Control/Dimming Thoughts on the 80% "Rule"
- Thread starterRick Brooks
- Start date
JD
Well-Known Member
Not missing a thing John! What you said is exactly correct when dealing with restive loads like conventional. Long lengths of cable REDUCE the current draw as they ADD resistance to the circuit.
There is (of course) one exception- Electronic ballasts and other switch-mode electronic supplies, which will actually increase their current draw if the voltage feeding them drops due to a long length of cable (or any other reason.) Of course, we wouldn't be operating these off a dimmer
DeadCheerios
Member
In a theatre or a building that is wired to run the equipment with installed dimmers I have zero issue with using what the dimmer is rated for, as it is built and setup in the space to be able to do that.
Setting up a gig in a community hall or a hotel I always stay close to 80% as I have no clue what the building is actually running at (voltage wise), especially those older buildings you never know if you get your full 120 or closer to 110.
Setting up a gig in a community hall or a hotel I always stay close to 80% as I have no clue what the building is actually running at (voltage wise), especially those older buildings you never know if you get your full 120 or closer to 110.
Will Elphingstone
New Member
I am aware that it will destroy them, but if its for a short period of time, and a last resort...
DeadCheerios
Member
Exactly.
For example if I have a 15amp circuit at 120v I could in theory run 1800 watts off that before the breaker flips. So if I am pulling 1800 watts and the voltage drops to 1650..POP. So if I use the 80% rule I will only put a max of 1440 watts on that circuit so even if the voltage drops 10v, I won't run into problems.
Except for incandescent units its the rests cd that stays the same so lower voltage gives lower current and wattage. Essentially the power grid is acting not as a dimmer.
This gets much more complicated with anything other than incandescent lights however.
Philip
BillConnerFASTC
Well-Known Member
Lot of variables there. If you're pulling a feed from the old hall, I'd be inclined to check or derate for that, but using that feed for your own dimmers - the dimmed circuits you run from there could all be loaded 100%. But if plugging lights into their dimmed circuits , I'll agree it it could be risky. When an undergraduate my college but a kids show on tour in rural upstate NY (lest I repeat myself) and we toured with a 6 X 1000 auto transformer board, which I fed form the nearest circuit breaker panel - probably breaking every conceivable safe practice and a few NEC requirements - but I had no hesitancy putting two 500 watt fresnels on a single dimmer - just might know to not turn on more than one dimmer on the same leg of the feeder. When you have 6 - 1k dimmers in a sunny cafeteria - you try to get away with all you can.
DeadCheerios
Member
Absolutely.
There are points where I will break my own rule as long as I know I won't be using all the lights at the same time. However when I am putting together packs to send out I will always assume everything I send out will be at full at the same time, just in case. Then if I end up needing to add more due to unforseen circumstances I have a bit of play room.
JChenault
Well-Known Member
Humm
So you are saying that in an incandescent situation, if I drop the voltage, that the amperage increases to maintain the same power output? Think about this. A resistance dimmer, an auto transformer dimmer, and an SCR dimmer work by reducing the voltage. If your theory were correct lamps would not dim when the voltage is reduced - indeed all breakers would pop at ( say) 10 volts as they would be drawing 12 times the normal load.
W=VA is a valid formula - but it is not the only one.
In your example, it is true that a 120 volt circuit drawing 1800 watts is pulling 15 amps. But what is the resistance of that circuit.
This is given by the formula P = (I*I) * R ( P = Power in watts, I = amperage, R = resistance. Plugging in the numbers you get a resistance of 8 ohms.
Now the resistance of the incandescent load doesn't change much as the lamp gets hotter and colder. So if you supplied ( say) 100 volts to the lamp, you would find the amperage by the formula I = V/R ( I - amperage, V = voltage, R = Resistance) In this case
I = 100/8 -OR- I = 12.5 amps.
At 100 volts - 12.5 amps would give you 1250 watts of power.
