Solid and Stranded Wire

• Manufacturers make 12-gauge wire in both solid and stranded wire types. Solid wire has a single, solid wire while stranded is made from multiple strands of smaller wires.
  
  Stranded wire has greater flexibility than solid wire which makes it easier to pull the wire through electrical conduit. Stranded wire also requires more care when making connections to screw type terminals. Stranded wire costs slightly more than solid wire.
  
  Solid wire costs less to manufacture and terminates easier on screw type terminals. Connections between solid wires require twisting the wires together before the connection is capped by a wire nut; stranded wires don't require twisting.
  
  

Sheathed Cable

• Sheathed cable bundles wires together in a plastic or metallic sheath. Plastic sheathed cable is often referred to as 'ROMEX®', a trademarked name for a specific brand of sheathed cable. Most residential single-family homes utilize sheathed cable wiring. The product installs quickly and easily and contains all the conductors for most wiring needs.
  
  Circuits that require hot and neutral wires use two conductor sheathed cable. Two conductor cable has one black and one white wire, plus a bare ground wire. Three conductor sheathed cable has a third, red wire. Although the bare wire is capable of conducting electricity, it is not referred to as a conductor since using it as a conductor is not allowed by National Electrical Code.
  
  Most manufactures color the plastic sheathing for 12-gauge sheathed cable yellow.
  
  

Spooled Wire

• Wire manufacturers supply spooled wire for use in raceways, often called conduit. The wire insulation comes in many different colors with black, white, red and blue the most common. Different wire colors help electricians differentiate wires while as they install it. Spooled 12-gauge wire is available in both solid and stranded wire types.
  
  Local building codes sometimes require the use of metallic conduit with spooled wire. Despite the higher cost of containing spooled wire within conduit, the wiring is easily changed or updated by simply running new wires through the conduits. Adding new circuits is much more difficult.
  
  

Insulation Temperature Rating

• Wires increase in temperature as current passes through them. In the past, most wire insulation was rated at 60 degrees Celsius, with special purpose wires having 75- or 90- degree Celsius rated insulation. Modern 12-gauge wire has the higher 90-degree Celsius rating. Wires types are defined in the national electrical code by their insulation rating: TW type insulation has a 60-degree rating, THW is rated at 75 degrees, and THHN is rated at 90 degrees.
  
  Older, lower temperature TW wire is common in older homes. Newer construction uses only the higher temperature THHN wire.
  
  The insulation temperature rating is important because lower temperature insulation may become brittle or melt at the higher temperatures. Wires with lower temperature ratings have greater restrictions on the amount of current they may carry.