Types of Wiring in a House: NM-B, MC, UF-B, and Conduit Explained

House wiring types refer to the specific combinations of conductors, insulation materials, and outer jackets approved for routing electrical power through different residential environments. The specific cable type you choose dictates exactly where the run can be physically installed (wet vs. dry, exposed vs. concealed), the required support intervals, and the temperature column you must use to calculate maximum ampacity. Most DIYers and junior electricians commonly confuse the outer cable assembly (like NM-B, often generically called "Romex") with the individual insulated conductors inside it (like THHN), or they mistakenly assume all metal-armored cables are identical when AC and MC have entirely different grounding mechanisms.

The Core Residential Cable and Wire Types

Before pulling any wire, you must match the cable assembly to the environment. The National Electrical Code (NEC) strictly defines where each jacket type is permitted. Below is the master reference table for the four most common residential wiring methods.

What Switching Cable Types Changes in Your Circuit

Switching from a pre-jacketed cable to individual wires in conduit doesn't just change your physical installation method; it fundamentally alters your ampacity calculations and material costs. To understand this, let's look at a worked numeric example for a common residential upgrade: running a 60A subpanel feeder to a detached garage 50 feet away.

Now, suppose you decide to run the feeder through the interior framing of an attached garage using NM-B instead of conduit.

The Result: You cannot legally or safely protect a 55A-rated cable with a 60A breaker. To use NM-B for a 60A subpanel, you are forced to upsize to 4 AWG NM-B (rated 70A at 60°C). This makes the NM-B cable significantly thicker, much harder to bend into boxes, and often more expensive per foot than pulling 6 AWG THHN through conduit, completely reversing the assumption that "Romex is always the cheaper, easier option."

Where You Meet This in Practice

Knowing the theory is only half the battle; recognizing where each type belongs on the jobsite prevents failed inspections and fire hazards.

• NM-B (Interior Dry Framing): This is your standard 14/2 (15A lighting) and 12/2 (20A receptacle) cable. It must be run through bored holes in wooden studs, secured within 8 inches of a single-gang box (or 12 inches for larger boxes), and stapled every 4.5 feet. It cannot be left exposed on the face of basement joists where it could be easily damaged.
• MC (Exposed Basement/Garage Walls): When wiring a finished basement where the drywall hasn't been hung yet, or surface-mounting along a masonry wall, MC cable is the standard. The metal armor protects against accidental impacts from storage boxes or tools. It requires specific MC anti-short bushings (redheads) and MC-rated connectors at every box entry.
• UF-B (Direct Burial & Outbuildings): If you are trenching power to a backyard shed or landscape lighting transformer, UF-B is designed to be buried directly in the earth (typically at a depth of 24 inches for a residential branch circuit without GFCI protection at the source, or 12 inches with GFCI). However, where UF-B emerges from the ground up the side of the shed, it must be protected by conduit from the burial depth up to at least 8 feet above grade.
• THHN/THWN-2 (Conduit Systems): Used when NM-B is prohibited. This includes runs inside concrete slabs (using PVC conduit), exterior surface mounts, or any location classified as "wet." You must calculate conduit fill capacity; for example, a 1/2-inch EMT conduit can only hold a maximum of nine 12 AWG THHN wires before you must apply ampacity derating factors.

Common Confusions and NEC Code Traps

Even experienced DIYers fall into specific traps when selecting and terminating these wiring types. Avoid these three critical errors:

1. Confusing AC (Armored Cable) with MC (Metal-Clad)

Older homes (and some modern commercial builds) use AC cable, often called "BX." AC cable contains no internal grounding wire; it relies on the metal armor and an internal aluminum bonding strip to carry fault current. MC cable, conversely, contains a dedicated green insulated (or bare) grounding conductor. Modern NEC heavily restricts the use of AC cable for new residential branch circuits, favoring MC. If you buy "metal armored wire" at a big-box store, verify the packaging explicitly says "MC" and check that a green wire is inside.

2. Stripping THHN Insulation Inside NM-B

Because the wires inside NM-B are technically THHN, some builders assume they can strip the outer PVC jacket off a 10-foot length of NM-B and pull those bare wires through a conduit to save money. This is a severe code violation. The individual wires inside NM-B are not marked with the THHN/THWN-2 printing required for conduit use, and pulling them through conduit can damage the insulation. Furthermore, you lose the 60°C ampacity restriction, but inspectors will fail the install because the wires lack proper conduit-rated markings.

3. Ignoring the 90°C vs 75°C Termination Rule

THHN wire is rated for 90°C, which allows it to be used for ampacity derating calculations (like bundling multiple circuits in one conduit). However, the lugs on standard residential breakers and receptacles are only rated for 75°C (or sometimes 60°C for older 15A/20A devices). You must always size your final breaker based on the lowest temperature rating in the entire circuit chain, which is almost always the 75°C termination limit.