As a filmmaker, when you’re part of a small crew, you want to be able to estimate what lights or equipment (and how many of them) you can use in any particular location.
Unfortunately, there isn’t a single tool that can give you the answer with a few clicks of a button. And, nobody wants to learn arcane formulas and graphs of electrical phenomena. Understanding electrical engineering concepts is hard. There are two possible solutions to this problem:
- Get assistance from a gaffer, or
- Do it yourself
If you’re on a well-budgeted production, there is no excuse for not involving a gaffer in your estimation of lighting and equipment. Hell, even on a zero budget movie, it might be better to do away with the director and keep the gaffer! One shouldn’t cheapen out. Gaffers know more about professional film gear (light fixtures, wind and smoke machines, video villages, etc.) than the average electrician. This experience matters.
However, on low-budgeted productions, tagging along a gaffer for estimation purposes might be expensive. And, no sane gaffer will tell you on the phone (or a forum) exactly what you can or cannot run without studying the electrical systems of the location first hand. So, let’s assume you can’t involve a gaffer for the location scout. What do you have to know to get the job done?
This is what this article is about – how to estimate your lighting and equipment requirements, given a portable generator or mains supply. My aim is to keep things as simple as possible. I’ve taken the liberty to eliminate anything that is not relevant to this purpose.
Instead of formulas, I’ll use analogies.
Speaking of formulas, throughout this three-part series, I’m only going to give you one formula – one formula to rule them all:
Power (W) = Voltage (V) x Amperes (A) x Power Factor (PF)
That’s it. No more formulas.
However, there’s a catch. The hard part is finding the right values for the variables. That’s what the rest of this article is all about.
This article will help you:
- Understand how an alternating current (AC from here on) power supply works.
- Understand why you might not get the full power you are ‘promised’.
- Estimate how much power a portable generator or mains power supply can really deliver.
- Estimate how many lights, computers, equipment or gear you can use on it.
- Learn to communicate effectively with your gaffer.
I will only be covering sources of alternating current (AC) in this guide. In this first part we’ll cover the basics. In Part Two we’ll look at how to deal with the mains power supply, and in Part Three we’ll look at dealing with portable generators.
|Important Warning and Disclaimer:
This article is for information purposes only. No action is to be taken based on any of the formulas, ideas, information or suggestions provided here. In all cases you must take the help of a certified and experienced professional electrician. You are totally responsible for your actions. Before you play with electrical equipment and power supplies, please ensure you know local laws and regulations concerning the use of electrical and electronic devices, emergency first aid procedures for burns and electric shocks, where the nearest hospital is and how to get assistance. Always wear protective clothing and shoes.
Remember, this ‘guide’ is just for a quick estimation, and does not substitute for practical expertise or recommendations. I have left out a lot of things, and have oversimplified concepts just to help understanding. Little knowledge is dangerous. Just understand, don’t apply it.
What is Voltage and what is Current?
Imagine your enemy running in the forest, and you want to set a trap for him. You dig a ditch so he’ll fall into it. The bigger the ditch, the better the chances of him falling.
This ditch is voltage. It has the potential to trap your enemy. The bigger the ditch, the greater the potential, or greater the voltage.
When your enemy falls into it, he generates current. Current flows.
The ditch (voltage) allows the possibility of current (fall). If you want to generate a current, your best bet is to ensure there is a voltage ready somewhere. A wall socket is one such ‘voltage-ready’ point in space, waiting for some device to be plugged in (enemy) so current can flow. An electrical device is one that needs current to work.
Pull out the device (or switch off the socket, in other words, break the circuit) and the current stops flowing. The device stops working. The wall socket is waiting for the next device.
The voltage at any point is measured in Volts (V). Current is measured in Amperes (A).
The most important thing to know about V and A is that they are different for DC (batteries and DC generators) and AC (portable AC generators and mains electricity). Both are measured in Volts and Amperes, but the way they are measured is different, and sometimes finding a one-to-one correspondence is impossible. It’s like men and women. Both are human, but try changing one to the other. The point is, don’t use DC formulas for AC, and vice versa.
