The different kinds of wire used in electrical work are defined by their conductor material, stranding, and insulation or jacket ratings, which dictate the maximum safe current (ampacity) and the physical environments where they can be legally installed. Choosing the wrong wire type changes your allowable ampacity, alters your physical installation rules (wet vs. dry, conduit vs. staple), and can lead to insulation meltdown or fire if the thermal limits are exceeded. Beginners commonly confuse wire gauge (the physical thickness of the copper, measured in AWG) with wire type (the insulation classification like NM-B or THHN), mistakenly assuming a 10 AWG wire is always rated for 30 amps regardless of its outer jacket.
The Core Differences: Insulation, Jacket, and Stranding
When you buy wire, you are actually buying two distinct components bundled together: the copper (or aluminum) conductor that carries the current, and the polymer insulation that contains the electromagnetic field and prevents short circuits. The National Electrical Code (NEC) classifies these based on thermal ratings and moisture resistance. According to the NFPA National Electrical Code guidelines, the maximum allowable ampacity is strictly tied to the temperature rating of the wire's insulation, not just the metal inside.
Here is a breakdown of the most common residential wire types you will encounter on a jobsite or in a home center:
| Wire Type | Max Temp Rating | Jacket / Insulation | Primary Application |
|---|---|---|---|
| NM-B (Romex) | 90°C (but limited to 60°C ampacity) | PVC jacket over individual THHN wires + bare ground | Indoor, dry locations; stapled to wood/metal studs |
| THHN / THWN-2 | 90°C | Thin PVC/Nylon, single conductor | Pulled through EMT/PVC conduit, panel pigtails |
| UF-B | 90°C (limited to 60°C ampacity) | Solid gray PVC encapsulating all conductors | Direct burial, outdoor damp/wet locations |
| XHHW-2 | 90°C (Wet or Dry) | Cross-linked polyethylene (XLPE) | Commercial conduit runs, high-heat environments |
Worked Example: NM-B vs. THHN in a 35A Continuous Load
To see exactly what wire type changes in a real circuit, let's look at a numeric example involving a continuous load. A continuous load is defined by the NEC as any load where the maximum current is expected to continue for 3 hours or more (like an EV charger, a hardwired heater, or commercial lighting). The code requires you to multiply the continuous load by 125% to size your wire and breaker.
The Scenario: You need to wire a 240V, 35-amp continuous load (such as a large shop heater or a Level 2 EV charger configured to 35A).
- Step 1: Calculate Minimum Ampacity. 35A × 1.25 = 43.75A. Your wire must have an allowable ampacity of at least 43.75A.
- Step 2: Evaluate 8 AWG THHN in Conduit. According to the 90°C column in NEC Table 310.16, 8 AWG copper THHN is rated for 55A. Since 55A > 43.75A, this wire passes. (Assuming your breaker terminals are rated 75°C, the 75°C column for 8 AWG is 50A, which also passes the 43.75A threshold).
- Step 3: Evaluate 8 AWG NM-B in a Wall Cavity. NEC Article 334.80 explicitly states that the ampacity of NM-B cable must be determined using the 60°C column, regardless of the fact that the internal conductors have 90°C insulation. In the 60°C column, 8 AWG copper is only rated for 40A. Since 40A is less than the required 43.75A, 8 AWG NM-B fails.
The Result: If you use THHN in conduit, 8 AWG is perfectly legal and safe. If you use NM-B Romex, you must step up to 6 AWG (which is rated 55A in the 60°C column) to handle the exact same 35A continuous load. This is why knowing the different kinds of wire is critical; using the wrong type forces you to buy thicker, more expensive copper to achieve the same electrical result.
Where You Meet Different Kinds of Wire in Practice
On the bench or the jobsite, you will select your wire type based entirely on the physical environment and the raceway method. Here is where each type earns its keep:
NM-B (Non-Metallic Sheathed Cable)
You will use NM-B for 90% of indoor residential branch circuits. It is fast to install because you only have to staple one flat cable rather than pulling three or four individual wires. However, it is strictly forbidden in wet locations, embedded in concrete, or run outdoors. If you are wiring a standard bedroom outlet or an interior subpanel in a dry basement, NM-B is your default.
THHN / THWN-2 (Single Conductors)
You meet this wire when you are pulling through metallic (EMT) or PVC conduit. Because the insulation is thin and slick (often coated with a nylon outer layer), it slides through conduit bends easily. You will also find THHN used as 'pigtails' inside your main service panel. As of the 2026 market landscape, buying bulk spools of THHN is often more cost-effective than NM-B for long runs, provided you are already installing conduit.
UF-B (Underground Feeder)
UF-B is the heavy-duty cousin of NM-B. Instead of a loose PVC jacket wrapping the wires, the conductors are fully encapsulated in a solid, tough gray plastic. You use this for direct-burial runs to detached garages, pole barns, or outdoor landscape lighting transformers. According to the Copper Development Association's building wire guidelines, UF-B must be buried at specific depths (typically 24 inches for direct burial residential branch circuits) and must transition to a protected wiring method before it emerges from the ground.
Frequently Asked Questions About Different Kinds of Wire
Can I use stranded wire for standard home wall outlets?
While stranded wire (like stranded THHN) is electrically capable of carrying the same current as solid wire, standard residential 15A and 20A duplex receptacles are designed with clamp-style or push-in terminations meant for solid 12 or 14 AWG wire. If you screw a stranded wire under a standard outlet side-screw, the strands can fray, splay out, and create a poor connection that leads to arcing. If you must use stranded wire for an outlet, you should crimp a ferrule or a pin terminal onto the end before terminating it, or use a receptacle specifically rated and listed for stranded conductors.
What is the difference between THHN and THWN-2 wire?
Historically, THHN (Thermoplastic High Heat-resistant Nylon-coated) was rated only for dry locations up to 90°C, while THWN (Thermoplastic Heat and Water-resistant Nylon-coated) was rated for wet locations but only up to 75°C. Today, nearly all modern single-conductor building wire manufactured in the US is dual-rated as THHN/THWN-2. The '-2' designation means it is rated for 90°C in both wet and dry locations. Always check the print legend on the wire jacket; if it says 'THHN/THWN-2', you are cleared for wet locations (like inside outdoor conduit that may accumulate condensation).
Why does my 10 AWG NM-B wire have a 30A limit but 10 AWG THHN has a 40A limit?
This comes down to heat dissipation. When individual THHN wires are pulled through a half-inch EMT conduit, there is air space around each conductor, allowing heat to dissipate efficiently into the metal raceway. NM-B cable, however, bundles three or four insulated wires tightly together inside a secondary outer PVC jacket. This tight bundling traps heat. To prevent the inner insulation from degrading over time, the NEC mandates that NM-B ampacity be restricted to the more conservative 60°C column, capping 10 AWG at 30A, whereas a single 10 AWG THHN wire in the 90°C column is rated for 40A.
Is aluminum wire still used in residential branch circuits?
For standard 15A and 20A branch circuits (outlets and lights), copper is the universal standard. However, for larger feeder circuits—such as a 100A or 200A run from your main panel to a subpanel, or a heavy 50A range circuit—aluminum (specifically XHHW-2 or THWN-2 aluminum alloy) is widely used and perfectly safe when installed correctly. Aluminum is significantly lighter and cheaper than copper. The catch is that aluminum expands and contracts more than copper under thermal cycling, and it requires specific anti-oxidant paste and torque-rated terminations to prevent loose connections and fires. Never use legacy 1970s-era solid aluminum branch wiring; modern aluminum feeders use advanced alloys and are highly reliable when torqued to manufacturer specs.
