Electrical wire types are categorized by their insulation material, outer jacket configuration, and intended environment, which collectively dictate their ampacity, voltage rating, and physical protection requirements. Choosing the correct type dictates whether your installation passes inspection, whether the insulation degrades in damp soil, and whether the conductors overheat inside a packed conduit. Before pulling any wire, it is critical to clear up the most common point of confusion: the difference between wire and cable. A wire is a single solid or stranded conductor (like THHN), while a cable is two or more insulated conductors bundled within an overall protective jacket (like NM-B or Romex). Confusing the two leads to buying the wrong materials for your framing or conduit runs.
The Core Taxonomy of Residential Wiring
When you walk into the electrical aisle, the different types of electrical wire are separated by their physical construction and the environments they are rated to survive. The National Electrical Code (NEC) strictly governs where each type can be installed based on its moisture resistance, flame retardancy, and physical armor. Below is the master spec sheet for the four most common residential wire and cable types you will encounter.
| Cable/Wire Type | Configuration | Wet/Dry Rating | Max Voltage | Common Use Case & Installation Method |
|---|---|---|---|---|
| NM-B (Romex) | 2+ insulated conductors, bare ground, paper wrap, PVC jacket | Dry locations only | 600V | Interior branch circuits; stapled directly to wood/metal framing inside finished walls. |
| THHN / THWN-2 | Single solid/stranded conductor, PVC insulation, nylon jacket | Dry (THHN) / Wet (THWN-2) | 600V | Pulled through EMT, PVC, or flexible conduit for exposed runs, subpanels, and commercial-style builds. |
| UF-B | 2+ insulated conductors, bare ground, fully encapsulated in solid PVC | Wet and dry (Direct burial) | 600V | Underground runs to detached garages, sheds, or landscape lighting; buried directly in trench. |
| MC (Metal Clad) | 2+ insulated conductors, green ground, interlocking aluminum/steel armor | Dry (standard) / Wet (with special jacket) | 600V | Exposed basement ceilings, commercial retrofits, or residential areas requiring physical damage protection. |
Where You Meet This in Practice: Framing, Conduit, and Burial
The type of wire you choose fundamentally changes your installation labor and derating math. If you are wiring a standard interior bedroom, NM-B is the undisputed king because you can staple it to studs and fish it through bored holes without needing to pull individual conductors. However, if you are running a feeder to a detached garage underground, NM-B is illegal; you must use UF-B for direct burial, or pull THWN-2 through buried PVC conduit.
The most critical practical difference arises when dealing with conduit fill and thermal derating. When you use individual THHN/THWN-2 wires in a conduit, the heat generated by the current cannot dissipate as easily as it does when a single NM-B cable is sitting in an open wall cavity. The NEC requires you to reduce (derate) the ampacity of the wire when you bundle multiple current-carrying conductors in a single raceway.
Imagine you are pulling three separate 120V/20A multi-wire branch circuits through a single 1-inch EMT conduit. This means you have 3 black (hot), 3 red (hot), 3 white (neutral), and 1 green (ground) wire. The ground does not count as a current-carrying conductor, leaving you with 9 current-carrying conductors.
According to NEC Table 310.16, 12 AWG THHN has a base ampacity of 30A in the 90°C column. However, NEC Table 310.15(C)(1) states that 7 to 9 current-carrying conductors require a 70% adjustment factor.
Calculation: 30A × 0.70 = 21A adjusted ampacity.
Because 21A is still greater than your 20A breaker, the 12 AWG wire is legally protected and safe to use. But, if you added a fourth circuit (bringing the count to 12 current-carrying conductors), the derating factor drops to 50%.
Calculation: 30A × 0.50 = 15A adjusted ampacity.
At 15A, your 12 AWG wire is now under-protected for a 20A breaker. You would be forced to upsize to 10 AWG THHN (40A base × 0.50 = 20A) to legally run that fourth circuit in the same pipe. This thermal reality is why electricians often prefer running parallel conduits or using larger wire gauges when bundling THHN.
Decoding the Alphabet Soup: Insulation Letters and Temperature Ratings
When buying single-conductor wire, you will almost exclusively see THHN and THWN-2 printed on the insulation. Modern manufacturing has largely merged these into dual-rated wire, but understanding the letters explains the physics of the insulation.
- T = Thermoplastic insulation (PVC).
- H = Heat resistance (rated for 75°C in wet, 90°C in dry).
- HH = High heat resistance (rated for 90°C).
- W = Water resistance (approved for wet locations like conduit outdoors).
- N = Nylon outer jacket (provides exceptional abrasion resistance when pulling through metal conduit).
- -2 = The modern standard allowing 90°C rating in both wet and dry locations.
A common trap for DIYers is looking at the 90°C column on the ampacity chart and sizing their breakers based on that number. While the wire insulation can handle 90°C, the termination points (breakers, receptacles, and switches) are generally only rated for 60°C or 75°C. Per NEC 110.14(C), you must use the 60°C column for circuits 100A or less (which covers almost all residential branch circuits), unless the equipment is explicitly marked for 75°C. The 90°C column is strictly used as your starting point for derating calculations, as shown in the example above.
For heavy feeders or service entrance conductors, you will also encounter XHHW-2. This uses cross-linked polyethylene (XLPE) insulation instead of PVC. It lacks the slick nylon jacket of THHN, making it slightly harder to pull in long conduit runs, but it offers superior thermal stability and a thinner insulation profile, allowing more conductors to fit in the same conduit diameter. You can find detailed physical specifications for these insulation types in the Southwire technical resource library.
Frequently Asked Questions on Wire Selection
Can I use indoor NM-B (Romex) if I run it inside a PVC conduit to protect it?
No. While you physically can pull NM-B through a short sleeve of conduit for physical protection (like dropping down a wall to a garbage disposal), you cannot use it for a continuous conduit run. NM-B is not rated for the heat dissipation characteristics of a sealed conduit, and pulling the bulky jacket around conduit bends often damages the internal paper and insulation. Use THWN-2 for conduit runs.
How deep do I need to bury UF-B cable?
Under standard NEC Table 300.5 guidelines, UF-B cable buried directly in the earth for a residential 120V/20A branch circuit must be buried at least 24 inches deep. If you run the same circuit through rigid metal conduit (RMC) or intermediate metal conduit (IMC), the burial depth requirement drops to 6 inches. If you use THWN-2 inside buried PVC conduit, the minimum depth is 18 inches.
Does Metal Clad (MC) cable require a separate ground wire?
Standard interlocking MC cable contains an internal green insulated equipment grounding conductor. You must pigtail this green wire to your device's ground screw and the metal box. You cannot rely solely on the metal armor of standard MC cable as the equipment grounding path, because the interlocking spiral design does not guarantee a low-impedance fault path. (Note: There is a specific variant called MC-AP with a special fitting that allows the armor to serve as the ground, but standard MC requires the internal green wire).
What is the difference between solid and stranded THHN?
Solid wire is a single thick piece of copper, making it stiffer but easier to terminate on standard residential push-in or screw-terminal receptacles. Stranded wire is made of many thin copper threads, making it highly flexible and much easier to pull through conduits with multiple bends. However, stranded wire can fray when stripped and requires more care when wrapping around terminal screws or using crimp ferrules.
