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Control/Dimming Questions about 3 phase - 1 phase load

Discussion in 'Lighting and Electrics' started by rockingyoursox, Mar 8, 2009.

  1. rockingyoursox

    rockingyoursox Member

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    My facility is an older style auditorium and i am having trouble determining the maximum load that i can pull. The reason is that though the facility is wired in three phase, the dimmers are only wired in single phase.

    Specifically, behind the dimmers, i can see phase one through three, and a ground. Each dimmer uses one phase. The dimmers are each 50 amp, and i have no trouble with their loads.

    However, the system as a whole has a label stating a 900 amp limit. For previous productions i have just assumed this to mean 900 amps at 120 volts, thus around 100,000 (900 amp * 115 volts) watt limit for productions. However, three phase voltage is 208. For my upcoming production, i would like to use significantly more power. Do i actually have 185,000 watts (208 * 900) or am i limited to 100,000 watts are previously calculated?

    And if i haven't given enough information, what other information is needed/what would i need to check?
     
  2. DaveySimps

    DaveySimps CBMod CB Mods Premium Member

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    I think you are treading on a bit of a forbidden topic here on CB. For safety reasons, I recommend consulting a licensed electrician to look over your system and give you the specific capacity rating you are looking for, especially if you intend to use a significantly higher capacity on your next show.

    ~Dave
     
  3. rockingyoursox

    rockingyoursox Member

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    i'm sorry, i guess i was unclear. I am not planning on rewiring or changing any part of my facility. My question was only what the limits on my facility were as currently constructed, specifically whether a three phase power limit of 900 amps applies as 120 Volts or as 208 volts.
     
  4. DaveySimps

    DaveySimps CBMod CB Mods Premium Member

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    For liability reasons, this could still cause some issues for Control Booth. If you take information you receive here from a member and apply that, and something hazardous happens, that is where problems come in, even if you are not rewiring. Overloading the system can be equally as hazardous. A licensed electrician can take the time and look over the system and give you absolute information in regards to your specific situation.

    ~Dave
     
  5. MNicolai

    MNicolai Well-Known Member Fight Leukemia

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    First of all, don't go opening things up and digging around in distro racks to determine your load ratings. If you reach that point, consult a qualified electrician who could probably find out much easier and faster.

    What you could look for though are ratings on branch panels or your dimmer rack. The potential difference between each leg is irrelevant unless your dimmers are wired to output 208v. If they output 120v, then that's the number you want to calculate with. If it's genuine 3Φ feed, then your pulling 120v on each phase, and if rated 900A/Φ, then you have 2700A @ 120v. HOWEVER, 900A could be an overall rating, thus giving you signicantly different numbers.

    If you can snap a photo of any load rating labels, be it on the inside of a branch panel, or at the dimmer rack, then we might be able to help you more. Don't go any further than opening the door to panels or racks. If you start to remove dimmer modules or open up the guts of dimming racks or branch panels, you may be putting your life at risk. A qualified electrician could find the answer to your question much faster than you could, and without bringing hazard to their health and safety in the process. It may cost a few dollars to bring one in, but consider a worthwhile investment in knowing you have accurate information and have the opportunity to ask them direct questions about how you can and can not load down your system safely.

    Additionally, if you find yourself limited on power, a licensed electrician could inform you of the best solutions on getting more power into your space, even if on a temporary basis.
     
  6. jmabray

    jmabray Active Member

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    Can we discuss theory here for a second though?

    Watt does indeed = volts times amps. However, normal three phase is 120/208. This means that between the hot and neutral you get 120 volts and then between phases you get 208 volts.

    So, for instance lets say you have a Sensor SR48+ dimmer rack fully loaded with d20 dimmer modules. That rack is then fed with 400 amps of three phase power.

    So 400*3 = 1200 amps. 1200 amps * 120 Volts = 144,000 total watts available.

    If you figure this by taking your dimmer module count 96 * 2400watts = 230,400 - you will be figuring wrong - so don't do that.
     
  7. jmac

    jmac Active Member

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    You don't specifically state, but I will assume the dimmers are mounted in a single rack or board of some kind, and again, for sake of discussion only, will assume this rack has a nameplate reading "120/208 volts, 3-phase, 4-wire, 900 amps". The phase bars (or busses) you mention are usually referred to as phases A,B,C, and the "ground" you mention is most likely actually the neutral bar. If it is not an older board, there may also be a ground bus. You mention yours is older. On older boards without a ground bus, generally only the steel rack enclosure serves as ground. As mentioned by you and others, the dimmers are generally connected from each of the three phases to neutral at 120 volts; and 208 volts is the voltage between any two phases).

