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Control/Dimming Electrical Question--Caution re: Loading

Discussion in 'Lighting and Electrics' started by MNicolai, Sep 1, 2008.

  1. MNicolai

    MNicolai Well-Known Member Fight Leukemia

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    Electrical Question:

    We just did an install, and I'm trying to figure out what label I should toss on the drawing for the space as to any necessary electrical advisements.

    Our system is fed via a 25KVA 120/240V (104A Max) transformer. The OCP breakers are 2-pole 120V/125A breakers. The feed to the dimmers is behind a 120/240V 100A 2-pole breaker, giving us two legs of 120V. There are 48 120V/20A dimmers in the Unison DR system, wired via a single phase strap kit. So 1/2 of the rack is on one leg, the other 1/2 if on the other leg.

    Which advisements should I place on the plot of the space regarding limitations on power? I need this to be an idiot-proof document, so the key is keeping it as simple as possible, but covering all of the bases.
     
    Last edited: Sep 1, 2008
  2. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    First, I advise against using the word "advisement," as I believe "advisory" is advisable.:) If I read your description and question correctly, I think "Caution: The total connected load across ALL 48x2.4Kw dimmers should not exceed 24,000 Watts!" would be sufficient. Unless you insist "Caution: The total connected load across dimmers 1-24 should not exceed 12,000 Watts, and the total connected load across dimmers 25-48 should not exceed 12,000 Watts!" is absolutely necessary. Since the Unison DR series in this case does not come in a "balanced rack"-type configuration, I hope you've distributed circuits such that the two legs should remain close to balanced.

    I realize one of the intents behind What's Connected? was to convince skeptics to allow sizing of supply power according to actual/anticipated load rather than total possible capacity, 200 total amps for a system with capacity of 960, or 21%, seems insufficient to me. One 575W lamp on every dimmer would overload the system! For more, see this article.
     
    Last edited: Sep 1, 2008
  3. Sony

    Sony Active Member

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    If I were you I would just put the Maximum Supply rating which from what I can make from what you said is 208 amps total and 100 amps per rack. Usually a transformer supplies the circuit breaker rating for each pole. Therefore 240 volts has 2 legs at 104 amps per leg would make it 208 amps total. This is how I know it works with 3 phase...but I'm not so sure about 240vAC however I believe it's the same (maybe someone can chime in and correct me if I'm wrong.) The 100 amp breaker on your dimmers limits your supply to the dimmers at 200 amps. I would just label the supply as 100 amps per rack and 200 amps total @ 120vAC.

    I'm more used to 3 phase and single phase 120vAC as this is what I use most.
     
    Last edited: Sep 1, 2008
  4. MNicolai

    MNicolai Well-Known Member Fight Leukemia

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    Yea, when I say advisements I'm aiming to be diplomatic. Understand, when these are on my drawing, they'll be more than just polite requests.

    One of my primary questions though is what does it take to trip a double-pole breaker? Is it an excess on either of the legs, or excess of both pole, combined?

    In a theatrical situation, it's not easy to keep a balanced load between both phases, which is why I'm asking questions on this. I can also add in that both phases and the neutral are wired via #1AWG, with a #6AWG ground.


    An somewhat unrelated question. Why would they wire in 125A OCP breakers on a transformer with a 104A/Ø max? My electrician told me that four OCP points would have to fail to bring harm to the transformer, but he hasn't had a chance to respond to that follow-up question yet.
     
  5. Sony

    Sony Active Member

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    An excess of 100 amps in just one leg will trip the breaker, that means if you overload just one dimmer rack, both your dimmer racks will shut down. This is why I personally hate having both dimmers on one breaker.
     
  6. MNicolai

    MNicolai Well-Known Member Fight Leukemia

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    The racks are little more touchy than that. They're bussed as follows...

    L1
    Dim 1-16, 41-48

    L2
    Dim 17-40

    I'm not too worried, because if every circuit is filled with one 575w fixture for 24 dimmers, it's only 2400w over, and the odds that each circuit has a light hooked up to it, much less that lights are on, much less all on at once on that particular leg, is rather slim. It's a small space, and the usual show has about 16 fixtures in it.
     
  7. MNicolai

    MNicolai Well-Known Member Fight Leukemia

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    You might be asking yourself the question, "Why would you install something without a more-than-adequate power feed?" Unfortunately there wasn't much choice. It would've cost an additional $20k to get it hooked up to a different transformer, which would've blown the entire project out of the water.
     
  8. Sean

    Sean Active Member

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    Yes, but you have no idea what, er, enterprising young LD will try in there.

    I did a couple of community theatre designs several years ago. They honestly didn't have enough gear for what I wanted to do. So, I borrowed a couple car-loads of gear from work.

    Let's just say I _easily_ hung more than twice the gear they'd ever used in the space. Now, I was ok for power but only because I _really_ looked. There was no documentation in the venue other than the ratings on the dimmers' breakers.

    "Build today for tomorrow....and next week."

    --Sean
     
  9. STEVETERRY

    STEVETERRY Well-Known Member

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    Are the houselights or other egress lighting on the same feed as the dimmer rack?

