ICEinLAVA's Domain

I know of nothing more important when it comes to providing a good, reliable sound system operation than having a great power distribution system. But before we dive into the ins and outs of cam-loks, genny connections, star grounding, and load sharing, let’s review the basics to bring everyone up to the same level of understanding.

Power distribution is not just a little rack mount unit that splits one plug into several receptacles, although you would never know it based on the sparse information about the subject. A little bit of knowledge in the hands of the inexperienced makes those individuals very dangerous, and their reference materials could be subjected to lawsuits (read: lawyer bait). So at the risk of baiting the legal community, here goes the information.

Safety is first, of course. Besides knowing how to implement a power distribution system, you should know that doing it wrong can result in shocks, unintentional welding, fires, and shrapnel flying about. If you have the slightest thought that what you are about to touch may shock you, make sure you touch it with dry skin and with the minimal amount skin surface area. I recommend a single knuckle, in case the current stimulates the closure of your hand muscles (think palm on mic windscreen). Your dry skin may have thousands to tens of thousands of ohms of resistance and is your primary protection against shocking currents. Once inside your body, the resistance drops to a few ohms.

Even before a Ground Fault Circuit Interrupter trips at six milli-amperes of current, a few milli-amperes will give you a tingle. If your muscles get vigorously stimulated from a shock, get yourself checked out by a medical professional. For the curious, send me an e-mail, and I will reply with a chart showing shocking current levels with physiologic effects from tingles to muscles ripping off of bones.

If you plan to start constructing power distribution gear like distro sub-panels, backline stringer AC cables, and feeder extensions, getting a copy of the 2002 National Electric Code handbook is a must. The book details the terminology, practices, components, and location-specific regulations needed for safe and legal distribution of electricity. Get yourself a personal copy at www.nfpacatalog.com, or borrow one from the local library until you can afford one.

While not widely enforced, all work on power distribution systems should be done by licensed electricians or licensed electrical engineers. The coursework to become an electrician is taught at vocational and technical schools as a 2-year degree, and would make an excellent credential to supplement your sound system experiences. While the license is only valid in the state you tested in, the experience and the credential will help on tour when dealing with other building electricians. If you will not pursue licensure, at least read the NEC handbook a couple of times, and have a licensed electrician/engineer double-check your work.

Watts Up Doc?

As an acknowledgement for James Watt’s work on steam engines and defining the amount of work (power), physicists have defined a standard unit of power as a “Watt,” instead of the previously used horsepower. For electricity and power distribution, knowing “Watt’s Law” is as crucial as knowing Ohm’s Law. Watt’s law is simply:

Power (Watts) = Voltage (Volts) x Current (Amperes)

Here in the good ol’ USA, our AC power for sound systems is mostly 120 Volts Alternating Current. Given that standard residential and commercial receptacles use 15 or 20 amperes, Watt’s Law would say that the circuits that feed the receptacles would then have either 1800 or 2400 watts in power availability.

Watt’s Law applies to many items, and the challenge is to distribute the various pieces of sound system gear so they do not add up beyond circuit power availability (capacity), or a potential “overload” may occur, resulting in a circuit breaker tripping or fuse blowing. There are obvious exceptions in sound system work, primarily in sizing audio power amplifiers to power distribution circuits. Because music reinforcement is not continuous in loading like DC or AC power circuits, audio power delivered to speakers does not directly transfer to continuous power supply consumption. For example, the average music power delivered to the speakers is typically one-eighth to one-third the rating of the amplifier. Then factor in the extra power lost in the form of heat in the amplifier circuits. That is why high efficiency 4000 to 6000-watt rated power amplifiers can be successfully supplied from one 2400-watt circuit. Consult the amplifier manufacturer’s specifications on power draw based on your nominal speaker loadings and intensity of operation.

Those Pretty Colored Wires

To understand how power is supplied from the electrical utility down to your gear, we need to draw a picture. Figure 1 shows a simple commercial or residential single-phase electrical service. While denoted as single-phase, it actually is two “hot” conductors out of phase with each other, sharing a common return wire that is called a neutral conductor. The electric utilities’ transformer converts the many kilo-volt level power line energy at the primary winding to a secondary winding of 240 volts with a center-tap point for the neutral feeder connection, which then creates two out-of-phase hot feeders. These beefy feeder wires exit the transformer vault or pole and run to the venue’s meter socket, which becomes the end of the electric utilities’ responsibility for power provision.

From the meter socket the feeders continue into the location to the main power distribution panel. Larger residential and commercial locations may have “sub-panels” to further group circuits by area, and have additional feeder wires between the panels. Besides being the single-point entrance for electrical service, the main distribution panel serves as the point where the neutral feeder wire gets attached to a safety ground conductor that makes a good electrical contact with the soil surrounding the building. This neutral-ground connection is only supposed to be done at one point in the electrical distribution system, and is made in the main panel neutral bus bar. The bus bar is usually a rectangular bar of steel with many holes serving as conductor inlets, with screws to fasten each neutral and ground conductor to the bar. The hot feeder wires also connect to hot wire bus bars that are connected to main circuit breakers or fuses to interrupt the circuit when very large overloads or short-circuits occur. From these safety circuit interrupters, the hot-wire current paths connect to branch circuit main busses to be tapped by breakers or fuses serving smaller branch circuits.

The branch circuit wires that transfer electrical service to parts of the venue are denoted as hots, neutrals, and grounds. Typically, the hot wire jackets will be black (phase A), red (phase B), blue (phase C), or violet (aux hot designator). The neutral wire jacket is white, and the ground wire jacket will be green or jacket-less. In residential situations, branch circuit wiring may be covered with an overall plastic jacket, sometimes called Romex, that is white, yellow, or gray in color. With some exceptions, commercial wiring is carried in metal or plastic piping called conduits, which lead to the outlet boxes that contain the final connections for light fixtures or receptacles.

The wire sizing, or gauge, helps determine the maximum amount of current, or ampacity, the circuit can handle. Typically 15-Amp residential service circuits use 14-gauge wires. The lowest commercial service circuits are 20-Amp rated and require 12-gauge wiring. At 30 Amps, 10-gauge is used. For 50 to 60-Amp sub-panels and portable distro panels, 6-gauge wiring is required. Larger wire sizes are used if long runs are required. The important thing to note is that the wiring, receptacles, and any other items in the circuit must be rated as good as the breaker trip rating. Too many times I have heard some maintenance dude say, “I replaced the 20-Amp breakers with 30-Amp breakers, so I do not have to run to the panel as much when bands plug in their gear.” The potential result is very warm wiring, smoking receptacles, and a fire hazard.

This is chapter one in what may become a periodic series of articles on nuances of power distribution practices (and mal-practices). In the future, I’ll try to cover topics like generator (genny) connections and operation, service (extension) cord types and stage power distro techniques, as well as common practices of sub-distribution of power to amplifier racks. It is my fondest wish to get everyone past the “dangerous stage” in working with high current power distribution, and practicing safe connections per the NEC handbook recommendations. For me, demonstrating competence in sound system power distribution is the biggest factor in discriminating the amateurs from the professionals.