    Based on the above nameplate info., the three-phase capacity of the rack in kilovolt-amperes is 900A x 208V x square root of 3, divided by 1000, or 900 x 0.36 = 324 KVA. This assumes the load is equally distributed among the three phases. That is, each phase is limited to 324/3 or 108 KVA.

    Note that the rack capacity may not be the limiting factor of your system. Another thing to investigate is the rating of the upstream fuses or circuit breaker feeding the rack. 900 amps is not a common rating these days. The rack may be fed by an 800 amp circuit breaker (or any size <= to 900 amps, perhaps 600 or 400 amps). Then your system capacity is based on the size of that circuit breaker or set of fuses. (If the upstream protection is > 900 amps, you have a problem...).

    There may also be other limiting factors further upstream in the system, or if the dimmer rack feeder and/or dimmer rack serve other equipment, etc....

    If the upstream feeder protection is 800 amps, the feeder capacity is 800 x 0.36 = 288 KVA. This number should generally be de-rated to 80% , leaving 288 x 0.80 = 230 KVA feeder capacity. Dividing by 3, leaves a maximum of 76 KVA per any one phase.

    To get kilowatts (KW), multiply the above KVA numbers by the feeder power factor, which might typically be approximately 0.90, depending on several factors.

    All the above is based on assumptions, and can only be taken as hypothetical. Repeating what others have stated, you are best advised to have a qualified electrical engineer and/or licensed electrician advise you on the specifics of your situation, after looking at your actual system.
     
  8. SteveB

    SteveB Well-Known Member

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    1) It's highly unlikely a dimmer rack is wired in single phase.

    2) The feeders are likely to be 3 phase, but the dimmers connect as follows - being a typical ETC Sensor 48 module/96 dimmer rack. Dim 1&2 - Phase A, Dimmer 3&4 - Phase B, Dimmers 5&6 - Phase C, etc... Thus, 208 volts is only a factor if you are operating a system at 208. Since the dimmers are only getting a single phase feed, 120 volts is the determining voltage for all calculations (assuming you are in the US or Canada). If all the dimmers are 6kw (you state 50 amp), then it's Dimmer 1 - Phase A, Dimmer 2 - Phase B, etc...

    3) There's not enough info about the dimmer system - I.E. is an ETC Sensor system, or similar Colortran D192 or Strand CD80 ?. Or a set of 12 packs in a rack. Or something really old with a patch panel. Different systems configure their phase assignments in different ways. Some REQUIRE a 3 phase feed for the control electronics to function (if you have control electronics).

    4) You state that there's a "900 amp limit" but you don't know if this means 900 amps PER PHASE, or 900 amps total, divided across 3 phases - so 300 amps PER PHASE, which seems more likely.

    5) If 300 amps power power per phase, or 900 amps total for all 3 phases is your limit, multiply that by 120 volts (115 is no longer used, unless the entire demand load is 115 volt lamps - but that's a ETCP question all to it's own) to determine total wattage capacity - 108,000 watts. Subtract about 20% for breaker de-rating if used for continuous usage of over 3 hrs. (a likelihood) , for about 86,000 watts (or 190 Source 4's @ 575 watts ea.)

    6) If you are uncertain as to the methods used to derive these figures, then you are not qualified to open up a dimmer rack and muck around to attempt to figure out system capacity. Best leave that to a qualified electrician who's conversant in theatrical systems.

    Steve B.
     
    Last edited: Mar 8, 2009
  9. MNicolai

    MNicolai Well-Known Member Fight Leukemia

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    A valid point is raised, which is that there are too many factors for what your system is rated for. From the transformer, to the branch panels, disconnects, and dimmer racks, there are a number of different figures to consider before you have a solid view of your system's capabilities. I can fully assure you that the dimmer rack's rating is hardly likely to be synonymous with your true load rating. The reason being that dimmer racks are rarely maxed out. A 288 (20A) dimmer install is simply never going to tap the 691,000W that it's capable of. The only important info about the dimmer racks is single phase v. three phase, and then the other relevant info comes from upstream at your feeders, probably at a disconnect.
     
  10. rockingyoursox

    rockingyoursox Member

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    SteveB, my dimmer system qualifys as "something really old with a patch panel". It is an 80s set of 32 dimmers, taking up a wall 8 feet tall by 10 feet wide, with an additional huge patch panel a little ways away. I believe it is an electro controls dimmer system, and this contributes to my trouble in finding quality information about their dimmer system. *Edit* specifically, its an electro-controls Slider Patch.

    jmac, i am a little confused as to your process. Admittedly, because my theater is wired entirely in one phase except at the dimmer bay level, i have very little experience with three phase. I follow all of your calculations except the addition of the square root of three. Does this come from the sinusoidal nature of three phase?

