A Basic Guide to Home Electrical Wiring

Electrical Wiring Sheathing

Most interior wiring is done with non-metallic, or NM, cable—sometimes referred to by a popular brand name Romex. NM cable is made of three or more wires wrapped inside a flexible plastic jacket, or sheathing. It is used for most interior circuits, such as those for outlets, switches, light fixtures, and appliances.

The color of the sheathing indicates the gauges of the wires within the sheathing:

• White: 14-gauge wires intended for 15-amp service
• Yellow: 12-gauge wires intended for 20-amp service
• Orange: 10-gauge wires intended for 30 amp service
• Black: Heavier gauge wires for devices that require 40- to 60-amp service

Electrical Wiring Safety Principles

Electrical wiring is safe to work on if you observe four basic principles:

• Work on dead equipment: If the electrical wiring or device is completely dead, it is safe to work on.
• Lock it out: Make sure the wiring or device, or anything upline (such as a circuit breaker), cannot be accidentally turned on.
• Test the equipment: Use a voltage tester or multimeter to test the wire or device before working on it.
• Be qualified or call someone who is: If you’re not knowledgeable or feel uncomfortable working on the electrical wiring, call a certified electrician.

Electrical Wiring Color Coding

Cable coloring relates to the type of wires inside the cable sheathing and their uses:

• Black and red: Typically are used for current-carrying or hot connections.
• White: Usually, grounded neutral conductors. 
• Green and bare copper: Grounding wires.

Electrical Wiring Labels

Electrical wires and cables have markings stamped or printed on their insulation or outer sheathing. These markings provide important information about the wiring and insulation, including the wire size and material, the type of insulation, the number of wires contained (inside a cable), and any special ratings or characteristics of the wire.

While looking at the color of wire or cable will help you narrow down the options at the store, reading and understanding the labels on wiring is the best way to ensure you get the properly rated material for your project.

Direct-Burial Cable

Standard electrical cable is designed to be run indoors, where it stays dry and is protected by wall, ceiling, or floor structures. For outdoor projects or when running wiring underground, you must use direct-burial cable, which can be installed underground with or without conduit (depending on local building code rules). With direct-burial cable, the individual conducting wires are embedded in solid vinyl to fully protect them from moisture.

Electrical Wire Stripping

Stripping electrical wire involves removing the plastic insulation surrounding the wire’s metal core. It’s important to do this carefully so there is no damage to the metal. The procedure is simple but requires a special wire stripping tool and an understanding of how to use it. This is a critical skill—and tool—for DIYers to have for any wiring project.

Electrical Wiring Box Fill Capacity

There is a set limit to the number of wires allowed in electrical boxes. Fill capacity is determined by the size of the box, wire gauges, and the number of devices installed on the box. For safety, it is important to remain at or below the fill capacity. One way to determine wiring fill capacity is to find the box’s volume (usually printed on the box or product literature) and divide this by the gauge of the wire in the box. For every device installed on the box, subtract two wires.

For example, a junction box with 18 cubic inches of volume can hold up to nine #14 AWG wires. Junction boxes have wires only, no devices. But if the box contains one electrical outlet, you must subtract two wires. The box could then hold up to seven #14 AWG wires (plus the outlet).