In Part One we understood the basics of power generation and how it arrives at a wall socket or outlet. In this part, we’ll focus on:
- What information you’ll need to collect for your estimation.
- How to calculate the available power, voltage and amperes of a circuit.
- The characteristics of various lighting fixtures, gear and equipment, and how they affect the power supply.
- How to estimate how many devices you can plug into a wall socket.
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. |
Inrush Current
Before we get to the meat and potatoes, we have two more concepts to understand. The first is inrush current (also called surge current).
Almost every device (including lighting fixtures and AC-to-DC converters that you find on chargers) requires an initial surge of current that is well over its rated capacity. E.g., tungsten fixtures sometimes generate 10-20 times the rated current on start up. The same applies to HMI and fluorescent lamps, but to a lower degree.
Let’s say a 15 A circuit from the DB feeds a 100 W tungsten light bulb in a room. If the voltage is 120 V, the Amperes drawn is 100 / 120 = 0.83 A (we know power factor for incandescent lights is 1). If the inrush current is 15 times the rated current, the bulb will need an initial surge of 0.83 x 15 = 12.45 A. This is lower than the specification of the breaker. But even if the bulb was 200 W, it wouldn’t make much of a difference. Why not?
It doesn’t trip because breakers normally require that current flow for a specific minimum amount of time before it is deemed dangerous. An inrush current subsides in less than a second. It’s like an elephant stole your pants right under your nose – while you were wearing them – and put it back on again – and you didn’t notice a thing!
This seems unfair, doesn’t it? Why put in a breaker at all? Well, for one thing it is already known that lighting fixtures and some appliances need the inrush current to start. The alternative is darkness. The second reason is that the inrush current only flows to the device that needs it, and its duration is so miniscule that it hardly matters to somebody’s electricity bill. On the flip side, if a device doesn’t play fair, and the inrush current is too large, it will destroy your circuit.
Now, how does inrush current affect large wattage lamps used in filmmaking? There are some neat tricks which the manufacturers don’t tell you about, but must be thanked for. One such trick to to preheat a tungsten filament to lower its resistance. It still looks switched off, but it’s being heated. When you switch on the lamp, the inrush current is well below dangerous levels. The coolest part is, you feel the light glows instantaneously and switches off instantaneously. Modern fluorescent and HMI electronic ballasts also have circuitry that greatly reduces the inrush currents.
Okay, enough with the technicalities of inrush current. Modern electrical systems, especially those with power factor correction, also have circuitry to limit the inrush current. For small devices, the inrush current is negligible. For larger wattage lamps, it is higher, but not high enough to cause the mains supply any problems. Very few manufacturers specify the inrush current on their lamps, and as far as I know they are not required to. If you can’t find the information, and the manufacturer doesn’t wish to share it with you, assume it’s 2-5 times the normal current.
You might have noticed that every circuit has a ‘maximum’ rating and a ‘normal’ rating. This is how electrical systems behave. The maximum rating is also called a ‘peak’ rating (as in: peak voltage, peak power, peak current, etc.). The normal rating is also called nominal rating or steady state rating or operating rating (as in: nominal voltage, steady state power draw, operating current, etc.). When we make estimations, we need both peak values and operating values.
The two major world ‘powers’
Luckily, countries believe that voltage should be constant. Therefore, most of the world offers either of these two standards:
Voltage | 120 V ±5% | 230 V ± 10% |
Frequency | 60 Hz | 50 Hz |
You have two choices:
- Live with it, or
- Buy a transformer and change the voltage for your house or facility.
There is one good reason for choosing the latter option, and that is our power formula:
Power (W) = V x A x PF
If a device is rated for a specific power and power factor, and the voltage is constant, the only variable that can change is current. Now look at the two voltages available for mains: 120 V and 220 V. For the same wattage, a circuit rated at 120 V will draw more current than a circuit rated at 240 V. This means, to get the same power, you’ll need thicker cables and higher rated breakers.
Sometimes, on set, you’ll find gaffers rigging portable generators with a transformer to raise the voltage. Some generators come with both 120 V and 240 V options for the very same purpose. Even in homes, devices that need more current (like heaters, etc.) might work on 240 V even if everything else is at 120 V. These are special cases. Most of the time, it’s either-or.
