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Need 3 phase neutral calculation help #184885 02/24/09 11:39 PM
Joined: May 2007
Posts: 55
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Up2code Offline OP
Member
Hello All. Working on renovation of banquet hall lighting. Total of 9 rows, with 15 fixtures each row(4 bulb 40w fluor.) Each row of 15 will be on own circuit & switch. 2400W per row. Service is 3ph. 120/208Y. 2400/120=20amps X 125%(continuous load)=25A. SO...30A breaker on #10THHN should work BUT if every 3 circuits share neutral, and all circuits are the same(balanced), what current will my neutral carry? And should I upsize each branch circuit neutral do to harmonics?

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Re: Need 3 phase neutral calculation help [Re: Up2code] #184886 02/25/09 12:16 AM
Joined: Apr 2002
Posts: 7,218
HotLine1 Offline
Member
Up2code:
Are you sure the fixtures are 4x40watt? Or did you take the info from the ballast?

Reason I mention it is most T-8, 4 lamp ballasts are under 1 amp @ 120 volts (<120 watts) which will bring your numbers down substantially.

Electronic ballasts have a 'THD' (total harmonic distortion) percentage on the label, which you will need to do the calcs.

Steve Fehr is the EE on site, & I'm sure he has a equation..

A basic 'rule of thumb', IMHO would be #8 neutral with your numbers.


John
Re: Need 3 phase neutral calculation help [Re: HotLine1] #184887 02/25/09 12:27 AM
Joined: Feb 2003
Posts: 939
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frenchelectrican Offline
Member
John.,

Not being harsh in here however there are few issue that need to be addressed in here.,

Genral lighting circuits it should not be more than 20 amp per circuit.

The THD percenetage it should be on the ballast for most electronic ballast IIRC they are either <20% or 10% one of the two if latter it will be not too bad but former yah you will have issue with netural conductor.

as far for sizing on netural conductor yeah go with #8 if long runs.

I will wait for Steve to see if he have more tibbits to add in here.

Merci,Marc


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Re: Need 3 phase neutral calculation help [Re: frenchelectrican] #184891 02/25/09 09:13 AM
Joined: Mar 2005
Posts: 1,213
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SteveFehr Offline
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Sorry, guys, I don't have a magic equation and, actually, I don't work much with lights either. I am familiar with the issues, though. The problem with high-THD devices like ballasts and switched DC power supplies is that you can throw your 3-phase neutral theory out the window.

Traditionally, the current on a balanced 3-phase circuit will cancel completely, leaving virtually no current on the neutral. So, we plan only for the unbalanced load, sometimes even putting in smaller neutrals. Switched power supplies, however, work differently. They basically lop off the peak of each phase in turn, and turn our nice pretty sine waves into square waves. So instead of ending up with sin-wave shaped current on the neutral, we end up with a giant square wave that's all peak and no trough. (If you looked at the neutral current on an unfiltered power supply, you'd see it as a 180Hz square wave on the neutral.) Ampacities are rated by RMS current, and rule-of-tumb power RMS assumes a sine wave. For square waves, however, RMS = peak, and is about 1.4x higher. There's some other voodoo goes into it, but the max theoretical current on a 3-phase neutral is somewhere around 167% IIRC.

We call the artifacts from this harmonics, since the net effect is a combination of waves at different frequencies. You can see it pretty clearly here as you add in prime frequency plus 3rd harmonic, 5th, 7th, etc, up to 25th, and all those harmonics added together turn a sine wave into a square wave. (This is mathematical, btw, and includes 2nd, 4th, etc, which aren't present on 3-phase systems, but the net is the same.)

http://en.wikipedia.org/wiki/Square_wave
[Linked Image from upload.wikimedia.org]

If these were computer racks with filtered switched DC power supplies, I'd convert watts to amps using the manufacturer's (or measured) power factor and call it a day. The filters on modern server power supplies *usually* compensate for the harmonics to the point where it's still a reasouable facscimile of a sine wave going out the back and neutral current is less than line current (which makes doubling the neutral unnecessary). If the ballasts are unfiltered, though, the easiest way to deal with it is to double the neutral. Make sure to account for the neutral when calculating voltage drop, too.

