Lighting Phase Cancelations & Amps

koncept

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Mar 6, 2005
Location
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Hello,

I got a questions about phase cancelations and amps... I have a 100 Amp three phase "company switch" and when doing shows we wire in a 8 channel nsi dimmer rack setup using single phase. When we threw an amp meter on there at hte last event leg1 (black) had 26ish amps on it (i was using an analog meter), leg 2 (red) had about 31 amps and the neutral had about 27 on it.

I thought that when you had a balanced load so say each leg in use had 30 amps on it, the neutral would have 0 amps on it. Am I wrong?

Thanks in advance
 

koncept

Active Member
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Mar 6, 2005
Location
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that is what i was worried about. we dont have the cable to make the dimmer three phase. we are looking to purchase it but have not done so yet. in the mean time we do a show once a week and need the rack (ideally we need double that). so until the cable gets ordered and everything else happends i wanted to get another opinion on the cancelation here.

also another question in a 110 single phase the neutral and the power leg should be ......
for us on a satelite pack they were the same
 

STEVETERRY

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Joined
Aug 12, 2007
Location
New York
Hello,
I got a questions about phase cancelations and amps... I have a 100 Amp three phase "company switch" and when doing shows we wire in a 8 channel nsi dimmer rack setup using single phase. When we threw an amp meter on there at hte last event leg1 (black) had 26ish amps on it (i was using an analog meter), leg 2 (red) had about 31 amps and the neutral had about 27 on it.
I thought that when you had a balanced load so say each leg in use had 30 amps on it, the neutral would have 0 amps on it. Am I wrong?
Thanks in advance
Actually, you did not have a balanced load because you connected to only two phases of a three phase service. Therefore, the C (blue) phase had nothing on it, so you did not get full cancellation.

Also, when using phase-control SCR dimmers like your NSI unit, you rarely get cancellation and in fact you may see neutral curents up to 1.3 times greater than the highest phase current, depending on dimmer settings. That's because an SCR dimmer is a non linear load, and does not draw current in pure sine wave, but in a chopped sine wave. This prevents the cancellation of current in the neutral. that's why the NEC requires a 130% neutral for portable dimming equipment using single conductor feeds.
Finally, you say you were using an analog meter. In order to get accurate readings of voltage on the output of an SCR dimmer, or current drawn by non-linear loads, you need to use a True-RMS-Responding meter,which would be very rare these days in an analog device.

Look for a digital meter that has True-RMS as a spec item. You can get one for well under $100. Otherwise, your readings are off by a large factor.

ST
 

SHARYNF

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Joined
Sep 3, 2006
If you look inside the dimmer and see how it is wired up, typically you will see that there are 4 lug bars, and that the individual dimmer circuts are distrubuted across the three hots and the neutral IF it is wired up in three phase, and across two hots and the neutral in split phase. Again TYPICALLY so to convert to three phase, you might have a built in transfer taking the third phase load off the two phase legs, or need to move things around.
On the NSI dimmers inside you will see these bars, and you will see that the wires are moved from the third leg when you move them back for three phase, you will have 3 3 and 2 so you are never going to get a completely balanced load. You will also see that there are small fused connection on all the phases, these are for the lights to show what legs are active.

I have a bunch of the 6 channel NSI dimmers, since most places don't have three phase I usually leave them on single phase since it is still a bit of a pain to open them up and move the connections around and you are not looking at a very high load. IF you always do use three phase then it makes sense to change them over.
Also remember that when you use a typical meter you are measuring the leg values, not the phase values, So you need to multiply the values you get by the square root of three (1.732)

www.radioelectronicschool.net/ files/downloads/3phase.pdf

Three phase power and or just using two of the phases gets confusing, since we are typically used to hwome power which is different since there is a 180 degree shift on home power but a 120 degree shift on three phase power

Sharyn
 
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STEVETERRY

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Aug 12, 2007
Location
New York
Also remember that when you use a typical meter you are measuring the leg values, not the phase values, So you need to multiply the values you get by the square root of three (1.732)
Err....not exactly. A dimmer pack with 120V outputs connected to a 208Y/120 service is WYE connected, not DELTA connected. Your statement about multiplying the phase current by 1.732 to get the line current only applies to delta-connected equipment that does not use a neutral. In a wye-connected system, the phase current measured on each of the three phases equals the line current.

So where might we encounter delta-connected loads in the theatre? The most common is a power distribution feeding three-phase chain motors or automated lights that operate at 208V single-phase, typically arc lights with 1200W sources.

