Home wiring types are distinct cable and conductor assemblies categorized by their insulation chemistry, outer jacket toughness, and environmental ratings, which strictly dictate where they can be legally and safely routed. Choosing the correct wiring type changes your physical installation method (stapling to studs versus pulling through conduit), your environmental limits (wet versus dry locations), and your ampacity derating requirements. The most common point of confusion among DIYers is conflating 'Romex' (a trademarked brand name for Non-Metallic Sheathed Cable, or NM-B) with individual THHN conductors, leading to the dangerous and code-violating practice of stripping the outer NM-B jacket to pull the inner wires through conduit.
The Core Wiring Types in Homes Explained
Before you buy wire, you need to understand the four primary assemblies used in residential electrical work. Each serves a specific physical and environmental purpose.
| Wiring Type | Construction | Max Environment | Typical Use Case |
|---|---|---|---|
| NM-B (Non-Metallic) | 2+ THHN conductors + bare ground wrapped in paper and a PVC jacket. | Dry, indoor, concealed spaces only. | Standard interior wall receptacles, lighting, and switches. |
| THHN/THWN-2 | Individual solid/stranded conductors with nylon-coated PVC insulation. | Dry (THHN) or Wet (THWN-2) locations; requires a raceway. | Pulled through EMT/PVC conduit for exposed runs or subpanels. |
| UF-B (Underground Feeder) | Similar to NM-B, but conductors are embedded in solid, moisture-resistant PVC. | Wet locations, direct burial, outdoors. | Trenching power to a shed, outdoor post lights, or well pumps. |
| MC (Metal-Clad) | THHN conductors wrapped in an interlocking aluminum or steel armor. | Dry indoor locations; high physical impact resistance. | Exposed basement walls, commercial builds, garage surface runs. |
What Changes When You Switch Cable Types?
Switching from a pre-assembled cable (NM-B) to individual conductors in a raceway (THHN in conduit) fundamentally alters your material costs, labor time, and derating math. Let's look at a worked numeric example for a 60-foot run to a new 30A RV receptacle in a detached garage.
Option A: 10/2 UF-B (Direct Burial / Wet Rated Cable)
- Material Cost: ~$110 for 60 feet of 10/2 UF-B.
- Installation: Trench 24 inches deep (or 12 inches if GFCI protected per NEC 300.5), lay the cable, backfill with sand, and install warning tape. No conduit bending required.
- Derating: Treated as a single cable assembly; no conduit fill or multi-conductor derating applies.
Option B: 1/2-inch Schedule 40 PVC with 10 AWG THWN-2
- Material Cost: ~$65 (three 60ft spools of 10 AWG THWN-2 at $15/each + $20 for PVC conduit and glue).
- Installation: Trench 18 inches deep (NEC 300.5 for non-metallic raceways), glue PVC fittings, and physically pull three stiff 10 AWG wires through the bends using fish tape and wire lube.
- Derating Math: If you decide to pull two 30A circuits (4 current-carrying conductors) through that single 1/2-inch PVC pipe to save trenching time, NEC 310.15(C)(1) mandates an 80% derating factor.
10 AWG THWN-2 is rated 40A at 90°C. 40A × 0.80 = 32A adjusted ampacity. Since 32A > 30A, you can still legally protect the circuit with a 30A breaker.
The cable type dictates whether you are doing 'trench and drop' labor (UF-B) or 'glue and pull' labor (Conduit + THWN-2), and whether you have the thermal headroom to bundle multiple circuits together.
Where You Meet This in Practice
You will encounter specific wiring constraints based on the physical environment of your project:
- Inside Finished Drywall: You will exclusively use NM-B. It is cheap, flexible, and staples easily to the sides of wooden studs. It cannot be used in masonry walls or wet areas.
- Unfinished Basements and Garages: If the walls are exposed, NM-B is vulnerable to physical damage. Here, you meet MC (Metal-Clad) cable or THHN in EMT conduit. If you run NM-B on the face of studs in a garage, local inspectors will likely fail it for lack of physical protection.
- Outdoor and Underground Runs: NM-B is strictly prohibited outdoors, even if you run it inside a conduit (conduit outdoors is considered a 'wet location' due to condensation). Here, you must transition to UF-B for direct burial, or pull THWN-2 (note the 'W' for wet-rated) through watertight PVC or rigid conduit.
Decision Tree: Which Wiring Type to Pull Today
Use this decision matrix to lock in your materials list before heading to the electrical supply house.
| If your installation environment is... | And the physical protection is... | Then pull this exact wiring type: |
|---|---|---|
| Indoor, concealed behind drywall | Standard wood framing | NM-B (e.g., Southwire SIMpull 12/2) |
| Indoor, surface-mounted / exposed | Unfinished basement or garage walls | MC Cable or THHN in EMT |
| Outdoors, buried in a trench | Direct earth contact (no conduit) | UF-B (Buried min. 24' deep) |
| Outdoors, buried in a trench | Inside Schedule 40/80 PVC conduit | THWN-2 (Buried min. 18' deep) |
| High-vibration or moving machinery | Flexible connection required | SOOW / SJOOW (Flexible cord, not permanent wiring) |
Common Mistakes and Code Traps
Can I strip the jacket off NM-B and pull the inner wires through conduit?
No. This is a massive code violation and a frequent DIY mistake. The individual wires inside NM-B are not marked with their individual insulation ratings (like THHN) on their surface, and they lack the necessary nylon outer coating to survive the friction of being pulled through a raceway. If an inspector sees bare wires pulled from a Romex jacket inside a conduit, they will fail the inspection and make you rip it out. Buy proper spooled THHN/THWN-2 for conduit runs.
Does UF-B cable need to be protected where it comes out of the ground?
Yes. While UF-B is rated for direct burial, NEC 300.5 requires that the cable be protected by conduit or a physical guard from the point it leaves the earth up to at least 8 feet above grade. A common failure point is running UF-B up the side of a shed without sliding it inside a piece of PVC or metal conduit to protect it from weed whackers, lawnmowers, and UV degradation.
Can I use THHN wire for my entire house instead of NM-B?
Technically yes, but practically no. You would have to install a continuous raceway (conduit) system from your breaker panel to every single outlet and switch box in the house. While this is standard practice in commercial construction and in certain municipalities (like Chicago, which mandates conduit for almost all wiring), the material and labor costs for a residential retrofit are astronomical compared to stapling NM-B to studs. Stick to NM-B for concealed residential walls unless your local AHJ (Authority Having Jurisdiction) mandates otherwise.
Always verify your local amendments to the National Electrical Code before purchasing materials, as some municipalities have strict bans on specific cable types or require AFCI/GFCI protections that dictate your wiring topology.
