200% neutral for dimmer rack?

Status
Not open for further replies.
J

jkim780

Senior Member
I have a project that has the auditorium with 400A 3ph 4wire dimmer rack for the lighting. The theater lighting consultant is asking for 200% neutral for dimmer rack. I know their will be some harmonics for the dimmers but is 200% neutral necessary? Have you ever used 200% neutral for dimmers? I asked the theater consultant why he needs 200% neutral and he said that's what they been doing all the projects.

Another thing, if I have 120/208V 3ph 4wire service, don't I have to have oversized neutrals at my service not to mention 200% neutral bus at main switchboard? If I have a feeder from MSBD to DP and to Dimmer rack, then that means I also have to have 200% neutrals & neutral buses all the way to dimmer rack. Doesn't it seem waste of money?

[ January 17, 2005, 01:05 PM: Message edited by: jkim780 ]
 
Re: 200% neutral for dimmer rack?

Waste of money? It depends upon who's assuming the cost. If it's a spec. and that's what he wants and is willing to pay for, personally I would not try to second guess him. Then, I would think that he has the responsibility and not you. If you had quoted the job already and are looking to reduct the cost that's a different issue. Another is if you are trying to be competitive in you bid looking to reduce your bid in order to be competive.
I would also think that you would have to determine the K-factor first based upon the components. Maybe the dimmmer manufacture has some data available that would give you some direction as to what harmonics one can expect when applying a given system of dimmers.
 
Re: 200% neutral for dimmer rack?

You definitely need 200% neutrals for any high harmonics load. The neutral can literally have twice as much RMS current as the phases. It is a safety measure and I would not scrimp on it. :eek:

Lawrence Lile, P.E.

Edited to remove private information

[ January 17, 2005, 08:58 PM: Message edited by: dereckbc ]
 
Re: 200% neutral for dimmer rack?

templdl,

For the cost, the owner will have to pay for it, eventually. And I have to right to know why he needs 200% neutral because it will be my reponsibility as an engineer. I have tried to get some information from the dimmer manufacturers but they don't have any data available.

lile001,

If I am not mistaken, you can have neutral currents as high as 173% max. at balanced 3ph, 4w system and practically, you are not gonna use your dimmers 100% at any time. I guess probably 50% max. That being said, "200% neutral for high harmonic" is not the right answer and "we have been doing that all the time" would be the worst.

My point is that why I have to 200% neutral if I really don't need it. Doesn't it seem like an overkill? Has anyone had problem with 100% neutral for staging lighting dimmers?
 
Re: 200% neutral for dimmer rack?

jkim780,
I understand. You're on the engineering design end of things and not on the installation end and it would be nice to know what the implications are rather than assume or just plain out take guess.
But I would venture to guess that the harmonics will probably vary all over the place as different combinations of dimming are adjusted.
 
Re: 200% neutral for dimmer rack?

lile001, do you have any documented cases where 200% neutral current was recorded you would like to share?
 
Re: 200% neutral for dimmer rack?

This may be a silly questions, but why do we treat dimmers differently from lighting ballasts or computers? I mean they are asking for a separate neutral for each circuits and 200% neutral for feeders. We don't usually provide a separate neutral for ballasts or computers, maybe oversize it a little bit if necessary.

lile001,

I would like to share that data if it is available.
 
Re: 200% neutral for dimmer rack?

Separate 200% neutrals? How many 3ph circuits would this involve? Or, are these 1ph loads? Is the neutral which you are referring to part of a 3ph circuit?? If it is a simple L-N 1ph load do you need a 200% neutral for that?
I guess a clarification of the feeder and/or branch circuit, 1ph or 3ph loads may be needed to have an idea.
What I'm saying as that you are referring to multiple 200% neutrals, which leads me to believe 1ph, L-N branch circuits loads.
If can give some insight of what voltage each light dimmer module is and how they are intended to be wired into the distribution system. Does each feed with its own branch circuit, etc.
If a 1ph L-N load I've never been aware that harmonics presented a neutral sizing problem.
 
Re: 200% neutral for dimmer rack?

Sorry for the confusion. Dimmer rack is 400A 120/208V 3Ph, 4wire. The consultant is asking for 200% nuetural for feeder. Dimmers are 120v, solid-state with SCR. Lighitng load is 120v 1ph, and he is asking for a separate neutral for each branch circuits.
 
