Electrical wiring types are categorized by their insulation material, jacketing, and intended environment, which dictates their ampacity limits and physical protection requirements. If you need the direct answer for your project: for standard indoor residential branch circuits behind drywall, use NM-B (Non-Metallic Sheathed Cable). For outdoor runs, wet locations, or long conduit pulls, use individual THWN-2 conductors inside a raceway.
The Core Wiring Types You Need to Know
Walking into the electrical aisle presents a wall of acronyms. Here are the four primary types of electrical wiring you will encounter in residential and light commercial work, along with their specific use cases.
| Wire Type | Full Name | Primary Use Case | Conduit Required? |
|---|---|---|---|
| NM-B | Non-Metallic Sheathed Cable (Romex) | Interior dry branch circuits (outlets, lighting, switches) | No |
| THHN / THWN-2 | Thermoplastic High Heat / Water-resistant Nylon | Conduit pulls, subpanel feeders, wet locations | Yes |
| UF-B | Underground Feeder Cable | Direct burial for outdoor lighting, sheds, and pumps | No (for burial) |
| MC | Metal-Clad Cable | Exposed runs in basements, garages, and commercial spaces | No (acts as its own armor) |
What Wire Type Actually Changes in Your Circuit
Choosing a specific type of electrical wiring changes two critical factors in your installation: physical protection requirements and allowable ampacity.
Physically, the jacket determines if the wire can survive being buried in damp soil (UF-B), if it can be pulled through sharp metallic conduit bends without the insulation tearing (THHN's nylon outer skin), or if it can be stapled directly to a wooden stud (NM-B). Running bare THHN wire through a stud cavity without conduit is a direct violation of NEC 300.4 and creates a severe fire and shock hazard from drywall screws or nail punctures.
Electrically, the insulation type dictates which temperature column you must use when sizing your breaker. Think of the temperature ratings like a vehicle's speedometer: the wire's insulation might be engineered to handle 90 MPH (90°C), but if the breaker terminal it connects to is only rated for 75 MPH (75°C), you must obey the lower limit to prevent the terminal from melting.
Worked Example: Sizing a 60-Amp Subpanel Feeder
Let's look at how the type of electrical wiring changes your material list and cost for a real-world scenario: running a 60-Amp subpanel to a detached garage 50 feet away.
Option A: Using NM-B (Romex) in the interior, transitioning to conduit outside.
NEC Article 334.80 strictly mandates that NM-B cable ampacity must be determined using the 60°C column, regardless of the fact that the wire's insulation is technically rated for 90°C. Looking at NFPA 70 (National Electrical Code) Table 310.16, 6 AWG copper at 60°C is only rated for 55 Amps. Therefore, 6 AWG NM-B cannot be used on a 60A breaker. You must step up to 4 AWG copper NM-B (rated 65A at 60°C). A 50-foot run of 4/3 NM-B is thick, difficult to bend, and costs roughly $250.
Option B: Using THWN-2 conductors pulled through PVC conduit.
Because THWN-2 is run in conduit and terminates in standard 75°C rated panel lugs, you are permitted to use the 75°C column. According to Table 310.16, 6 AWG copper at 75°C is rated for 65 Amps. This is perfectly legal and safe for a 60A breaker. Pulling four individual 6 AWG THWN-2 wires (two hots, one neutral, one ground) through 1-inch PVC conduit is physically easier, and the wire itself will cost roughly $90 for the same distance, plus about $60 for the PVC conduit and fittings.
The Verdict: For feeder runs over short distances inside a house, NM-B is faster. For a detached garage feeder, THWN-2 in conduit is the superior choice due to lower material costs, easier pulling, and the ability to utilize the 75°C ampacity column.
Where You Meet These Wiring Types in Practice
You will encounter these wiring types in specific phases of a build or renovation:
- Rough-in for interior walls: You will exclusively use NM-B (14/2 for 15A lighting circuits, 12/2 for 20A receptacle circuits). It is fast to staple and requires no conduit threading.
- Kitchen and Bathroom upgrades: You will use 12/2 or 10/3 NM-B for 20A small-appliance branches and 30A dryer ranges, respectively. If the wall is open, NM-B is the undisputed king of speed.
- Finishing a basement with exposed ceiling joists: Local inspectors often forbid NM-B on the underside of joists where it is subject to physical damage. Here, you will meet MC (Metal-Clad) cable or EMT conduit with THHN wires.
- Running power to a landscape water feature or hot tub: You will use THWN-2 inside liquid-tight flexible metallic conduit (LFMC) or Schedule 80 PVC, as these locations are classified as wet or damp environments where NM-B is strictly prohibited.
Decision Tree: Picking the Exact Wire for Your Project
Use this decision-tree-table to terminate your planning phase and pick the exact wire type and gauge for your next trip to the supply house.
| If your project is... | And the environment is... | Then buy this exact wire type |
|---|---|---|
| Standard 15A/20A wall outlets or lights | Interior, behind drywall, dry | 14/2 or 12/2 NM-B |
| Electric range, dryer, or EV charger | Interior, dry, short run (<30ft) | NM-B (e.g., 6/3 or 8/3) sized to 60°C column |
| Subpanel feeder to detached building | Underground or exterior conduit | THWN-2 individual conductors in PVC conduit |
| Buried line to a gate motor or shed | Direct burial (no conduit) | UF-B Cable buried at code-mandated depth (usually 24") |
| Exposed wiring in a garage or workshop | Surface mounted, subject to impact | MC Cable or THHN inside EMT metal conduit |
Common Confusions and Code Caveats
Confusion 1: Mixing up Wire Gauge (AWG) with Wire Type
A common mistake among DIYers is assuming that because a wire is 12 AWG, it is automatically good for 20 Amps. Gauge is just the size of the copper; type is the insulation. A 12 AWG wire with older 60°C insulation (like TW) is only good for 20A, but a 12 AWG THHN wire in a raceway with 4 current-carrying conductors must be derated by 80%, dropping its effective ampacity to 20A * 0.8 = 16A, which means it can no longer be used on a 20A breaker. Always check both the AWG and the insulation type lettering printed on the wire jacket.
Confusion 2: The 'Romex' Brand Name vs. NEC Terminology
Electricians universally say 'Romex,' but the NEC and your local inspector will use the term NM-B (Non-Metallic Sheathed Cable). Romex is simply a trademarked brand name owned by Southwire. When pulling permits or speaking to Mike Holt's NEC Resources or your local AHJ, refer to it as NM-B to ensure you are communicating the exact legal standard.
Confusion 3: Assuming THHN can be run bare through studs
THHN is cheaper per foot than NM-B, leading some beginners to buy spools of it and fish it through wall cavities without conduit to save money. This is a massive code violation. THHN lacks the outer abrasion-resistant jacket required for pulling through wooden framing without a protective raceway. The friction of pulling it, or a stray drywall screw, will easily compromise the thin nylon skin, creating an arc-fault hazard inside your walls. If it is inside a wall, it must be inside a continuous conduit, or you must switch to NM-B.
