Over this past summer in FRONT of HOUSE, we embarked on a three-part series that touched on some aspects of electrical power distribution in the context of portable generators. Those articles covered a broad range of generator topics, from the important distinctions between bonding and grounding to the upcoming code changes for small generators. In light of the positive reception for that series, we undertake a series on portable power distribution (a.k.a., AC distro) downstream of the generators.
I repeat the disclaimer from the generator series. I am not a licensed electrician, and implore readers to consult with an experienced electrician or their Authority Having Jurisdiction (AHJ) for further clarification about electrical matters. Equally important is realizing that knowledge gained from an article like this one is no substitute for direct experience in practicing safe handling of electrical equipment in the field. A solid conceptual understanding of portable electrical power distribution should be a goal for all pro audio professionals. This will give a basis for safe shows and good relationships between the industry and AHJs.
This article begins with a brief conceptual overview of the common methods of power distribution, followed by an introduction to the NFPA 70: National Electrical Code® (NEC) definitions for terms related to power distribution methods. The NEC has its own specific terms and is rather dense to digest in absence of knowing the underlying vocabulary. As no group of articles in a magazine can hope to be as comprehensive as the code, a reasonable approach is to equip FRONT of HOUSE readers with tools to understand what the code is saying. Next month we’ll build on this article to apply directly to power distribution in the portable context using the terminology from the NEC.
Power Distribution: Big Concepts
Whether for stage, home or industry, the underpinnings of modern power distribution rises from hundreds of years of physics and mathematics. Much of this came to a head in the Victorian age with the advent of the telegraph. Arguably the most influential figure of this period is Oliver Heaviside (1850 -1925), and the interested reader is encouraged to read about his influence. Here, for brevity, we’ll swoop over some broad concepts that govern the electrical systems we use.
At a high level, electricity consists of a “push” that causes electrons to move. The push is transmitted between electrons via something that we call the electric field. The field is the electrical property that causes one electron to be aware of another. That push can either drive the electrons away, or pull them closer. A common way that we measure this push is called voltage. The degree of this push is compared between points. If the points have different values of the electric field, a voltage will be measured. When voltage attracts electrons in one direction, we call it positive voltage; if the electrons are repelled the other way, we call it negative voltage. It is fairly straightforward to produce a voltage that alternates smoothly between defined positive and negative values. In common electrical parlance this is called an AC voltage.
When we want to characterize how the push from voltage causes electrons to move, we are now talking about current. Current is a way of measuring the movement of electrons. To allow current to flow, we must create a continuous conductive path. While the electric field that gives rise to voltage is an inherent property of electrons, current is only driven to flow when a voltage difference exists over a conductive path.
When generating AC voltages, it is common to generate multiple discrete voltages spaced out in time. The typical electrical generator has a rotating section that generates either the positive or negative voltage, based on the stage of its rotation. By spacing multiple windings around the generator, each one is at a different stage of the voltage-producing process. Each winding has a different angle to the generator’s rotating section relative to its neighbors at any given point. We call each of these discrete voltage sources a phase. The term phase comes from the mathematics used to describe the relative timing of a signal that repeats over a set period of time, like our voltage(s). If an electrical system utilizes a single voltage supply from the generating system, it is termed a single-phase service. If an electrical system uses multiple voltage sources, each with a fixed time relationship, it is termed a polyphase service. A common case of polyphase power distribution is a three-phase service.
Since current flow requires a completed circuit, there must be a conductive path back to the service on which the voltage originated. In a single-phase system, this return path typically takes the form of an un-energized conductor (i.e., no applied voltage) that conducts current only when a load (e.g., power amplifier) is placed between the voltage source (i.e., phase) and the other conductor. For three-phase services, loads may either be connected between two (or more) phases, or between a phase and an additional un-energized conductor.
Both single- and three-phase services can, under certain circumstances, exhibit a unique property where the net voltage at an un-energized conductor sums to zero, and thus provides no push to drive current flow. When these conditions are realized, the un-energized conductor was historically deemed a neutral conductor. A requirement for no current in the neutral is equal balancing of the load resistance between each connection of a voltage source and the non-energized conductor.
In practice, the load “across” each phase is rarely perfectly balanced, so the neutral has some voltage on it, and therefore normally has some current flowing on it. The magnitude of the current in the neutral depends on how well the loads are balanced across the phases of the power distribution service. With careful balancing, the neutral is responsible for carrying much less current than would otherwise be the case without the unique voltage summing properties. The modern definition for the neutral allows for it to carry current as a result of unbalanced loads. With broad conceptual descriptions of voltage, current, polyphase distribution, and the neutral completed, we now turn to defining terms from the NEC useful for portable power distribution.
