Electrical Wiring in North America - Design and Installation Conventions

Design and Installation Conventions

For residential wiring, some basic rules given in the NEC are:

• Phase wire in a circuit may be black, red, orange (high leg delta) insulated wire, sometimes other colors, but never green, gray, or white (whether these are solid colors or stripes). Specific exceptions apply, such as a cable running to a switch and back (known as a traveler) where the white wire will be the hot wire feeding that switch. Another is for a cable used to feed an outlet for 250VAC 15 or 20 amp appliances that do not need a neutral, there the white is hot (but should be identified as being hot, usually with black tape inside junction boxes).
• The neutral wire is identified by gray or white insulated wire, perhaps using stripes or markings.
• With lamp cord wire the ribbed wire is the neutral, and the smooth wire is the hot. NEC2008 400.22(f) allows surface marking with ridged, grooves or white stripes on the surface of lamp cord. With transparent cord the hot wire is copper colored, and the neutral is silver colored.
• Grounding wire of circuit may be bare or identified insulated wire of green or having green stripes. Note that all metallic systems in a building are to be bonded to the building grounding system, such as water, natural gas, HVAC piping, and others.
• Larger wires are furnished only in black; these may be properly identified with suitable paint or tape.
• All wiring in a circuit except for the leads that are part of a device or fixture must be the same gauge. Note that different size wires may be used in the same raceway so long as they are all insulated for the maximum voltage of any of these circuits.
• The Code gives rules for calculating circuit loading.
• Ground-fault circuit interrupter (GFCI) protection is required on receptacles in wet locations. This includes all small appliance circuits in a kitchen, receptacles in a crawl space, basements, bathrooms and a receptacle for the laundry room, as well as outdoor circuits within easy reach of the ground. However, they are not required for refrigerators because unattended disconnection could cause spoilage of food, nor for garbage disposals. Instead, for refrigerators and other semi-permanent appliances in basements and wet areas, a one-outlet non-GFCI dedicated receptacle is generally used. Two-wire outlets having no grounding conductor may be protected by an upstream gfci and must be labelled "no grounding". Most GFCI receptacles allow the connection and have GFCI protection for down-stream connected receptacles. Receptacles protected in this manner should be labeled "GFCI protected".
• Most circuits have the metallic components interconnected with a grounding wire connected to the third, round prong of a plug, and to metal boxes and appliance chassis.
• Furnaces, water heaters, heat pumps, central air conditioning units and stoves must be on dedicated circuits
• The code provides rules for sizing electrical boxes for the number of wires and wiring devices in the box.
• In a fixture, the brass screw is hot, and the silver screw is neutral. The grounding screw is usually painted green.

The foregoing is just a brief overview and must not be used as a substitute for the actual National Electrical Code.