Wiring cable types refer to the specific combination of conductor insulation, outer jacket material, and assembly configuration designed for distinct environmental and electrical conditions in a circuit. Selecting the correct wiring cable types dictates your maximum allowable ampacity, whether the run can be buried or exposed to moisture, and if your installation passes local AHJ (Authority Having Jurisdiction) inspections. Getting this wrong doesn't just mean a failed inspection; it means latent fire hazards from overheated conductors or premature insulation breakdown from UV and moisture exposure.
The Core Residential Wiring Cable Types
When you walk into the electrical aisle, you are primarily looking at three distinct categories for branch circuits and feeders. While the copper inside might be identical, the insulation and jacketing change the legal and physical rules of the installation.
| Cable / Wire Type | Insulation Temp Rating | NEC Ampacity Column Used | Primary Application |
|---|---|---|---|
| NM-B (Romex) | 90°C | 60°C (Per NEC 334.80) | Indoor, dry, concealed in walls |
| UF-B | 90°C | 60°C (Per NEC 339.5) | Outdoor, direct burial, wet locations |
| THHN/THWN-2 | 90°C | 75°C or 90°C (Termination dependent) | Conduit, raceways, panel wiring |
NM-B (Non-Metallic Sheathed Cable): Often generically called Romex (a Southwire trademark), this is the standard for indoor residential wiring. It consists of THHN-insulated conductors wrapped in a paper separator and a PVC outer jacket. It is strictly limited to dry, indoor locations.
UF-B (Underground Feeder): Think of the outer jacket like a weather-rated raincoat: NM-B is a light windbreaker for dry indoor use, while UF-B is a heavy-duty rubberized coat meant to be buried in wet soil. The conductors are individually insulated but embedded directly in a solid, moisture-resistant PVC jacket without the paper wrap. It is rated for direct burial and wet locations but is stiffer and harder to strip than NM-B.
THHN/THWN-2 (Individual Conductors): These are single wires pulled through conduit (EMT, PVC, or flexible). Because they dissipate heat better in a raceway and lack an insulating outer jacket trapping thermal energy, the National Electrical Code (NEC) allows you to utilize higher ampacity columns for these wires, provided your terminations are rated for it.
Where You Meet Wiring Cable Types in Practice
You will encounter the practical limits of these wiring cable types the moment you transition from indoor framing to outdoor or high-load projects.
- Running a subpanel to a detached garage: You cannot use NM-B once you leave the main house's dry envelope. You must transition to individual THWN-2 wires in buried PVC conduit, or use UF-B if you are direct-burying the cable without conduit (minimum 24 inches deep per NEC Table 300.5).
- Wiring a 240V electric baseboard heater: If the manufacturer's termination block is only rated for 60°C, you must size your THHN wire using the 60°C column, negating the high-temperature advantage of the wire.
- Fishing wires through existing walls: NM-B is the undisputed king here. Modern variants like Southwire's SIMpull ROMEX feature a lubricated jacket that drastically reduces friction when pulling through tight stud bays, saving your knuckles and the drywall.
Worked Example: The 60°C vs 90°C Ampacity Trap
The most common mistake DIYers and junior apprentices make is looking at the 90°C column on the wire ampacity chart and assuming they can push that much current through NM-B cable. Let's look at a real-world scenario where this misunderstanding causes a code violation.
The Scenario: You are installing a hardwired 40A Level 2 EV charger in your garage. The manufacturer specifies a 40A continuous load.
Step 1: Calculate the Required Ampacity
Per NEC 210.20(A), continuous loads (those running for 3 hours or more) must be sized at 125% of the load.
40A × 1.25 = 50A required ampacity.
Step 2: Evaluate 8 AWG NM-B Cable
You grab a roll of 8/2 NM-B. Looking at the standard copper ampacity table, 8 AWG in the 90°C column is rated for 55A. You might think this passes. However, NEC 334.80 explicitly states that the ampacity of NM-B cable shall be determined using the 60°C column, regardless of the 90°C insulation inside.
Looking at the 60°C column, 8 AWG is only rated for 40A.
Result: 40A < 50A. This fails. The cable will overheat under continuous load.
Step 3: Evaluate 8 AWG THHN in Conduit
You decide to run EMT conduit and pull three 8 AWG THHN wires. Because THHN in conduit is governed by Article 310, and assuming your breaker and EV charger terminations are rated for 75°C, you use the 75°C column.
In the 75°C column, 8 AWG is rated for 50A.
Result: 50A = 50A. This passes code.
Step 4: The NM-B Fix
If you absolutely must use NM-B for this run, you have to step up to 6 AWG NM-B. In the 60°C column, 6 AWG is rated for 55A, which safely covers the 50A requirement.
Common Confusions: Wire vs. Cable and Jacket Ratings
People frequently confuse wire and cable. In electrical terminology, a wire is a single solid or stranded conductor (like a spool of THHN). A cable is an assembly of two or more conductors bound together inside an outer jacket (like NM-B or UF-B). You buy THHN by the wire, but you buy Romex by the cable.
Another major point of confusion is low-voltage data cable jackets, which often get mixed up with line-voltage wiring. You will see terms like CMP (Plenum), CMR (Riser), and CM (General). These refer to fire-retardant and smoke-generation ratings for network cables, not their electrical ampacity. Never use low-voltage CMP/CMR cable for 120V/240V mains power; the insulation will melt and cause a short circuit at mains voltages.
Frequently Asked Questions About Wiring Cable Types
Can I run NM-B Romex inside a PVC conduit underground to my shed?
No. NEC 334.12 explicitly prohibits the use of NM-B in wet or damp locations, and the interior of an underground conduit is classified as a wet location due to condensation and groundwater seepage. You must use individual THWN-2 wires (the 'W' stands for water-resistant) or UF-B cable inside the conduit. For a detailed breakdown of underground feeder rules, refer to EC&M's guide on Article 339.
What is the actual difference between 12/2 and 12/3 wiring cable types?
The numbers refer to the count of current-carrying insulated conductors, not the bare ground wire. A 12/2 cable contains two 12 AWG insulated wires (one black, one white) plus a bare copper ground. A 12/3 cable contains three 12 AWG insulated wires (black, red, white) plus the bare ground. You use 12/3 for 3-way switch loops, split-receptacle circuits, or 240V appliances that require a dedicated neutral.
Why does my THHN wire have a slick nylon outer jacket?
That nylon jacket is what gives the wire its 'THWN-2' dual rating. While the base PVC insulation provides the primary dielectric strength and 90°C heat resistance (THHN), the thin nylon outer layer provides superior resistance to moisture, chemicals, and oil. More importantly for the installer, the nylon acts as a dry lubricant, drastically reducing the pulling tension when you are fishing multiple wires through long conduit runs.
Do I need to derate ampacity when bundling multiple NM-B cables through a single stud hole?
Yes, under specific conditions. According to NEC 310.15(C)(1), if you bundle more than three current-carrying conductors together for a continuous distance of more than 24 inches, you must apply a derating factor. If you stack three 12/2 NM-B cables tightly through a fire-blocked stud bay without maintaining spacing, you have six current-carrying conductors bundled together. This requires an 80% derating factor, which effectively drops the ampacity of 12 AWG wire below the standard 20A breaker limit. Always use fire-rated caulk or maintain physical spacing when running multiple cables through framing.