    ST
     
  10. MNicolai

    MNicolai Well-Known Member Fight Leukemia

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    The houselights are on dimmers, but the egress is not. There are also 3 banks of fluorescents in the room, one of which is on egress, and that's tied in to our emergency generator.
     
  11. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Quoting from the article referenced above:
    Section 220-10, referenced in the Fine Print Note, speaks to the requirements for continuous and non-continuous loads and the application of any applicable demand factors. Since there are no published demand factors applicable to theatres, the bottom line is that a dimmer rack can be fed with enough power to control the inventory of fixtures (present and future) in a given facility. The one caveat is the requirement that if you are going use a "derated feeder" to feed a dimmer rack with less than its nameplate (full load) rating, then you must ensure that the houselights and emergency lights are not affected if the main breaker trips. [Emphasis mine.] In practical terms this dictates a separate main breaker for houselights and emergency lights if the system has a derated feed. The result is that if you turn on too many stage lights and trip the main breaker, the audience will be able to sit in comfort reading their programs while you reset the breaker and relight the show!

    I guess we have to admit the man knows what he is talking about.;) [user]MNicolai[/user], it sounds like you have that covered.

    Back to your original question (sorry I dragged the thread off track): "Maximum Load: 24,000 Watts!", pro or con? Remember this is a High School Little Theatre, seating ~100. While there may only be 16 fixtures assigned to the space, many more fixtures are readily available. A professional MIGHT look at the supply rating for the dimmer system; it's almost guaranteed that an amateur would not. (I blew a few screw-in fuses while in HS: Xrays containing 16x 150PS30 lamps don't fit so good on a 1000w autotransformer dimmer.)
     
  12. MNicolai

    MNicolai Well-Known Member Fight Leukemia

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    I think extra precaution has to be taken though because of the OCP the electricians used. They put a 125A two-pole breaker on as OCP for the transformer, which has only a 104A max. So back to another question I asked, why would they do that?
     
  13. TimOlson

    TimOlson Member

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    Hi MNicolai. if you have 2 phases represented on your dimmer, I would first color code the front part of each dimmer so anyone could tell, just by looking, what phase was being loaded.

    I would also recommend de-rating the stated load of each phase by 15 or 20%. that gives you some head room for things such as heat rise over time. I always derate services like this when I'm calculating worst case power requirements, and if I'm forced to get closer to the actual rated load on the circuit breaker I know I'm gettingrinto risky territory. THe sign on the dimmer could then read "85 A max on Red, 85A max on Blue"

    I would then post the formula for calculating Amps directly underneath the load statement. this should cover most bases and a somewhat intelligent newbie should be able to do a fairly good job of balancing loads.

    Lastly, I would either install an amp meter for each phase in the cabinent, or lacking that, someplace to safely use an amp clamp type of meter. I would build the habit of measuring phase loading for each scene into each production until your show electrician was very familiar with each scene's requirements. you could even make a place on your tech checklist for each scene's power requirements. that way, any adds equipment wise could be done in a framework of available resources, and not that of the "blank check" method: "well there's more fixtures in storage, so I should be able use them all"

    it is indeed unfortunate that your system was designed in such a way that it could be easily overloaded. asking how THAT happened is a very different question than how to lable the service.

    will you ever be putting moving lights on the same transformer as these dimmers? if so then it's more important to keep that headroom available -- as voltage drops, those instruments draw more amperage and a lot of them make harmonics on the neutral.

    peace, Tim O
     
  14. STEVETERRY

    STEVETERRY Well-Known Member

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    The Code allows the next standard size breaker up from the transformer rating.

    ST
     
  15. STEVETERRY

    STEVETERRY Well-Known Member

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    Good--you can do whatever you want with the feed size, as long as it matches the equipment inventory and the likely diversity factor.

    Question--what is the equipment inventory of the space, with wattages?

    ST
     
  16. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Last edited: Sep 1, 2008
  17. STEVETERRY

    STEVETERRY Well-Known Member

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    hahaha

    ST
     
  18. MNicolai

    MNicolai Well-Known Member Fight Leukemia

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    On average, 16 S4/S4Jr. Our first plot will have 52 S4's, because I have that many that I need to find a home for until November when the Arts Center opens and we'll finally have ample storage. Some are 750w, most are 575w; I'm trying to standardize down to 575w. That said, there will be over 150 S4's in the department, but to light the average show in that space shouldn't take more than ~16 fixtures, give or take. Actually, ST, in a couple weeks Ellen will be out here to give her workshop and she'll see what the entire space looks like.

    With that many fixtures up though, it'll be very simple that I will be the only person using the system, doing lots of mental math for each submaster and cue I run, until the excess fixtures are relocated.

    I will certainly label the racks so people know which dimmers are on which phases. @Tim, how that system was installed the way it was; our final electrical budget was $20k, and they said just getting a 3Ø feed and mounting the racks on the wall would cost the full $20k, not including new power distribution, rigging, new lights, lamps, cables, arch controls, or anything else. That said, the project wouldn't have happened if we had to set it up any way else. And in regards to moving lights; it is extremely unlikely, if anything we'd be most likely to have a couple rented scrollers here or there.
     

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