Home Wiring: Standards, Codes, and Safety


WE DEPEND ON ELECTRIC POWER so extensively in our homes and businesses, on our farms, and in industry that it would be difficult to list all its uses. Electric power serves in countless ways while normally presenting little danger, and we often take it for granted. Yet when not used properly electric power can cause fire, destroy property, seriously injure people, and even cause death.


Electric power is safe for us to use only when it’s under control. We control it by using wiring and electrical equipment that is of dependable quality. Equally important, the wiring and electrical equipment must be properly installed and maintained.

Electrical parts and devices are manufactured according to specific safety standards. They should be installed using methods that are uniform throughout all the states—the methods proved by experience to be both practical and safe.

Product listings: Reputable retailers and manufacturers sell only merchandise that is listed by a qualified electrical testing laboratory. The oldest and most commonly recognized testing organization is Underwriters Laboratories (UL). When a manufacturer submits its product to such an organization, the product is investigated and subjected to performance tests. If it meets the safety standards, it’s then “listed by (name of laboratory).” Many people say “approved by... “but that is not correct terminology. To determine that the product continues to meet requirements in the product standards, trained field representatives periodically visit the factory where the product is made to audit production controls. Additionally, the testing laboratory regularly tests factory and store samples. If these samples meet the requirements, the product continues to be listed and will bear a listing mark.

Some items (wire, large switches, fixtures, conduit) have a listing mark similar to those shown in FIG. 1—1 on each coil or piece. Cord sets (extension cords) have labels as shown in FIG. 1-2. Devices like toggle switches carry the name of Underwriters Laboratories Inc. (may be abbreviated) and the word LISTED molded or stamped on each piece. Still other items such as receptacles, sockets, outlet boxes, and toasters have no label, but the nameplate, tag, or the item itself bears the listing mark of FIG. 1-3. Each piece is marked in some way so that the inspector or purchaser can identify the manufacturer by referring to a directory listing manufacturers that have proved their product meets the applicable requirements. The listing mark on the product indicates compliance. In addition to UL, other certification laboratories include ETL Testing Laboratories, Inc. (now part of Intertek Testing Services), Factory Mutual (FM), and the Canadian Standards Association (GSA); however, UL is the most widely used.

Listed” usage requirements: The term “listed” indicates the merchandise is suitable and safe if used for the purpose and under the conditions for which it was designed. Electrical inspectors will turn down listed merchandise if improperly used. For example, the inspector will refuse to accept listed armored cable used in a barn if the barn is a corrosive location, because armored cable is for use only in permanently dry locations. Listed lamp cord will be turned down if used for permanent wiring because such cord is listed only for use on portable equipment.

Listing by UL does not mean that two similar pieces of merchandise are of the same quality. It merely indicates that both pieces meet the UL safety requirements. One piece may far surpass those requirements, while the other just barely meets them. For example, of two brands of toggle switches, one may average 25,000 ons and offs at full load before breaking down, and the other only 6,000. Use your own good judgment in making your choice among listed brands just as you would in selecting other merchandise.


Listed electrical parts of high quality that are carelessly or improperly installed might still present a risk of both shock and fire, so rules have been developed regulating the installation of wiring and electrical devices.

National Electrical Code: For a safe installation, listed devices must be installed as required by the National Electrical Code (abbreviated NEC). The NEC is simply a set of rules specifying the installation and wiring methods that over a period of many years have been found to be safe and sensible. The NEC permits installations to be made in several different ways, but all wiring must be done in one of the ways described in the NEC. A revised edition of the National Electrical Code is published every three years, with the next edition to appear in 2009. Every reader is urged to obtain a copy of the NEC and to use it as a reference. It contains detailed regulations and extensive tables. Because the NEC is almost 700 pages long, only its most important general points can be included in this guide.

In our discussions, all reference to NEC section numbers will be to sections in the 2009 NEC, which contains a large number of changes from previous versions of NEC. Note that effective with the 2001 NEC the word “section” is no longer used to refer to specific provisions of the NEC. All references formerly written as “Section 90-1” For example, now simply reference “90.1.” The familiar hyphens between article and section number have been converted to a dot everywhere. Other changes consist of : (a) renumbering a section without a change in text, so that the only change is in the number of the section; (b) renumbering a section with changes in the text, sometimes of minor but often of major importance; and (c) new section numbers covering material not in the previous edition. Therefore, a 2001 section number mentioned in this guide may or may not be about the topic covered by the same section number in an earlier or later NEC. Vertical lines in the margins of some NEC pages indicate material that differs from the previous edition. Beginning with this edition, the vertical lines denote all changes, including deleted material. The marginal bullets that used to indicate deleted material are no longer used.

All methods described in this guide are in strict accordance with the latest National Electrical Code as interpreted by the author. Every effort has been devoted to making all statements in the guide correct, but the final authority on the NEC is your local inspector. This guide covers only the wiring of houses and farm buildings. Any statement that some particular thing is “always” required by the NEC means “always so far as the type of wiring described in this guide is concerned?’

