Tuesday, October 21, 2014

Juicer Math, Pt. 2 (or Why We Can't Just "Add More Cable" To A Run).

We all know (or at least we should) that Wattage = Volts x Amps. (Also known as Ohm's Law if you wanna get technical about it.) If you don't know what that means or why it's important, I suggest you look it up.

But how many of us know about line loss and voltage drop? And I mean, really know it?

Basically, your voltage is tied to the length and size of your cable and the amperage you draw. So the longer your cable run and/or the more amps you need, the lower your voltage will dip.

There are various problems associated with low voltage, such as equipment not running properly (HMIs won't run if voltage is too low and tungsten will dip in color temperature) and some safety issues as well. In fact, for our movie making purposes, the National Electric Code only allows us to a maximum drop of 3%. Math time: 3% of 120v* is 3.6v. Which means the lowest we could go voltage wise is (120 - 3.6 = ) 116.4v.

Now how do we calculate whether or now we're within those limits? By using the Voltage Drop Formula!


Where...   √3 = 1.73 (Duh)
                  K = Resistance of the conductor being used
                  I = Amperage
                  L = Length or Distance in feet
                  ACM = Area of Circular Mills (aka: a really scientific way of saying how thick the cable is. This number is based on the gauge of the cable being used)

Since the "K" we're usually dealing with is copper, and the resistance of copper is 10.8, we'll simplify the formula a bit. √3·10.8 = 17.82, so the new formula becomes...


There. Isn't that better??

Okay. So what does that mean? It means that if you're running 400ft of banded cable (aka: #2 gauge wire; aka: ACM of 66360) and pulling 100amps/leg, your voltage drop is...

(17.82·100·400) / 66360 = 10.74 volts

Guess what? That's way more than the 3.6 volts we've already determined you're allowed to drop and the fire marshall can totally shut you down.

But what if you switched to using 4/0 (which has an ACM of 211600)?

(17.8·100·400) / 211600 = 3.37 volts

Congratulations! You're still within code and you get to run 400 feet of 4/0!

But let's say your show sucks in the sense that it doesn't have the money for a rigging crew, the 4/0, or the money to pay you for a pre-call to lay out all that cable if you just happened to have the 4/0 anyway. Basically, you're stuck with whatever banded you have on the truck. And by doing some basic algebra...

(17.82·100·L) / 66360 = 3.6
[yadda, yadda, yadda...]
L = (3.6·66360) / 1782
L = 134'

... you know you can go 134 feet before being out of code.*** You lay in 150' of cable anyway because 1) you only carry 50' lengths on the truck and 2) when all is said and done, you've been averaging 90 amps/leg anyway which puts you right on the edge (at 90 amps/leg with 3% voltage drop, it comes down to be 148.9' if you want to be exact about it). So you're good! Yay!

So why am I giving you this mind-numbing lecture in mathematics (especially when there's surely an app for all this)? Because I'm trying to show there's a reason why we run the cable the way that we do. That the choices we make in placing the generator isn't based on our own personal whims, but is dictated by what's in or not in the shot, what production can afford, the amount of manpower we have, and what can be done safely. It's not that we don't give a shit about the sound department, but more often than not, this is as far as I can get the generator away from set based on the above criteria. We do the best with what we have, which is why it irks me to no end when conversations like this happen...

Locations/Sound Guy/Etc.: "We need to move the generator."
Me: "That's a little easier said than done..."
Them: "Why not? It's just another 50 feet."
Me: "Well, it's not just 50 feet. I have to go around that wall/building/can't just cut across that yard where the owner's sitting on his porch holding a shot gun. It may even only be 75 feet, but all my cable is in 50 feet lengths, so really, it's be at least 100 feet of cable."
Them: "Well, don't you have two more pieces of banded? I saw some on your truck when I passed by a minute ago."
Me: "Yeah, but-"
Them: "So what's the problem?"
Me: "The problem is that with banded and the amperage we're using on set, I can only go 134 feet before I'm breaking code and I'm already at 150 feet, so-"
Them: "Oh no no no no. Don't pull that number mumbo-jumbo shit on me. Just run the cable. Is that so hard?"


Don't people think there's a reason why we sometimes need 2/0 or 4/0? For Pete's sake, if it was always as simple as just slapping on another piece of cable regardless of the gauge, I'd just be running stingers with a cube tap on the end straight out of the generator to run the entire set!

Stupid mutherfuckers. (Can you tell I've been through this a few times?)

