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Home Wiring Code
In the previous installment we discussed the nature of electricity, how it gets into your house and the use of circuits to distribute it throughout the house (see How Home Wiring Works, Nov./Dec. 2004). We also considered the all-important subject of safety, including grounding, circuit breakers, ground-fault circuit interrupters and arc-fault circuit interrupters. In this issue we'll look at some of the specific requirements for residential wiring dictated by the National Electrical Code.
It's important to understand that the NEC is not a law. However, many state and local governments have adopted some or all of its recommendations and made them requirements for building. The code is a 600+-page book that specifies safe methods for the installation of electrical wiring. For any specific task it often offers several solutions, requiring only that you use a solution exactly as specified.
With few exceptions, local jurisdictions allow homeowners to wire their own homes. One is never allowed to wire another person's home, however, unless licensed or working under the direct supervision of a licensed electrician. All new wiring, whether it is installed by a licensed electrician or the homeowner, should be inspected either by a code official or the electric utility.
If all your lights, receptacles and appliances were on a single circuit protected by a single fuse or circuit breaker, you would find yourself in total darkness every time you overloaded the circuit. And to carry the massive total current, the wire would have to be as large as the service entrance cable. So the NEC recommends breaking the total electrical load into several categories and many individual breaker-protected circuits, such as circuits for lighting, appliances and various areas of the home.
Lighting circuits The NEC requires a lighting allowance of 3 watts per square foot of living space. A 15-amp circuit can carry a total of 1,800 watts (15 amps x 120 volts). Therefore, the requirement is one 15-amp circuit for every 600 square feet of living space. So that you won't find yourself totally in the dark, you should have at least two lighting circuits on each floor.
Because much of the lighting in a typical home is provided by lamps plugged into receptacles, lighting circuits also might include the general-purpose receptacles around the perimeters of rooms. Although receptacle height is not specified, horizontal spacing is. In any habitable room, no point along a wall can be more than 6 feet horizontally from a receptacle. This includes any wall 2 feet or more in width, as well as peninsulas, islands and free-standing railings. The intent is to eliminate the need for extension cords.
Small-appliance circuits Small kitchen-type appliances (toasters, blenders and mixers, for example) draw more power than lamps and radios, so the NEC requires two 20-amp circuits in the kitchen, pantry and dining area. With the exceptions of a kitchen clock and the oven light/ignition system of a gas range, no lighting may be on these circuits. Although the circuits are rated 20 amps, receptacles may be rated either 15 or 20 amps.
Individual appliance circuits According to code recommendations, larger, power-hungry appliances should have their own separate circuits. These appliances include the clothes washer, clothes dryer, dishwasher, waste disposer, water heater, water pump, electric range, electric wall oven, electric cooktop, oil burner, furnace blower, any permanently connected appliance that draws more than 1,000 watts, and any permanently connected motor rated at more than 1/8 horsepower.
Safety circuits In locations where the presence of water, metal plumbing or the earth increase the possibility of a potentially lethal electric shock, receptacles are required to be protected by GFCIs (ground-fault circuit interrupters). The NEC specifies the following GFCI locations: kitchen counters within 6 feet of a sink, bathrooms, garages (whether attached or separate), unfinished basements, crawlspaces, around swimming pools, and on outdoor decks, patios and porches. Protected exterior receptacles are required at both the front and rear of the house, at a minimum.
As a precaution against fire, all receptacles in sleeping areas (bedrooms) must now be protected by arc-fault circuit interrupters (AFCIs).
The NEC lists a number of other requirements for specific rooms in the house
Kitchen Receptacles should be located 6 to 10 inches above counter surfaces, with no point of the counter more than 24 inches from a receptacle. Peninsulas and islands are exceptions, requiring just one receptacle each.
Lighting in a kitchen should be both general (one or more overhead fixtures) and task-oriented (under wall cabinets, for example) to illuminate work surfaces. If there are two entrances to the kitchen, the overhead lights should be controlled by a three-way switch at each door.
Dining room and pantry At least one of the two kitchen small-appliance circuits must make an appearance in any adjacent dining room, breakfast area or pantry.
