Introduction: This blog is intended to provide information to persons in the live entertainment industry to enhance their technical knowledge and provide a better understanding relating to many electrical aspects of equipment, wiring and safety.

Disclaimer: Although the vast majority, of the information provided, is indisputable, there are cases where some of the information is “open to interpretation”, especially in the area of the National Electrical Code (NEC).

In such cases the “final say” is up to your local “Authority Having Jurisdiction” (AHJ), usually the electrical inspector. Some of the information provided is based on the 2014 National Electrical Code (NEC), however, these requirements may be accepted as written or modified by local authorities (by law), especially in large cities. This information does not necessarily represent the opinion of the editorial board or Motion Labs and its owners.



The National Electrical Code (NEC) outlines and defines the requirements for the permanent and temporary power distribution equipment installation for live entertainment.

The NEC articles that relate to these types installations are described in the following:

(518) Assembly Occupancies;

(520) Theaters, Audience Areas of Motion Picture and TV Studios Performance Areas, and Similar Locations;

(525) Carnivals, Circuses, Fairs and Similar Events;

(530) Motion Picture and TV Studios and Similar Locations.

We will be mainly focusing on “temporary” installations in “Theaters, Audience Areas of Motion Picture and TV Studios Performance Areas, and Similar Locations” (NEC Article 520). Keep in mind that there are other articles in the code that may apply, but if your equipment and installation methods conform to the parameters described in Article 520, that’s the code article you should comply with. In cases where Article 520 is moot on any subject or equipment you will be installing, you may have to revert to the “general sections” of the code, described in Chapters 1 through 4, these chapters pertain mostly to permanent installations however.

It is absolutely necessary that you know what the connected load (amps) will be. That’s up to you to figure-out before starting any power distribution system design. We will not be discussing these requirements herein.

For the sake of clarity, we will arbitrarily break-down these temporary (portable) systems into three sizes:

Small : 20 to 50 amps, single phase.

: 20 to 50 amps, single phase. Medium : More than 50 amps, up to 200 amps, single & three phase.

: More than 50 amps, up to 200 amps, single & three phase. Large: 200 to 400 amps (or more), three phase.

One of the limiting factors, pertaining to system size, is the type of connection method you will use to connect your temporary power distribution system (PD) to “house power”. It can be as simple as a 15 amp Edison plug up to 400 AMP “Cam-Locks” (in some cases multiple Cam-Lock sets).

The power distribution systems we will be discussing, will most likely fit somewhere in between the two scenarios depicted below (but you never know, so be prepared):

Real small:



Real Big:



Small size systems:

The “smallest system” may only consist of a 15A “power strip”, and a couple of extension cords. As simple as it is, it’s very common, and is typical for a three or four-piece band with no stage lighting and a minimal sound system. There is no need to go into any more detail …. Let’s move on to a little larger system:

As previously stated, the connection method to “house power” is the first thing to consider. Small Power Distribution Systems (PD) usually incorporate 30A or 50A plugs on the input. There are various configurations, however, most typical are: (2) Line conductors, (1) Neutral conductor and (1) Grounding conductor.

The common designation is: 125/250V, XXA, 4 wire. The voltage designation is the “maximum” voltage allowed for use with these connectors, although the actual voltage will typically be 120/240V.

Since most (if not all) the connected load will be 120 volts, these arrangements will supply 60 amps @ 120V on a 30-amp connector and 100 amps @ 120V on a 50-amp connector. Another thing to check is whether the receptacle configuration is “Twist-Lock” or “Straight Blade” (provided by the “house”).

30A Twist-Lock





30A Straight Blade



50A Twist-Lock



50A Straight-Blade



The next thing to do, after the determination of the input connector, is to figure-out the quantity and type of receptacles you require for your load equipment and the location, on stage, of these receptacles. The cord coming from the house receptacle is typically connected to a “breakered stringer box”, which breaks-out the 30A or 50A power into several 120V, 20A outlets. Most of the time, these outlets are NEMA 5-20R, but can be “Neutrik Power-Cons”, “Twist-Locks”, “Stage Pin Connectors”, or whatever you require.