Please note that I am not arguing for or against derating your dimmers in a real world situation - just trying to make sure the math / and understanding of the formulas are correct. As always if someone can explain this better please feel free.
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When you wrote: "and it had to be fully loaded to not ghost." Was this during the resistance dimmer days, early dual SCR's, early TRIAC's OR ? ? ? In my road house days, I can recall this being a problem with touring piano boards, and their associated four packs of smaller sub dimmers, coupled with a number of ghost loads back stage out of sight purely for loading, but I don't recall it ever being a problem once solid state dimmers and "Scrimmers" hit.
Toodleoo!
Ron Hebbard
In my non-rep producing theatre days, I had a variety of co-op students thrust upon me. One actually had a flair for seriously appropriate non distracting effectively supportive lighting; stuff that established a mood, showed the actors to their best, layers of gobos, great colors from fabulous angles: None of the bump buttons set to 100% and bop to the beat that most of his age were into. He passed preliminary exams and was accepted into Canada's National Theatre School but was one year too young to attend. The production manager found a way for him to run the much smaller second space for a year to fill in time. One day he too came up with the theory that FELs had to consume more current as the dimmers went down. I sat him down, explained the math, and concluded by pointing out if his theory were correct, LD's contracted on a per production basis would need to negotiate for how many B.O.'s the theatre was prepared to pay for. At the end of his year he opted to do a proper construction and maintenance electrical apprenticeship, joined the IBEW and eventually became lead foreman on several major oil drilling rig projects in northern Alberta doing extremely well financially 'til the bottom fell out of our western Canada oil industry. Out of the blue, he cold-called me mentioning how explaining that simple math had changed his whole career path. I always giggle when I read folks assuming a one K lamp will always draw 1Kw.
Thank you for continuing to lead the good fight against the folly of some posters notions of electricity.
Toodleoo!
Ron Hebbard.
It's very easy to quote chapter and verse of rules and formulas. It's not always so easy to know what those rules and formulas actually mean, and our rich heritage of dubious descriptors doesn't help. When does a 575w lamp draw 575w? Possibly never if your theater's voltage is too high or too low. When is an 8Ω speaker actually 8Ω? Rarely. What load can a 20A breaker handle? Why did I blow up my 1000w subwoofer with a 800w amp?
It's very tempting to want to skip past all of the background knowledge and have an attitude of "Just tell me the answer". Unfortunately, that's a great way to get burned in life, and in the case of electricity, possibly literally.
Thank you profusely!! You've hit upon so many of my pet peeves in a single post.
How many times have you heard the "local expert" kid working after school in a local music store exclaiming to the gullible that speaker X can handle TWICE the power of speaker Y without ever mentioning a word about either speaker's efficiency. Being able to survive twice the applied wattage without smoking does little good if it's producing the same, or less, output. When have you ever heard any discussion of 'I Watt at I Meter' in a music store? (or doubling the power gains an extra 3 DB while doubling the distance costs me 6 DB.) I've never spent much time hanging around music stores, perhaps the little time I have has been in the wrong stores.
Rant off. Over and out.
Toodleoo!
Ron Hebbard.
Dionysus
Well-Known Member
The other important factor to remember for voltage drop is KIRCHHOFF'S CIRCUIT LAWS, wherein in a series circuit supply voltage equals the sum of the dropped voltage over the entire circuit (conductors included). Since these circuits are also not purely resistive (thus resistance), to be accurate one must account for all sources of IMPEDANCE (Impedance is Resistance and Capacitive and Inductive Reactance together) so Z should be used instead of R.
I don't have time right now to go fully into it right now however.
BillConnerFASTC
Well-Known Member
I was thinking piano boards - 14 plate 3K specifically. (I understand 24/48, 512, and some other "standards" but could never figure out how 14 - 3Ks became the standard. Probably had to do with box car doors or something. I believe the 14 -3k was about the same physical size as the 12 - 6K but memory is foggy.) Several photos of 12 X 6 but can't find one of a 14 X 3. Bad google fu his morning I guess.
