As I've been building my craft and career as a semiprofessional gaffer and DP on narrative film and video projects, I have powered sets using everything from ungrounded wall sockets in a circa-1920 house to a modern 3-phase, 600-amp set distribution system. More often than not, I find myself lighting a location with as much as 10,000 watts of light using only 120-volt fixtures plugged into household outlets. Although modern homes are capable of providing power for this much light, when you're working with this much wattage, some guidelines and safety precautions are in order to ensure a smooth shoot, no downtime, and no loss to property or life.

If you regularly manage the powering of a set with a dedicated distribution system, then powering a set with the branch circuits of a location is a familiar (if somewhat frustrating) exercise. However, if your next lighting setup will be larger than your usual 2,000-watt interview rig, it might help to review some of the basics of electricity as well as examine specific location power practices and safety procedures. This overview assumes a typical single-family house built within the last 30 years as a reference. Such a house typically has a 100-amp, single-phase, four-wire,120/240-volt, alternating current feed from the power grid; a dual-bus breaker box with 120-volt and 240-volt sub-breakers; and copper wiring throughout.

(A caveat: I am not a certified electrician and this is not meant to be a definitive guide to safe electrical practice. As with the use of any electrical power, consult a qualified electrician familiar with national and local electrical codes and practice if you are unsure about anything.)

In any powering scenario, a finite amount of electricity at some maximum capacity is supplied to a set. On the set, lighting instruments, cameras, monitors, and other tools create a load that consumes electricity at a predictable rate. The amount of available capacity and consumed load is expressed using four useful measurements: volts, amperes, watts, and resistance (see the "Fundamentals" sidebar).

Most modern, middle-class homes of around 1,800 square feet have electricity delivered from a nearby step-down transformer to the home's breaker box by three feed wires: two 120-volt hot legs and a neutral leg (correctly called a grounded leg, but because this may be confused with a grounding wire, we'll use the "neutral" convention).

Two hot lefs (one marked with red tape) connect to the main breaker at top. The main breaker is actually two parallel breakers whose switch levers are physically ganged together. (Image courtesy Michael Morlan.)

The two hot legs (one marked with red tape) connect to a main breaker that is really two parallel breakers whose switch levers are physically ganged together and are each rated at around 100 amps. The main breaker is usually the larger pair of breakers centered at the top of two rows of smaller breakers.

The neutral leg connects directly to a neutral bus. The neutral leg is also electrically connected to the grounding leg and the earth ground (a water pipe or buried rod). Each of the two main breakers feeds a hot bus, which then feeds banks of smaller sub-breakers.

One bus serves the left bank of smaller sub-breakers, the other the right bank. The electricity of each hot bus is distributed by the sub-breakers into 120-volt branch circuits (typically 15 or 20 amps), feeding lights, outlets, and appliances within the home.

The two 120-volt hot buses may also be summed together by paired and ganged sub-breakers into a 240-volt branch circuit for systems requiring that voltage, including clothes dryers, oven ranges, hot water heaters, and air conditioning and heating systems.

Each of the two main breakers feeds a hot bus, which then feeds the left and right banks of smaller sub-breakers. Each hot bus is distributed by the sub-breakers into 120-volt branch circuits (typically 15 or 20 amps). (Image courtesy of Michael Morlan.)

Each branch circuit then delivers power into the house. After passing through a light or other electrical device, the electricity drains from the house by returning to the breaker box, traveling through the common neutral bus, and back out of the house through the neutral feed wire coming from the power grid. The single neutral bus serves both hot buses.

Each branch circuit commonly delivers power to multiple locations within the house. You shouldn't assume that a single branch circuit is delivering power solely to the outlets or light fixtures in a single room. Conversely, you shouldn't assume that each outlet in a single room is on a different branch circuit. More often, to save on electrical cable lengths, home builders will assign a branch circuit to all of the outlets and/or light fixtures within a single wall. That means the branch circuit will support outlets on both sides of that wall in two or even three rooms. There are no specific rules of how the branch circuits are distributed throughout a house, only how many total outlets and light fixtures they may serve.