These breakered stringer boxes always have an input connector (usually Twist-Lock), and may also have an output connector (feed through), which enables several units to be “daisy-chained”. When converting 30A or 50A to 20A (to feed 20A receptacles) a circuit breaker is required, which is integral in the “Breakered Stringer Box”. Several of these boxes may be fed from separate outputs on the main power distribution unit.

If a reduction in the ampere rating of connected devices is not required, a “Non-Breakered Stringer Box(s)” is all that’s needed. These units may take the form of a single box or several outlet boxes mounted “in-line” on a single cord (depicted below). These cord-mounted box assemblies allow placement of receptacles where you need the power, and eliminate the need for additional extension cords.

Breakered Stringer Box



Single In-Line Box



Multiple Boxes on a Single Cord



Medium Size Systems:

50A to 200A systems can be single phase or three phase. Power distribution equipment can be manufactured to specifically operate on either system. This does not exclude the use of a single phase system on 3-phase, or a 3-phase system on single phase. When this is done, however, it does not use the PD equipment to its full capacity or may not use all the house power that may be available.

Single Phase (Grounding conductor not shown)



Three Phase (Grounding conductor not shown)



There are several types of system components that may be used for medium sized systems. The selection and quantity of these components vary and are dependent on your requirements.

Cam-Lock type, single conductor, separable connectors and cables are the most common type of main feed input connection. Cable sets can be manufactured in any length. Pin & Sleeve connectors are available, but are less common.

Cam-Lock Connectors



Cam-Lock Cable Sets



Pin & Sleeve



Connection to house power may take the form of mating Cam-Lock connectors on a “company switch”, or may be directly hard-wired to a circuit breaker panel or disconnect switch.

Company Switch





Circuit Breaker Panel





Disconnect Switch



Main Power Distribution Units (medium size)







Satellite Branch Circuit Power Equipment (fed from the main PD)

Breakered Stringer Box



Multiple Boxes on a Single Cord



Rack-mounted PD



Large Size Systems:

These systems are generally 400A, (or more), 3-phase. Larger systems may require multiple 400A units. Output Cam-Lock connectors (feed through) can be provided to allow several PDUs to be daisy chained together. If more than 400A is required, several 400A units may be fed by individual 400A feeder circuits.

There is a vast variety of “sub-main” and “satellite” power units that may be selected to distribute power to different portions of the stage area; these areas may include: center stage band gear, front of house mixers, lighting control consoles, stage lighting & dimmers, chain hoists & controllers, special effects, powered speakers, monitor mix station, amplifier racks, and the like. The power equipment and associated cable assemblies should be selected to put power where you need it and must have the capacity to handle the electrical requirements, with some “head room”. The main PDU (depicted below) is only an example. There are multiple power output devices (outlets) the can be incorporated, in both type and quantity.

400A Main Power Distribution Unit



Amplifier Racks



Front of House Equipment



Stage Lighting Truss Hoist



Stage Lighting Dimmer Racks



Center Stage Equipment



Conclusion:

Unfortunately, there is no “one size fits all”. Each job will have different requirements.

Consideration should be given to the “modularity” of your system. You can start small and add equipment as expanding requirements present themselves. As an example, what was your 100A main system, may become a satellite for a new 400A system. Conversely, the 100A “sub-main” portion of your 400A system may become the “main” for a smaller job.

There is no way of describing all the possible scenarios that you may encounter, and configurations that may be required. There are literally hundreds of Power Distro configurations available and an infinite number of potential job requirements. Check-out Motion Labs web-site to view a multitude of possible options. www.motionlabs.com

Motion Labs is here to help.

We encourage you to ask questions, provide constructive criticism, and suggest subjects for future blogs. Remember some of the subject matter may be argumentative in nature. If you disagree or need clarification, let us know.

Thanks:

Jim Herrick

About the author:

Jim Herrick, over 40 years’ experience in the electrical field. Licensed in the State of New Jersey as an Electrical Contractor (License #6748) and Electrical Inspector (License #7702). Of the 40 years’ experience, 30 of which, included work in the theatrical industry, which also included electrical equipment design for major companies in the industry.