In this article, we will only refer to AC power, and the formulas only apply to AC.
What is Power?
If your ditch is large enough, you can trap a bigger enemy, or many enemies of the same size. If you do this, you have power over your enemy. If your ditch is too small, you have no power over your enemy. This is one way of using the word ‘power’.
In the world of mechanical engineering, a horse has power – it has power to pull a cart. Steam engines and fighter jets have the power to carry and deliver payloads. Electricity has the same kind of power. It can move things too – table fans, motorized sliders, trains, computers and even your brain.
For this reason, we equate the power of electricity to the power of mechanical forces. It makes comparison easier. If a horse can pull a carriage 20 feet in 10 seconds, how many seconds will it take for an electrical locomotive to pull the same carriage over the same distance, and so on. This power, the ability to move things, is measured in Watts (W).
As we have seen, power, voltage and current are related like family:
Power (W) = Voltage (V) x Current (A) x PF
What the $#@%! is PF?
The Power Factor (PF)
The power factor (PF) will always be between -1 and +1. For our purposes, its value will always be between 0 and 1.
What happens when you multiply a number with a number that lies between 0 and 1? Yes, the value of the number reduces. The only way to get the full value back is if the power factor is 1. Anything lower is a losing proposition.
Why is there a power factor? Trust me, you don’t want to know.
Real power and Apparent power
This one you’ll want to know.
Theoretically, you want all the power possible. Why waste any? It’s like building a ditch to trap an enemy but finding a small elephant sleeping in a corner of your ditch taking up space. The bigger the elephant the lower your chances of trapping your enemy.
Let’s look at our formula again (the very same one, no more!):
W = V x A x PF = [V x A] x PF
V x A is called the Apparent Power. W is the Real Power. I know your head’s hurting. Here’s the balm:
You speak to your loved one over the phone, and time flies. It felt like you just spoke for ten minutes. The bill says you spoke for five hours, and the amount reflects that.
- Real power is the power a device thinks it draws.
- Apparent power is the power it really draws.
The device says: “But I’m only rated for X Watts, and that’s all I’ve used!” The socket says: “You took Y Current, and now you must pay.” Since the device is like a spoiled brat who broke somebody’s iPad, the father (you) pays.
The villain in the piece is the power factor. He’s Mr. Ugly in the Good, Bad and Ugly. And just like in the movie, Mr. Good has to suffer Mr. Ugly. After all, the power factor is introduced into the transaction from the device itself, and not from the socket.
For our purposes, real power can never be greater than the apparent power, only lesser or equal.
What’s the point of knowing terms like real power and apparent power? It is important because some specifications display the real power while others display the apparent power:
- When you see a device with a power rating in Watts (W) or KiloWatts (KW, meaning 1,000 Watts), the rating reflects the real power.
- When you see a device with a power rating in VA or KVA, the rating reflects the apparent power.
It gets worse. Some manufacturers (even professionals) interchange the two, due to ignorance, laziness or marketing needs. Some countries have strict regulations on how electrical devices need to be rated, and how these ratings should be displayed. Other countries might not. The goal of understanding real power, apparent power and the power factor is so you are not taken for a ride.
Let me give you an example. Why do we say a light is 1K or 2K or 4K? K-what? Is it KW or KVA? See the problem?
Power factor of some lighting fixtures and gear
There are five main types of lighting fixtures: HMI, Tungsten, Fluorescent, LED and Plasma. The other devices you might need powered on set are PCs, laptops, battery chargers, monitors, wind or smoke machines, etc.