ST
 

SteveB

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Joined
Mar 20, 2004
Location
Brooklyn, NY
Phases and Harmonics

Also, when using phase-control SCR dimmers like your NSI unit, you rarely get cancellation and in fact you may see neutral curents up to 1.3 times greater than the highest phase current, depending on dimmer settings. That's because an SCR dimmer is a non linear load, and does not draw current in pure sine wave, but in a chopped sine wave. This prevents the cancellation of current in the neutral. that's why the NEC requires a 130% neutral for portable dimming equipment using single conductor feeds.
ST
Steve

A question about harmonics. And a thread hijack warning.

I was under (the potentially incorrect) impression that harmonic distortions as you have described above - and in other articles over the years, do not become an issue if the demand load is under 70% of feeder size. As in, my total and typical demand load is around 500 amps per phase (architecture 'kinda limits us to anything more), with feeder sizes on my system being split across 2 - 3 phase/5 wire feeds ea. at 800 amps per phase (1600 amps total - 3 phase).

The neutrals were not upgraded (during renovation) to double conductors as it was deemed cost prohibitive to replace all the existing feeder cableing and conduits back to service entrance at the time of upgrade, as well as the existing service entrance 800 amp fused switches, AS WELL as the existing Con-Ed service entrance feeders back to the street vault transformers - which unless these feeds are upgraded as well, makes the whole neutral upgrade pointless.

According to what I was told (names of the potentially guilty parties withheld) I had to see over 560 amps per phase, on each of the feeder systems - or a total of about 1160 amps per phase total for the entire system, before I might see harmonic issues.

Any thoughts appreciated.

Steve Bailey
Brooklyn College
 
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STEVETERRY

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Location
New York
Re: Phases and Harmonics

Steve
A question about harmonics. And a thread hijack warning.
I was under (the potentially incorrect) impression that harmonic distortions as you have described above - and in other articles over the years, do not become an issue if the demand load is under 70% of feeder size. As in, my total and typical demand load is around 500 amps per phase (architecture 'kinda limits us to anything more), with feeder sizes on my system being split across 2 - 3 phase/5 wire feeds ea. at 800 amps per phase (1600 amps total - 3 phase). The neutrals were not upgraded to double conductors as it was deemed cost prohibitive to replace all the existing feeder cableing and conduits back to service entrance at the time of upgrade.
According to what I was told (names of the potentially guilty parties withheld) I had about 560 amps per phase before I might see harmonic issues.
Any thoughts appreciated.
Steve Bailey
Brooklyn College
You are right. Harmonics and the resultant neutral overcurrents only become an issue where the total load in use is close to the total amount of service available.

Note that my previous comment about 130% neutrals only applies to single conductor portable feeders. That's because 4-wire feeders in conduit heat as a system and the only time we see neutral overcurrents in phase control dimming systems is when the phase currents are less than full. Single conductor portable feeders are rated at their free-air ampacity, so the "system heating" model does not work--hence the 130% requirement.

In past articles, I've often recommended the use of a 4-display, true RMS ammeter on the service feeding a dimmer system, especially systems with feeds derated below the system nameplate rating. This lets one know precisely what is going on with neutral overcurrent.

I also recommend Harmonic Mitigating Transformers for feeding dimmer systems (see upcoming article in the Fall ESTA Protocol).

ST
 

JD

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Jan 1, 2005
Location
North Wales PA
Having a balanced load is more important then having low current in the neutral. The neutral is speced to be the same gauge as the hots because it is expected to handle current. Depending where you have your dimmers set, the neutral current will be all over the world during a live show anyhow. The only time you have near zero current on the neutral would be if you had three non-dimmed equal loads, one on each leg. At that point the current draw would be true delta on a wye feed. (very rare) Design balance into your show, but never expect it! SCR / Triac dimmers all work by chopping the waveform, so even with a balanced load there is going to be current on the neutral between the chop points on different channels. The trick is that it is only going to be flowing during fragments of the waveform, so I am not sure any meter is going to tell you the truth. If you wind a couple of turns of wire around the outside of the neutral line, and feed it to an oscilloscope, it looks pretty wild! (Done it, been there.)
 

STEVETERRY

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Location
New York
Having a balanced load is more important then having low current in the neutral. The neutral is speced to be the same gauge as the hots because it is expected to handle current. Depending where you have your dimmers set, the neutral current will be all over the world during a live show anyhow. The only time you have near zero current on the neutral would be if you had three non-dimmed equal loads, one on each leg. At that point the current draw would be true delta on a wye feed. (very rare) Design balance into your show, but never expect it! SCR / Triac dimmers all work by chopping the waveform, so even with a balanced load there is going to be current on the neutral between the chop points on different channels. The trick is that it is only going to be flowing during fragments of the waveform, so I am not sure any meter is going to tell you the truth. If you wind a couple of turns of wire around the outside of the neutral line, and feed it to an oscilloscope, it looks pretty wild! (Done it, been there.)

