Portable Power Distribution System




Electrical power can come from several sources; batteries, generators, or transformers from utility services.

Batteries are considered a DC power supply usually 10 units combined in a portable power pack.

Generators are portable power units ranging in size from 5500 watt units to 1200 amp or larger units. The larger units are switchable from AC to DC and from single phase to three phase.

Motion picture type generators are rated by the total amperage output, each leg is fused or circuit breaker protected at the appropriate size for the generator. Typically a 1200 amp generator has 400 amp fuses on each leg; a 900 amp generator is fused at 300 amps per leg.

AC Generators are designed to operate at 120/240 volt single phase or 120/208 volt three phase at 60 cycles.

Transformers are a device for transferring electrical energy in an AC circuit by means of electromagnetic induction. Each circuit is represented by a winding around a common magnetic core, the ratio of the voltage in the two circuit being equal to the ratio of the number of turns – the product of current and voltage is the same in each circuit.

HMI lights are designed to operate at 120 or 240 volts AC and most have a built in compensating factor within the ballast to accommodate voltages as low as 100 or 200 volts respectively, anything lower will have adverse reactions to the lights usually causing the light to drop off line.

Incandescent lights are designed to operate at 120 or 240 volts, lower voltage causes the light to turn yellow.

Some units such as 20kw lights have multiple voltage globes available 208v - 220v - 230v - 240v choose the proper globe to match the power supply.


Switches, dimmers and ballasts are considered control devices; these devices or a combination of these devices connect or disconnect the light from the circuit.

These devices are rated for use as AC only or AC/DC use. A switch closes the circuit allowing current to flow from the power source to the load (light), this causes a momentary arc between the contacts of the switch and must be rated at the proper size for the load.

Dimmers are usually AC units designed to control multiple lights at one time through a control board. These units are rated from 1000 watt units its to 12,000 watt units and come in a variety of packs from 6 units to one hundred or more on a dimmer pack.


Conductors are insulated copper wire that varies in size from #12 to 4/0. The insulation of the cable is rated in degrees centigrade and rated for the use intended. The motion picture industry uses extra hard usage rated cable, identified as types; E.I.S.L., S.O., W, and others rated for hard usage.

Using the proper size cable is important for two reason; ampacity and voltage drop. When current flows through a wire it creates a certain amount of heat; the greater the amperage the greater the heat. The larger the load the larger the cable must be to eliminate heat.

Voltage drop is determined by the length of the run of cable and the amount of current demanded by the load, a rule of thumb is a 4 volt drop per 100 feet using 4/0 with a 400 amp load or 1 volt per 100amp per 100 feet.

Splicing devices such as lug spiders or cam spiders are used to connect multiple cable runs or paralled cable runs.


For all practical purposes, the load for motion picture application refers to a lighting load. Light used in motion picture application fall into 3 categories; DC discharge units, AC discharge units and incandescent units.

Incandescent units can be used on AC or DC current and require 120 volts or 240 volts depending on the design of the globe. The globe contains a tungsten filament that when heated glows causing light to be reflected away from the filament and is rated in degrees Kelvin (color temperature). At the proper voltage the lamp produces 3200 degrees Kelvin rating rises.

HMI, C.I.D., Xenon and other discharge type lights require AC or DC converted to AC in order to operate. These units require 120/208/240 volts at 60 cycles to operate efficiently and produce a daylight light source rated at 5600 degree Kelvin.

The HMI type units consist of a fixture and ballast system connected by a feeder cable to the head and power feeder to the ballast. There are two types of ballast; magnetic and electronic. The magnetic type ballast must be fed with voltage set at 60 cycles; the electronic ballast converts the system to high frequency, which compensates for fluctuating voltage.

The carbon-burning arc operates on DC power or AC rectified to DC. The arc system consists of a light fixture and grid connected by feeder cable from the head to the grid and power feeders from the grid to the main line.


All circuits must be protected by fuses or circuit breakers rated at the capacity of the cable to insure the cable will not overheat and to protect the operator from a faulty circuit.

Fuses are safety devices constructed of material such as lead that will melt and open a circuit when overloaded. These devices must be replaced once they have blown.

Circuit breakers are devices designed to open when overloaded and can be reset once the problem has been corrected. The entire distribution system must be protected by a main circuit breaker or fuses in the generator or at the main source of power.