Here’s a quick list of the approximate power factor (Not accurate! It varies from device to device) for these types of gear:
|Gear||Approximate Power Factor*|
|HMI Electronic Ballast||0.5 to 0.6|
|HMI Magnetic Ballast||0.5 to 0.6|
|LED||0.6 to 0.9|
|Fluorescent Electronic Ballast||0.6 to 0.9|
|CFLs||0.6 to 0.9|
|PCs||0.5 to 0.7|
|Laptops||0.7 to 0.9|
|Variable speed fans||0.5|
|If you don’t know, use this||0.6|
|*Uncorrected Power Factor|
What is a good power factor? Anything greater that .85 (85%) is good. As you can see, most things we want powered have a poor power factor. A power factor of 0.5 (50%) means that the device draws twice the amperes it is rated for! What a waste.
Of course, all is not lost. Many modern power supplies come with something called Power Factor Correction (PFC). It is an additional circuit that is added to the guilty party to bring it as close to 1 as possible. Usually the result is above .9 (90%) and this is okay.
To find the power factor of a fixture, look at the electrical specifications on the device. It must be mentioned explicitly. If you have to ask, then there’s something wrong.
Rule of thumb: If any of the specifications I’ve listed in this guide are not explicitly available from a manufacturer’s written tech specs, then don’t trust that manufacturer.
For every device you need powered, find its power factor and write it down. Great! We’ve already knocked down one variable from our four-variable formula. Now, on to something totally different.
What is Single-phase and Three-phase power?
Imagine a battery:
Now imagine two batteries connected to each other in this way:
If each battery is 1.5V, then together, they provide 3V. Now, imagine this quirky scenario:
Don’t worry about the polarity of the batteries, it’s just an analogy. If you measure between any two points you get 3V:
Why would anyone want to create power in this strange ‘Y’ design? That’s not for us to know at this time. All you need to understand about three-phase electricity is: If current travels through one wire in single-phase, it travels through three in three-phase. Don’t forget that three-phase power is AC.
The area where the three batteries meet is called Neutral (N). If you measure from the tip of each battery to its rear (N), you’ll get 1.5V. This is single-phase.
I might have given the wrong impression that three-phase is always twice the value of single-phase, but that’s not the case. In fact, three-phase is 1.732 (?3) times that of single-phase, voltage-wise. What we usually get on our wall sockets is single-phase – at homes (Residential) or in offices (Commercial). Three-phase is usually left for transmission and for large factories, machinery, etc. (Industrial). There are exceptions, of course. If you have a three-phase supply in your bedroom, don’t freak out.
Electrical Distribution in a nutshell
Electrical distribution is one of the great triumphs of the human race. Without it we’d be having only candle-lit dinners, and be spending most of our time in meaningful face to face human interactions.
Here’s how electrical distribution works: First, something must move. When it does, it generates electricity. The thing that moves is a turbine (think: ultra large ceiling fan). How would you move a ceiling fan without electricity? You could turn the blades with your hands. Or, you could:
- Blast the fan with water, and the blades will move – Hydroelectric power, usually carried out by building dams and using the power of moving water from a great height.
- Blow air – Wind turbines.
- Put rockets on it – Burning coal or other fossil fuels.
- Put a small atomic bomb between the blades and set it off – Nuclear power.
Together, these are called power stations or power plants. For some strange reason, when turbines rotate, electricity is born. The power plant generates three-phase electricity at a certain voltage.
Power begins at the power plant:
From here, quickly, before power is lost, it is transmitted via power lines over large distances:
The power transmission lines end at substations. The most important thing about sub-stations is that they are able to divide power into different ‘groups’ for different people:
The power you receive in your home is single-phase power. Some households or large apartment complexes might get three-phase power, but it is usually split into single-phase lines for each building. It also depends. There’s a world of a difference between a two-storied structure in a village and a skyscraper.
But don’t worry. Regardless of whether you find single-phase or three-phase sockets on location, this guide covers both, and you should be okay.
The substation might also cater to really large industries, small industries, further transmission, etc. Electrical power of every kind imaginable can be created from the substation. The weapon of choice is called the Transformer. Its job is to convert voltage from one level to another.
