[SML] Computing a lighting load

[smashwolf at gmail.com]

Be careful on assumptions of instrument load!

The reason I say this is that I actually come from the computer industry 
doing
load calculations in enormous 1M Sq Foot data centers like Equinix, and DRT.
Modern digital electronics do not load a system like conventional dimmers
and lamps.

Modern lighting equipment, like LED lights, and intelligent lights use 
switch
mode power supplies that result in high levels of odd positive, 
negative, and
3rd order zero sequence harmonic current.

In short, you do not end up with even loading of your neutral return, 
and you
MUST plan your distribution system both for the additional return load, AND
a method for handling the harmonics, or you will have much sadness during
high load productions.

A typical symptom of this is if you have GFI breakers, and you keep 
tripping the
system WAY below your system's planned load limit.

A VERY comprehensive 4 page paper on the subject:
http://www.academia.edu/7277579/EFFECT_of_SINGLE-PHASE_NON-_LINEAR_LOADS_AS_SOURCES_of_HARMONIC_CURRENTS_in_LOW_VOLTAGE_ELECTRICAL_DISTRIBUTION_SYSTEM

YOu may need to over-build your system in some cases, just to use all 
these modern
fixtures that each "use less power" and a poorly planned system ,ay 
indeed use less
power, but as a result be able to deliver less power safely, or properly 
due to the
overall contamination of these harmonics.

When purchasing systems of instruments, pay close attention to the 
engineering
of the power supply to see if the manufacture has addressed his concern 
in some way.
There are switch mode power supplies emerging that address this in 
clever ways
but I can promise you that there is a lack of concern for this in the 
design of
most economy instruments from China.

-smash

-- 
Bryan Manternach, President, Smashwolf Productions & Studios
2883 Aiello Drive, Suite #3, San Jose, CA, 95111
408-363-8324, 408-209-3099 (mobile), smash at smashwolf.com


On 2/11/15 10:30 AM, Lou Poppler via Stagecraft wrote:
> On 2/11/15, PATRICK MCCREARY via Stagecraft <stagecraft at theatrical.net> wrote:
>> I have always used a 50% assumption. Then divide by 3, assuming you're
>> using a 3-phase 5-wire feeder. Then assume you want a somewhat standard
>> size - in your case, 600A 3-phase. We have a 300A 3-phase for 176 2.4K
>> dimmers, and have never even come close. I'm not sure I would have room
>> to hang enough fixtures to load the system fully.
> A quick back-of-the-envelope sanity check says your system trips when
> any phase exceeds 36KW.  If your users are always careful to balance
> the load over all 3 phases, then you are good up to 108KW.  But if
> some show winds up with 37 1Kpars accidentally all on the same phase,
> and all at full at the same time, then not so good.  Many modern
> electric rooms would use a common-trip 3ø breaker, so you might expect
> to lose all dimmer power when one phase is overloaded.
>