Study of the NEC is necessary and helpful, but the NEC alone won’t teach you how to wire buildings. Read NEC 90.1 for the exact wording of its purpose, which is essentially the practical safeguarding of persons and property from hazards arising from the use of electricity. These hazards are fire and electric shock. The NEC contains provisions considered necessary for safety. Compliance with the NEC together with proper maintenance will result in an installation essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion. The NEC is not intended as a specification for design, nor is it intended as an instruction manual for the inexperienced. A lay reader picking up a copy of the NEC without specialized training will likely be hopelessly confused. Electricians generally spend at least four years in training, including extensive work with code instructors, before they work in the field without supervision.

Legal aspects: Neither the NEC nor the product standards as enforced by the testing laboratories have the force of law. However, most states (and/or municipalities if so authorized by the state legislature) pass laws requiring that all wiring must be in accordance with the National Electrical Code. Usually power suppliers won’t furnish power to buildings that have not been properly wired, and insurance companies may refuse to issue policies on buildings not properly wired. In addition, faulty work may complicate insurance reimbursements in the event of a loss. You have no choice except to follow the law, and in doing so conscientiously you will automatically produce a safe installation.

Local codes: The National Electrical Code is sometimes supplemented by local codes or ordinances, which seldom differ from the national code in general terms but which frequently amend specific provisions. For example, armored cable wiring is one method permitted by the NEC but sometimes prohibited by local codes.

Local permits: In many places it’s necessary to get a permit from city, county; or state authorities before a wiring job can be started. The fees charged for permits generally are used to pay the expenses of electrical inspectors, whose work leads to safe, properly installed jobs. Power suppliers usually won’t furnish power until an inspection certificate has been turned in.

Local licenses: Many areas have laws that no one may engage in the business of electrical wiring without being licensed. Does that mean that you cannot do electrical work on your own premises without being licensed? In some localities the law is interpreted that way, but not in others. Consult your power supplier or local authorities.

Remember that if a permit is required, you must get one before proceeding with your work. Before applying for a permit, be sure you understand all problems in connection with your job so that your wiring will meet national and local code requirements. If it does not, the inspector must turn down your job until any errors have been corrected.

FIG. 1-1 Examples of labels applied on merchandise listed by Underwriters Laboratories Inc.

FIG. 1-2 Listed cord sets (extension cords and appliance cords) are tagged with a flag label.

FIG. 1-3 If because of size, shape, material, or surface texture the product bears only the round symbol at left, a complete listing mark with the four elements shown at right will be found on the smallest unit container in which the product is packaged.

National Electrical Code and NEC are registered trademarks of the National Fire Protection Association Inc., Quincy Massachusetts. Everyone is urged to study the NEC.


Electrical systems in buildings must be installed in such a way that they present the least possible hazard to the occupants or the property. Any unique conditions and uses, such as encountered on farms, must be carefully evaluated for wiring requirements that promote safety for people, animals, equipment, and buildings. In addition, the work itself must be performed safely for the protection of the installer. Unless you feel confident that you can satisfy these safety objectives, it would be best that you use this guide as a guide to understanding your wiring system and leave the actual installation to a professional. At the very least, this guide will allow you to have a more intelligent conversation with your electrical contractor, and get more for your money.

Protect your eyes: For your personal safety whenever you are doing any electrical work, protect your eyes at all times by wearing safety glasses. Bits of copper can fly out when cutting wire, and hot solder, flux, and plaster dust have a way of making right for the eyes. If an arc is inadvertently started when you must test on an energized circuit, molten metal can be thrown out too quickly for you to escape it.

Be aware of electrical fire hazards: Short circuits, overheating, and the arcing or active flow of electricity through an ionized path in the air or other gas (and usually where it doesn’t belong)—these are hazards that can result from faulty wiring and cause fire. Electrical arcs, For example, typically run at temperatures considerably above that on the surface of the sun. Be careful to use wires with the proper ampacity and check for wires or cords with damaged insulation. In Section 7 you will find a thorough discussion of the essentials of proper grounding. Protecting your electrical installation through the use of overcurrent devices—fuses and circuit breakers—is the focus of Section 5. Limiting vulnerability to lightning damage is a special concern for rural residents and is discussed in Section 17, “Farm Wiring" Safe wiring practices for fire prevention are emphasized throughout the book.

Take advantage of hazard-warning detectors: Faulty wiring is just one of many possible sources of fire or other hazards in a home. Installing appropriate alarm equipment is a relatively small investment that can save lives. Regularly check to see that your warning alarms are functioning. Get in the habit of replacing the batteries in a smoke detector or CO (carbon monoxide) detector every fall at the same time as you reset your clocks to standard time. Remember that modern designs of even permanently wired detectors usually depend on battery backup.

