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Control/Dimming Help w/ Spec'ing Power for New Dimmer Installation

Discussion in 'Lighting and Electrics' started by ML.Rice, May 16, 2019 at 10:57 AM.

  1. ML.Rice

    ML.Rice Member

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    Hello Collective,
    I am speccing some dimmers for a small performance space currently being built. I'm looking at getting a used Sensor+ touring rack with 24 D20 modules. Where I am getting tripped up is that I am being asked to give a general idea of how much power will be needed for the rack. 48 x 20A = 960A, right? The dimmer rack has a 400A service mains switch. I feel like I'm missing something very simple here.
     
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  2. egilson1

    egilson1 Well-Known Member Premium Member

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  3. DrewE

    DrewE Member

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    How many phases? If it's three phase, it's 960A / 3 phases = 320A. If it's single (split) phase, it's 960A / 2 legs = 480A, assuming the dimmer racks support such a configuration. If, for some perverse reason, you have only 120V power available (and not 120/240V split phase), the maximum would indeed be 960A...but I highly doubt the dimmers could be configured to be used in such an environment anyhow.

    Of course both are worst-case values, with every dimmer fully loaded and fully on simultaneously--something that will never ever happens in practice in virtually all cases. I suspect there may well be codes and standards for sizing the main feed for the dimmers (which I do not know offhand), but as a practical matter I think an effective starting point is to figure what the largest lighting load you'd expect to power with them is and then work up from there...of course, I am in no way suggesting one ignore applicable codes or standards!
     
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  4. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    Feed should be based on schedule with loads, not capacity of dimmers. Are you likely to put 4 -575s on each circuit?
     
  5. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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  6. egilson1

    egilson1 Well-Known Member Premium Member

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    That's what we get when the rigger tries to do electrical stuff. :snooty:
     
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  7. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    Yeah. Add up your loads or read five pages and then add up your loads.

    I do a schedule and for a typical system show 1150 watts in each circuit when I did dimmers. Still over kill for several 96 dimmer racks but not crazy. For this one, maybe 1725. Depends and needs more info.

    LED is tougher but I tend to put 1200 watts on each switched circuit. Probably will reduce that going forward. That's like 500 CS fixtures and will never be half that, but some circuits will see 10.
     
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  8. STEVETERRY

    STEVETERRY Well-Known Member

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    But then you'd miss all the other pearls of wisdom in those five pages. :)

    ST
     
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  9. ML.Rice

    ML.Rice Member

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    Winner Winner Chicken Dinner. I was forgetting to factor in the phases. We'll be 3-phase power so 400A service would cover our butts should we somehow manage to get every dimmer fully loaded.

    Thank you!
     
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  10. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    I guess if someone doesn't mind paying for it. I always have budget considerations.
     
  11. derekleffew

    derekleffew Resident Curmudgeon Senior Team Premium Member

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    Sounds like a "portable" rack being permanently "installed." If the outputs are connected to other-than-portable-cable, i.e. installed building wiring, you'll need a road show connection panel with a circuit breaker for each permanent circuit. Something to consider.
    .
     
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  12. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    Not everyone likes to shuck oysters.
     
  13. Robert

    Robert Active Member

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    When and where does a max load of 80% capacity come into play? I've had some facility electricians bring that up to me when they run disconnects for our pds'. They're telling me we can only use up to 80% of the fuse rating in the disconnect.
     
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  14. SteveB

    SteveB Well-Known Member

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    It’s a calculation based on continuous loading. Most circuit breakers can only handle 80% of certain loads (incandescent fixtures being one type) if the load is on for more than 3 hrs. Note that while ETC dimmers use continuous load rated breakers on the modules, the feeder breaker is likely an 80% rated.
     
  15. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @Robert In my little portion of Canada, our inspectors are often more familiar with inspecting steel mills where 80% ratings are common to allow for motor starting currents. In one venue a 1200 amp three phase 120 / 208 motor actuated 3 pole main breaker was decreed perfect to power 100 six KW dimmers plus 6 20 Amp non-dims. A few years later, a full height Century Strand CD80 rack was added up stage right above the road connect transfer panel for 96 20 amp FOH circuits; in this case the AHJ who happened to arrive for this rack's initial inspection decreed an 800 amp three phase 120 / 208 volt supply was necessary for this lone rack. His rationale? 48 slots, each loaded with dual 2.4 Kw dimmer modules equated to 96 x 20 amps for a maximum load of 1920 amps across 3 phases equated to 640 amps per phase times the 125% allowance (for starting the motors [we didn't have]) equated to 800 amps per phase; thus he decreed we required an 800 amp three pole breaker to power this one rack. Done deal, the three pole 800 amp breaker was installed and supplied by feeders he decreed needed to be capable of supplying 125% more than the breaker was rated for; 1000 amps of feeders supplied the 800 amp breaker.
    Suffice it to say that rack NEVER came up shy for available power and its 800 amp 3 pole breaker NEVER tripped due to overloading. My boss, and my boss's boss debated the inspecting AHJ's requirements but he stuck to his calculations and the feeds and breaker were installed per his requirements. Most times it's less problematic to politely bow to their wishes than to attempt to debate with someone higher up their chain of command; more costly, but less problematic.
    Toodleoo!
    Ron Hebbard
     
  16. Robert

    Robert Active Member

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    Wasn't there some conversations years back about oversizing feeds to dimmer racks to reduce harmonics on the neutral and ground lines? Is that still in play? I know when I had two full CD 80 racks installed there was only 800amps 3 phase provided for the two. Never had a problem and still running strong.
     
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  17. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    80% is for continuos load. It would rarely be applicable to a stage lighting system.

    The 800 amp feed - that's good for 500 - 575 watt units - all on at full. Are your hangs near 500 units? And if so, many cues all at full?
     
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  18. SteveB

    SteveB Well-Known Member

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    Typically they run double neutral conductors. Thus a large 24 or 48 module Sensor touring rack has 2 neutral CamLok connections.
     
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  19. RonHebbard

    RonHebbard Well-Known Member Premium Member

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    @Robert In my neighborhood there were problems when racks which previously held a lesser quantity of larger (6 and / or 12 K ) dimmers powering telco-style hard patches were replaced with a larger quantity of much smaller (2.4 K) dimmers configured as dimmer per circuit and no longer via any manner of hard patches. Neutrals were overheating due to the increased harmonics even though there was no increase in the connected loads. Several installations suffered overheating of neutrals and neutral lugs before engineers understood the fault and doubled (and in some cases tripled) the quantity / size of the neutrals.
    Toodleoo!
    Ron Hebbard
     
  20. BillConnerFASTC

    BillConnerFASTC Well-Known Member

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    What? No one read the article for Steve's pearls or wisdom? I am so disappointed.
     
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