Wiring types refer to the specific combinations of conductor material, insulation temperature rating, and outer jacketing engineered to safely carry electrical current under distinct environmental and physical conditions. What this changes in a real installation is everything from how the wire is physically supported (stapled to studs vs. pulled through conduit) to its legal burial depth and its ampacity derating when bundled. The most common confusion among DIYers and junior apprentices is conflating a cable assembly (like NM-B, commonly called Romex) with the individual conductors inside it (like THHN), or assuming that all 12 AWG copper wire automatically yields 20 amps of capacity regardless of the insulation's temperature rating or the termination limits of the breaker.

The Core Wiring Types You Actually Need to Know

Before you can size a breaker or dig a trench, you need to know the physical and thermal limits of the four primary building wires used in residential and light commercial work. According to the National Electrical Code (NEC), each type has a specific governing article that dictates its use.

NM-B (Nonmetallic Sheathed Cable): The standard indoor residential cable. It contains two or more insulated conductors and a bare ground, wrapped in a flexible PVC jacket. Rated for 90°C (194°F) but ampacity is strictly limited to the 60°C column per NEC 334.80. Use only in dry, protected, indoor locations.
THHN/THWN-2 (Thermoplastic High Heat-resistant Nylon-coated): Individual, single-conductor wires with a tough nylon outer skin. THWN-2 means it is also rated for wet locations. Rated for 90°C in dry/wet locations. This is the wire you pull through EMT, PVC, or flexible metal conduit.
UF-B (Underground Feeder): Looks like NM-B but the gray PVC jacket is solid and completely encapsulates the conductors (no paper filler). Rated for direct burial and damp/wet locations, but still limited to the 60°C ampacity column.
MC (Metal-Clad Cable): Individual THHN conductors wrapped in an interlocking aluminum or steel armor jacket, plus a dedicated green insulated grounding wire. Used where physical damage is a risk or in commercial drop ceilings where NM-B is prohibited.

Where You Meet These Wiring Types in Practice

Theory is fine, but you will encounter specific physical constraints on the jobsite that force your hand on which wiring type to pull.

  • Finishing a dry basement or bedroom: You are stapling to wooden studs behind drywall. You will use NM-B. It is cheap, fast to strip, and the flexible jacket protects the wires from standard wood friction.
  • Stubbing up into a surface-mounted metal junction box: You cannot staple NM-B to a cinderblock wall or leave it exposed. You will mount EMT conduit and pull individual THHN/THWN-2 wires through it.
  • Running power to a detached garage or shed: You are digging a trench. If you are direct-burying the cable without conduit, you must use UF-B. If you are burying PVC conduit, you can pull THWN-2 through the conduit (which is often cheaper and easier to pull for long runs).
  • Wiring a garage workshop where walls are left open: Exposed NM-B is vulnerable to physical damage from tools, bikes, and shelving. You must use MC cable or run THHN inside rigid/EMT conduit to protect the conductors.

The Math: Ampacity, Derating, and Temperature Columns

This is where most mistakes happen. Wire manufacturers print the 90°C ampacity on the spool, but NEC 110.14(C) requires you to size your overcurrent protection based on the lowest temperature rating of any connected component (breaker, receptacle, or splice), which is almost always 60°C or 75°C for residential gear.

The Golden Rule: You can use the 90°C column for derating adjustments, but your final adjusted ampacity cannot exceed the 60°C/75°C base ampacity for termination purposes.

Worked Numeric Example: Derating in Conduit vs. Cable

Imagine you need to run a 240V circuit (2 hots, 1 neutral, 1 ground) to a subpanel. That is 3 current-carrying conductors (the ground does not count for derating). Let us look at 10 AWG copper.

Scenario A: Pulling 10 AWG THHN through EMT conduit

  1. Base ampacity in the 90°C column (NEC Table 310.16): 40A.
  2. Because you have 3 current-carrying conductors, no derating is required (derating starts at 4 conductors). Let us add one more circuit to the conduit, bringing the total to 4 current-carrying conductors.
  3. NEC Table 310.15(C)(1) derating factor for 4-6 conductors is 80%.
  4. Adjusted ampacity: 40A × 0.80 = 32A.
  5. Because 32A is below the 75°C termination limit for 10 AWG (35A), you can safely protect this circuit with a standard 30A breaker.

Scenario B: Running 10/3 NM-B Cable

  1. Base ampacity for NM-B is strictly locked to the 60°C column per NEC 334.80.
  2. 10 AWG in the 60°C column is exactly 30A.
  3. Even though you only have 3 current-carrying conductors, you cannot apply 90°C derating math to bypass the 60°C hard lock on the cable assembly itself.
  4. You protect this with a 30A breaker.

As noted by industry experts at EC&M Magazine, misunderstanding the difference between the conductor's thermal rating and the termination's thermal rating is the most common cause of failed rough-in inspections.

Decision Tree: Which Wiring Type to Pull for Your Project

Use this decision matrix to select the exact part number and wiring type for your next run. Do not guess; let the physical environment dictate the wire.

Installation Environment Physical Protection Required? Moisture / Wet Location? Concrete Pick (Wiring Type)
Inside finished drywall, wooden studs No No (Dry) NM-B (e.g., Southwire 12/2 Romex)
Inside EMT, PVC, or Flexible Metal Conduit Yes (Conduit provides it) Depends (Use THWN-2 if wet) THHN/THWN-2 (Individual conductors)
Direct burial in soil (no conduit) No Yes (Wet) UF-B (Buried min. 24' deep, or 12' if GFCI protected)
Exposed masonry, garage walls, drop ceilings Yes (Armor provides it) No (Dry) MC Cable (Metal-Clad with green ground)
Inside HVAC ducts or plenum air spaces No No CMP (Plenum-rated, low smoke/zero halogen)

Common Mistakes and Code Violations to Avoid

Can I strip the jacket off NM-B and use the inner wires in conduit?

No. This is a major NEC violation. While the individual wires inside NM-B are often manufactured as THHN, they lack the individual surface printing required by NEC 310.120(A)(2)(a) to identify their insulation type and rating once the outer cable jacket is removed. An inspector will immediately fail this, and you will have to pull the wires out and buy properly marked THHN spools.

Does UF-B require a grounding wire if I am using it for a 240V load?

Yes. Modern NEC (since the 1996 cycle for ranges/dryers and 2008 for subpanels) requires a separate equipment grounding conductor. You must buy 3-conductor UF-B (e.g., 10/3 UF-B with ground), not 2-conductor. The ground is not a neutral; it is a fault-current path.

Can I mix NM-B and THHN inside the same junction box?

Yes, but with conditions. You can transition from NM-B to THHN inside a sufficiently sized junction box. The NM-B must enter the box through a proper cable clamp (to secure the outer jacket), and the THHN must exit through a conduit fitting. Ensure your box fill calculations (NEC 314.16) account for all conductors, clamps, and devices.

When purchasing building wire, always verify the jacket printing for the UL listing, AWG size, and voltage rating (typically 600V for standard residential types). Refer to manufacturer specification sheets from Southwire or Cerro Wire to confirm the exact outer diameter for your conduit fill calculations before heading to the supply house. Buy solid copper for standard 15A and 20A branch circuits to ensure compatibility with standard back-wired receptacles, and reserve stranded THHN for long conduit pulls where flexibility is required to navigate sweeping bends.