Electrical wiring types are categorized by their insulation material, outer jacket rating, and intended environment, which collectively dictate their maximum ampacity and safe routing methods. When you select a specific wire type for a new circuit, it changes three critical factors in your installation: the allowable current capacity (due to temperature rating columns in the NEC), the physical routing method (stapled directly to framing versus pulled through conduit), and the environmental hazards the cable can survive (UV exposure, moisture, or physical impact). Hobbyists and DIYers most commonly confuse the cable assembly (like a multi-conductor NM-B sheath) with the individual conductor insulation (like THHN), or mistakenly assume all copper wire shares the exact same ampacity regardless of its temperature rating.
The Core Categories of Residential Wire
To make safe, code-compliant decisions, you must understand the four primary wire and cable assemblies used in residential and light-commercial work. Each serves a distinct physical and electrical purpose.
NM-B (Non-Metallic Sheathed Cable)
Commonly known by the brand name Romex, NM-B is a flat cable containing two or more insulated conductors and a bare copper ground, wrapped in a PVC jacket. It is strictly for dry, indoor locations. The individual conductors inside are typically rated for 90°C, but NEC Article 334 mandates that the overall ampacity of NM-B must be calculated using the 60°C column of Table 310.16, regardless of the internal wire rating.
THHN / THWN-2 (Individual Conductors)
These are single, nylon-coated wires pulled through conduit (EMT, PVC, or flexible). THHN stands for Thermoplastic High Heat-resistant Nylon-coated. Modern manufacturing almost always dual-rates these as THWN-2, meaning they are approved for wet locations and high heat. Because they are installed in conduit, you can utilize the 75°C or 90°C columns of NEC Table 310.16 ampacity charts for sizing and derating, provided your terminations are rated for those temperatures.
UF-B (Underground Feeder)
UF-B looks like NM-B but features a solid, gray PVC jacket that completely encapsulates the conductors. It is rated for direct burial underground without conduit and can withstand moisture and sunlight (UV). However, like NM-B, its ampacity is locked to the 60°C column.
MC (Metal-Clad Cable)
MC cable contains THHN/THWN-2 conductors wrapped in an interlocking aluminum or steel armor jacket. It provides physical protection against nails, screws, and impact, making it ideal for exposed basement walls or garage shops where NM-B would be vulnerable to damage.
Where You Meet This in Practice
The wiring type you choose fundamentally alters your physical labor and conduit fill calculations. If you are roughing-in a standard 20A bedroom circuit, you will likely use 12/2 NM-B. The physical constraint here is support: NEC Article 334.30 requires NM-B to be secured within 12 inches of every electrical box and at intervals not exceeding 4.5 feet along the studs. You cannot simply lay it across ceiling joists without running a running board if it is smaller than 8 AWG.
Conversely, if you choose to run THHN through 1/2-inch EMT conduit for the same circuit, you eliminate the staple requirement but introduce conduit fill limits. According to NEC Chapter 9, Table 1, when pulling three or more conductors through a conduit, you are limited to 40% cross-sectional fill. For 1/2-inch EMT, that means a maximum of nine #12 AWG THHN wires. If you need to run ten wires, you must step up to 3/4-inch EMT. Furthermore, pulling THHN through multiple 90-degree bends requires calculating conduit body placements to avoid exceeding the 360-degree bend limit between pull points.
Environmentally, if a cable passes through a masonry wall that is in direct contact with the earth (like a below-grade concrete block foundation), NM-B is prohibited because masonry retains moisture. You must transition to THWN-2 in conduit or use UF-B for that specific segment.
Worked Numeric Example: Sizing a 60A Subpanel Feeder
The most dangerous mistake DIYers make with electrical wiring types is ignoring the NEC temperature columns. Let us calculate the exact wire size needed for a 60A feeder from your main panel to a detached garage subpanel, assuming copper conductors, 30°C ambient temperature, and standard 75°C rated breaker lugs.