There’s another reason why you might want to change the voltage, and this when you’re traveling to a country whose electrical specifications are different from your own, and you are unfortunately carrying devices that won’t work. Finally, you’re using transformers whether you know it or not. Ballasts for HMI and fluorescent lamps are transformers too.
Anyway, using transformers to raise voltage is beyond the scope of this article, so let’s ‘live with it’.
Voltage drop
I told you there were two things we needed to know to start estimating our needs. Voltage drop is the second.
The farther away the distribution board from the wall socket, the greater the fall in voltage. If you know the resistance of the cable being used you can calculate the voltage drop, but I promised no formulas. So, feel free to skip this next box.
The formula to calculate voltage drop is:
Voltage Drop = [Length (in feet) x Resistance (in Ohms/1000 feet) x Rated Amperes (A) ] / 500 If the distance between the DB and the wall unit is 20 feet, and you’re using a 10 AWG cable (Resistance = 1 Ohm/1000 feet), and the rated Amps is 15 A, the Voltage Drop = 0.6 V. That’s hardly anything. But for fun, if you want to take the same cable to the top of the Empire State Building (1,250 feet), the voltage drop is 37.5 V. This means the percentage drop is 31.25%. Is this okay? No. In most countries, the voltage drop is never allowed to exceed 5% (USA) or 4% (UK). As a rule of thumb, you can assume the maximum is 5%. Just in case you’re wondering, the maximum length that would satisfy the 5% drop for the above example is 200 feet. |
The simplest and most accurate way to know the voltage drop is to measure it with a meter! But we’re not electricians, just filmmakers. Most countries forbid a voltage drop of greater than 4-5%. Therefore, in our calculations, we assume a voltage drop of 5%.
What is the maximum power you can get from a mains supply, period?
Every circuit created in the distribution board is rated for a current, in Amperes. The engineer who designed it ensures people follow this limit by selecting:
- A cable that will only work till that limit,
- A circuit breaker that will trip beyond that limit, and
- A wall socket that can only handle that limit.
E.g., if the engineer has decided that a circuit should only supply 15 A, then the socket will be a 15 A socket, the breaker will be a 15 A breaker, and the cable will be of sufficient thickness just enough to handle 15 A. If you want to change the ampere rating, you’ll need to change everything.
If voltage is constant, and the power factor is constant for your device, the one factor that decides how much power you can draw is the current rating.
So, what options do you have for circuits? In most cases, you have two options:
- The normal option – most of your appliances, lights, chargers, fans, etc., will run on this.
- The ‘higher-up’ option(s) – more demanding appliances like ovens, refrigerators, air conditioners, etc., will run on these.
This system has been followed since the dawn of electricity distribution. Depending on the country you live in, the normal and higher-up option(s) will vary. Here are two examples for the US and the UK:
UK | Frequency (Hz) | 50 | |||
Voltage (Volts) | 230 V ± 10% | ||||
Max Current (Amperes) | 5 | 13 | 15 | 30 | |
Max Apparent Power (W) | 1200 | 3120 | 3600 | 7200 | |
USA | Frequency (Hz) | 60 | |||
Voltage (Volts) | 120 V ± 5% | ||||
Max Current (Amperes) | 15 | 20 | 30 | 50 | |
Max Apparent Power (W) | 1800 | 2400 | 3600 | 6000 |
It is obvious, that at the same Ampere rating, you get less power (about half) on a 120 V system than a 240 V system.
Probably the most common ampere rating circuit is 15 A (sometimes also 13 A or 16 A). As you can see from the above table, a 15 A socket can only deliver a maximum of 3.6 KW in the UK, and 1.8 KW in the US.
The 80% rule
This theoretical maximum is not always available to you. Some laws and regulations state that a circuit must not be loaded to greater than 80% of its capacity. This means 2.9 KW in the UK and 1.5 KW in the US – on a 15 A circuit. A 20 A or 30 A will give you more, as shown above. Whether or not you follow this practice, depends on the regulations. The regulation is there for a good reason: Many of the wiring we have around are older than our parents. It is not wise to run 15 A through a 15 A rated wire from another era.
Therefore, as a rule of thumb, if you’re in an old location, and the distribution board looks like a spider-terminator’s web, then apply this rule for safety.
What to do when you walk into a location
This is detective work. I’m going to explain this with an example. Let’s say you’re interviewing the CEO at his swanky office. You have two 5 A circuits, two 15 A circuits and a 20 A circuit to play with (this is imaginary, so play along).