    Additionally, thank you all for your warnings about electricity. To put your mind at ease, my knowledge that the system is three phase comes from the dimmer bay being opened by an electrician previously, however was intent on fixing what of the system could be fixed and did not explain the specifics of the electrical limits involved.

    As per what the 900 amps is stating a limit to, the short answer is that i don't know. There is a metallic imprint in the center panel beneath several circuit breakers for other parts of the facility that says Maximum Load: 900 amps, and nothing more. Clearly the makers thought this was self explanatory, however, the three phase component made me curious. Additionally, if it is a 300 amp limit per phase, then i have most likely come very close to blowing the limit before, as i had not been sure to distribute phases evenly, as i understand is a must.
     
    Last edited: Mar 8, 2009
  11. icewolf08

    icewolf08 CBMod CB Mods

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    Best thing to do is take a couple photos of the dimmer rack, patch panel, related breaker panels, and any other relevant gear or signage. At best we will be able to tell you what you are working with. Though you are not asking for it, just know that we cannot give you any wiring or similar "how-to" advice, though we may be able to help you determine good safe usage limits for your system. The more details you can provide, the more insight we can give you.
     
  12. rockingyoursox

    rockingyoursox Member

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    hey, googling around electro controls, and reading another thread on this site, i found this:
    [media]http://www.johnsonsystems.com/firing_cards/EC-Mod-Dimmer-wFC-233_large.jpg[/media]

    the right half of the picture highly reseambles what my dimmer bay has 32 of, except that mine have 50 instead of 20, and i believe that they do not have A, B, and C lights, but a single power light instead. I need to check that however. I will try to post pictures next week sometime.
     
  13. jmac

    jmac Active Member

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    Yes- square root 3 is a essentially a factor used because the system is 3-phase. It is just the ratio of the 208V phase-phase voltage to the 120V phase-neutral voltage. The handy .36 factor can be obtained by taking P-P voltage .208 x SqRt 3, or the P-N voltage .120 x 3.

    I'm sure it is related to the sine wave and some vector analysis which I have long since forgotten from college days...

    Today, for nearly all 3-phase equipment such as dimmer racks, panelboards and switchgear, the nameplate ampere rating is the rating per phase. A 120/208 volt, 3-phase 4-wire, 200A panel can handle 200A flowing on each of the three phase busses.

    However, given the age of your equipment and the ambiguous NP, it's anyone's guess what was meant. If there are main bus bars (as opposed to just terminals), and your electrician could determine the material (copper likely) and estimate the cross-sectional dimensions of the bars, one could figure the approx. ampacity of the bars (at least tell you if 300A or 900A is more likely accurate.)

    Now obviously, don't go stick your hands in any hot panel or place a tape measure on any live bus bar....
     
  14. TimMiller

    TimMiller Well-Known Member

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    I will do the same thing, i will flash the load on, let my meter (to the best of its ability) attempt to catch an inrush current rating (for a ball park amount), I will leave the entire rig on for a while, to let everything get a little toasty, and meter my feeder to see how balanced i am, along with making sure i'm not about to pop anything. I worry more about being balaced across all 3 legs of power when i am running off of generators, they really dont like it when you are pulling hard on 2 legs but not the 3rd.

    Back on topic, the dimmers themselves are single phase, but what they will do commonly is stagger the breakers across the three legs of the 3 phase power as stated before, dimmer 1 for example is tied to leg 1, dimmer 2 to leg 2, dimmer 3 to leg 3, dimmer 4 back to leg 1 and so on. This distributes the load pretty evenly. Maybe this will help answer your question, if the breaker handle on your 3 phase breaker is labeled 900 amps, then it will handle 900 amps a leg, which will give you a total of 2700 amps. And if you load all of your dimmer up to their max handling capacity of 50 amps x 32 dimmers, you will be pulling 1600 amps, but even if it is wired single phase, that will be split across 2 legs, so 1600/2=800A.
     
  15. gordonmcleod

    gordonmcleod Active Member

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    There will be a fused disconnect somewhere that feeds the rack check the size of fuses and its nameplate
     
  16. JD

    JD Well-Known Member

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    Regarding a prior comment:
    A 300 X 3 fused disconnect will supply the same total power if you are loading it Wye (3 legs of 120v) or Delta (3 legs of 208v) This is due to the fact that the phase legs lag each other (on Delta) so each fuse sees more than the one leg of current. In other words, the concept of 300 amps X 208 volts goes out the window. (Otherwise known as NFR - "no free ride") ;)

    About the only time a load is Delta in lighting is if you are running movers at 208v, or a 3 phase hoist.
     