Bathrooms A GFCI-protected receptacle must be located within 36 inches of all lavatories (bathroom sinks). A switch-operated ceiling fixture and lights to either side of each lavatory generally provide effective lighting. An important prohibition bars pendant fixtures, ceiling paddle fans, and track lighting within 3 feet horizontally of a tub or shower, and 8 feet vertically from the floor of a tub or shower.
Living rooms The no more than 6 feet from a receptacle rule applies here. But with a little planning, you can ensure that receptacles dont fall behind couches, where they cant be accessed, and that there will always be a convenient receptacle for vacuuming. Lighting in the living room can be provided either by a switch-controlled ceiling light or by switch-controlled receptacles into which lamps are plugged.
A common practice is to wire receptacles so that only the top or bottom halves are controlled by wall switches, so you can switch table lamps on or off when you enter or leave a room without cutting power to the clock plugged into the same receptacle. Again, if the room has more than one entrance, use three-way switches, so the lights can be switched on and off again as you pass through the room.
Bedrooms The 6-foot rule for receptacles applies here also. However, receptacles in bedrooms must be protected by AFCIs. Lighting can be set up the same as in living rooms.
Closets There should be a light in every closet, but incandescent bulbs must be enclosed in fixtures (though bare fluorescent bulbs are okay). In any case, the light fixture must be separated from stored clothes and other objects by a minimum of 12 inches horizontally and vertically. Separation may be reduced to 6 inches for recessed fixtures.
Stairways For safety's sake, make sure stairways are well lit, so you can see every step. Without exception, stairway lights should be controlled by three-way switches at the top and bottom landings.
Garages The NEC requires garage receptacles to be GFCI-protected (a minimum of one), except those for garage-door openers or dedicated to appliances. At least one switch-controlled light should be provided; preferably, it should be on a three-way switch so it can be controlled from both the house and the entrance door to the garage.
Except for the rules above, the NEC does not specify the total number of circuits a home must have. As anyone who has lived in an older home can testify, however, you can never have too many. To get an idea of how many circuits you should install in your home, refer to Recommended Numbers of Circuits.
The code lists the types of cable (wires encased in a protective sheath) that are acceptable in different locations in the home. Because of low cost and ease of use, Type NM (plastic-sheathed cable such as Romex) is by far the most used. A minority of jurisdictions still require Type AC armored cable for exposed locations and farm buildings. Armored cable is often referred to as BX, but this is just the trade name of one such product. See Cable Types for Residential Wiring on page 60 for more info.
Staple cable at least every 4.5 feet.
Staple within 12 inches of metal boxes having cable connectors (clamps).
Staple within 8 inches of nonmetallic boxes having no cable connectors.
Holes less than 1.5 inches from a stud edge must be protected by an 18-gauge steel plate.
Following these rules will reduce the chances of some unfortunate soul someday driving a nail or screwing through the cable, shorting it out. Worse would be intercepting a cable of unknown location with a saw blade when cutting a hole in the wall. At least the rules give you a hint as to where cables are most likely located.
Both for convenience in rewiring/
repairing and the fact that wires are most likely to arc where they are spliced together, all wiring connections must be made in electrical boxes. These metal and nonmetallic boxes (most shapes and sizes are available in both materials) come in many sizes and shapes, each designed for a special purpose.
Rectangular boxes are for single receptacles and switches in walls.
Beveled boxes are for wall switches.
Handy boxes are for surface-mounted receptacles and switches.
Square and octagon boxes are primarily for splicing cables.
Ceiling boxes are for light fixtures with canopies (extra room for splices).
If you have ever tried to add a branch circuit to an already crowded box, you understand the reason there is also a rule that stipulates the number of wires, clamps, switches and receptacles allowed in a box. Refer to Number of Conductors Allowed in Electrical Boxes on page 61. (A conductor can be a wire end, through-wire, switch or receptacle, cable clamp, or fixture stud. Grounding wires are lumped as one.)
Some homeowners try to exceed the code allowance, blindly cramming wires and devices into the box in a way that invites circuit shorting. Don't do it!
In the next installment, we'll take a look at that other major system in your home: the plumbing. First we will see why it is actually three separate systems in one, then we will see how the plumbing code says the job must be done.