Re: Need 3 phase neutral calculation help [Re: SteveFehr] #184892 02/25/09 09:58 AM
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Up2code Offline OP
Member
Thanks for the info guys. After reading Steve's response, I feel inadequate as an electrical contractor now smile I will take actual amperage readings today at jobsite, and will check ballast for labeled THD%. Thanks again

Re: Need 3 phase neutral calculation help [Re: Up2code] #184893 02/25/09 10:14 AM
Joined: Apr 2002
Posts: 7,218
HotLine1 Offline
Member
Marc:
Thanks, I missed the 30 amp cb, guess I stopped reading at the CD calc.

Up2code:
Check your calcs and heed what Marc pointed out re: 20 amp MOCP

BTW, is this job already installed? You can get all the info you need off of the ballast label, wattage, amps, voltage, THD, etc.




John
Re: Need 3 phase neutral calculation help [Re: Up2code] #184907 02/25/09 02:04 PM
Joined: Mar 2005
Posts: 1,213
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SteveFehr Offline
Member
Originally Posted by Up2code
Thanks for the info guys. After reading Steve's response, I feel inadequate as an electrical contractor now smile I will take actual amperage readings today at jobsite, and will check ballast for labeled THD%. Thanks again
lol, I feel the same way in about half the threads on here! We all have our areas of expertise, and I come here to learn smile There's a lot of overlap between engineers, technicians and electricians, but a lot of specialized knowledge, too. I might be able to tell you precisely what cable you need to pull, but would be lost on how actually to do it myself... You guys probably would have laughed to watch me try to pull in a couple pieces of #2 THHN through PVC when I wired up a new panel for my addition. (Lube? What's that? :p)

Re: Need 3 phase neutral calculation help [Re: Up2code] #184914 02/25/09 04:20 PM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
Member
You have started off with a basic error. Bulb wattage is irrelevant.

Instead, look at the ballasts. The current marked on the ballast is the figure you use for your load calculations. Also look at the ballast for the 'power factor.' While 'power factor' is technically different from 'harmonic distortion,' it still seems to work out that ballasts with high power factors (greater than .93) typically do not have harmonic issues - even if the fixtures have lables warning of the risk.

The other 'ace in the hole' that you have is that the NEC understates the ampacity of #14, #12, and #10 wire. This means that your neutral is already slightly oversized ... I'd use #10 on a 30A circuit without fear.

For a final check, hook up one string of lights. Measure your amp draw (on the neutral) two ways: with a 'normal' meter, and with a "True RMS" meter. If the readings differ by more than 15%, you need to take harmonics into consideration. Less than 12%, you can disregard the issue.

Re: Need 3 phase neutral calculation help [Re: renosteinke] #184916 02/25/09 05:27 PM
Joined: May 2008
Posts: 4
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mb95307 Offline
New Member
Be careful, your installation may have a code violation. Fluorescent luminaires are typically not allowed on 30 amp circuits. You need to verify that your lampholders are rated greater than 750 watts. NEC Section 210.23(B) states that to be used on a 30 amp circuit, a lampholder must be of the heavy-duty type. NEC Section 210.21(A) states that in order to be a heavy-duty type, a lampholder that is not of the admedium type needs to be rated not less than 750 watts and most fluorescent lampholders are not rated this high.

Re: Need 3 phase neutral calculation help [Re: SteveFehr] #184922 02/25/09 08:00 PM
Joined: Nov 2002
Posts: 790
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wa2ise Offline
Member
Originally Posted by SteveFehr
If the ballasts are unfiltered, though, the easiest way to deal with it is to double the neutral. Make sure to account for the neutral when calculating voltage drop, too.


Simple computer switching power supplies, and probably electronic ballasts use a bridge rectifier and a filter capacitor. In this image:
[Linked Image from dranetz-bmi.com]

the red trace is the voltage, and the purple trace is the current into a switching power supply (as the power supply's load changes, the current spike tallness changes as well). Note that the current is zero most of the time (2/3 of the cycle or more), and large spikes occur at the time of the peaks of the voltage waveform. This shows only one phase. Now think of the next phase, and the timing of the spikes on that phase happens during the zero current period of time on the 1st phase. Likewise on the 3rd phase. None of the current spikes on the 3 phases overlap in terms of time, and thus the neutral gets hammered with current spikes 6 times per 60Hz cycle, while one hot phase wire only sees 2 current spikes per 60Hz cycle. Thus the need for a heavy neutral wire.

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