Transmission & Distribution: Electrical wiring 101

What engineer out there hasn't been called upon to replace a light switch, circuit breaker or other electrical device? Most of us have, I'll bet.
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What engineer out there hasn't been called upon to replace a light switch, circuit breaker or other electrical device? Most of us have, I'll bet. Unfortunately, as knowledgeable as we broadcast or post-production engineers think we are, the worlds of power and audio/video are vastly different. And, making a mistake at 120 V AC can have far different consequences than at audio or video levels.

Fortunately, some of the basics you learned in electricity 101 apply. Once you understand some of the terminology, you're off in the right direction. Let's begin with a review of some basic terms.

It's elementary, Dr. Watson

Receptacles, switches, plates and cord connector bodies are available in a wide range of sizes, ratings and styles with specific features and characteristics to meet most design/application requirements.

Have you ever wondered what the difference is between a 48-cent light switch and one that costs three dollars? The terms “economy,” “competitive,” “intermediate” or “residential” are sometimes used to indicate that the device is economically priced or designed for light-duty applications. Terms such as “specification” or “super-specification” would indicate devices that are of better quality, designed for greater reliability and usually higher priced. You favorite hardware store may use the terms “residential” for the cheap switch and “commercial” for the more expensive switch. Are the commercial versions different? Well, they certainly cost more.

None of these terms has an official status with standardizing agencies such as Underwriters Laboratories (UL), Factory-Mutual (FM), Electrical Testing Laboratories (ETL), or the National Electrical Manufacturers Association (NEMA).

Presently, UL lists wiring devices for only two grades, standard and hospital grade. All devices, whether termed intermediate, economy or specification, must meet identical UL requirements (although as mentioned above, specification grade devices are typically of better quality construction).

Select your switch

There are two basic types of snap switches - AC general use and AC-DC general use. A “T”-rated AC-DC is also available for 125 V, tungsten filament lamp loads.

The AC-DC type is designed with a quick-make/quick-break action requiring rugged springs and components to assure dependable operation. If T-rated, its contacts are designed to handle the high inrush current of incandescent lamps. The AC-only type has a somewhat slower make/break action.

For example, AC-DC general-use switches are rated 3 A, 5 A, 5 A or 6 A, 10 A, 20 A, 30 A and 40 A, and 60 A at 125 V; AC general-use switches are rated 15 A, 20 A and 30 A at 120 V. Ratings for other voltages are also provided.

All AC general-use switches are marked AC in addition to their electrical rating. AC-DC general-use switches usually are not marked AC-DC, but are always marked with their electrical rating.

Mercury switches have an AC rating of 125 V, 15 A or 20 A, but they are also T-rated at 10 A, 125 V DC.

Flush snap switches are available in the following types: single-pole; double-pole; three-way; four-way; two-circuit; maintained contact, SPDT, DPDT; and momentary contact SPDT, DPDT; SP normally open, SP normally closed; DP normally open and normally closed; and three-way. Most of these switches can be obtained with key-lock design and ratings vary among manufacturers.

Use the right terminal

How many times has someone asked you which wire goes to which terminal? The terminals on AC power devices are typically labeled “white” and “black.” Boy, does that cause the poor neophyte problems if he runs into a three-way circuit using a red wire.

Switches may also be identified for the type of wire they are rated for. Line terminals of 15 A and 20 A switches marked CO/ALR are for use with aluminum, copper and copper-clad aluminum conductors. Terminals of switches rated 30 A and above marked AL/CU also are suitable for aluminum, copper and copper-clad aluminum conductors. Also, switches furnished only with screwless pressure terminal connectors are acceptable for use with copper and copper-clad aluminum conductors, but are not suitable for use with aluminum conductors.

Switches provided with push-in screwless pressure terminal connectors have a particular advantage when they are ganged in the same box. Sec. 380-8(b) of the NE Code prohibits ganged snap switches with exposed live parts if the voltage between adjacent switches is over 300 V. Because switches with pressure-type terminals have no exposed live parts, they can be used to satisfy this Code rule.