Smoke detectors: Self-contained smoke detectors that sound an alarm when visible or invisible combustion products are sensed have been credited with saving many lives by alerting occupants early enough for them to escape from afire. Smoke detectors in homes are required by local building codes. There are three types of detectors. Ionization alarms are especially effective at detecting fast-flaming fires that quickly consume combustible materials such as burning paper or grease. Photoelectric alarms are more effective at detecting slow, smoldering fires such as a cigarette burning in a couch, which may take hours before bursting into flame. For your maximum protection, install both types or select the third kind— a dual detector that combines both features. Building codes in each community specify the required locations but generally not the type of alarm. In general, never place smoke detectors where they will be subject to nuisance alarms. For example, an ionization detector placed near a bathroom door will likely go into alarm when the door opens following a shower. Some local codes require a smoke detector in the hallway outside bedrooms or above the stairway leading to bedrooms on an upper floor, while other localities require a detector in each bedroom, the kitchen, and one on each floor of the dwelling.

Battery-operated, plug-in, and direct-wired smoke detectors are available. Battery-powered models may be acceptable in an existing dwelling, but in new construction most building codes require connection to the electrical source, with a battery backup and all alarm units interconnected by wires so that if one responds, they all respond. The required location is on the ceiling, or on the wall not more than 12 inches from the ceiling. Don’t place the detector in the path of ventilation that would move air past the detector faster than in other parts of the room. To install a direct-wired detector, choose a circuit with often-used lights on it, such as the bathroom light, to assure the circuit could not be off without being noticed. Wire the detector directly across the two circuit wires, unswitched.

Carbon monoxide (CO) detectors: Many local ordinances also require permanently wired carbon monoxide detectors with battery backup to alert occupants to the presence of this invisible, odorless, deadly gas. Check with your building official or fire marshal for specific requirements. CO detectors are highly recommended even if not required by local codes. Battery-only styles are available. CO detectors can usually be found in stores near the smoke detectors.

Prevent electric shock: Safe installation practices are emphasized throughout this book. Shock—the accidental flow of electric current through the body—is the principal hazard that can result from unsafe practices. Observing the following cautions will help you avoid injury.

Disconnect the circuit: Whenever possible, work should be done only on circuits that have been de-energized (disconnected from the source of power). If the disconnecting means is out of sight from the work location, you must take positive steps, such as taping the switch to the OFF position or removing the fuse, to make sure someone else cannot inadvertently energize the circuit while you are working on it. Some testing can be done only on energized circuits. Be extremely careful when testing that you don’t contact live parts. As explained on page 63, receiving a shock is more dangerous when the victim is standing in water or a damp location than on a dry surface. For your protection, stand on dry boards or a rubber mat and be sure that neither you nor the floor is wet. Remember that electricity and water are a dangerous combination.

Avoid shocks from small appliances: Even when a building has been wired in strict accordance with the NEC, shocks can still occur from the use or misuse of appliances. Fatal shocks have occurred in bathtubs or showers when an appliance such as a radio or hair dryer has accidentally fallen into the water. In that situation, even an appliance that is not defective can cause a fatality. Touching a defective radio or heater or other appliance while in a tub or shower can also be fatal. A child might chew on the cord of a lamp or appliance; in doing so, if the child punctures the insulation of the cord (or if the cord is defective) while he or she is also touching a grounded object such as a radiator or plumbing, a fatal shock can result. A person using a defective appliance while touching a grounded part of plumbing can easily receive a shock.

A toaster can present extreme danger if the toast sticks rather than popping up when it’s done because when that happens people sometimes use forks to release their toast. Don’t do it without first unplugging the toaster. The live heating elements are exposed in the toaster. If your fork touches a heating element and you are touching a grounded object (such as a water faucet or a sink) at the same time, you could receive a dangerous or even fatal shock. Unplug the toaster before reaching inside it for any purpose, or use wooden tongs to extract the toast.

Know how to help shock victims safely : If you discover a shock victim, the most important point is to be careful not to become a second victim yourself. This guide cannot possibly give you all the needed information, but it can give you a few hints. We’ll first discuss shocks caused by the ordinary 120/240-volt wiring in a house or a farm building. If you find a victim and it appears that the shock was caused by an appliance or similar equipment, disconnect that equipment. If it has a wall switch, turn it off. If it doesn’t, pull the plug out of its receptacle; don’t touch the cord because it may be defective, but rather grasp the plug and pull it out. Don’t touch the appliance itself.

In cases where the source of the shock is not apparent, hurry and open the main switch of the building, or throw the main circuit breaker to OFF if your building is equipped with breakers. Then turn your attention to the victim. If the victim is unconscious or appears not to be breathing, phone 911 or the emergency service in your area for assistance. If you know CPR (cardiopulmonary resuscitation), start using it while you are waiting for medical help. (You’ll be better prepared for emergencies if you have taken a class in CPR. Contact your local Red Cross Section for information.) If the shock has been caused by a high-voltage line that has been knocked down by a storm or accident, the situation is quite different and many times more dangerous. In such a case, phone 911 and the power company if you know the number. If the high voltage wire is touching the victim, don’t touch him or her; if you do, you could become the second victim by trying to help somebody who quite possibly has received a fatal shock. Theoretically you can use a completely dry wooden pole at least four feet long to push the fallen wire off the victim, but such accidents seldom occur near your home and you might not have such equipment with you. You might be able to rig a sling or rope (only if absolutely dry) to separate the wire from the victim. Time will be a critical factor. If at all possible, depend on experts from your power company to perform the rescue.

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Monday, 2008-12-29 16:58 PST