW are you going to safely break the neutral without burning things up on the other circuits?

Handle tie, or not - You have to turn off all 3 circuits to get the job done right.

~Matt
 

iwire

Moderator
Staff member
Location
Massachusetts
Hold on a sec. Lets take the new 2008 rule out of the equation for a moment.

You have a 3 phase MWBC. The 3 conductors are NOT handle tied. Your job is to install a new receptacle, and you have turned off the circuit you want to add on to. HOW are you going to safely break the neutral without burning things up on the other circuits?

Handle tie, or not - You have to turn off all 3 circuits to get the job done right.

~Matt

Matt, your not thinking this through. Sometimes what you said would be correct, many times it is not.
 

Ragin Cajun

Senior Member
Location
Upstate S.C.
No comment on the new 2008 requirement.

As a design engineer I can not in good conscience require a client to shut down three circuits to work on one or have three go down because one tripped!!!

In the grand scheme of things, two extra neutrals is minor in comparison to the other issues.

I have a job now where the client is changing out cubicals with smaller ones to fit in more people. He would have a fit if I didn't have independent circuits!!!

JMHO

RC
 
For Data Center Wiring--This is the best way

For Data Center Wiring--This is the best way

I'm bidding one now that states not only seperate neutral but also seperate ground

We work on projects where we did the original project 10-15 years ago. Our SOP for: Data (IT) Rooms having majority of loads being non-linear (Switch Mode Power Supply) loads. 3/4" EMT, 1 phase, 1 neutral, 1 IG #12 per circuit. All 3 (yes, all 3) with circuit numbers applied (wiremarkers) at both the panelboard end and the IG receptacle or IG plugmold strip end. Max. 4 circuits (12 #12 THHN) per homerun. No MC cable allowed. Derate #12 THHN to 70% (.7 x 30 = 21A). Completely flexible for future MAC's, upsizing to #10's for the occasional 30A ckt., etc. Very common to change 2 120V ckts. to 1 208V circuit, as many IT chassis are now coming in 208V, eliminates neutral current and harmonics on the neutral.

But for a rec center? Very costly.
 
Hold on a sec. Lets take the new 2008 rule out of the equation for a moment.

You have a 3 phase MWBC. The 3 conductors are NOT handle tied. Your job is to install a new receptacle, and you have turned off the circuit you want to add on to. HOW are you going to safely break the neutral without burning things up on the other circuits?

Handle tie, or not - You have to turn off all 3 circuits to get the job done right.

~Matt
On MWBC: Always pigtail the phase and neutral, never use the device for neutral continuity. We also only use WAGO pushin splices for the neutral conductor and install a WAGO with one open hole, (requires having various hole counts in your pouch, and costs more, but we often cannot even shut off the branch circuit).
 

iwire

Moderator
Staff member
Location
Massachusetts
Very common to change 2 120V ckts. to 1 208V circuit, as many IT chassis are now coming in 208V, eliminates neutral current and harmonics on the neutral.

This does not eliminate harmonics, it only eliminates the branch circuit neutral, the harmonics will still show up on the feeder neutral. Assuming a 208Y120 supply.
 
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