Wall-mounted occupancy (motion) sensors can replace standard wall switches, and many of these models incorporate a switch mechanism for override capability. The area they can cover depends on several factors, including the mounting location and height, room configuration, furniture, the sensor's sensitivity setting and type of motion detection. While they may sound neat and are often recommended by architects, I've found them unreliable in my conference rooms. It's embarrassing to have the lights go off in the middle of a client demonstration.

X10 to the rescue

Electronic enhanced switches (typically using X10 technology) provide a number of features that are increasingly being used in residential and institutional occupancies. For example, fully automated lighting controls allow any indoor or outdoor light to be remotely controlled from virtually any point within the system, which consists of controllers and receivers.

Configured as wall switches and receptacles, the receivers accept command signals sent through the existing AC branch circuits. In addition, a wall-mounted programmer can be used or a pocket-sized, handheld controller can control these devices.

A word of warning from firsthand experience. These controlled circuits can be unreliable. I've found my X10 circuits turned on by lightning and children pushing the wrong buttons on universal remote controls. It may sound cool to program your coffee pot to come on automatically in the morning. But, what happens if it's accidentally turned on while you're away. Some circuits are better left to manual activation.

Receptacles

Receptacle ratings range from 10 A to 400 A, with some sizes available at 125, 250, 277, 480 and 600 V. Most popular are the 15 A and 20 A flush type, rated at 125 V and 250 V. These are available with several different slot configurations in grounding and nongrounding type. NE Code rules require that receptacles installed for the attachment of portable cords shall be rated at not less than 15 A, 125 V or 10 A 250 V. Sec. 210-7 states that the receptacles on 15 A or 20 A branch circuits must be of the grounding type.

Receptacles located outdoors but protected from direct contact with rain by a roof or overhang are considered to be damp areas. These receptacles must be protected by a closable cover when the plug cap is not being used.

In wet areas, the receptacles must be protected by a cover that will not be affected by rain when the plug cap is installed. Any cover acceptable for use in wet locations (outdoors without protection of roof or cover) also is acceptable in damp areas.

Grounding-type receptacles are available with special grounding means designed into their box-attachment screws that automatically ground the grounding terminal to the box when the receptacle is installed.

Special receptacles

Split-bus receptacles are available in flush duplex types with ratings of 15 A or 20 A, 125 V or 250 V. Typical units consist of duplex assemblies with one parallel-blade receptacle (125 V) and one tandem blade receptacle (250 V), or two parallel-blade receptacles. Split bus receptacles make it possible to wire separate circuits or controls to each set of line terminals.

Many standard parallel-blade duplex receptacles contain a break-off feature that permits the connection of both receptacles on a singe circuit, or a jumper can be removed on the line terminal bus to permit two-circuit operation.

GFCI devices

Receptacle-type ground fault circuit interrupters (GFCI) can, in many instances, be used in place of circuit-breaker type GFCIs to provide protection from shock hazard. A receptacle-type GFCI may be wired as a terminal device or as a feed-through unit to protect additional downstream receptacles. When properly installed, the device automatically shuts off power when it detects current leakage to ground of a few milliamps, preventing serious injury or electrocution.

These GFCI devices are available with a number of installer-friendly features. For example, one manufacturer offers a device with two back wire holes per termination to allow for a multiple of wiring options without having to pigtail and use wire connectors. It also has easy-to-read Line/Load markings. The NE Code specifies those areas where GFCIs are required. However, GFCI protection should be provided for any type of circuit where there is a danger to personnel from ground faults.

Isolated ground circuits

Isolated grounding receptacles are for use where electronic equipment can be adversely affected by pickup of transient signals, interference or RF from surrounding equipment.

Rather than relying on a mechanical ground path through the metallic housing of the raceway system, these devices use an insulated grounding conductor in the raceway to provide a separate “pure” grounding path. This separate grounding conductor is run with the circuit conductors in the same raceway and is connected with the normal equipment, but connects to ground only at the service equipment ground terminal.

So, there you have it, the basic rules for wiring the boss's office. Or not! Just remember the cardinal rule, keep one hand in your pocket and the other on your favorite beverage. That way you're sure to stay out of trouble.

Acknowledgement: This article was adapted from the original written by Joseph Kinsley for Broadcast Engineering's sister publication, Electrical Construction and Maintenance.