Seperate neutrals for every circuit?!

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joedirt

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Currently doing a job where the engineer is requesting a seperate neutral conductor for each circuit.... he is saying it is because the transformer has a phase displacement... I understand a phase displacement but is there a situation that requires an individual neutral for every single circuit?
The panels are simply feeding receptacles etc and not specific equipment.
I tend to believe that from the load side of the Tranny it shouldn't matter. Any help to clarify this would be appreciated.
 
IMO this is a typical install for the isolation of harmonics to prevent ground loops in a audio application. On top of that all the circuits pertaining to the audio would also be on the same phase. You then use the remaining phases for "dirty power"
 
joedirt said:
Currently doing a job where the engineer is requesting a seperate neutral conductor for each circuit.
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He can do that if he wishes. I had a client that insisted on that. If the client is willing to pay the extra costs, so be it.
joedirt said:
he is saying it is because the transformer has a phase displacement.
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That is nonsense. Yes, there will be a phase difference between the primary and secondary voltages. That does not matter, since the loads are connected to the secondary, not to the primary. The phase angle of the load, with respect to the phase angle of the primary, has no relevance to the way the circuits operate, nor to how they need to be wired.
joedirt said:
I tend to believe that from the load side of the Tranny it shouldn't matter.
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You are correct.
 
Mr. Bill said:
It's a solution to computer harmonics on receptacle loads. This is a summary from Mike Holt's book.
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I would not call it a solution, the harmonics are still present now you just have to worry about them on the feeder panel and transformer.
 
Well the engineer is requesting it now... after 3 levels are pulled! There is nothing in the specs and the conduits are not sized for that.
Like I mentioned... I believe that I understand harmonics and I understand how electronic loads are creating the need for larger neutrals. The load side of the Tranny feeding the panel has 200% neutral, but again... I didn't think each circuit would require it's own.
 
joedirt said:
I didn't think each circuit would require it's own.
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NEC wise it is not required, beyond that it is a design decision.

I would be excited about it, I would price up the extra and turn it in.
 
joedirt said:
Well the engineer is requesting it now... after 3 levels are pulled! There is nothing in the specs and the conduits are not sized for that.
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#1. Do not change anything without a healthy change order in writing and signed! (after you make SURE it's not hidden in the specs somewhere, AND the engineer acknowledges the fact)

#2. See #1.
 
This recently came up on one of our projects. The first page of our electrical spec calls for separate neutrals. It's been that way since I started at this company. After the building is 80% done one of our field guys notices at the panel that the EC did not use separate neutrals. Mentions to the EC that the spec calls for additional neutrals. The EC drags this out and argues this for 3 weeks while the rest of the building is being finished putting him further behind. He says it's wasteful, they've never seen this before, and schedule will be delayed. The EC asked me to work with them on this. So the owner, myself, and EC have a sit down. His arguement was that this was hidden in the spec because it was not in the wiring section. I said it's not hidden when it's on the first page. If I believe you honestly didn't see this in your bid then how fair is it next EC who bid on this job and he had this in his bid but didn't get the work. He offers to put meters on the conductors after occupancy to measure peak load and to add conductors if needed. I told him I'm concerned about the life of the building. What will the loads be in 10-20 years as people and equipment shift around. He says some of the neutrals were sized at #10 awg because of the distances and #10's can carry 35 amps. The neutrals never going to see this much current. I say if the conductor is derated then you can't use it's full capacity. Worst possible case of high harmonic current on a shared neutral is +72% or 34.4 amps on 20 amp circuits with no overcurrent protection on the neutrals. Adding these additional conductors will ruin the schedule. Then you shouldn't have wasted the last 3 weeks arguing. I think the EC and owner ended up working something out between them when the EC treatened to walk off the site.

My general aim is always to enforce work per plans & specifications. Allowing exceptions on projects only encourages more exceptions in the future. I understand people make mistakes, I make them too, but this is the fairest I can be to all parties and it always seems to make me the bad guy.
 
Mr. Bill said:
My general aim is always to enforce work per plans & specifications. Allowing exceptions on projects only encourages more exceptions in the future. I understand people make mistakes, I make them too, but this is the fairest I can be to all parties and it always seems to make me the bad guy.
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Probably a good practice, but for things that ultimately don't matter one way or the other, I might be inclined to accept some kind of change just to keep the schedule on track, especially if it came with a hefty price deduct.
 
Well I have scavenged the specs and see nothing about it. There are no notes on the prints either. The only thing I could find in the specs was "not too share neutrals on the load side of a dimmer".... I wouldn't anyway... but saying not to share them there alludes to the fact that you can share them elsewhere.
I am curious though... Mr Bill... where did you come up with the +72%?
 
joedirt said:
I am curious though... Mr Bill... where did you come up with the +72%?
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I've heard this number in different places over the years. I did a search and here's some of what I found.

APC white paper. See Page #6 (1.7 times phase current)
http://www.apcmedia.com/salestools/SADE-5TNQZ5_R0_EN.pdf

AC Power Systems Handbook, page 163 (1.73 times phase current)
http://books.google.com/books?id=a988UyrJttYC&printsec=frontcover&source=gbs_summary_r&cad=0#PPA163,M1

Some field measurements. See page 16 (171% of phase current)
http://www.pserc.org/cgi-pserc/getbig/generalinf/presentati/psercsemin1/psercsemin0/enjeti_slides.pdf
 
joedirt said:
Well I have scavenged the specs and see nothing about it. There are no notes on the prints either. The only thing I could find in the specs was "not too share neutrals on the load side of a dimmer".... I wouldn't anyway... but saying not to share them there alludes to the fact that you can share them elsewhere.
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It sounds like you're entitled to charge for any changes they want, including delays on what has not yet been installed if they want new work done differently.

It all has to be re-engineered, drawn, approved, priced, purchased, installed, etc.
 
Result...

Result...

Sorry for taking so long to get back but here goes.....
It was not in the specs... the engineer said we should "just know" so we argued it is not yet an industry standard and we installed everything to code. We currently have to turn over level 1,2,3... so... what has been done is basically accepted... but they want number 8's or seperate neutrals for level 4,5,0 and penthouse. It's a joint venture here so I'm not sure how billing goes... its some type of budget. Thanks for the help and input guys.
PS GO BILLS!!!!
 
joedirt said:
Sorry for taking so long to get back but here goes.....
It was not in the specs... the engineer said we should "just know" so we argued it is not yet an industry standard and we installed everything to code. We currently have to turn over level 1,2,3... so... what has been done is basically accepted... but they want number 8's or seperate neutrals for level 4,5,0 and penthouse. It's a joint venture here so I'm not sure how billing goes... its some type of budget. Thanks for the help and input guys.
PS GO BILLS!!!!
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Were there any company practices or specs referenced outside of the job specs and NEC? There are a few things in my plant that the regular contractors "just know" - no 1/2" conduit, no 1-1/4" conduit, no EMT unless specifically called out, and no shared neutrals are a few examples. But we have a set of Refining Practices that covers all of this and the electrical section alone is 4" thick.
 
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