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Why two neutral camlok inputs?

Discussion in 'Question of the Day' started by McCready00, Sep 11, 2008.

  1. McCready00

    McCready00 Active Member

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    On many dimmer/distro racks, there's two neutral camlok inputs. I read somewhere that the second camlok was used when you had dimmers. Actually, I am not quiet sure of what I am talking about. :rolleyes:


    Finally, I was just wondering why there was a second neutral camlok and where do you connect this one back to the main power distribution.

    thanks
     
  2. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Re: why two neutral camlok inputs

    McCready00, promoting your post to a "Question of the Day." [user]STEVETERRY[/user] and others are not eligible to answer.
     
  3. LekoBoy

    LekoBoy Active Member

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    Serves no purpose at all, other than to sell more CamLoks and allow manufacturers to charge more for the racks. No one ever uses the second one, because as you said, there's no place to connect it back at the service panel.
     
  4. soundman

    soundman Well-Known Member

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    Not sure if I am one of the others or not but it has to do with how three phase dimmers work. In a perfect world anytime you use 3 phase power (for 120 V operation) you draw on each phase equally and they cancel out giving you a draw of 0 amps on the neutral leg. (it has to do with the phases being out of phase from each other) When you use SCR dimmers you chop up the waves and do all sorts of horrible things to them. I am not in the math mood right now but if one phase is fully loaded and on at 100% and another phase is fully loaded but only on at 33% you could end up with 133% on the neutral leg depending on the circumstances. going over 100% means you are going to have some issues so they distribute the load amount 2 cables to be sure.
     
  5. porkchop

    porkchop Well-Known Member

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    I've never seen this but I have to guess that soundman is correct. There is definitely a chance that you could run more than 100% of the allowed load down that neutral line if you have very very poor balancing of phases. That would be the only reason that I could see you having to have the second neutral. If I really remember my EE classes this would only effect Y pattern three phase hookups as Delta Hookups use their ground a there effective neutral (admittedly theres a good chance of me having the switched)
     
  6. dramatech

    dramatech Well-Known Member

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    It is because of the SCR/triac/SSR dimmers switching into the phase after the "0" crossing. The exact effect has been explained to me several times, but I can't repeat the exact technical explanation. As best I remember, it creates some sort of back load on the neutral that is slightly out of phase with the hot line. In any case, Dimmer manufactures post instructions in their permanent rack installations, to increase the size of the neutral wire. If I am not mistaken, different manufactures post different values. In my EDI rack it states 110% of the hot. In the Teatronics rack it states 115% of the hot lines. Camloks being for portable installation, and most feeders already being 4/0, which is the largest wire to fit into a standard camlok connector, it would be impossible to install a larger wire, and so the need to run the second neutral.
    Yes, I am aware that there are camloks larger than the 400 amp units, but the larger ones are rarely seen in in locations other than carnivals, where they are used for running amusement rides such as roller coasters.
     
  7. Footer

    Footer Senior Team Senior Team Premium Member

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    Just to tack onto this... say you are running 4 sets of feeders for "the big rock show", how do you run them? All neutrals together, grounds together, phases together or do you do ABCNG?

    And... the final question.... when you run the feeder and have some left over, what should you do with the excess.
     
  8. Sean

    Sean Active Member

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    The 5 (or6) wires should stay together next to one-another. Through the same hole in the wall, hole in the floor, etc.

    Figure-eight the cable. Do not make a big round coil of feeder.

    --Sean
     
  9. philhaney

    philhaney CBMod CB Mods

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    It doesn't go back to main power distribution, on a dimmer rack it goes to the lighting control board/panel (the old analog ones) to prevent ghosting.

    (OK, that's my semi-informed guess. :) )
     
  10. ship

    ship Senior Team Emeritus Premium Member

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    Its called a six wire feeder and at times used and will more so hopefully in the future especially with 120v loads due to as above the unbalanced loading conditions where by like 2/3 or more you see at times a load on a neutral over that of a power conductor. Even these days now seeing beyond GFCI breakers, four pole breakers, the fourth pole being at 50% loading for the neutral to shut it down if the neutral is over normal even half way acceptable amperage. But the two conductor neutral is a sort of ESTA standard these days and even if rare anyone uses the six wire system, it's still standard.

    No, not some way to make more money... sorry but really not the answer. Six wire.. were it my choice I would be using it.
     
  11. LekoBoy

    LekoBoy Active Member

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    The questions remains as to what to do with that extra neutral back at the service panel which is fed with a neutral conductor the same size as the hot legs.

    Upgrading every company switch, or even requiring a double neutral for new construction, would be more than a little cost-prohibitive, now wouldn't it? Incredulously, if adding a useless extra connector on road dimmer racks helps some sleep better at night, more power to them.
     
  12. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Um, [user]ship[/user], I see nothing in the ESTA TSP documents addressing this issue.

    Applying Demand Factor for the Neutral
    is discussed in NEC 220.22.

