Power distribution, generators and tie-ins

Lesson 2 explained how electricity behaves. This lesson follows the power from its source to the light. On most location shoots that source is a portable generator; on stages it is house power through studio distribution; occasionally it is a tie-in to a building's electrical service. From the source, feeder cable carries power to distribution boxes, which split it into smaller circuits for individual fixtures. Every link in that chain must be correctly sized, protected and placed. A generator in the wrong place can poison a crew with carbon monoxide; an unbalanced load can overheat a neutral; a feeder run that is too long can drop the voltage so far that fixtures misbehave; and an improper tie-in can electrocute someone or set a building on fire. This lesson covers the distribution chain, generator selection and placement, load balancing, voltage drop, fuel and run time, and the rules around tie-ins.

The distribution chain

A typical set power system has several stages.

• Source: a generator, studio house power or a tie-in to a building service.
• Main disconnect and overcurrent protection at the source, so the whole system can be switched off and protected.
• Feeder cable: heavy single-conductor cable, often 4/0 or 2/0 copper, carrying three hot legs, a neutral and a ground from the source to the set.
• Main distribution box, sometimes called a spider box splitter, "lunch box" for small circuits or a "company switch" when built into a building.
• Sub-distribution: smaller distribution boxes with breakers that feed 100 A, 60 A or 20 A circuits through stage pin, bates, twist-lock or Edison connectors.
• Branch circuits: extension cables (stingers) and fixtures.
• Dimmers and DMX control may sit between the distribution and the fixtures, letting the gaffer adjust levels from a console.

Each stage has overcurrent protection sized for the cable it feeds. As a rule, a smaller cable must never be connected downstream of a breaker rated for a larger cable unless there is protection sized for the smaller cable.

Generators for film

Film generators are designed for the job: quiet enough for sound recording, with stable voltage and frequency, sound-attenuated enclosures, built-in distribution panels and electronic governors that cope with electronic loads.

• Sizes range from small portable inverter generators of 2 to 7 kW for small shoots to tow-behind and truck-mounted units of 100 to 1,400 kW for features and large commercials.
• Most produce 120/208 V three-phase, with panels for Cam-Lok feeder connections and receptacles.
• Sound-blimped "movie" generators are designed to be quiet at a distance; sound crews still prefer them as far away as practical.
• Many productions now add battery energy storage systems or hybrid generator-battery units, which reduce fuel, noise and emissions and can run small loads silently.
• Generators are operated by trained crew, often a dedicated generator operator, who monitors load, voltage, frequency, fuel and temperature.

Generator placement and carbon monoxide

Where a generator sits is a safety decision as well as a sound and logistics decision.

• Carbon monoxide (CO) from exhaust is colorless, odorless and deadly. Never run a generator indoors, in a garage, under a tent, near open windows, doors or air intakes, or in a low area where exhaust can pool.
• Place generators downwind of the set, base camp and dressing rooms where possible, with the exhaust pointed away from people.
• Keep fuel storage, refueling areas and the generator away from ignition sources, with fire extinguishers nearby and spill kits ready (US Bulletin 13 covers combustible and flammable fuels).
• Keep generators on stable, level ground, chocked and secured, with barriers to keep the public away.
• Ground generators as required by code and manufacturer instructions; the US Bulletin 23E covers generator grounding.
• Keep cables from the generator out of traffic and walkways, using cable ramps where they must cross.
• Consider noise: hearing protection near large units, and placement that keeps noise away from neighbors (Lesson 8 covers noise dose).

Balancing the load across phases

On a three-phase system, the gaffer aims to spread single-phase loads evenly across the three legs. A badly unbalanced system overloads one leg while others sit idle, raises neutral current and can make a generator run poorly.

• Each fixture or circuit is assigned to a leg, often color-coded on the distribution (in North America, commonly red, black and blue for legs, white for neutral and green for ground on film feeder).
• Electricians keep a running log or plot showing which fixtures are on which leg and the current on each.
• Clamp meters measure actual current on each leg and the neutral, since real loads differ from nameplate ratings.
• When fixtures are added or switched off during the day, the balance is checked again.

For resistive loads, the current in the neutral of a three-phase system depends on the imbalance between the legs:

When the three currents are equal, the neutral current is zero. Harmonics from electronic loads add extra neutral current on top of this, as Lesson 2 explained.

Cable sizing and voltage drop

Feeder cable must carry the load without overheating, and long runs must not drop the voltage too far.

• Ampacity is the current a cable can carry continuously without exceeding its temperature rating. It depends on conductor size, insulation type, ambient temperature and whether the cable is in free air, bundled or covered. Film distribution uses tables from the electrical code and cable manufacturer; 4/0 single-conductor portable cable, for example, is commonly rated in the region of 400 A in free air, but the exact value must come from the code and manufacturer tables.
• Voltage drop occurs because even copper has resistance. Over long runs at high current, fixtures receive less voltage. Tungsten lamps shift warmer and dimmer; electronic ballasts may shut down or behave erratically.
• A common design target is to keep voltage drop below about 3 to 5 % at the fixture.

For a balanced three-phase feeder, the line-to-line voltage drop is approximately:

Here \( I \) is the current per leg, \( R_{\text{per length}} \) the resistance of one conductor per unit length and \( L \) the one-way length of the run.

Sizing distribution and generators

The gaffer sizes the generator and distribution from the lighting plan, adding margin for changes on the day.

• List every fixture and its power, including practicals, video village, chargers and support loads.
• Convert to current per leg, accounting for power factor of electronic loads.
• Apply the continuous load guideline of about 80 % of the rating of breakers, cables and generators.
• Add a margin for creative changes; DPs often ask for "one more light".
• Choose a generator whose rated output, not peak output, covers the load, and consider electronic load compatibility.

House power and studio distribution

On sound stages, power comes from studio distribution with fixed outlets and panels designed for film lighting. Studios have their own rules about who may connect and which outlets to use. In ordinary buildings on location, using wall outlets is limited to small loads on circuits whose capacity and wiring are known. Plugging large fixtures into domestic outlets risks overloading old wiring that may already be shared with other loads.

Tie-ins: connecting to a building's service

A tie-in connects film distribution directly to a building's electrical service, usually at a panel or disconnect, instead of using a generator. It can save fuel and noise but is among the most hazardous electrical tasks on a production.

• Tie-ins involve working on or near energized conductors with very high available fault current, creating serious shock and arc flash risk.
• Only qualified and, where required, licensed electricians may perform tie-ins, using proper equipment, protective gear and procedures.
• The building owner's permission and an assessment of the service capacity are needed.
• Many jurisdictions require permits or inspections; many studios and productions prohibit tie-ins except by approved electrical contractors.

In the United States, connections to building power fall under the NEC as adopted locally, OSHA electrical standards and local permitting. Many cities require a licensed electrician and a permit. OSHA and NFPA 70E require an energized work justification, arc flash risk assessment and protective equipment for work on live equipment. US Safety Bulletin 23 and its supplements address portable power distribution, including off-lot work.