Using the correct sized wires and conduits prevents electrical shorts, electrocution and fires. The size of the wire is particularly important because the wire's diameter (gauge) determines the amount of amps it can handle before overheating and melting. To pick the correct size wire you should understand the electrical needs of the circuit you are working on. Conduit size is dependent on the gauge of wire you want to install inside of it. The right size conduit will make fishing the wire through the conduit a simple process.
Wire Size (Gauge)
Decide if you want to use aluminum or copper wire. If you are unsure go with copper as it is the most commonly used in modern construction. Aluminum is still used for multi-stranded wires and is safe if installed correctly.
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Determine the electrical needs for the circuit you want to wire. The most important number is the amount of amps. Almost 3/4 of all residential electrical circuits are 15 amps. This services lights, switches and outlets. Baseboard heaters and kitchen outlets typically use wires designed for 20-amp capacity. Larger appliances like dryers, water heaters and AC units use 30-amp conductors. Ovens and stoves may need up to 40 amps of capacity.
Select a wire size using the wire type and circuit needs from the chart below: 15-amp wire: copper 14 gauge or aluminum 12 gauge 20-amp wire: copper 12 gauge or aluminum 10 gauge 30-amp wire: copper 10 gauge or aluminum 8 gauge 40-amp wire: copper 8 gauge or aluminum 6 gauge
Use the "Wire Size" section first to decide the gauge of wire you need to use for a safe circuit.
Decide the number of wires that need to be protected by the conduit.
Follow the link in the resource section to determine the size of conduit you need for your application.
If you are unsure of the amp requirements of the circuit, consult the owner's manual for the device it is powering.