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#138440 - 09/10/03 05:53 AM Three phase lighting....... neutral too small ?  
Steve|Uk  Offline
Member
Joined: Jul 2003
Posts: 18
Horley, Surrey, UK
Hi there

So.... I have a question.......

I'm on a job where the lighting is 600x600 grid lights (4 28w tubes each fitting) and there are some motors around, so the lighting is spread over three phases - it's switched by a single pole switch running a 4 pole contactor (3 phases 1 neutral).

The contactor is remote - the supply cable from the contactor to the "splitter" if you will is ~30m then three flexes come out of this box to the three groups of lights. The flexes are, I would say, around 50m each.

Each "phase" is running at ~4 amps.

The supply SWA is 2.5 XLPE 4 core. The flex is 3183Y 1.5mm (3 core 1.5mm)

So whats the question?

Isn't the SWA neutral going to be working too hard ?

I've always been cautious about running a three phase supply to run 3 single phase light groups, even for stroboscopic reasons, and would love to have some comments thereon......


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#138441 - 09/10/03 04:47 PM Re: Three phase lighting....... neutral too small ?  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
I don't see a problem here.

If the lighting load is balanced between the three phases at the "splitter" junction box, and all three groups are switched simultaneously by the contactor, then the neutral current on the SWA feeder will be negligible.


#138442 - 09/10/03 08:29 PM Re: Three phase lighting....... neutral too small ?  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,217
SI,New Zealand
Steve,
I'd agree with Paul there.
In a Three phase system, the Neutral only carries any out-of-balance current, which in this case will be negligible. [Linked Image]


Let's face it, these days if you're not young, you're old - Red Green grin

#138443 - 09/10/03 11:03 PM Re: Three phase lighting....... neutral too small ?  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
I am not familiar with UK practices, but I don’t the there will be a significant heating problem if the neutral is the same size as the phase conductors in branch circuits. Discharge lighting {fluorescent, mercury, sodium} does have the capability of producing triplen current harmonics in the neutral of wye circuits where feeding phase-to-neutral loads, but it seems to be overrated in a lot of cases. It wouldn’t hurt to check things with a clip-on ammeter with the finished installation. [There are oversized-neutral multiconductor cables available in the US, but I don’t think they are specified or even marketed for lighting.]

I agree that using 3ø to minimize flicker problems with discharge lighting to eliminate strobe effects [perception of ‘freezing of observed motion’] over rotating machinery is good to consider.


#138444 - 09/11/03 12:39 PM Re: Three phase lighting....... neutral too small ?  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Scott,
For sake of comparison, the 2.5 sq. mm conductors in that armored cable are about 20% larger than AWG #14.

Unless there are severe derating factors to consider due to temperature etc., there's no danger of overloading with only about 4A per phase and a very small amount of imbalance current on the neutral.


#138445 - 09/11/03 01:55 PM Re: Three phase lighting....... neutral too small ?  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,217
SI,New Zealand
Just as an aside, a Neutral conductor is only required to be the same size as one of the phase conductors, over here. [Linked Image]


Let's face it, these days if you're not young, you're old - Red Green grin

#138446 - 09/12/03 06:35 AM Re: Three phase lighting....... neutral too small ?  
PaulCornwall  Offline
Member
Joined: Jun 2003
Posts: 112
England
you havnt said if the neutral cable is getting hot??

I came across a problem in a McDonalds restautant once where the neutral bar had burnt out in a dis board..

this controlled all the sign lighting in and out of the shop,, but none of the flourescent fittings had capacitors in, (there were lots), and bingo..


#138447 - 09/12/03 01:51 PM Re: Three phase lighting....... neutral too small ?  
Steve|Uk  Offline
Member
Joined: Jul 2003
Posts: 18
Horley, Surrey, UK
Thanks for all the replies guys.

In retrospect, I feel a complete bafoon that I asked that question - I know I havent done much 3 phase in the past 10 years - but to have let **that** slip my memory........ [Linked Image]

/me goes and sits in the corner......

Steve


#138448 - 09/12/03 03:48 PM Re: Three phase lighting....... neutral too small ?  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,217
SI,New Zealand
Hey Steve!,
That's what ECN is for mate.
Any problems big or small are solved here,
I wouldn't feel bad about posting that question if I were you, a lot of electricians (that aren't here) wouldn't even know what we are talking about. [Linked Image]


Let's face it, these days if you're not young, you're old - Red Green grin

#138449 - 09/14/03 05:06 PM Re: Three phase lighting....... neutral too small ?  
lyledunn  Offline
Member
Joined: Jun 2002
Posts: 159
N.Ireland
Hi,
I agree somewhat with Bjarney, especially if these fittings are high frequency. Given the phase currents are only 4 amps, the triplens could only summate to 1.738 times the phase current. Thus, I see no problem with the size and length of cable.


regards

lyle dunn


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