Follow these steps:
The sockets
Count the number of sockets. In our example, we have five of them. Some sockets have the ampere rating written on it. If nothing is written on it, you might want to call an electrician to know exactly what the rating is. You’ll also want to know two other things:
- Whether or not the socket has a fuse. If yes, what is its rating? Does it match the socket’s rating? It should.
- What is the thickness of the cable used (more later)?
You write down this information clearly, and then ask for the distribution board or panel. If it’s a commercial office, it most likely will be on the same floor. For older offices and homes, it might be on the ground floor.
One other thing you want to inspect is the socket type – is it a 2-pin or 3-pin socket? What kind, exactly? You need to ensure your gear has plugs that will match the sockets.
Measure the distance of the walk
From the socket to the distribution board – how long is it (in feet)? Don’t forget that cables travel through walls, the ceiling or the floor, and not straight as an arrow. If the distance is just a few hundred feet, the voltage drop is negligible.
Finding the distribution board
First, find out where you are in the chain:
- Electrical room of a complex or area (multiple buildings)
- Main Distribution Board (MDB) of a building – apartment or commercial
- Distribution Board of an apartment, house or office or floor
- Panel or distribution board of an apartment or home
The goal is to get the distance between the DB and the socket as close as possible What do I mean by this?
If you start at an electrical room of a large complex but are only interested in two sockets in one room of a single apartment, you should look for the MDB of the building, or even better, the DB on the floor or the apartment. How do you know when you are close enough? Simple. Where do you have maximum control to read and manipulate power if it came to that? That’s your ideal beginning location. E.g., if you have access to the electrical room, but are not allowed to draw power from it directly, what the hell are you doing there anyway?
The distribution board
Open the distribution board. If it’s a branded one it’ll have a drawing on the back of the cover (or some other place) with information on how it is wired. Unfortunately, most of the DBs will look like this (on a good day!):
Your job is to find which sockets go to which circuit breaker (MCB) on the distribution board. If the information isn’t written on the board, and no one knows, the simplest way to test it is to get someone to plug in something at the socket and switch off the breaker to see if it stops. If you’re the only one, then use a radio! If you want a tool that will do the job for you, look for a Circuit Breaker Finder.
Every location presents its own challenges. Don’t assume just because a building is ultra-modern on the outside and sits in the heart of the business district it will have good wiring. The same is true in reverse. Never assume a run-down home or location has bad wiring. In short, don’t make assumptions, find out the answers. If you can’t, stop this exercise and call a professional.
At the distribution board, you’re also checking to see if the same cables that you saw in the socket end up here as well. If not, then you know there’s a joint somewhere in between. If there is, you might want to investigate where it is to see if it’s just a simple joint or whether there’s something else going on that shouldn’t be happening.
The cables
Cables are important things. No matter what the ratings say, if a cable can’t support a certain ampere level, current will not flow.
Here are the approximate (Please don’t use!! Please refer to your local standards and laws. This is just a rough indication for information purposes) values of cable sizes against Ampere draw:
Cable required | |||
UK | USA | ||
Required Amps (A) | Square mm | AWG | Square mm |
6 | 0.75 | 14 | 2.08 |
8 | 1 | 13 | 2.62 |
12 | 1.5 | 11 | 4.17 |
17 | 2.5 | 9 | 6.63 |
22 | 4 | 8 | 8.37 |
29 | 6 | 7 | 10.5 |
40 | 10 | 5 | 16.8 |
54 | 16 | 4 | 21.2 |
71 | 25 | 3 | 26.7 |
87 | 35 | 2 | 33.6 |
105 | 50 | 1 | 42.4 |
135 | 70 | 1 (1/0) | 53.5 |
165 | 95 | 00 (2/0) | 67.4 |
190 | 120 | 00 (2/0) | 67.4 |
220 | 150 | 000 (3/0) | 85 |
You don’t have to do it yourself, but a professional electrician would study the cable size and refer to the manufacturer’s data sheets for exact measurement. There is no easy way. The good news is, once you’ve successfully estimated your requirements on many projects, you’ll start to get a hang of things.
There is one kind of circuit where the cable size can throw you off, and that is a Ring Circuit, which is mostly found in the UK. In this case, the cable is usually only about 60-70% of the total, because two wires share the load equally.
