Residential Types of Wiring: NM-B, THHN, and MC Cable Compared

Types of wiring refers to the specific combinations of conductor material, insulation temperature rating, and outer jacketing used to safely route electrical current through a building based on environmental and physical constraints. In a real installation, the wiring type you choose dictates your allowable ampacity, physical protection requirements, and whether the run can survive damp environments without degrading. Beginners commonly confuse the cable assembly (the outer jacketed bundle like NM-B) with the individual conductors inside it, wrongly assuming a 12 AWG THHN wire pulled in conduit has the exact same physical protection rules and ampacity limits as a 12 AWG NM-B cable.

The Physics of Insulation and Heat Dissipation

Every time current flows through a wire, it generates heat due to the resistance of the copper (I²R losses). The insulation wrapped around that copper acts like a winter coat: a higher temperature rating (like 90°C THHN) allows the wire to run hotter without the insulation melting or catching fire, but it still traps heat inside the bundle. This is why the National Electrical Code (NEC) ampacity tables assign different current limits based on both the wire gauge and the insulation type.

Worked Numeric Example: Conduit Derating

Let's look at what happens when you pack multiple wires together, which changes the real-world capacity of your circuit. Imagine you are running a feeder to a subpanel using 8 AWG wire, and you need to pull 6 current-carrying conductors through a single piece of EMT conduit.

• Scenario A (THHN in Conduit): The 90°C column for 8 AWG THHN is 55A. Because you have 6 conductors in one raceway, NEC Table 310.15(C)(1) requires an 80% adjustment factor. 55A × 0.80 = 44A. Since 44A is greater than the 40A standard breaker size, you can safely protect this circuit with a 40A breaker.
• Scenario B (NM-B Cable): If you somehow bundled three 8/2 NM-B cables together (yielding 6 current-carrying conductors), you must use the 60°C column for NM-B, which is 40A. Applying the same 80% derating factor: 40A × 0.80 = 32A. You are now forced to drop down to a 30A breaker, losing 25% of your circuit capacity simply because of the cable type's lower base insulation rating.

Where You Meet This In Practice

Understanding these types of wiring moves from theory to reality the moment you start planning a physical route through a house. Here is how these cables are deployed in standard residential jobs:

The Garage Workshop Transition

You are wiring a 20A, 120V receptacle circuit in an unfinished garage. The walls are open, but the ceiling has exposed joists. You cannot staple NM-B (Romex) to the face of the ceiling joists where it could be physically damaged. The standard practice is to run 12/2 NM-B up the inside of the drywall to a junction box near the ceiling, then transition to individual 12 AWG THHN wires pulled through 1/2-inch EMT (metal conduit) along the exposed ceiling joists to the drop-down outlets. The NM-B handles the concealed, protected wall cavity, while the THHN/EMT combo provides the physical crush-resistance required for exposed runs.

The Backyard Shed Feeder

For a 50A subpanel feeder to a detached shed, you are burying the line underground. NM-B is strictly prohibited here. You have two main choices:

1. UF-B Cable: You can trench 24 inches deep and lay 6/3 UF-B cable directly in the dirt. It's fast, but UF-B is expensive and stiff to work with.
2. THWN-2 in PVC: You can trench 18 inches deep, lay Schedule 80 PVC conduit, and pull individual 6 AWG THWN-2 conductors. This is the professional preference because pulling individual wires through a glued PVC sweep is significantly easier on your hands and the wire insulation than wrestling a rigid UF-B jacket through dirt.

Material Costs and Labor Trade-Offs

When deciding between wiring types for a large renovation or new build, material costs and labor time heavily influence the choice. Below are typical 2026 retail pricing benchmarks for 12 AWG equivalents:

While NM-B is the undisputed king of cost and speed for standard interior walls, industry professionals increasingly use MC (Metal Clad) cable in high-end custom homes or multi-family dwellings. MC cable eliminates the need for separate conduit bending while providing the metal armor that protects against rogue drywall screws and rodent damage, justifying the higher upfront material cost through reduced call-backs for damaged wires.