Another stupid request? "Can't you just bump up the voltage on the generator?" I can, but not as much as you think. 1) Going too high with the voltage on the generator can mess up the generator, which is never good; 2) Voltage goes up when amperage goes down, so when we start turning lights on and off, someone with some very expensive and sensitive equipment plugged into our system may get a nasty surprise along with a repair bill; and 3) Upping the voltage at the generator is not an appropriate way to make up for the fact that you're running the wrong sized cable to begin with. You should do it right the first time. And again, if cranking up the voltage regulator was an option, don't you think we'd say fuck it to 2/0 and 4/0 every time and just run a damn stinger to run the whole set and just "bump up the voltage"??

So no, it's not that hard to physically lay out another two pieces of banded. But electricity is a bit more complicated than just laying out pieces of cable, asshole.

Juicer Math, Pt. 1

* 120v being the standard here in the good ol' U.S. of A.
** This is a formula for a 3-Phase system only. Hence, the √3. The Single-Phase formula is 2KIL/ACM. If you don't know the difference between a 3-Phase system vs a Single-Phase system, you probably shouldn't be the one planning all this out...
*** And that's just on a good day. Heat causes resistance, so if it's particularly hot out and/or the cable is laying on some sun-baked asphalt, the voltage drop can be higher.

Sunday, October 5, 2014

The Filmmaker's Test.

The following is a simple test to see if you really are a filmmaker. Proceed with caution.

Raise your hand if you've ever...

...watched an action movie and wished you could kick ass like the hero.

...watched a scene unfold on screen and dissect how the camera was set up and where the lights were placed.

...felt bad for the crew when you watch a show that takes place mostly at night. Or in the rain. Or both.

...been so excited the night before your first day on a shoot that you can't sleep.

...known when Crafty would bring out new snacks and hover around the table in anticipation.

...hoarded the kind of soda or fizzy water you like from the coolers.

...skipped over the veggies at catering.

...played with props when the Propmaster wasn't looking.

...cleaned your sunglasses with supplies found on the camera cart.

...sat down on set and flipped through a "magazine" or book used for set dressing.

...sprawled across a bed on a bedroom set.

...gone through the breakfast line more than once.

...raided the set cart for batteries for your T.V. remote at home.

...taken a nap on company time.

...snuck a picture of a celebrity on set.

...sat in a pass van just for the air conditioning.

...gone "grocery shopping" at the craft service table.

...borrowed a belt or jacket from wardrobe because you forgot yours at home.

...had to explain to your mom a few dozen times what it is you do at work.

...had to explain to your dad what you do at work in hopes that maybe he can explain it to her (note: this never works).

...splurged on a phone or a tablet with the thought of, "It'll come in handy at work!"

...shown up for work an hour early because you looked on the wrong line of the callsheet.

...shopped for a present for someone while at work (thanks, cellphone!).

...stopped what you were doing, looked around and thought, I have a pretty cool job.

...put off a doctor or dentist appointment because of work.

...rolled from one shoot right on to another.

...never met your neighbors because you never see them.

...forgot your friend's birthday because you were so busy, you lost track of the days.

...lost a girlfriend/boyfriend/significant other because you never got to see them.

...missed out on a family event because you're booked on a job.

...gave up a pet because you're never home to take care of it.

...ever wondered how you'd be able to balance a career and a family with a job like this.

...had a day so good, you knew this is what you wanted to spend the rest of your life doing.

...had a day so kick-you-ass-brutal, you wonder if it's all worth it.

...been told that Hollywood's a hard town and you won't make it.

...left behind family, friends, and all you've ever known to move to a place you've never been to because you wanted to make movies.

...purposely forged ahead without the safety net of a stable bank account or back-up career because you want that fear to push you to succeed.

...come to a halting realization that this is harder than you ever thought.

...reached a stagnant point in your career and start to wonder what else you could do for money.

...wondered what'll happen if the calls for work stop coming in.

...have this nagging fear that maybe they were right and you'll never survive in this industry.

...ever wonder if you're good enough to rise out of the low paying gigs.

...been scared that this is as good as it gets and you've peaked in your field.

...feared that you'll have to return home to your parents' house because you couldn't make it and it's time to "grow up." 

...feared that you sacrificed it all for nothing.

...kept on forging on anyway because you have faith in yourself that it will all work out in the end because despite all the uncertainties, the pain, and the fear, you're passionate about what you do.

If your hand is still raised, congratulations. You are a filmmaker.

Bonus test.

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