Scenario A: Using NM-B Cable (Indoor, attached garage route)
Because NM-B is legally restricted to the 60°C column of NEC Table 310.16, we look at the 60°C row. A 6 AWG copper wire at 60°C is only rated for 55 Amps. Since 55A is less than our 60A breaker, 6 AWG NM-B is a code violation and a fire hazard. To safely carry 60A using NM-B, you must step up to 4 AWG copper, which is rated for 70A at 60°C. You would need to purchase and pull a very stiff, expensive 4/3 NM-B cable.
Scenario B: Using THHN/THWN-2 in PVC Conduit (Outdoor/underground route)
Individual THHN wires in conduit allow you to use the 75°C column (matching the 75°C rating of your breaker lugs). Looking at the 75°C row, a 6 AWG copper wire is rated for 65 Amps. This safely exceeds the 60A breaker requirement. You can use 6 AWG THHN, which is significantly cheaper, more flexible, and easier to pull through 1-inch PVC conduit than a massive 4/3 NM-B cable.
Bench Tip: Always check the voltage drop for long feeder runs. While 6 AWG THHN is legally sufficient for 60A at 75°C, if your garage is 150 feet away, you should upsize to 4 AWG THHN to keep voltage drop under the recommended 3% threshold for feeders.
Decision Tree: Picking the Right Wire for Your Project
Use this decision path to terminate your planning phase with a concrete material pick. Do not mix and match these environments.
| Installation Environment | Circuit Type | IF Condition... | THEN Default Pick (Concrete Value) |
|---|---|---|---|
| Indoor, dry, concealed behind drywall | 15A or 20A Branch (Lights/Outlets) | Standard residential framing | 14/2 or 12/2 NM-B (Southwire Romex) |
| Indoor, exposed walls (basement/garage) | 20A Branch (Workbench outlets) | Risk of physical impact or abrasion | 12/2 MC Cable (Metal-Clad armor) |
| Underground, direct burial to detached building | 60A to 100A Subpanel Feeder | No conduit desired, trenching to 24 inches | 4/3 or 2/3 UF-B (Direct Burial rated) |
| Long runs, conduit, or high-amp feeders | 50A+ Subpanel or EV Charger | Pulling through PVC/EMT conduit | #4 or #6 AWG THHN/THWN-2 (Individual strands) |
Common Confusions and Code Caveats
Can I use the 90°C column to size my NM-B wire?
No. While the individual wires inside NM-B are often stamped '90°C', the NFPA 70 National Electrical Code Article 334.80 explicitly restricts the final ampacity of NM-B to the 60°C column. The 90°C rating is only useful if you are applying derating factors for bundling multiple cables together, but the final derated ampacity cannot exceed the 60°C baseline value.
Is THHN the same as THWN-2?
Historically, THHN was for dry locations and THWN was for wet locations. Today, almost all major manufacturers like Southwire and Cerrowire dual-rate their building wire as THHN/THWN-2. If the spool says THHN/THWN-2, it is approved for both dry conduits and wet locations (like underground PVC conduit that inevitably collects condensation).
Why can't I use NM-B in a wet location if I put it inside a conduit?
Conduit is not considered a waterproof seal; it is a raceway that will eventually accumulate condensation or groundwater. NM-B paper wrappers and PVC jackets are not rated for continuous moisture exposure. If a cable is inside a conduit in a wet location, you must use individual THWN-2 conductors or a wet-rated cable assembly.
When planning your next electrical project, default to NM-B for all standard indoor, concealed branch circuits, and reserve THHN in conduit for subpanel feeders, outdoor runs, and exposed commercial-style installations. Always verify your final wire size against the 60°C or 75°C column of NEC Table 310.16 based on your chosen cable type, as local Authorities Having Jurisdiction (AHJ) will fail an inspection if the temperature column math does not align with the physical jacket type installed.