As long as you use A True-RMS-Responding meter with a reasonable "crest factor" (the ability to measure a non-sinusoidal waveform), you can rely on the currents you read.

ST
 

SHARYNF

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Joined
Sep 3, 2006
Err....not exactly. A dimmer pack with 120V outputs connected to a 208Y/120 service is WYE connected, not DELTA connected. Your statement about multiplying the phase current by 1.732 to get the line current only applies to delta-connected equipment that does not use a neutral. In a wye-connected system, the phase current measured on each of the three phases equals the line current.
So where might we encounter delta-connected loads in the theatre? The most common is a power distribution feeding three-phase chain motors or automated lights that operate at 208V single-phase, typically arc lights with 1200W sources.
ST
This is an interesting area, and while I am not an expert in it, and my posted reply is an over simplification, here is my understanding

First is that the transformers used in buildings are infact delta wye typically since the power company does not supply a neutral usually, but the inside connection is typically wye

second is that with single phase loads on three phase supply, the thinking now is that you should provide an independant neutral for each of the three phase legs when they are being used in a single phase setup

third is that you should spec the amp rating of the neutral leg at a minimum to 1.73 times the rating of the other legs.

So again my understanding, and an over simplification is that you cannot simply take the amp ratings of the feeds and add them up since your neutral in an unbalanced configuration could have 1.7 times the current of a leg in a wye setup where you are using the legs as individual 120 volt connections, or when you are using two of them in a 208 connection.

Common wisdom was that the neutal would be zero, and again in a perfect world with all balanced etc, it is, but typically now in wiring for feeders from a company switch practice seems to be moving to over rating the ampacity of the neutral by almost a factor of 2. This typically was not the practice, if you look at prosoundweb for instance there has been a lot of discussion re neutral ratings and the need to change what we use on feeders for the neutral in unbalanced situations (Which is really just about all our setups)

This is typically NOT how most electricians have done it
So as far as I understand it, in a three phase wye connection that is NOT
balanced, you cannot simply add up the amp readings on the hots, but take into consideration the amps on the neutral.

Again not claiming to be an expert, but my understanding is that a lot of the thinking on this has recently changed

http://www.itic.org/archives/articl...computers_and_electronic_office_equipment.php

Sharyn
 

STEVETERRY

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Location
New York
First is that the transformers used in buildings are infact delta wye typically since the power company does not supply a neutral usually, but the inside connection is typically wye

Correct.

second is that with single phase loads on three phase supply, the thinking now is that you should provide an independant neutral for each of the three phase legs when they are being used in a single phase setup

Especially with non-linear loads, multi-wire branch circuits with a common neutral are not recommended.

third is that you should spec the amp rating of the neutral leg at a minimum to 1.73 times the rating of the other legs.

No. The NEC requires that you take non-linear loads into account when designing the power systems, but there is no requirement that I know of for a 1.73 neutral multiplier.

So again my understanding, and an over simplification is that you cannot simply take the amp ratings of the feeds and add them up since your neutral in an unbalanced configuration could have 1.7 times the current of a leg in a wye setup where you are using the legs as individual 120 volt connections, or when you are using two of them in a 208 connection.

Neutral currents can be raised by nonlinear loads like phase control dimming systems and switchmode power supplies from PC's and office equipment. However, with a dimming system, the neutral overcurrent only occurs when the phase currents are less than full. Since the four wires in the conduit (A-B-C-N) heat as a system, there is no requirement to upsize the neutral in permanent feeds to dimming systems. But with portable single conductor feeders between the company switch and the dimmer rack, the neutral is required to be 130% of the phase conductors, since these cables are rated at their free-air rating and have no "overhead" to deal with neutral overcurrents. Note that the code does require an automatic 20% derating of feeder conductors with a non-linear load, since the neutral must be considered a current-carrying conductor. That causes a 20% de-rating of conductor ampacity when there are four wires in a conduit (A-B-C-N).
ST
 

David Ashton

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Sep 8, 2007
Location
perth W Australia
At risk of being pedantic the dimmers are not "a non-linear load" it is the incandescent lamp which is the non-linear load, and as stated you cannot balance a dimmer load except when all channels are at full as the dimmers are switching at different times on different phases, trying to get this concept across to the average electrician is always problematic, here in Australia we have the stupid situation where our standard 3 phase plug has a neutral pin with half the c.s.a. of the active pins which continually burn out.
 

SHARYNF

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Sep 3, 2006
From what I hear, there is a definite move on the part of some touring co's to completely re think the setup, and more and more are looking at distro's with independant neutrals for each leg, or doubling the size of the neutral. Most typical electrical setups are just not designed with the large unbalancing that can occur with lighting dimmer loads especially since people rarely think of what phase each instrument is connected to.