The cable must be protected by fuses or circuit breakers rated at the proper amperage for the cable, anytime you step down in cable size, and you must provide suitable protection.

Portable power distribution centers provide branch circuits from the main power feeders through receptacles rated at 20 amp, 60 amp, and 100 amp.

Plugs and receptacles are rated at specific voltage and amperage. These units are configured differently so they cannot mate with a plug or connector of a different voltage or amperage.


The purpose of grounding is safety. In all discussions concerning electrical wiring you will regularly meet the term ground, grounded and grounding. They all refer to deliberately connecting parts of a wiring system to a grounding electrode or grounding electrode system.

Grounding falls into two categories:

Systems Grounding and Equipment Grounding

Systems Grounding – is accomplished by attaching one current carrying conductor of an electrical system to ground or earth at the source of power, this is called the neutral or common leg.

Equipment Grounding – is accomplished by attaching all of the non-current carrying metal parts of a system together and connecting to the same systems ground at the source of power (neutral and ground).

The ground can be the earth or in the case of portable generators, the frame of the generator which will serve as a large conducting body that serves in place of the earth.

Equipment grounding it the intentional connecting of all metal parts of a system together through a ground wire. Its purpose is to have all exposed conducting surfaces at the some potential, so that some touching any two metal surfaces will not experience any difference in potential; in other words, so that the individual will not feel a shock and get knocked on his tail or worse.

The other intentional grounded conductor of the system is the neutral or common conductor. This is a circuit conductor, not an equipment ground, and it is grounded to keep it at the some potential as earth, or ground as with a generator. It should be grounded at the source and nowhere else.

An unintentional ground is known as a ground fault. This occurs when a live conductor accidentally comes into contact with a metal surface. This type of ground fault is usually arcing and is extremely destructive. When a ground fault occurs in a grounded system the safety device (fuse or circuit breaker) will activate which opens the circuit and current will not continue to flow.


How dangerous are shocks? Most people think that high voltage causes fatal shocks, this is not necessarily so. The amount of current flowing through the body determines the effect of a shock.

A milliampere is 1/1000th of an amp; a current of one milliampere through the body is just barely perceptible. One to eight milliampere causes mild to strong surprise. Current from eight to fifteen milliampere are unpleasant, but usually the victim is able to free himself or to “let-go” of the object that is causing the shock. Currents over 15 miliamperes are likely to lead to “muscular freeze” which prevents the victim from letting go and often leads to death. Currents over 75 milliamperes are almost always fatal; much depends on the individual involved; how much muscle mass, body condition and condition of the heart.

Rule of thumb; “One volt won’t hurt you, one amp will kill you”.

Of course the higher the voltage, the higher the number of milliamperes that would flow through the body under any number of circumstances. A shock from a relatively high voltage while the victim is standing on a completely dry non-conductive surface will result in fewer milliamperes than a shock from a much lower voltage while standing in water.

The purpose of the ground wire in a circuit is to provide a path for an unintentional hot wire that comes in contact with the case or exterior housing of a lamp, ballast, power distribution unit or other devices to travel to earth or grounding plane and trip the circuit breaker or blow the fuse in the circuit. A grounding wire will not prevent you from being shocked. The degree of shock will depend on the surface on which you are standing your general physical condition, and the condition of your skin at the contact point. The ground wire is meant to carry the majority of the current if properly installed, do not undersize the grounding conductor.

Circuit breakers in a system are designed to trip when a ground fault occurs. They are rated as thermal magnetic or magnetic breakers, most portable power distribution systems use magnetic breakers due to the high ambient temperatures the equipment is subject to.

GFCI. (Ground Fault Circuit Interrupter) breakers are designed to sense the difference in current between two circuit wires and trip at four to six milliamperes in as little as 1/40th of a second. A fault current, much too low to trip a normal breaker or blow a fuse, could possibly flow through a person in contact with the faulty equipment or grounded surface. If the surface a person is standing on is wet it greatly increases the likelihood of a severe shock, the use of a GFCI in this instance is desirable. The GFCI will not prevent a person who is part of a ground fault circuit from receiving a shock, but it will open the circuit so quickly that the shock will be below levels which will inhibit breathing or heart action, or the ability to “let-go” of the circuit. A GFCI is not a substitute for grounding and should be considered a supplemental part of the circuit.