The further you get from the substation, the lower the voltage gets. This is because, due to various reasons, energy is always lost during travel. Every transmission line or cable (even the one going from your mouse to the computer – wired or wireless) has a loss.
Therefore, at every end point in the electrical distribution chain, you’ll find a transformer that is waiting to raise the voltage to acceptable levels. Non unlike mic amplifiers, or an energy drink at the end of a long run.
As you can see, the entire chain of events is handled by two kinds of tools:
- Generators – they convert ‘something’ into electricity.
- Transformers – they raise or lower the voltage.
Single-phase into your home or office
Near a cluster of homes you’ll find a transformer (No, it does not look like a car). From the transformer a cable travels (either overhead or underground) into your home, and ends up in a box called the electrical Distribution Board (DB) (Sometimes also ‘panel’):
The mains supply is first connected to a Circuit Breaker (CB). There are many kinds of circuit breakers, each with its own advantages and disadvantages. We need not be concerned as to their names at this point. Just note that the role of a circuit breaker is to trip (or stop or break) the circuit if something goes wrong. This usually happens if current flows through something that it is not supposed to. Circuit breakers save lives and equipment. And, their use is mandated by law in most countries.
After the main breaker, each part of the cable is split to its core functions. These are:
- Line or Phase – the wire that carries current to the board.
- Neutral – this is zero volts. It exists close the circuit (the bottom of the ditch you dug for your enemy. If there is no bottom, he’ll fall forever).
- Earth or Ground – this is also ideally at zero volts. It exists to provide a quick path for any unwanted current. To where? To the ground or earth! If the mains supply has an earth wire, it will be earthed or grounded at the transformer to the substation all the way back to the power plant. If there is no earth wire, we’ll need to create it, but that’s outside the scope of this article.
In the above image, the three wires terminate at rectangular plates called busbars. Busbars are metal plates that conduct electricity. Their job is to reduce wiring inside the DB, and keep things neat and tidy. Their other job is to ensure every circuit gets an even voltage.
From the busbars, you can start building circuits. A circuit is a closed loop. Notice the direction of the arrows in the above image – the line/phase will take current from the mains and supply it to the circuit. Neutral should ideally remain at zero. If a device uses up all of the current, then nothing should flow through neutral. In any case, just in case it does flow, then neutral is connected to Earth. This way, both are ensured zero volts under normal conditions.
Earth has two directions. This is because when a short circuit happens, the circuit is not confined to the line and neutral wires, but beyond it, like this:
Let’s say for some bad reason some current touches the outer frame of a tungsten fresnel light (many of the cheap Arri knock-offs are dangerously wired this way), and a poor soul happens to touch it.
There are three ways for this extra current to flow:
- If the fixture is grounded, it will flow through that.
- If the circuit is grounded, the current will flow back to the DB and out. If not, the ground wire from the fixture must be connected to a rod and driven into the earth.
- If neither are grounded, the poor soul will get an electric shock. This is because we are conductors of electricity, and our contact with the ground allows a path for the current to flow (Ground is always assumed to be zero volts). This is why it is a good idea to wear protective boots on set.
The direction of current flow depends on how a fixture or device is earthed or grounded. But you might ask: Why is it the other way? Well, the distribution board itself acts like a fixture, and if a loose wire or whatever touches the frame it will have a potential. All it takes is for someone or something to touch it. This is why I showed the flow of ground current in two directions.
By the way, the parallel lines on the ground (the one that looks like an upturned tree) is the universal electrical symbol for earth or ground. The universal color of the cable for earth or ground is green+yellow or green. The universal color for neutral is black (but not always). The line or phase wiring can be of different colors. Sometimes, different colors are used to distinguish between different circuits, for easy maintenance. Each country has its own set of color schemes for single-phase and three-phase line cables.
So, what’s the best way to ground something? Well, you must have a grounded distribution board. You must have excellent circuit breakers that don’t compromise on anything. Every device capable of giving you an electric shock must be grounded as well. Usually, this means wiring an earth cable to a metal rod and piling it into the ground.