Re: 200% neutral for dimmer rack?

Thanks jkim780.
If you are referring to each branch circuit being 120v, 1ph, then it is of my opinion that you are correct in questioning the request for 200% neutrals for those branch circuits as harmonics aren't an issue and, as such a 200% neutrals are a waste of money. It's when those branch circuits combine at the 3ph panel and the feeder to it that it that becomes the problem. So, 200% neutral on the panel and 200% neutral for the feed.
But as I rereading your last reply before sending this reply I picked up on the fact that A 200% neutral has been specified which is correct. But you went on to say
Lighting load is 120v 1ph, and he is asking for a separate neutral for each branch circuits
Click to expand...
which omitted the request for a 200% neutral. I then went back and reviewed your original post which stated
then that means I also have to have 200% neutrals & neutral buses all the way to dimmer rack
Click to expand...
again, it is somewhat confusing. But, I do think we're getting closure to understanding your application.
You still didn't describe how the 120v, 1ph circuits would be wired.
You would think that a 120v L-N branch circuit would have its own neutral unless you are considering a home run type circuit where you may have (2) L-N circuits sharing a neutral where there would be 208 L-L which I still doesn't think would be a problem. But if you were to have (3)L-N branch circuits sharing a neutral then that's a problem in by opinion.
But that may be a moot point, and I may be wrong, being a public facility I don't believe home run circuits are allowed anyway.
Dave
 
Re: 200% neutral for dimmer rack?

Dave,

You are right on the money. 200% neutral on the panel and 200% neutral for the feed.

AS for the 120v 1ph circuits, a 120v L-N branch circuit will have its own neutral at least that is what my consultant wants.

That being said, my question is

1) Do we really need 200% neutral for the feeder and the panel bus?

2) Why does a 120v L-N branch circuit need its own neutral? Why not homeruns like you mentioned?
 
Re: 200% neutral for dimmer rack?

It took a while to get where we're at and I've just about out of gas on this one.
My understanding of your application is limited to what you have presented so far and my assumption that the dimmers are switching something like the switching power supply in computers, ballasts, and similar electronic devices except that each dimmer if variable and not fixed.
A) Yes, based upon if you have harmonic content on the feeder and the panel. It is only my assuptin that you will have harmonic content based upon the desription of your load of SS dimmers.
B) Not sure. If it were considered to be similar to a 1ph 3w 120/240v with branch circuits incorporating a home run technique, then, No.
If it is from a 208y/120 where you had (2) L-N loads sharing a common neutral which is essentially single phase, since if is feed from a 3ph wye, I'm not familiar with the effect switching type power supply loads would have on the neutral. I would like to say there would be be inclined to say that harmonics would not be a problem in this application also but that's not good enough answer.
I would be a definite No if you were attempting to apply a(3) L-N branch circuit loads with a common neutral.
After my quick review of art 210.4, it doesn't appear that the NEC prohibits home run circuits in commercial and industrial facilities.
 
Re: 200% neutral for dimmer rack?

jkim780, I have not read all the post so help me out here. Where is this 400-A 208/120 being supplied from? A service? K rated transformer? What?

I do a lot of design on high harmonic loads with 208/120 in data centers, no exp with theater lighting. Where I am going with this is; unless you are coming from a K rated transformer with a 200% neutral bus, following uo with a 200% feeder is kind of pointless.

It would be like towing a vehicle with a 1/4 inch nylon rope followed by a chain because the rope is not long enough to begin with. Hope that makes since.

However, it is a common practice to use a K rated transforer with 200% feeder circuits, followed by individual neutrals on the single phase 120 branch circuits. But I must say I convinced my company to only use a 150% neutral from compiled data collected over the years measuring neutral current. Never seen one go over 120%. Not saying it isn't possible to go higher, just never witnessed it happening, nor do I know anybody who has documented it. If someone has ever seen it please advise.
 
Re: 200% neutral for dimmer rack?

Dereckbc, that is exactly my point. To help you understand my situation, I have a existing 2000A 120/208V 3ph 4wire serive. Off of that main switch board(MSBD), I am feeding a new 800A 120/208V 3ph 4wire distribution panel(DP). Then this DP feeds my 400A 3ph 4wire dimmer rack.

Now, if I have to have a 200% neutral for my dimmer rack feeder, I need a 200% neutral bus at DP and feeder to DP, and 200% neutral bus at MSBD and 200% neutral at service lateral. Do I not?