My house has a branch circuit that supplies the downstairs half-bath's socket, and then splits to the two upstairs bathroom sockets and the sockets outside the front and back doors. You might assume this is an awfully odd way to wire a house until you realize the builders saved $60 dollars by installing one GFCI (Ground Fault Circuit Interrupter) in the downstairs bath to protect all five outlets.

To ensure the smoothest possible shoot, no blown breakers, and no lost time, you should perform a tech scout of the location before shooting begins. A major part of your scout trip will involve making a map of the branch circuits. To get the most accurate circuit map, consider the following procedures: You can obtain construction blueprints--a good start, but prone to inaccuracies. Or, with a partner and the homeowner's permission, progressively shut off each breaker and document which outlets and lights have lost power with an A/C power indicator. But this is disruptive to the property owner, so consider using a circuit tracer with a frequency generator that plugs into an outlet and a "listener" at the breaker box that identifies which sub-breaker the generator is on (see the figure below).

Using a tone transmitter plugged into an outlet (at left) and a detector at the breaker box (at right; breakers shown with cover off), you can trace branch circuits in a location with a minimum of disturbance to the owners. (Image courtesy of Michael Morlan.)

The other half of your tech scout is to hire a certified electrician to measure the power typically consumed by the normal use of the house. For this, your electrician will need access to the individual feed wires headed toward or contained within the property's breaker box. He will use a clamp-on ammeter to measure the amperage being drawn by the property on each hot leg. Although this is essentially dangerous (the electrician may be exposed to lethal, powered surfaces), the procedure is rather simple.

Clamping an ammeter around each hot lead tells you how much amperage is being consumed by a location. Here, a reading of 9.9 amps tells us the location is consuming 1,188 watts on the right-hand hot leg. (Image courtesy of Michael Morlan.)

Here's an example: The property's main breaker is rated at 100 amps. The electrician clamps the ammeter around one hot leg feed wire (typically marked with red tape) and a reading of 40 amps is taken. The electrician clamps the ammeter around the other hot leg feed wire (typically black and unmarked) and a reading of 30 amps is taken. The two readings are then subtracted from the total rating of the main breakers to arrive at an available capacity for your set. For example:

Main breaker rated: red bus=100 amps, black bus=100 amps

Ammeter reading: red bus=40 amps, black bus=30 amps

Available for set: red bus=60 amps, black bus=70 amps

Safety margin: red bus=20 amps, black bus=20 amps

Used by the set: red bus=40 amps, black bus=50 amps

Note that the ammeter readings should be performed at a time similar to your shooting time so the property's load will be similar. Talk with the property owners to determine if they are using a typical amount of electricity at the time you are taking your reading. If your shoot will run between night and day, the property load could change drastically as room lights are turned on or off.

Cycling electric dryers, ranges, water heaters, and air conditioning and heating systems should all be taken into account when performing a reading. In addition, a safety margin should be maintained to account for the surge of electricity when compressor motors start up. Making note of the 240-volt ganged sub-breakers will help you anticipate which bus such use will appear on.

Make a big note of the actual available amperage capacity on your circuit map.

You should avoid loading any outlet or breaker at 100 percent of its capacity. Leave a margin for error of around 20 percent to allow for latent failures and other uncontrolled loads like cycling refrigerators.

Avoid loading a single 120-volt outlet with more than 1,500 watts. The typical household electrical outlet (Edison socket) is rated for 15 amps, while the breaker protecting the circuit may be rated at 20 amps: It is possible to overload the outlet before popping the breaker. At full load, a 15-amp outlet may overheat, melt, even explode, especially as it ages.

Avoid loading a 20-amp breaker with more than 1,900 watts. Again, leave room for unexpected loads.

Power delivery from a transformer to a typical house looks something like this. Power travels from a step-down transformer through a breaker box, which splits out two legs of 120 V, which feeds higher-voltage appliances like air conditioners or heating systems. (Image courtesy of Michael Morlan.)

As I previously mentioned, there are two hot legs served by a single neutral leg. When an electrical engineer designs the circuit layout of a house, he or she takes into account how the house will typically be used--which lights may be on simultaneously, which outlets may be used at the same time--and then balances the assignment of elements in the house between the sub-breakers on each hot leg.