Understand the circuits
It’s only in rare cases (air conditioner, heater, etc.) that a complete circuit is dedicated to one socket. For general appliances and lights, you have multiple points per circuit. E.g., a 15 A circuit can be divided between two sockets (outlets) or more; so, if they’re both being used, they will have a total capacity of 15 A, and must share that. Some older homes might have 5-10 sockets per circuit. This is obviously bad because it reduces the power of each outlet assuming all of them are being used at the same time.
You’d want to isolate the outlets according to their circuits. This gives you the freedom to choose what lamp (or device) goes where.
In our example, let’s assume the two 5 A outlets and the two 15 A sockets share the same circuit. The 20 A socket is on its own breaker.
Circuit breakers
Single-phase circuit breakers are rated at standard values, the most common being: 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A, 100 A and 125 A. Look on the breaker itself, and you will see it in big bold lettering. Any breaker that falls within these limits is a Miniature Circuit Breaker (MCB). Household and Office circuits needn’t go over 100 A (total) normally.
Now look at the main breaker (that attaches to the mains supply cable). It will be rated higher. Let’s not worry about its name, but only two things:
- Is it single-phase or three-phase?
- What is its Ampere rating?
Knowing the total power coming into the house or office will tell you whether the circuits made from it are okay or disasters waiting to happen.
Whatever you do, don’t assume the main breaker’s ampere reading must be the sum of all the individual breakers’ readings. That’s not how it works.
If you add up the amps on each MCB, you’ll notice it is always greater than the amps on the main breaker. Does this mean the circuit is overloaded? Of course not. Just like Internet and mobile bandwidth is always oversold (because not everyone uses all of it at the same time), electricity is also a scarce and expensive commodity. Every socket is not always connected to a device, and every light in a building is not always turned on at the same time. Some devices, like heaters or air conditioners, are turned on at the same time depending on what season it is.
The bottom line is, look at how many houses or offices the distribution board is feeding. Here’s a rule of thumb: The more the houses the more the difference between the sum of the breakers and the main breaker. E.g., if you live in a hut in a small village, then even 100 Watts is precious and you can bet you’ll use every last drop of it. On the other hand, if you’re staying in a modern high-rise, with more than 100 apartments, then the difference can be almost 50%. In any case, the ratio of the sum of amps of each MCB and the main breaker must be between 40% to 100%, and never less than 40% – as a rule of thumb. If this ratio holds good, you can forget about the main breaker and move on.
Note down the ampere rating on the breaker – for each circuit you care about.
In our example case, let’s say we’ve identified three breakers (three circuits), and the ampere ratings are 5 A, 15 A and 20 A (just an example).
Putting it all together
Okay, if the breaker, socket and cables are all okay, you know the circuit is capable of carrying the ‘advertised’ ampere rating. If any of the links in the chain is not capable of carrying the rated current, then the circuit must be ‘down-rated’ to what it can carry.
In our test case, we know our outlets will give us 5 A and 15 A (but not at the same time, because they are shared), and 20 A.
Our takeaways from this investigation are these values:
- The ampere ratings of each circuit
- The voltage ratings of each circuit
- The 80% rule
Do you want an easier and more accurate way to do this entire thing? Simple, just measure everything at the outlets with a Multimeter or Power Meter:
You’ll have all your answers in ten minutes.
How to use the power formula
The formula for power applies to every possible thing in a circuit – separately. It applies separately to the circuit breaker, the line wire, the socket, the ballast and the lamp, etc. I’m going to simply things, and only divide it into two parts:
- What the outlet is capable of giving
- What the device wants
This means you will be using the formula twice – one for the socket and one for the device. The goal is to get the power ratings for the device lower than the socket. If it’s higher, you can’t use that device on that socket.
At the socket
The formula first:
W = V x A x PF
Now that we know about the voltage drop and the 80% rule, we can slightly modify our formula (ignore the power factor because the outlet isn’t drawing any current from your lights):
W = (V x 95%) x (A x 80%)
The values for 120 V and 230 V are as follows:
Country | Voltage (V) | Amperes (A) | Max. Power (Watts) | |
All outlets together | If shared equally | |||
US | 120 | 5 | 456 | 228 |
120 | 15 | 1368 | 684 | |
120 | 20 | 1824 | n/a | |
UK | 230 | 5 | 874 | 437 |
230 | 15 | 2622 | 1311 | |
230 | 20 | 3496 | n/a |
What do we know? We know how many Watts of real power each outlet is capable of delivering.