So, my point is that if I don't have a 200% neutral from service to DP, what is the point to have a 200% neutral from DP to dimmer rack?

So, back to my original question, do I really need a 200% netural feeder to dimmer rack? Can I just get away with 100% neutral? And like I mentioned before, considering the diversity, the phase conductors never will have full 400A at any given time. 1/3 of branch circuits are house lighting and 1/3 is stage lighting, and 1/3 is work lights. So, I think worst case, dimmer will have about 60% loads. So, 400A X .6 = 240A and assuming 150% harmonics, then 240A X 1.5 = 360A.
So, if I have 100% neutral which is 400A, isn't it good enough for this dimmer rack feeder?

P.S. I still don't understand why the shared neutral is not allowed for each dimmer circuit.
 
Re: 200% neutral for dimmer rack?

Jkim780, I hate to second-guess a fellow engineer. We train drivers tend to error on the cautious side, and would rather explain extra up front installation cost rather than explaining why we need to retrofit our oversight. From your description I would have probable used a K rated transformer feeding the panel for the lights and eliminated the neutral altogether and all the other baggage that comes with it.

But here is my two?cents worth. Lets start at the main panel. If the 800-amp neutral feeder cable were 2-750 MCM feeding the panel, every thing is OK (forget de-rating for now), Then if you feed the 400-amp panel with another 2-750, that is OK, maybe overkill, but allowable. But if your 400-amp panel is being fed by 4-750 MCM, and the 800-amp panel is being fed by 2-750 MCM, then you have a valid argument. Or in other words you do not normally feed 3000KCM cable with a 1500 MCM cable unless you are trying to control voltage drop, not harmonics.

So if I were you look at the cable progression if you are feeding a large pipe with a smaller pipe, then ask questions. If the pipe is the same size from MDP or progresses to smaller sizes down stream build as spec, the engineer is covering his rear. If you wanted you could ask the engineer would a K rated transformer be warranted on the dimmer panel to control harmonics. IMO that would be the smart approach.
 
Re: 200% neutral for dimmer rack?

My (very basic ) understanding of this is that the dimmers are electronic, and will produce harmonics. Also you could possibly plug the loads in so you would have the most unbalance possible, with the most dimming, ( I am guessing that the more you dim the more you create harmonics?). So I guess that may be the reason for that. I wonder if the man. had trouble with overheating with a 100% neutral?
 
Re: 200% neutral for dimmer rack?

jkim780
I've done a couple of theatrical dimmer systems, where there was a lighting consultant doing the theatrical lighting design and I was the specifying electrical engineer for the rest of the job.
1.) P.S. I still don't understand why the shared neutral is not allowed for each dimmer circuit. It is important to run individual neutrals with each l-neutral dimmer branch circuit. Combining individual dimmed branch circuits one one neutral often causes flickering lights on the combined circuits.
2.) Now, if I have to have a 200% neutral for my dimmer rack feeder, I need a 200% neutral bus at DP and feeder to DP, and 200% neutral bus at MSBD and 200% neutral at service lateral. Do I not?. Any place you upsize a neutral you will decrease the voltage drop in that section. Also, If you have a 1000 amp service feeding a 400 amp feeder to your dimmer rack, the 1000 amp neutral, if present is already more than double the 400 amp feeder neutral, so you wouldn't necessarily need to double the neutral all the way back to the service to achieve what the engineer wanted. In all fairness, a 200% neutral was probably called for as a safety net just to be sure there was no neutral problem.
Jim T
 
Re: 200% neutral for dimmer rack?

First, let say thank you all for the comments.

dereckbc,

Exisitng building service is already 120/208V. If it were 277/480v, I would use a k-rated transformer.

jtester,

If you have a 1000 amp service feeding a 400 amp feeder to your dimmer rack, the 1000 amp neutral, if present is already more than double the 400 amp feeder neutral, so you wouldn't necessarily need to double the neutral all the way back to the service
Click to expand...
I thought of that, too. But, my DP (800A not 1000A) is feeding (3)-200A panels and 400A dimmer rack. That is what makes me concerned.

a 200% neutral was probably called for as a safety net just to be sure there was no neutral problem.
Click to expand...
That would be my guess, too.
 
Status
Not open for further replies.
Top