A typical North American 120-volt sine wave. Every 60th of a second, alternating current electricity swings from +180 volts to -180 volts and back again. The actual usable power of this sine wave is 120 volts, as calculated by a root mean square (RMS) algorithm. (Image courtesy of Michael Morlan.)

If the demand for electricity--the load--is the same on both hot legs of a breaker box, the sum of electricity on the neutral leg is nothing.

Let's examine that statement more closely. In North America, alternating current cycles from positive voltage to negative and back to positive 60 times a second (60 hertz or 60 Hz).

As I previously stated, the two 120-volt, alternating current hot legs may be summed together to create a single 240-volt supply. This is done by delivering the two 120-volt AC legs 180 degrees out of phase with each other. That is, when one leg has cycled to a negative 120 volts, the other has a positive 120 volts. Together, they sum to a 240-volt differential.

So, when a 120-volt 1,000-watt (8.3-amp) light is loaded on each 120-volt hot leg, the combined load on the neutral leg is 1,000 watts, 180 degrees out of phase with another 1,000 watts, or 0 watts.

The two lights have effectively cancelled each other out when their respective electrical drains reach the neutral bus. This is an ideal electrical load scenario.

The wattage passing across each of the two hot buses is essentially summed together on the neutral bus. So, when the same load is present on each hot bus, consuming the same wattage, the opposing loads cancel each other out. There is virtually no current on the neutral bus, an ideal electrical load scenario. (Image courtesy of Michael Morlan.)

If our electrical engineer has designed the house correctly, the homeowner's typical use of lights, outlets, and appliances on both of the hot legs will result in a load on the neutral leg at or near 0 amps.

Armed with your circuit map, the available amperage capacity available for the set, and the wattages of your set gear, you are prepared to distribute your lighting load effectively and safely.

One can't write enough about the proper and consistent use of safety grounds for all electrical equipment that provides them. Safety grounds are designed to drain away lethal electricity when a failure occurs in the isolation between hot lines and the frame of an electrical device. You should never disable these features of set gear.

That being said, at times you may find yourself working in a location that predates the use of ground wires in its construction: the dreaded two-prong socket. That's when you typically whip out the ground-lifters and add them to the ends of all your stingers.

In an ideal world, you would run a separate wire from the ground wire tab of each ground-lifter to a buried copper rod in the yard. This would provide a proper safety ground for your setup. But that would be incredibly cumbersome and present all kinds of new hazards, like tripping. So, there are occasions when running without a ground wire is unavoidable.

I was moving a 1 K fresnel light on its stand to a new position when the light housing touched a metal ceiling lamp and instantly popped a breaker. Apparently the ceiling lamp was wired incorrectly and hot current was present on the frame of the lamp. It was fortunate that my fresnel fixture was still plugged in and the short between ceiling lamp and fresnel was safely drained away by the fresnel's ground wire. If the fresnel had not been plugged in (read: the ground wire was lifted), the hot current would have passed through me and my sneaker laces, touching the floor and resulting in an unpleasant shock!

Remember, even though you are essentially borrowing the use of someone's house for your location, someone is going to have to pay the electrical bill when it comes due.

I shot a 16 mm short film at a friend's house. Many of the scenes, including nighttime moonlit interiors and night-for-day spaces, used as much as 8,000 watts of light burning for 10 hours a day. After we wrapped, I asked for my friend's electrical bills from the prior three months and for the month I was there. The difference was $300. A big thank you and a personal check assured we would remain friends afterward.

It might be a good idea to offer compensation--whether a fruit basket or cash--for the excess electrical power that your production pulls from your host's house.

Whether you're using the electrical power of a residential home or the purpose-built, set-distributed electrical delivery of a portable generator and distro-boxes, the rules of capacity and load are the same:

- Know your fundamentals. Be able to calculate capacity and load in your head or calculator.

- Know and match capacity to load, and vice-versa.

- Understand and document each location's specific electrical delivery capabilities and limitations.

- Balance the load.

- Respect the limits of your gear.

- If you don't know, ask an expert.

- Above all, be careful.

Have fun out there!

Electricity is the travel of electrons between atoms of a material. At the human scale, we use several measurements of this sub-atomic, electrical activity to quantify it.