Sharyn
 

STEVETERRY

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Location
New York
At risk of being pedantic the dimmers are not "a non-linear load" it is the incandescent lamp which is the non-linear load, and as stated you cannot balance a dimmer load except when all channels are at full as the dimmers are switching at different times on different phases, trying to get this concept across to the average electrician is always problematic, here in Australia we have the stupid situation where our standard 3 phase plug has a neutral pin with half the c.s.a. of the active pins which continually burn out.
Even when all channels are at full, there is a zero-cossing deadtime in each current waveform that prevents full cancellation of neutral current because the current waveform is never a true sine wave.

And yes, the dimmers themselves are not the non-linear load, the combination of the lamps and the dimmers is the non-linear load.

ST
 

STEVETERRY

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Location
New York
From what I hear, there is a definite move on the part of some touring co's to completely re think the setup, and more and more are looking at distro's with independant neutrals for each leg, or doubling the size of the neutral. Most typical electrical setups are just not designed with the large unbalancing that can occur with lighting dimmer loads especially since people rarely think of what phase each instrument is connected to.
Sharyn
Well, if the dimmer rack uses single conductor feeders, the code requires a 130% neutral. In the US rental market, it's dealt with as follows:

--At 400A, double the neutral to 2x 4/0 conductors
--At 200A, use a 4/0 neutral
--At 100A, use a 2/0 or 4/0 neutral

The same rule would apply if a distro were feeding non-linear loads like movers with switch mode power supplies.

I've never heard of using a single neutral for each phase in a feeder to a portable system, only on branch circuits leaving such a system..

Don't forget that the need for an oversize neutral is actually dependent on the connected load. There's no point in doubling the neutral if the feed to the dimmer system is only fractionally loaded, which is often the case in the theatre. This point goes to Steve Bailey's earlier questions.



ST
 

SteveB

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Mar 20, 2004
Location
Brooklyn, NY
Well, if the dimmer rack uses single conductor feeders, the code requires a 130% neutral. In the US rental market, it's dealt with as follows:
--At 400A, double the neutral to 2x 4/0 conductors
--At 200A, use a 4/0 neutral
--At 100A, use a 2/0 or 4/0 neutral
The same rule would apply if a distro were feeding non-linear loads like movers with switch mode power supplies.
I've never heard of using a single neutral for each phase in a feeder to a portable system, only on branch circuits leaving such a system..
Don't forget that the need for an oversize neutral is actually dependent on the connected load. There's no point in doubling the neutral if the feed to the dimmer system is only fractionally loaded, which is often the case in the theatre. This point goes to Steve Bailey's earlier questions.
ST
Exactly. The double neutral setup is common on that most common of portable touring dimmer racks - the ETC Sensor 96 dimmer rack.

To be clear, even in a touring rack, if the demand load is less then roughly 70% of rack breaker and feeder capacity, then double neutrals are not required to be hooked up. For safety, the rack/pack should have such capability to allow full usage of the rack as desired.

And a thanks for clarifying the issue of permanent feeder systems in conduits and portable systems not in conduit. It was never clear in my mind the differences, way-back-when, when you wrote an article in a trade publication about harmonics on Broadway and other such setups.

Steve B.
 

SHARYNF

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Sep 3, 2006
Over on prosoundweb there have been a lot of discussions. The comments re a neutral for each leg was in regards to distro's that can be switched from three phase to single phase, and in doing so the recommendation was to go with split neutrals and one of the designers was recommending going with an indivdual neutral for each leg
Sharyn
 

STEVETERRY

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Aug 12, 2007
Location
New York
Over on prosoundweb there have been a lot of discussions. The comments re a neutral for each leg was in regards to distro's that can be switched from three phase to single phase, and in doing so the recommendation was to go with split neutrals and one of the designers was recommending going with an indivdual neutral for each leg
Sharyn
Can you give us a link to the specific discussion?

ST
 

SHARYNF

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Sep 3, 2006
http://srforums.prosoundweb.com/index.php/m/223248/24440/0///15773/#msg_223248

http://srforums.prosoundweb.com/index.php/m/223663/24481/0///15773/#msg_223663

Here is a link re the very confusing situation where you have a distro, two 120 volt legs and are trying to determine the amp load, down the thread, is the explanation that if say you have 40 amps on one leg and 3 amps thru the other you will have 37 amps on the neutral

http://srforums.prosoundweb.com/index.php/m/200335/22030/0///15773/#msg_200335

the links are to the first post in the thread, Lee P is usually the electrical guy
Sharyn
 

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