How To Buy Electrical Wire and Cable

Get the right type home wiring to power all your appliances and lighting fixtures

Wire and cable must be sized and installed correctly to pass an electrical inspection and keep electricity running safely through your home. This guide will teach you the difference between wire and cable and considerations for choosing between the two.

Tip: To calculate max wattage your wire or cable can hold, multiply amps by voltage. So Amps x Voltage = Watts.

Wire vs. Cable

While the terms wire and cable are often used interchangeably, technically wire is one electrical conductor and cable is a group of wires (or conductors), encased in sheathing.

The National Electrical Code (NEC) and local building codes regulate the types of wire and cable that can be used in specific electrical applications, as well as the manner of installation.

Check with your local building inspector before you start any electrical wiring and cable project and be sure to obtain required permits. When your work is complete, always have it inspected for compliance with local codes to ensure that it has been safely installed.

Wire Labeling

Label information printed on wire coverings provides the information necessary to help you choose the right wire for your home.

The information printed on the sides of wire indicates its THHN/THWN, material, max voltage rating and gauge.

• THHN/THWN: These letters represent the most common types of individual wires used in residential applications.
     o T – Thermoplastic insulation
     o H – Heat resistance
     o H – High heat resistance up to 194 degrees Fahrenheit
     o W – Rated for wet locations
     o N – Impervious damage from oil or gas

• Material: Most wire will be marked “CU” for copper, the most common conductor of residential electricity. Because electricity travels on the outside of copper wire, wires are insulated to protect against fire and shock. Older wire could be aluminum or copper-clad aluminum.

• Maximum voltage rating: This will be a number such as 600, which indicates the maximum voltage the wire can carry.

• Gauge: This indicates wire size, as defined by the American Wire (AWG) system. The most common gauges are 10, 12 or 14. Larger numbers represent smaller sizes or gauges of wire.

It is recommended for safety purposes that you stay around 80% the max wattage your gauge is capable of carrying. This chart shows both the recommended 80% wattage load and the absolute maximum 100% wattage load for the most common wire and cable gauge sizes.


Wire and Cable Gauge Amps Recommended 80% Wattage Load Max Wattage Load Common Uses

14-gauge

15 amps

1440 watts (120 volts)

1800 watts (120 volts)

Common residential wiring:
• Light fixtures
•Household receptacles

12-gauge

20 amps

1920 watts (120 volts)
3840 watts (240 volts)

2400 watts (120 volts)
4800 watts (240 volts)

Common residential wiring:
• Light fixtures
• Household receptacles
• Small appliances

10-gauge

30 amps

2880 watts (120 volts)
5760 watts (240 volts)

3600 watts (120 volts)
7200 watts (240 volts)

Large household appliances:
• Window a/c units
• Clothes dryer

8-gauge

40 amps

7680 watts (240 volts)

9600 watts (240 volts)

Large household appliances:
• Electric ranges
• Central a/c

6-gauge

55 amps

10560 watts (240 volts)

13200 watts (240 volts)

Large household appliances:
• Central a/c
• Electric furnace


Tip: Remember -- amps = the amount of current a wire can safely carry. Wattage = the rate of electrical energy used by an appliance. Amps x Voltage = Watts

Wire Colors

A wire’s color indicates what it can safely carry.


White insulation: typically considered neutral, but can sometimes be used as a hot lead in certain situations, such as switch loops.
Green insulation and bare copper: ground wire
All other insulation colors: hot wire that carries a current

Safety: If you have any confusion as to whether a white wire is used as neutral or hot in your particular project, check with a professional electrician before working.

Cable Types

NM-B, UF, AC, metal-clad, coaxial and category 5e cables each have specific purposes and applications.  

NM-B cable

• “NM” = non-metallic, which refers to the flexible, typically PVC sheathing surrounding the cable; “B” = a heat rating of 194 degrees F, ensuring that wires can operate at certain levels without overheating.
• Most common form of indoor residential electrical wiring.
• Inside the sheathing are at least two thermoplastic insulated wires of the same gauge, though different cables can have different gauges.
• For indoor use only, in spaces free from moisture and away from any heat sources. Do not bury or run outside of a wall. Best used behind walls and ceilings and inside floor cavities.

UF cable

• “UF” = underground feeder; rated for in-ground and damp-area installation.
• Looks like NM-B cable, but the wires are embedded as a group in solid thermoplastic (rather than individually encased in flexible thermoplastic).
• Like NM cable, UF cable comes in a variety of gauges to meet all electrical code requirements and is labeled with the same information carried on NM cable plus the designation UF.

AC

• “AC” = armored cable, also known as “BX”; consists of insulated hot and neutral wires and a bare bonding wire, all wrapped in paper.
• Wire enclosed in metal sheathing which acts as the grounding conductor.
• Relatively expensive and difficult to work with; often found in older homes but not used in new builds.
• For indoor use only.

Metal-clad cable

• Similar to AC, but wires are wrapped in plastic instead of paper.
• Has green grounding wire because its metal sheathing can’t be used as a ground.
• For indoor use only.
• Not often used in residential applications.

Coaxial cable

• Usually called “coax.”
• Metallic cable often used to carry television signals and connect video equipment.
• Features central wire conductor covered with a dielectric or non-conducting insulator surrounded by mesh or a metal sheath and covered by a thin plastic layer for protection.

Category 5e cable

• Usually called “cat-5e,” aka ethernet cable
• Industry standard for unshielded twisted-pair cables (UTP) for connecting phone, computer, home automation and A/V networks.
• Copper cabling typically consists of 4 pairs of wire (8 total conductors) wrapped in a single jacket.


Tip: Both NM and AC cable require special cutting tools that cut the sheathing but not the wires, bushings inserted in the ends to prevent the sharp edges of the metal from damaging the wires, and special electrical boxes and connectors.

As with wire, all the information you need to know about NM-B or UF cable is printed on the sheathing.

Type: This label will most commonly read NM-B.
Gauge: The gauge of the individual wires inside the cable, such as 14, 12, 10, etc.
Number of wires: This number follows gauge. For example, 14/2 indicates that there are two 14-gauge wires (a ground wire, if part of the cable, is not included in this number) within the cable.
Grounding: The word “GROUND” or the letter “G” indicates the presence of a ground wire.
Voltage rating: The most common rating for residential use is 600 volts, though this can vary. The number indicates the maximum voltage the wire can safely carry.
UL: Indicates that the cable is safety certified and approved for use by Underwriters Laboratories.