Alternating current (AC): like that from the power grid; the polarity of the circuit cycles between positive and negative 60 times a second (in the United States).

Amperes: the number of electrons flowing between two points in a circuit.

Direct current (DC): like that from a battery; has a steady flow of electrons from a negative terminal, through the circuit, to a positive terminal.

Resistance: measured in ohms, is the opposition to electrical flow in a circuit. Tungsten lights are essentially specialized resistors enveloped in a vacuum or gas.

Volts: the potential (or pressure) between two points in a circuit.

Watts: the amount of power being consumed by a load.

There is a mathematical relationship between amps, volts, and watts that may be expressed with the following equations.

The wattage (also expressed as volt-amperes) available in a circuit may be calculated by multiplying volts and amps, as in: watts = volts x amps.

Conversely, if wattage and voltage are already known, amperage may be calculated with: amps = watts divided by volts.

And voltage may be derived with: volts = watts divided by amps.

Similarly, there is a mathematical relationship between resistance, volts, and amps:

resistance = volts divided by amps

amps = volts divided by resistance

volts = amps x resistance

Although measuring resistance is very useful and important when working with larger, more complicated set distribution systems, it's of less use when powering lights of known wattage from household circuits of known volt-ampere capacity. However, if you encounter a light bulb, monitor, or other device that lacks wattage markings, you can measure the resistance across its power terminals and use the following calculation to determine the device's wattage: watts = (volts divided by resistance) x volts.

A household electrical outlet (Edison socket) is rated at 15 amps, so 15 amps x 120 volts = 1,800 watts capacity.

A household main breaker is actually two breakers rated at 100 amps at 120 volts each, so 100 amps x 2 x 120 volts = 24,000 watts or 24 kiloWatts (kW) capacity.

Examples of Electrical Load

A Mole-Richardson Tweenie Fresnel has a 650-watt bulb, so 650 watts divided by 120 volts = 5.4-amp load.

A Mole-Richardson Baby Fresnel has a 1,000-watt bulb, so 1,000 watts divided by 120 volts = 8.3-amp load.

A camera's 12-volt, 3-amp power supply draws 100 watts from the 120-volt circuit, so 100 watts divided by 120 volts = 0.83-amp load.

Let's say you have a bulb of unknown wattage, and you measure resistance of 14.4 ohms. To calculate wattage, do this: (120 volts divided by 14.4 ohms) x 120 volts = 1,000 watts.

Most electrical conductors and connectors have their amperage ratings marked on them on the jacket. If the markings are missing, there are some general rules determining how much electricity a power cord can handle without overheating or burning.

Three-wire stranded-copper stinger capacities:

Gauge: 16, rated amperage: 13, rated wattage: 1,500, 80% rated wattage: 1,200, 50% rated wattage: 750

Gauge: 14, rated amperage: 18, rated wattage: 2,100, 80% rated wattage: 1,700, 50% rated wattage: 1,050

Gauge: 12, rated amperage: 25, rated wattage: 3,000, 80% rated wattage: 2,400, 50% rated wattage: 1,500

Gauge: 10, rated ameperage: 30, rated wattage: 3,600, 80% rated wattage: 2,800, 50% rated wattage: 1,800

Note these ratings are for 3-wire stingers being used under ideal conditions: less than full load, room temperature environment, free air flow around full length of cable, and runs of 100 feet or less.

Connector capacities:

Edison 120-volt (3-prong) sockets common in the United States are rated at 15 amps (1,800 watts).

Less-than-ideal conditions:

Running a stinger near maximum capacity causes it to heat over time and become less efficient. If you're using a stinger for over 1 hour, derate its amperage by 20 percent.

If you are working in particularly hot conditions, or if part or all of a cable is insulated from a free flow of air, it should be derated by 50 percent.

If powering HMI lights, motors, or light dimmers, the cable should be derated by 25 percent or more.

If running a cable farther than 100 feet, consider trading up to a larger gauge to support your load.

As a cable gets longer, internal resistance steals power from the line and heats the cable. It is possible to actually burn a cable before a breaker pops if the cable run is too long.

Exceeding these ratings can result in damage and even fire. Be careful in the assignment of lighting load, not only to the household branch circuits, but also to the stingers that connect load to circuit.