At the fixture or device
Our DP (or inner DP) tells us we need a couple of Arri 4K HMI PARs, a couple of 1.2K HMI PARs, four 650 W tungsten fresnels, and a few LED 1x1s. We also need one outlet to charge two Anton Bauer Dionic 90 batteries and run a Macbook Pro at the same time.
Can all this run on the five outlets we have?
The formula first:
W = V x A x PF
What we want are two values of A – the operating current and the inrush current. We start with our biggest fixtures, the 4K HMI PARs. If you’re using an electronic ballast, the power factor (PF) = 0.98.
The maximum power of the 4K fixture is 4,650 VA, or 4,557 Watts. Looking at our table above, we can immediately see there isn’t a single socket that can deliver this power. For this reason, you can forget about the 4K fixtures. As a note – you can connect a 4K fixture in a 30 A socket if the voltage is 230 V, but not in 120 V.
The maximum power of the 1.2K fixture is 1,390 VA, or 1,362 Watts. It will just about ‘fit’ in a 15 A circuit (US). It will also fit in a 20 A circuit (US) and the 15 A and 20 A circuits in the UK. Important note: The inrush current for HMI fixtures isn’t very high, for reputable manufacturers. If the power factor is low, or if there’s a high inrush current, these numbers will change.
In this scenario, you might decide to use four 1.2 K HMIs to make up for not having 4K fixtures.
Can you plug in four 1.2K HMI fixtures? Not in the US. At best, you can have two 1.2K HMI fixtures, one in the 15 A socket and the other in the 20 A outlet. In the UK, you can power three of them (two in the 20 A circuit and one in the 15 A circuit*).
*You might be able to squeeze in another 1.2K fixture in the 15 A circuit because it’s very close, and the power rating we’re taking into account is the maximum power rating, not the lamp rating which is lower. E.g., a 1,200 Watt lamp will have an apparent power of 1,225 VA, which is 90% of the maximum power. However, what might spoil your party is the inrush current, which might not allow you to strike the fourth lamp.
With a sigh, you decide to use just two 1.2K HMI fixtures. You need them. This leaves you with these options:
- In the US – one 5 A circuit to play with.
- In the UK – one 5 A circuit and one 15 A circuit to play with.
What about your idea of four 650 W tungsten fresnels? Since the power factor is 1, each 650 W fixture will eat up 650 Watts of power. If you’re in the US, it’s a no-go, because your remaining 5 A circuit can only accommodate 456 Watts. Even if you tried to squeeze one of them in the 20 A circuit, you can’t (1,824 – 1,362 = only 462 W).
In the UK, you can accommodate all four 650 Watt fresnels into a 15 A circuit, provided you switch them on one at a time (to give each one its inrush current cushion). This also goes to all the lights – you must switch them on one at a time, in the order from largest to smallest, current draw-wise.
Both teams have one 5 A circuit left, in which they can plug in**:
- Dual Dionic 90 charger – about 200 Watts
- 15″ Macbook Pro Retina – about 100 Watts
**Assuming a PF of 1. In the real world, you might not get that.
What about the LED 1 x 1s? Each 1×1 is rated for 40 Watts. However, their power factor (PF) is about 0.54. This means, the real power draw is 74 Watts. Four 1x1s would need 296 Watts. In both countries, you can connect the 1x1s in one of the outlets that have not maxed out.
Here’s how things pan out for our production:
US | UK | |
Wishlist | 2x4K HMI, 2×1.2K HMI, 4×650 W Tungsten, 4x1x1 LEDs, Charger and Laptop | |
Reality | 2×1.2K HMI, 4x1x1 LEDs, Charger and Laptop | 3×1.2K HMI, 4×650 W Tungsten, 4x1x1 LEDs, Charger and Laptop |
Life isn’t fair on one side of the Atlantic, that’s for sure! Team UK has 3 1.2K HMIs and 4 650 W tungsten fresnels. Team US has 2 1.2K HMIs and no fresnels.
This is how you estimate if your lighting fixtures and equipment will work on a single-phase mains supply. Remember, I’ve been conservative with my ‘guesstimating’, and you should be, too. A good electrician might help you squeeze in an extra light or two, but that’s something you shouldn’t assume at the beginning.
In Part Three we’ll look at dealing with three-phase circuits and portable generators.