[SML] Computing a lighting load

[e-mail frank.wood95]

This is s surprisingly difficult problem. As Lou Poppler very
correctly points out, a phase imbalance can easily occur. The software
I use for my work as an LD will warn me of the possibility, but it
should always be checked. It is always a possibility that an
inexperienced operator may enter "1 through 200 at full" for a
lighting check.

For my own designs, I have a policy of 'one luminaire per dimmer',
apart from the cyc lighting. Since nearly all of our gear is 1KW or
less, there is a reasonable safety margin if you allow for a 50%
maximum load. Not all LDs are so cautious. One of ours quite regularly
overloaded the supply to our black box space. You might also consider
visiting companies, who may well travel more and heavier gear than you
own.

If it is not too costly, I should go for at least 70% of potential
full load. The cost of the heavier cables and breakers is probably a
fraction of the manpower cost to install them.

On 11 February 2015 at 18:30, Lou Poppler via Stagecraft
<stagecraft at theatrical.net> 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.
>
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-- 
Frank Wood