One of the biggest dilemmas when tackling any electrical wiring job is how to choose the correct electrical wire. Electrical current creates heat--the larger the amperage, the greater the amount of heat. Too small of a wire size and too much heat can cause the insulation to melt off the wire. The heat of the electrical current running through a bare wire increases the chance of an electrical fire. Wire must be sized according to the amperage that feeds it and must be large enough both to carry the amperage and dissipate the heat. Choosing the right wire for indoor or outdoor use is also important and should be considered.
Determine what type of circuit you are installing. You could be installing a 110-volt or a 220-volt circuit. Plus, it might be a dedicated circuit or an undedicated circuit. A good example of a 110-volt circuit would be the circuits that power your small appliances, lights, television and washing machine. This example would also be an undedicated circuit because several items per circuit can be powered at any given time. An example of a 220-volt circuit would be the circuit that powers your electric dryer or the circuit that powers your electric water heater. These are also examples of dedicated circuits, as only one item will ever be powered on that circuit.
Determine the amperage of the circuit you are installing. A good example would be an electric water heater; it requires a dedicated 220-volt circuit and is rated for 30 amps per industry standards. This information is located on a permanent plate attached to the water heater.
Use the information on the type and amperage of circuit to determine the correct electrical wire. Electrical wire size is standard and must be labelled according to the National Electrical Code (NEC). Charts available online at sites like www.cerrowire.com are based on NEC standards for the US. The charts list the gauge wire and the maximum amperage for that gauge of wire. Choose wire that equals or exceeds the amperage of your circuit.
For example, the correct wire needed for a 30-amp, 220-volt circuit feeding a water heater would be 10-3 NM cable. A 220-volt circuit always needs four wires: two hot wires, a neutral and a ground. A 30-amp circuit needs a 10-gauge wire. A 110 circuit always needs three wires: one hot, one neutral and a ground. Your amperage for receptacles will always be 20 amps on a 110 circuit.
Choose non-metallic (NM) cable for indoor use. NM cable is normally found in residential attics and walls. This cable is constructed with an outer sheath that surrounds the conductor, or wires, inside. NM cable does not have to be encased in electrical conduit and can literally be laid out across your attic with no safety concerns unless there are rats. Rats have been known to chew through NM cable, but it is rare.
Choose underground feed or direct burial (UF/DB) cable for outdoor use. UF/DB cable is normally buried underground. The conductors are encased in a heavy flexible plastic that insulates each conductor individually, surrounding it in a waterproof insulation. UF cable does not have to be encased in electrical conduit. However, before burying the cable, you need to check with your local building codes to determine the acceptable depth of burial. Most codes ask you to bury it 18 inches or more.
Choose thermoplastic high heat nylon (THHN) wire for additional residential wiring. THHN is the type of insulation that surrounds the copper conductor and is the same conductor that you will find encased in NM cable and UF cable. You must run THHN inside electrical conduit if you choose to use it. THHN is also recommended for short wire runs of 6 or 8 inches and is commonly used for pigtails inside electrical boxes that house your switches, receptacles and lights.
Running NM cable will be the least expensive option when wiring inside your home because the NM does not have to be encased in electrical conduit.