Wiring cables types refer to the standardized categories of insulated conductors and outer jackets engineered for specific environmental, thermal, and physical installation conditions. Choosing the correct type dictates your allowable ampacity, determines whether the run is legal in wet or dry locations, and establishes the physical protection required by code. The most common mistake DIYers make is confusing a cable (a factory-assembled group of wires with an outer jacket, like NM-B) with a wire (a single insulated conductor, like THHN), or assuming all non-metallic sheathed cables are rated for outdoor burial.

What This Changes in Your Installation: Swapping a cable type isn't just about the jacket color. It changes the temperature column you must use for ampacity derating (e.g., NM-B is limited to the 60°C column per NEC 334.80, even if its internal wires are rated 90°C), alters the required burial depth for underground runs, and dictates your bending radius limits.

The Core Wiring Cable Types in Residential Work

Before pulling any wire, you need to know the baseline specifications for the four main categories you will encounter in home electrical work. As of the 2026 NEC cycle, the distinctions between these remain strict, particularly regarding temperature ratings and physical protection.

Cable / Wire Type Full Name Primary Use Case Max Temp Rating (Ampacity) Wet Location Rated?
NM-B Non-Metallic Sheathed (Romex) Interior dry walls, ceilings, floors 60°C (for ampacity) No
UF-B Underground Feeder Direct burial, outdoor exposed runs 60°C (for ampacity) Yes
MC Metal-Clad Cable Exposed interior walls, commercial, physical protection 90°C No (unless specifically marked)
THHN/THWN-2 Thermoplastic High Heat / Water Resistant Individual conductors pulled inside conduit (EMT/PVC) 90°C (dry) / 75°C (wet) Yes (THWN-2)

Think of the insulation as the wire’s skin and the outer jacket as its winter coat. THHN is just the skin; it must be housed inside a conduit (the coat) to be protected from physical damage and UV light. NM-B and UF-B have the jacket built-in from the factory, but UF-B's jacket is formulated to resist moisture and soil acids, whereas NM-B's paper-wrapped PVC jacket will wick water and fail if buried.

Where You Meet This in Practice

Theory only matters when it hits the job site. Let’s look at a real-world scenario where picking the wrong wiring cable type—or the wrong gauge within that type—results in a failed inspection or a tripped breaker.

The Scenario: You are running a new 20A, 120V dedicated circuit from your main panel to a detached garage workbench. The total one-way cable distance is 60 feet. You plan to bury the cable in a trench.

First, we eliminate NM-B because it is strictly for indoor, dry locations. Burying NM-B is a direct violation of NEC Article 334 and will result in an immediate inspection failure. We must use either direct-burial UF-B or individual THWN-2 wires inside a schedule 80 PVC conduit.

Next, we calculate voltage drop. The NEC recommends a maximum 3% voltage drop for branch circuits. Let's run the math for 12 AWG copper (the standard size for a 20A breaker):

  • 12 AWG Resistance: 1.93 ohms per 1,000 feet.
  • Total Loop Distance: 60 ft out + 60 ft back = 120 feet (0.12 kft).
  • Loop Resistance: 1.93 × 0.12 = 0.2316 ohms.
  • Voltage Drop at 20A: 20A × 0.2316 ohms = 4.63V.
  • Percentage Drop: (4.63V / 120V) × 100 = 3.86%.

At 3.86%, 12 AWG exceeds the 3% recommended limit. Your power tools at the workbench will run hot and slow. To fix this, we step up to 10 AWG (1.21 ohms/kft). The new voltage drop is 2.9V, or 2.4%, which passes perfectly. Therefore, the correct material for this run is 10/2 UF-B, not 12/2.

Common Confusions That Fail Inspections

When discussing wiring cables types, a few specific misconceptions cause DIYers to fail inspections or create fire hazards. According to industry analysis from Electrical Contractor Magazine, these are the most frequent code violations related to cable selection:

1. The 90°C Ampacity Trap with NM-B

The individual wires inside a standard Southwire SIMpull NM-B cable are often printed with '90°C'. However, NEC 334.80 explicitly states that the ampacity of NM-B cable must be determined using the 60°C column of Table 310.16. You cannot use the 90°C column to derate NM-B, even for ambient temperature adjustments. A 12 AWG NM-B is strictly capped at 20A, regardless of the 90°C printing on the inner THHN insulation.

2. THHN vs. THWN-2

Older stock might be marked only as THHN (dry locations only). Modern dual-rated wire is marked THHN/THWN-2. If you are pulling wire through an underground PVC conduit where condensation is guaranteed, you must use THWN-2. Using strictly THHN in a wet conduit will eventually lead to insulation breakdown and a ground fault.

3. Exposed NM-B in Garages and Basements

NM-B cannot be run exposed on the surface of studs in an unfinished garage or basement where it is 'subject to physical damage.' If the cable is exposed below 8 feet on a wall, it must be protected by conduit, or you must switch to Metal-Clad (MC) cable or EMT conduit with THHN.

Decision Tree: Picking the Exact Cable for Your Run

Use this matrix to terminate your decision process and pick the exact material you need to buy. Do not guess; follow the path to the concrete pick.

Installation Condition Required Cable / Wire Type Concrete Pick (Standard 20A Circuit)
Inside finished, dry interior walls (living room, bedroom) NM-B (Non-Metallic Sheathed) Southwire SIMpull 12/2 NM-B with Ground
Buried directly in a trench (no conduit) to a shed or post light UF-B (Underground Feeder) Cerro Wire 12/2 UF-B (or 10/2 if >50ft)
Exposed surface wiring in an unfinished garage or workshop MC (Metal-Clad) or EMT + THHN Southwire 12/2 MC Cable (Armorlite)
Long underground run (>60ft) or complex conduit bends Schedule 40/80 PVC + THWN-2 Cerro 12 AWG THHN/THWN-2 (Black, White, Green)
High-heat areas (attics >120°F ambient) or bundled tightly THHN/THWN-2 in Conduit Cerro 10 AWG THHN/THWN-2 (allows derating headroom)
The Default Recommendation: If you are wiring standard 15A or 20A receptacle circuits inside the dry, finished walls of a residential home, your default pick is always NM-B (commonly called Romex). Specifically, buy 14/2 for 15A lighting circuits and 12/2 for 20A receptacle circuits. It is the most cost-effective, code-compliant, and installer-friendly option for 90% of indoor home wiring.

Frequently Asked Questions

Can I use UF-B cable inside my house instead of NM-B?

Yes, UF-B is legally permitted anywhere NM-B is allowed, as it is a superior, moisture-resistant jacket. However, it is significantly more expensive, harder to strip, and the solid plastic jacket doesn't bend as easily around tight stud corners. Unless you have a specific reason (like running through a damp crawlspace), stick to NM-B for interior walls to save money and knuckle skin.

Why does my MC cable have a green grounding wire AND a bare bonding strip?

Metal-Clad (MC) cable often includes both an insulated green equipment grounding conductor (EGC) and a bare aluminum or copper bonding strip. The bonding strip must make continuous contact with the metal armor to ensure the armor itself acts as a ground path. When terminating into a metal box, the bonding strip must be folded back over the armor and secured with the MC fitting to maintain the ground continuity. Always refer to the manufacturer's installation guidelines for the specific MC brand you purchase.

Is it safe to mix different wiring cable types in the same junction box?

Yes, you can transition between cable types in a properly sized, accessible junction box. A common example is transitioning from indoor NM-B to outdoor THWN-2 in a conduit body (like an LB fitting) right where the wire exits the exterior wall. The box must remain accessible (not buried in drywall or concrete) and must have adequate volume for the number of conductors entering it.