    Last edited: Mar 9, 2009
  17. rockingyoursox

    rockingyoursox Member

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    thank you all for the responses,
    i took a couple hours going circuit box hunting and found the upstream box
    it contains a large 400 amp breaker, and its a labeled three phase 4 wire box (more modern than my dimmer system though).

    I know that i have drawn over 500 amps previously, so i suspect 400 amps refers to the capacity of each leg. The lower rating on the actual dimmer rack leads me to believe that the breaker has been replaced and was replaced with one of a higher wattage upon later inspection
     
  18. rockingyoursox

    rockingyoursox Member

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    on further thought, i still have a remaining question

    the breaker had a 400 written on the lever, and the box was labeled 4 wire, 3 phase. Additionally, the labeling paper said Phase 3 followed by a greek letter (i believe it was phi). Does 400 refer to the total capacity of all phases at 120 Volts, the total capacity of all phases at 208 volts, or the capacity of each individual phase at 120 volts?

    note that this entire box was wired in 3 phase.
     
  19. jmac

    jmac Active Member

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    You most likely did not actually draw 500A continuously for a long time. Even at an overload condition of 500A, a 400A breaker will not trip instantaneously (only a short circuit will cause that). Perhaps you had 500A connected load, but never ran everything at full for a significant time.

    Unless really old, the actual breaker trip rating should be indicated on the breaker handle (often dust covered) or on a sticker, which you might not see w/o your electrician taking the enclosure cover off. Or you may see a model number. Ratings on the enclosure may be less reliable. Again, use your electrician to investigate this.

    The 400A means each phase is rated to handle max. 400A. If any phase draws over 400A (+/- tolerances), the breaker will eventually trip. It is really not useful to think you have 1200A (400 x 3) here to work with, even though you sort of do... You have a 3-phase system designed to carry a max of 400A on each leg. You can't put 600A on one phase and 300A on the other two...

    Further, if anyone replaced a breaker with a higher rated breaker, this should not have been done w/o also increasing the wire size commensurately.

    I'm still not clear if your DR is rated 900A or 300A per phase. The way equip. is rated now days, it would almost always mean 900A per phase. If it is only good for 300A per phase, then protecting it with a 400A breaker or anything more than a 300A breaker is a code violation and unsafe condition.

    Greek "phi" (circle w/ slash) means "phase". 4-wire means you have a neutral in addition to the three phase wires, so you can connect things (like dimmers) up 120V or 208V. (A three-phase, 3-wire system could only be used to connect at 208V, and might be used for mechanical equipment or an elevator). No need to carry the neutral for them.

    For me at least, capacity is best thought of in terms of KW or KVA, because KW and KVA can be directly translated across different system voltages and phases, whereas amps cannot.

    You seem to have 400A at 120/208 volts, 3-phase, 4-wire. Total capacity per previous post calc. is 400 x .36 x .80 = 115 KVA (80% de-rating required to allow for possible continuous loads). To get to KW (so we can talk dimmers), assume .90 power factor, which gives total 104KW or 35KW per phase. You have 104KW to work with, regardless of voltage (e.g. 120V dimmers on 208V, 3-ph. system)..

    If you have 50A dimmers connected at 120V, 50 x .120 (remember single-phase now, no sqrt 3!)) = 6.0. These are 6KW dimmers. The system could handle 5 x 6 = 30KW, or five 6 KW dimmers on each phase.

    With five 6KW dimmers on each phase, you have total 90KW used of available 104KW on your DR feeder. This allows for running all dimmers at full, or the connected load. Typical theater will not run everything at full, so you may be able to increase the connected load, but now we're getting into a bit more serious engineering...

    Ok, to get back where we started, how many amps does the 90kW draw? Two ways to figure- As a 3-phase system- 90 / .36 = 250A per phase, out of max. 320A (400 x .8). Or look at it as three 1-phase systems- each 30 / .120 = 250A (again per phase). Three wires, with each carrying 250A. But don't think of it as 750A, or that you need an 800A breaker.

    Better quit now, I think I may be starting to go in circles... Hope this helps.

    The above being hypothetical, as always, seek the services of your local engineer and electrician for the specifics of your system.
     
  20. TimMiller

    TimMiller Well-Known Member

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    FYI NEVER EVER RUN A MOVER OFF OF DELTA POWER. THEY GET REALLY MAD AND LET OUT THE MAGIC SMOKE. I will try to post pics as to what happens. They want two legs of 120, not a leg of 208. Theoretically i guess you should be ok going between one leg of 120 and one of 208, but i really dont want to try it. Delta does make motors really happy though.
     

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