12/4 NM-B Romex is an electrical cable with four conductors (three insulated and one ground) and is typically used in 20-amp circuits. This cable type is often utilized in applications requiring multiple conductors in a single cable run. These include:

• Multi-way lighting circuits
• Appliances or equipment circuits
• Complex residential wiring

"NM-B" wire stands for "Non-Metallic Type B." This is a specific type of electrical wire commonly used in residential indoor wiring.

A 12-gauge Romex wire is usually yellow.

The letter "B" in NM-B wire indicates that the cable was designed for 90 degree C ampacity ratings. NM-B wire without B is an older type of an electrical cable that is no longer in use. The modern NM-B wire was introduced in 1984.

NM-B wire can be run through conduit, despite any stereotypes that suggest otherwise. It is run in conduit in areas where it can be subjected to physical damage, such as exposed locations. Its typical indoor in-wall installations already protect it by its non-metallic sheathing.

Romex NM-B wire can only be exposed under particular circumstances, such as one- and two-family dwellings, their attached or detached garages, and their storage buildings. This is specified in NEC 334.10 and 334.15. If exposed, the cable has to be protected from physical damage.

To strip Romex wire, remove the outer jacket to expose the conductors. 

It's crucial not to damage the conductors beneath the jacket. A tool like the L'il Ripper Stripper is handy for this, as it can cut the jacket without harming the conductors. You can also use a utility knife, but you must be careful. 

Once the jacket is removed, decide how much insulation you want to strip off (usually about 1 inch). Insert the conductor into the right-sized notch in a wire stripper, squeeze the handles, and pull the insulation off the wire. Be cautious when stripping lamp cords made of stranded copper wire, as the individual strands are easily damaged​​.

Running NM-B Romex in the attic is acceptable, but it requires protection. If you run Romex in the attic, you should staple the cable or secure it with joists or rafters.

Splicing NM-B Romex is quite simple. You just have to:

• Turn off power: Always start by turning off power to the circuit you're working on.
• Strip wires: Remove the outer sheathing of the Romex cables at the point of splice and strip the insulation from the individual wires.
• Align wires: Align the same-colored wires from each cable.
• Twist wires together: Twist the ends of each set of similar wires together using pliers.
• Secure with wire nuts: Place wire nuts over the twisted wire ends and twist until tight.
• Insulate and protect: Insulate the splice with electrical tape and place it in a junction box.
• Cover junction box: Cover the junction box with a solid cover plate.

The difference between splicing Romex and splicing other types of cables is that you have to splice the outer sheath first and then strip the insulation of individual conductors.

Romex NM-B Wire can only be run through bored holes between the studs as a protection.

Romex (NM cable) can be used in commercial buildings, but specific restrictions and requirements must be adhered to. According to the National Electrical Code (NEC) 334.12(A)(1), NM cable is not permitted as open runs above drop ceilings in non-dwelling occupancies. This means that Romex cannot be used in areas like above drop ceilings in commercial buildings. However, it may be used in industrial building open runs if installed with additional protection.

A building that uses Romex must be of type III, IV, or V construction. In buildings of combustible construction, Romex may not be used. Additionally, the local jurisdiction might have its prohibitions or restrictions on the use of Romex, regardless of the type of construction.

NM-B Romex may not be the best for commercial buildings as it lacks required specific fire-resistant ratings.

No, you cannot run Romex underground or in conduit. Is the cable you are looking for an UF-B wire.

NM-B Romex is not plenum-rated.