In electrical terminology, a wire is a single solid or stranded conductor, while a cable is a group of two or more wires bundled together inside a single outer sheath. Understanding the different kinds of wires and cables is the foundation of any safe installation because the specific type you choose dictates the allowable ampacity, the permitted installation environment (wet, dry, buried, or in conduit), and the temperature column you must use when sizing breakers.
Choosing the wrong type doesn't just violate code; it changes the thermal limits of your circuit. For instance, running a high-load appliance wire through insulation without the proper rating causes heat buildup, degrading the insulation and creating a fire hazard. Below is a breakdown of the primary wiring types you will encounter on the jobsite or in a residential panel.
The Core Differences: Wire vs. Cable and Insulation Types
While hardware stores often use the terms interchangeably, the National Electrical Code (NEC) treats single conductors and multi-conductor cables very differently. Single wires (like THHN) must be run inside a raceway or conduit for physical protection, whereas nonmetallic sheathed cables (like NM-B) can be stapled directly to framing. A standard cable consists of current-carrying conductors (hots and neutral), a bare or green equipment grounding conductor, paper or plastic filler for shape, and an outer PVC jacket.
Residential Wire and Cable Specification Sheet
| Type | Configuration | Max Temp Rating | Primary Use Case | NEC Article |
|---|---|---|---|---|
| NM-B (Romex) | 2+ w/ Ground (Solid) | 90°C (Ampacity limited to 60°C) | Interior dry-wall branch circuits | Article 334 |
| THHN / THWN-2 | Single Conductor | 90°C Dry / 75°C Wet | Conduit runs, subpanel feeders | Article 310 |
| UF-B | 2+ w/ Ground (Solid) | 90°C (Ampacity limited to 60°C) | Direct burial, outdoor wet locations | Article 339 |
| MC (Metal Clad) | 2+ w/ Ground (Solid/Stranded) | 90°C | Exposed runs, commercial, retrofit | Article 330 |
Where You Meet This in Practice
You will select your wire or cable type based entirely on the physical path the circuit must take from the panel to the load. The physical environment dictates the jacket material and the installation hardware required.
- Interior Dry Walls (NM-B): When wiring a standard 15A or 20A bedroom or living room receptacle circuit, you will pull 14/2 or 12/2 NM-B cable. It is cost-effective, easy to strip, and the bare copper ground is automatically included. NEC 334.30 requires NM-B to be secured within 8 inches of the electrical box and at intervals not exceeding 4.5 feet.
- Conduit and Subpanels (THHN): If you are running a 50-foot underground PVC conduit to a detached garage subpanel, you cannot use NM-B because it is not rated for wet locations and the jacket traps moisture. Instead, you pull individual 6 AWG THHN/THWN-2 conductors (two hots, one neutral, one ground) through the pipe. This also makes future upgrades easier, as you can pull new wires without digging a new trench.
- Outdoor Burial (UF-B): For a direct-burial run to a landscape lighting transformer or a post light where digging a trench for rigid conduit is impractical, UF-B is required. Its solid, gray plastic sheath completely encapsulates the conductors, preventing moisture ingress and resisting soil acids. It must be buried at least 24 inches deep unless protected by a GFCI breaker, which reduces the depth requirement to 12 inches.
- Exposed Retrofit (MC): When finishing a basement where walls are left exposed, or running power in a commercial drop-ceiling, Metal Clad (MC) cable provides the necessary physical crush protection that NM-B lacks. The interlocking aluminum or steel armor acts as its own raceway.
Worked Example: Sizing a 40-Amp Subpanel Feeder
Let's look at how wire type changes your sizing math and thermal limits. Assume you need to feed a 40-amp, 240V subpanel located 60 feet from the main breaker. We will assume copper conductors and an ambient temperature of 30°C.
Scenario A: Using 8 AWG NM-B Cable
According to NEC 334.80, the ampacity of NM-B cable must be determined using the 60°C column of NEC Table 310.16, regardless of the fact that the insulation itself is rated for 90°C. In the 60°C column, 8 AWG copper is rated for exactly 40A. This is acceptable for a 40A breaker, but leaves zero headroom for voltage drop or future expansion.
Scenario B: Using 8 AWG THHN in Conduit
If you pull individual 8 AWG THHN wires through a conduit, you are permitted to use the 75°C column (assuming your breaker and panel lugs are rated for 75°C, which most modern equipment is). In the 75°C column, 8 AWG copper is rated for 50A. This gives you a 10A buffer and runs significantly cooler under load.
Voltage Drop Check: Using the standard single-phase formula VD = (2 x K x I x D) / CM where K=12.9 (copper), I=40A, D=60 ft, and CM=16,510 (circular mils for 8 AWG):
VD = (2 x 12.9 x 40 x 60) / 16510 = 3.75V
At 240V, a 3.75V drop is 1.56%, which is well under the NEC recommended 3% maximum for feeders. However, if the run was 120 feet, the drop would exceed 3%, and you would need to upsize to 6 AWG regardless of the wire type to maintain power quality at the subpanel.
Common Confusions and Mistakes to Avoid
What do people commonly confuse about wire types?
1. THHN vs. THWN-2: Older stock might only be labeled THHN (dry locations only). Modern manufacturing almost universally dual-rates wire as THHN/THWN-2. If your wire only says THHN, it cannot be pulled through a wet conduit or used outdoors. Always check the jacket printing before pulling wire into an underground or exterior raceway.
2. Solid vs. Stranded: Electricians often confuse the physical flexibility of the wire with its electrical rating. Solid wire is standard for residential branch circuits (NM-B) because it terminates cleanly under standard receptacle screws and pushes easily into back-wire clamps. Stranded wire is used in flexible cords, automotive applications, and large-gauge conduit pulls because it bends easier, but it requires crimp lugs or pig-tailing to terminate properly on standard residential devices without fraying.
3. The 90°C Column Trap: A frequent mistake is looking at the 90°C column on the ampacity chart because the wire jacket says '90°C'. Unless every single termination point (breaker, lug, receptacle) in the entire circuit is explicitly rated for 90°C, you must derate your final ampacity to the lowest rated component in the chain. In residential gear, this is almost always 60°C or 75°C. The 90°C column is only used as a starting point for applying ambient temperature or bundling derating factors.
4. Aluminum vs. Copper Cables: While copper is the standard for branch circuits, aluminum (specifically AA-8000 series alloy) is frequently used for large feeder cables (like 2-2-2-4 SER) due to cost savings. Aluminum requires larger wire gauges for the same ampacity, larger conduit fill space, and the use of anti-oxidant paste (like Noalox) at terminations to prevent galvanic corrosion and high-resistance heating.
Safety & Code Caveat: Always de-energize the panel, lock out the main breaker, and verify dead with a tested multimeter before working inside any enclosure. The NEC-style guidance provided here assumes standard copper conductors in a 30°C ambient environment. Your local Authority Having Jurisdiction (AHJ) has final authority on all installations and may have local amendments that restrict certain cable types (e.g., banning NM-B in specific multi-family dwellings or requiring AFCI protection on all 120V branch circuits).
