Types of wiring in homes refer to the specific cable assemblies and individual conductors classified by their insulation materials, outer jacketing, and NEC-approved installation environments. The wire type you choose dictates exactly where the circuit can be physically routed (inside dry walls, buried in dirt, or pulled through metal conduit) and determines the maximum safe current (ampacity) based on its temperature rating. The most common confusion among DIYers is mixing up a cable assembly (like a roll of yellow NM-B) with the individual conductors inside it (like THHN), which leads to the dangerous and code-violating practice of stripping the outer jacket to pull the inner wires through a conduit.
The Core Cable Assemblies and Conductors
When you walk into a hardware store, you are looking at four primary categories of residential wiring. Understanding the physical construction of each is the first step to picking the right one.
1. NM-B (Non-Metallic Sheathed Cable)
Often called by the brand name Romex, NM-B is a flat, jacketed cable assembly containing two or more insulated conductors and a bare copper ground. The outer PVC jacket is rated for dry, indoor locations only. The individual conductors inside are typically rated for 90°C, but NEC Article 334.80 mandates that the entire assembly's ampacity must be calculated using the 60°C column of NEC Table 310.16. This is why 14 AWG NM-B is capped at 15A and 12 AWG NM-B is capped at 20A, regardless of the inner wire's higher thermal tolerance.
2. THHN / THWN-2 (Individual Conductors)
These are individual, single-core wires with a thin, tough nylon outer coating over PVC insulation. THHN stands for Thermoplastic High Heat-resistant Nylon-coated. They are rated for 90°C in dry locations and 75°C in wet locations (THWN-2). You cannot use these loose in a wall cavity; they must be pulled through a raceway (like EMT metal conduit or PVC Schedule 80). Think of NM-B cable like a pre-assembled, jacketed garden hose, while individual THHN wires are like bare pipes that require you to build a protective conduit trench around them.
3. UF-B (Underground Feeder)
UF-B looks like NM-B, but the conductors are encased in a solid, grey, moisture-resistant PVC jacket rather than wrapped in a hollow sheath with paper fillers. It is rated for direct burial in the earth without conduit (though conduit is required where it emerges above ground). Like NM-B, its ampacity is limited to the 60°C column.
4. MC (Metal-Clad Cable)
MC cable features individual THHN conductors wrapped in a continuous, interlocking aluminum or steel armor. It provides physical protection against nails and screws, making it ideal for exposed basement walls, garages, or commercial-style residential builds where NM-B would be vulnerable to damage.
Where You Meet This in Practice
In a standard home installation, you will rarely use just one type of wiring. A single circuit often transitions between types based on the physical environment.
- Inside the Main Panel: You will see individual THHN/THWN-2 wires or large stranded THHN used for service entrance conductors and heavy feeder lines (like a 50A range circuit) routed through short sweeps of metal conduit into the panel lugs.
- Inside Dry Interior Walls: Once the circuit leaves the panel and enters the wall cavities to feed outlets and switches, it transitions to NM-B. It is stapled to the sides of wooden studs, protected by the drywall.
- Running to a Detached Garage or Shed: When the circuit leaves the house and goes underground, you transition to UF-B buried at the proper depth (typically 24 inches for direct burial, or 18 inches if in PVC conduit), or you pull THWN-2 individual wires through a continuous underground PVC conduit sweep.
- Exposed Garage or Basement Ceilings: If the wire is running across the bottom of joists where it could be struck by stored items, code requires physical protection. Here, you use MC cable or run THHN inside EMT conduit.
Worked Numeric Example: Ampacity and Conduit Derating
Let’s look at how wire type changes your math when building a 20A kitchen appliance circuit. We need to run four current-carrying conductors (two hots, two neutrals for a multi-wire branch circuit) through a single 3/4-inch EMT conduit in a wall.
Scenario A: Using 12 AWG NM-B
If you try to strip four NM-B cables and stuff them into a conduit (which is a code violation in itself due to fill capacity and heat), you are legally bound to the 60°C column. 12 AWG at 60°C is 20A.
Math: 20A × 0.80 = 16A.
Result: You cannot put this on a 20A breaker. You would have to upsize to 10 AWG.
Scenario B: Using 12 AWG THHN in Conduit
Because THHN is installed in a conduit, we are allowed to use its 90°C rating for derating calculations (even though the breaker terminates at 75°C). 12 AWG at 90°C is 30A.
Math: 30A × 0.80 = 24A.
Result: 24A is greater than the 20A breaker requirement. You can safely use 12 AWG THHN for this run, saving space in the conduit and money on copper.
This numeric example highlights exactly why electricians pull individual THHN through conduit for complex runs rather than trying to force NM-B into tight spaces.
Decision Tree: Which Wire Type to Pull
Use this decision matrix to select the exact wiring material for your next project. Follow the path down to your concrete pick.
| Installation Environment | Physical Protection Needed? | Decision Path | Concrete Pick (Part/Type) |
|---|---|---|---|
| Inside standard dry interior walls (stud cavities) | No (Drywall provides protection) | If indoor & dry → Use NM-B | Southwire 12/2 or 14/2 NM-B (Romex) |
| Exposed basement ceiling or garage walls | Yes (Vulnerable to impact) | If exposed & indoor → Use MC or Conduit | AFC Cable Systems 12/2 MC Cable |
| Underground direct burial (no conduit) | N/A (Earth provides protection) | If buried & no conduit → Use UF-B | Cerro Wire 12/2 UF-B (Grey jacket) |
| Underground inside PVC conduit | Yes (Conduit provides protection) | If in conduit & wet → Use THWN-2 | Cerro Wire 12 AWG THWN-2 (Individual) |
| Complex panel-to-panel runs with tight bends | Yes (Requires raceway) | If multiple bends & high fill → Use THHN | Southwire 12 AWG THHN (Individual) |
Frequently Asked Questions
Can I use NM-B (Romex) inside a conduit?
Technically, the NEC allows it in very specific, short sleeves for physical protection (like dropping down a wall to a garbage disposal). However, you cannot use NM-B for long conduit runs. The outer PVC jacket traps heat, and the flat shape makes it nearly impossible to pull through bends without violating conduit fill capacity rules (NEC Chapter 9, Table 1). For conduit runs, strip the jacket and use individual THHN, or just buy THHN to begin with.
Why is the ground wire in NM-B bare copper?
The equipment grounding conductor (EGC) in NM-B is left bare because it only carries current during a fault condition (a short circuit). It does not generate heat during normal operation, so it does not require insulation. Furthermore, a bare ground can make incidental contact with metal boxes and fixtures, ensuring a continuous ground path. For more on grounding topology, consult Southwire's technical resources on grounding and bonding.
Is 12 AWG always better than 14 AWG?
12 AWG handles 20A, while 14 AWG handles 15A. While 12 AWG has less voltage drop over long distances and allows for future circuit upgrades, it is stiffer, harder to fold into standard single-gang outlet boxes, and more expensive. Default to 14 AWG for standard lighting circuits and 12 AWG for kitchen, bathroom, and garage receptacle circuits.
The Default Recommendation
While the correct wire depends entirely on the routing environment, we do not leave you with an open-ended "it depends." For 95% of standard indoor residential branch circuits—running from the panel through wooden studs to standard outlets and switches—buy NM-B (Romex). Specifically, stock up on 12/2 NM-B for 20A receptacle circuits and 14/2 NM-B for 15A lighting circuits. Only switch to individual THWN-2 in conduit when you are leaving the protective envelope of the home's drywall to run underground or through exposed masonry.