    The magic factor is 1.33.
     
  13. David Ashton

    David Ashton Active Member

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    I think it's time for one of "the others" to get some facts on to this thread.
    It is the fact that a perfectly balanced dimmer load will run a current of 130% of the full load/phase current when the dimmers are in the 40-60% range.
    While there may be the same load on each phase they are switched on at different times because of the phase difference so there is no phase cancellation in the first 60 degrees of wave chopping and limited cancellation in the next 60 and only in the last 60 degrees does the phase cancellation down to near zero happen.
    The important point is that load balancing has no importance in this issue.
    If you have problems think about us poor Australians, our "standard" 3 phase plug has neutral pins half the size of the actives, but it's "too hard" to change.
     
  14. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Are you saying that a dimmer rack with a full load of 200A per phase will show a load of 260A on the neutral when all dimmers are at 40-60%?
     
  15. David Ashton

    David Ashton Active Member

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    Absolutely, that is what you will get, but please do not believe what I say, get a reasonable rms meter and see for yourself, of course its easier to do at 10a on a little dimmer to experiment, but the result is the same.
     
  16. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Please see the documents attached to this post: http://www.controlbooth.com/forums/lighting/6582-clamp-meters-3.html#post80618. At the time, the focus was on non-RMS vs. true-RMS meters, but the data appear to dispute your statement.

    With dimmers at 50% the neutral current is ~133% of the least loaded phase, but is NOT close to 133% of the "dimmers at 100%" load, (in fact, always less).
     
  17. David Ashton

    David Ashton Active Member

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    In my shed I have a 3 phase box with 4 meters on it 3ph and neutral, as you bring up a balanced load the currents on each phase sum as there is no cancellation and continues to rise until around 50% when they are well in excess of the peak/phase current.
    This is a simple fact which is clearly demonstrable, unless the laws of physics are different in te US you will get the same result.
    This is a basic simple experiment and you do not need expensive meters, simple moving iron meters work perfectly well, make up a rig to show students, this is of fundamental importance.I will bet that any data which seems to disprove the fact of high neutral currents is wrong.
    This is not rocket science.
     
  18. STEVETERRY

    STEVETERRY Well-Known Member

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    Re: why two neutral camlok inputs

    The Code calls for the neutral terminal and neutral feeder of a portable 3-phase phase-control dimmer rack to be sized at 130% or 200% of the phase conductors See below for more discussion of 130% vs. 200%). This is because of the non-linear nature of a phase-control dimmer rack that produces neutral overcurrents.

    On a 400A feed, there is no readily available portable conductor above 400A, since table 400.5(B) gives 4/0 in free air a rating of 405A. In addition, an E1016 Cam-Lok is only rated at 400A.

    So, a second parallel neutral conductor is required, hence the second Cam-Lok. Generally, electricians just add another 4/0 neutral, which is more conductor than is needed, but helps if the two neutrals are not of exactly the same length.

    Why isn't a second neutral required in the conduit feeding the 400A company switch? Because those conductors have a lower ampacity to begin with (and are therefore larger) and in addition are de-rated to 80% ampacity because there are four current-carrying conductors in the pipe. The neutral is always considered a current carrying conductor when feeding a non-linear load. The four wires in the conduit heat as a system, and the neutral experiences overcurrent only when the phase conductors are at less that full current (when the system is dimmed and the current waveform is most non-sinusoidal).

    On the other hand, the portable single conductor feeders are pushed to their free-air rating, are not in a pipe, and are not in contact with each other, so they heat as individual conductors. The neutral conductor therefore has no overhead to deal with neutral overcurrents. So, we add a neutral.

    Note that a three-phase dimmer rack without internal conversion to single phase (240/120V 3W+G) requires a double size neutral terminal. Why? The assumption is that when the road electrician is faced with only a single phase 3 wire +G service, he or she is going to connect two phases of the rack to one phase of the company switch. This means that the neutral could be loaded to 2X the phase current.

    BTW, sine wave dimmers do not require oversize neutrals because they are not non-linear loads.

    Clear as mud?


    ST
     
  19. Franklights

    Franklights Member

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    as many have mentioned it is to make up for an unbalanced load on the neutral. California code began this trend and many dimmer rack manufacturers are implementing it to be compliant and safe. it is a matter of time before the rest of the states start to implement it as well. As always, most production companies are behind the trend due to a lack of national support for it, the expense on the extra cabling, connectors, labor, etc. every dimmer I have bought recently has the double neutral although we haven't been using it.
     
  20. STEVETERRY

    STEVETERRY Well-Known Member

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    Err...when the right combination of phase currents and waveforms occurs, the neutral will get to about 130% of the phase load when all is at full. That may not occur, however, when fading the whole system on the grand master. To hit the worst case, patch 3 subs for the A phase, B phase, and C phase dimmers. Then play with them. You'll get to around 130%.



    ST
     

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