Ultimately, all this is too much work for the filmmaker, technician or DP. It is best left in the capable hands of an electrician, also known by his or her alter-ego superhero name – Gaffer.
We’ve seen how power comes from a power plant to the distribution board. Let’s not stop there.
First, just so there’s no confusion, let’s define some terms:
Why do we need plugs? Two reasons: Safety and Convenience. You have something to hold on to without risking electric shock, and you can plug or unplug something easily in one motion.
There are only four ways a cable can be wired:
- Line + Neutral (two wire system)
- Line + Neutral + Earth (three wire system)
- Line 1 + Line 2 + Line 3 + Neutral (three-phase four wire system)
- Line 1 + Line 2 + Line 3 + Neutral + Earth (three-phase five wire system)
Earth or ground need not travel within the same cable, but can be a separate cable. This is preferable.
You might be wondering why, sometimes, you have plugs with just two pins, while on others there are three? Who decides if there should be two or three? Shouldn’t all sockets be earthed (hence have three pins)?
Yes. All sockets should have three pins, but sometimes, you have devices that only offer you a two-pin plug:
Why? Here’s a general rule of thumb: Any device that converts AC into DC immediately to power a low voltage DC device usually has only two pins. Of course, by law, they are allowed to do this for two reasons:
- Voltage is low, but more importantly –
- Their devices are insulated, so even if line touches something, nothing on the outside (where you’ll touch it) will ever be effected. Sometimes, they have double insulation to ensure this is so.
Examples of such devices include mobile phones, laptops, camera battery chargers, etc. Examples of devices that need DC but are not completely insulated include computers, printers, etc.
There various “stamps of approval” that show a device adheres to certain legal standards, like the CE mark or FCC mark, etc. However, many products manufactured outside these jurisdictions have the “mark” but don’t meet the required standards. This is a dangerous practice, and the only way you can be sure an equipment is safe is by getting it thoroughly tested by a professional. In some countries using unsafe equipment will land you in trouble with the law.
This means opening up the equipment for systematic testing. E.g., if you have bought lighting fixtures from countries where such legal standards are not strictly followed, you must open it up and get each connection, cable and grounding checked. If you don’t, it might prove lethal. No joke, playing with AC.
Back to wiring. There are some rule of thumbs for wires that you should be aware of:
- The thicker the wire (greater the diameter) the more voltage it can carry.
- The longer the wire, the more it will lose voltage.
This is why transmission cables are super thick – they have to travel over hundreds of miles and they’ll lose a lot of their power (how inefficient!). To counter this, the voltage at the source is raised to mind-numbing values, so they’ll reach the other end with what’s required. Then they get the energy drink.
The same happens on a smaller scale in a house or office. If a cable has to travel further than what science dictates, you must use a transformer to raise the voltage. If the transformer is attached to the source, the cables will get thicker.
Before we move on, let’s cover the power surge. When lightning strikes, current might flow through cables, and this will pass on to your equipment if it isn’t protected somehow. Some equipment comes with internal surge protectors, but the best way to protect your equipment is by using a high quality surge protector:
Bottom line is, the thickness of cables are decided by how much current they have to carry, and for how far.
There are complicated formulas for calculating how much load a cable can take. Each manufacturer publishes their own tables so you can estimate which type of cables are right for which scenario. One would hope that the person who wired a facility would have used the appropriate cables instead of shortchanging its tenants to lower costs. Unfortunately, the only way to know for sure is to open the distribution board and study it.
Sometimes, you’ll find the cables are too small for the equipment you want, but the mains can supply the power you need. In that case, the only way to get power is to get separate cables (long enough to run from the DB to your location) and connect them to the busbars (carefully, so as to not affect any other circuit – you’ll need permission for this).
Enough of the basics.
In Part Two we’ll look at how to read ratings for mains power supplies, and from there we’ll figure out how to estimate how many lights you can use on it. Don’t worry, we’ll do it in baby steps.