Dedicated or Oversized Neutrals?

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Smart $

Esteemed Member
Location
Ohio
Situation: Powering computer-based workstations

I would like to know which is the better wiring method--dedicated circuit neutrals vs. oversized neutral MWBC's--and why? I am of the impression the latter method is better but I continually get EE'd drawings spec'ing dedicated neutrals.
 

iwire

Moderator
Staff member
Location
Massachusetts
What are you suppling?

If it is GP receptacles than IMO neither is needed.

With non-linear loads normally I would rather go over sized neutral and MWBCs.

In a data center I would rather use all two wire circuits as MWBCs can be a pain when you need to work on one leg of it but the other one or two are critical.

IMO this is the type of thing that should be decided on a case per case basis not a cover all specification.
 

ron

Senior Member
When it is a pod of workstations (grouped), often you don't have a choice, it is multiwire branch circuits built into the furniture whether you want it or not.
When dealing with critical circuits, I prefer dedicated branch circuits for troubleshooting purposes.
 

cpal

Senior Member
Location
MA
Harmonic currents are generated within the utilization eq. and cycle at frequencies higher than 60 hertz. These freq, are generally multiples of 60 and called triplins. 60 X 3= 180 and so on.

High freq current tends to send the electrons to the outer circumfrence of the conductor which accounts for less conductive material for the current to follow. Less conductor CMA the higher the resistance per foot causing a higher I sq R loss.

I'm not sure that this is a problem with the smaller BC conductors but it is an argument used when considering the larger neutral Vs individaul neutrals

Charlie
 

Smart $

Esteemed Member
Location
Ohio
iwire said:
What are you suppling?

Umm... that would be computer-based workstations. No outlets for a toaster and coffee pot, or the vacuum sweeper, or... well anything not computer related.

Though the question was generalized, the current project is a high school. One type of classroom has computer workstations around the perimeter of the room, say 36' by 30', with one every 4' and a print station on three of the four walls. Receptacles are in wiremold, one quad per workstation/print-station, which is only broken by say the 10' of an inset entry and 10' for a SmartBoard? (system powered separately via 2 circuits). Off the top of my head, something like 11 or 12 circuits are run to and through the wiremold in this one classroom.

Troubleshooting is of no concern here, but your point is well taken. Besides, my installations never need troubleshooting :lol:
 

iwire

Moderator
Staff member
Location
Massachusetts
Honest answer.

I would not worry about harmonic current given your description.

You will not be operating the circuits anywhere near loaded.

Just a personal opinion nothing more...
 

Smart $

Esteemed Member
Location
Ohio
cpal said:
Harmonic currents are generated within the utilization eq. and cycle at frequencies higher than 60 hertz. These freq, are generally multiples of 60 and called triplins. 60 X 3= 180 and so on.

High freq current tends to send the electrons to the outer circumfrence of the conductor which accounts for less conductive material for the current to follow. Less conductor CMA the higher the resistance per foot causing a higher I sq R loss.

I'm not sure that this is a problem with the smaller BC conductors but it is an argument used when considering the larger neutral Vs individaul neutrals

Charlie
Ummm... Saturday, day off, brain dead temporarily...

CMA is...?
BC is...?

I'm well aware of harmonics and skin-effect, but consider this:

Using an individual branch circuit neutral, the current with all harmonics generated have to travel the entire length of the circuit.

In a multiwire branch circuit using an oversized neutral, only the uncancelled harmonics and the unbalanced portion of current travel the neutral. Voltage drop on the circuit is essentially halved (balanced load), I?R loss is decreased, less required conductors, less conduit, less man-hours, etc.
 

Smart $

Esteemed Member
Location
Ohio
iwire said:
Honest answer.

I would not worry about harmonic current given your description.

You will not be operating the circuits anywhere near loaded.

Just a personal opinion nothing more...
I'm not!

...Actually I'm obligated to wire the circuits as designed, i.e. 1 hot, 1 neutral per circuit. That is not the intent of my OP: Which is the better wiring method... and why? See my response to Charlie's post...
 

iwire

Moderator
Staff member
Location
Massachusetts
Smart $ said:
That is not the intent of my OP: Which is the better wiring method...

Which is better?

Good luck with that, it will all be personal preferences as all the methods you mentioned will work.

Which item is 'better' to you?

1) Least material?

2) Fastest installation?

3) Lowest voltage drop?

4) Highest reliability?

5) Best looking panel?

6) Maximum ability to handle harmonic currents?

IMO every choice we make is merely a trade off.

Maybe someone else has an answer, I will shut up now.;) :D
 
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infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
As an installer I would prefer over-sized neutrals. As others have said many times when feeding office partitions you're stuck with how the partition is wired. I have seen jobs where we pulled separate neutrals as per the drawing only to have shared neutrals in the furniture. This ends up with a bunch of unused neutral conductors buried in the partitions' junction boxes.