NEC: Not Universal
The NEC is not adopted universally in all U.S. jurisdictions. Some locations have exceptions to the NEC; others have additional requirements beyond the code. Because of the way the code is adopted, your local AHJ should be considered the ultimate resource for understanding code compliance of your pro audio endeavors. Further, the NEC is an evolving document and has been revised extensively over the years. When reading the code for yourself, please reference code cycle 2008 or later. My code reference for this article is either the 2011 code or the proposed 2013 code draft to be officially released in 2014. A digital version of the NEC 2008 is an affordable download ($9.99 U.S.) for Apple devices on the iTunes store, and I encourage readers to purchase a digital copy of it. Having a digital copy of the code will allow you to read the code more thoroughly. Your local AHJ can clarify what code cycle is the current one for your area.
In many cases we are paraphrasing the code definitions and statements; please see the NEC for the full definition. If we quote the code directly, we’ll indicate that by placing “quotes” around the text. Code quotes with brackets [ROP x-x] after them refer to proposed changes for the 2014 code cycle. When we chose to quote the code draft is it because the revised definitions have improved clarity, and are worth referencing even if their wording changes slightly in the final 2014 code cycle.
NEC: Key Terms
All definitions for the NEC are in article 100 (NEC 100). There are many more definitions inside NEC 100, but these comprise the bulk of the terms used to describe polyphase power distribution. While the NEC lists definitions alphabetically, these definitions are listed here in order such that they build on each other conceptually.
Energized — “Electrically connected to, or is, a source of voltage.”
Ampacity — “The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.”
Bonding — “Connected to establish electrical continuity and conductivity.” Connection of two electrically conducting elements together achieves two main purposes: First, It holds the points at the same voltage potential, and second, it serves as a path for current in the event that a voltage potential difference (e.g., from an external voltage source) creates a current flow.
Grounding — Physically connecting an electrical system to the contents of planet earth.
Grounding Electrode — The physical conductor that ties to planet earth to facilitate grounding.
Grounded — An electrical conductor physically connected to the earth by nature of it being bonded to a grounding electrode or grounding electrode system.
Grounded, Solidly — “Connected to ground without inserting any resistor or impedance device.”
Ungrounded — “Not connected to ground or to a conductive body that extends the ground connection.”
Neutral Point — “The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system, or midpoint of a single-phase portion of a 3-phase delta system, or a midpoint of a 3-wire, direct current system. Informational Note: At the neutral point of the system, the vectorial sum of the nominal voltages from all other phases within the system that utilize the neutral, with respect to the neutral point, is zero potential.”
Grounding Electrode Conductor (GEC) — A conducting element used to connect a grounded conductor to a grounding electrode.
Bonding Jumper — “A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected.”
Ground Fault — “An unintentional, electrically conductive connection between an ungrounded conductor of an electrical circuit and the normally non—current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth. [ROP 5—11]”
Equipment Grounding Conductors (EGC) — “The conductive path(s) that provides a ground-fault current path and connects normally non—current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both. [ROP 5—14a]”
“Note: It is recognized that the equipment grounding conductor also performs bonding. Also: See 250.118 for a list of acceptable equipment grounding conductors.”
Service — “The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.”
Service Equipment — “The necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s) and their accessories, connected to the load end of service conductors to a building or other structure, or an otherwise designated area, and intended to constitute the main control and cutoff of the supply.”
Separately Derived System — “A premises wiring system or portion of a premises wiring system other than a service. Power for such systems is derived from a source of electric energy or equipment with no direct connection from circuit conductors of one system to circuit conductors of another system, other than connections through the earth, grounding electrode(s), grounding electrode conductors, bonding jumpers used to connect grounding electrodes, equipment grounding conductors, metal enclosures, or metallic raceways. [ROP 5—20]”
Overcurrent — “Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault. Informational Note: A current in excess of rating may be accommodated by certain equipment and conductors for a given set of conditions. Therefore, the rules for overcurrent protection are specific for particular situations.”
Overcurrent Protective Device (OCPD) — A device that opens a circuit if the current rises above a certain amount. Think of a circuit breaker or fuse.
Circuit Breaker — “A device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.” A circuit breaker acts as an OCPD.
Effective Ground-Fault Current Path — “An intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors. [ROP 5—6]”
Note that the NEC, especially in older editions, plays somewhat loose with the terms grounding or grounded where the authors meant to refer to bonding. The new, upcoming code cycle draft for 2014 is more consistent in its wording, but grounding terminology might still be found in circumstances that are actually discussing bonding.
Conclusion
So far, we’ve reviewed the underlying concepts of AC voltage generation and how that results in polyphase power distribution. We also introduced the unique possibility of a neutral conductor that has no net current flow under conditions of balanced loads on the service. And the definitions for terms related to polyphase power distribution used throughout the NEC are directly applicable to portable power distribution.
Next month, we’ll discuss the common portable power configurations, both single and polyphase, using the vocabulary of the NEC. We suggest that you familiarize yourself with the definitions of the terms above, and keep this article handy as a companion for the next article. It can be frustrating to learn new terminology in a way that sticks, but doing so will make working with the NEC much easier, and allow the pro audio practitioner to work through the code to a depth where they can have detailed discussions with any AHJ or electrician