This brings up a question. Do the power supplies in todays PC's still produce large amounts of harmonic currents or have the manufacturers found a way around the problem?
 

ron

Senior Member
Many new PC power supply manufacturers have adopted the European standard regarding harmonic limits and power factor correction. Since they want to produce one product and sell it to everyone, we (USA) get that benefit.
So .... high harmonics and low power factor are not as much of a problem anymore for new equipment.
 

cpal

Senior Member
Location
MA
my understanding of harmonics would suggest that they will not cancel out so if you have info indicating that they will cancel on a mwbc I,d like to hear it.
cma is circular mil area it is the bases for calculating resistance of a conductor R=KL/CMA??? as well as other things.


BC is branch circuit?? as in MWBC ??

any way !!

as I understand it some harmonis are generated by DC switching power supplies (internal to the utilization equipment). they are not likely to cancel inf act the information I have read suggests that they add lots of stray current lots of heat. This is why neautrals overheat. These currents get into the premises wiring system and can not find their way back to the sourse (in the equipment). All currents seek their supply. They also get into the windings of transformers and create overheating. K rated transformers compensate for this by using smaller conductors per phase and parallel them up to reduce the cross setional area of the conductors. All of the study material I have access to does not support your idea of canceling harmonic currents.

Charlie
 
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Smart $

Esteemed Member
Location
Ohio
cpal said:
All of the study material I have access to does not support your idea of canceling harmonic currents.
That's because most study materials only discuss the harmonics that don't cancel out, especially when concerned with transformers (especially if it is sales literature :). Vaguely stated, the even numbered harmonics cancel each other out at the common neutral point. In the case of dedicated neutral circuits, that would be the panel. In mwbc's these harmonics are cancelled at the common point in the circuits' wiring just like the base frequency, and in the same manner, i.e. only the balanced portion. Literature on such topic usually discusses only triplens, as they are the basic and strongest form of harmonics that don't cancel out.
 

iwire

Moderator
Staff member
Location
Massachusetts
cpal said:
inf act the information I have read suggests that they add lots of stray current lots of heat. This is why neutrals overheat.

Charlie have you ever seen neutrals damaged from harmonic currents?

Most of the literature out there on this topic is published by people with an agenda.

Like the Copper Organization etc.

I am not saying that harmonics do not exist only that the issue has been overblown for sales purposes.
 

cpal

Senior Member
Location
MA
Bob

Neutrals burning off are not common to my experince. But I am aware of elevated terminal temperature (infa-red) atrributed to harmonic currents. I have also observed modular petition connections failing on mwbc.

I do not have a proble with increased neutral sizing. It is one recognized method to safe guard conductor over heating. I believe I mentioned mwbc appear to function adaquately (ref my op).

The op questioned why EE spec a configuration. I suggested the argument that smaller conductors provide increase conductivity by reducing cir mil area for the higher freq currents.

Charlie
 
I have seen the neutral conductor discolored, sometimes burned and temperature scans showing very hot spots on the bc conductors/terminals, but the worst damage that I have seen is usually on the feeder supplying the panel and transformers supplying those feeders. Of course this has been (IMO) drastically reduced since the advent of individual neutrals and oversized neutrals.
 

iwire

Moderator
Staff member
Location
Massachusetts
Pierre I have seen those same issues on residential panels.

Do you have any reason to believe that it was Harmonic currents causing what you observed?
 
Unfortunately you are correct, degraded conductors are found in all locations.


It was determined in the instances that I am talking about that triplens did cause the elevated current in the neutrals and that was the cause of the burned conductors.
 

Smart $

Esteemed Member
Location
Ohio
Pierre C Belarge said:
It was determined in the instances that I am talking about that triplens did cause the elevated current in the neutrals and that was the cause of the burned conductors.
In those instances, did the wiring method use same-size individual circuit neutrals, same-size mwbc neutrals, or oversized mwbc neutrals?
 

iwire

Moderator
Staff member
Location
Massachusetts
Here is the problem I have understanding this problem when it comes to branch circuits.

I have read from multiple sources that the harmonic currents in the neutral are not going to exceed 1.73 the phase current.

12 AWG is rated 25 amps at 75 C

So if I run a standard 12/4 MC to supply equipment that generates harmonic currents each phase could be loaded to about 14 amps before we would see an overload on the neutral.

Now the question I ask is this.

When was the last time you saw a print that showed the loading of circuits supplying non-linear loads so high?

The jobs we work we generally supply two or three office cubes or three or four receptacles in two or three offices. per circuit. If you amp clamp them later it is unusual to find more than 5 amps of load. (Except for those pesky laser printers)

On feeders I can see it being more of an issue but for branch circuits IMO the engineers are trying to 'fix' a problem that is only a problem on paper and not out in the real word.
 
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