Electrical wiring types for a house refer to the specific combinations of conductor insulation, outer jackets, and shielding designed to safely route current through distinct physical environments like drywalls, underground trenches, or exposed joists. What you choose dictates your physical routing method, environmental limits, and final allowable ampacity. The most common mistake DIYers make is confusing a cable assembly (like a roll of NM-B) with the individual conductors inside it. They strip a 12 AWG NM-B jacket, see 'THHN' printed on the inner wires, and assume they can pull those wires through a conduit and use the 90°C ampacity column. You cannot; the assembly's outer rating limits the entire run.
The Core Cable Types Every DIYer Must Know
To make the right pick, you need to know the baseline specifications of the five cables that make up 99% of residential work.
- NM-B (Non-Metallic Sheathed Cable): Often called by the brand name Romex. It contains hot, neutral, and bare ground wires wrapped in a PVC jacket. It is strictly for dry, indoor locations. While the inner wires are rated for 90°C, NEC Article 334 mandates that NM-B ampacity must be calculated using the 60°C column.
- THHN / THWN-2: Individual, heavily insulated single conductors. They have no outer jacket and must be pulled inside a raceway (like EMT metal conduit or PVC). Because they are in a raceway, you can use the 75°C or 90°C ampacity columns (depending on termination ratings).
- UF-B (Underground Feeder): Looks like NM-B but has a solid, moisture-resistant gray PVC jacket that embeds the wires. Rated for direct burial underground without conduit, though it still requires conduit where it emerges above grade to protect it from physical damage (like weed whackers).
- MC (Metal-Clad Cable): Individual THHN wires wrapped in an interlocked aluminum or steel armor jacket. Used in commercial builds or residential areas where cables are exposed and vulnerable to physical impact, like unfinished basement ceilings.
- SER (Service Entrance Round): A heavy-duty, multi-conductor cable used primarily for feeding subpanels or large appliances (ranges, HVAC). It features a bare, concentric neutral wire wrapped around the insulated hots.
Where You Meet These Wiring Types in Practice
Mapping the right cable to the right physical zone prevents failed inspections and premature insulation breakdown.
- Interior Drywall Cavities: NM-B. Staple to the side of studs, at least 1.25 inches back from the face to avoid drywall screws.
- Exposed Garage/Shop Walls: EMT conduit with THHN, or MC cable. NM-B is prohibited where it is 'subject to physical damage' (NEC 334.15).
- Underground Trench to a Shed: UF-B direct buried at 24 inches, or THWN-2 pulled inside Schedule 40 PVC buried at 18 inches.
- Main Panel to Indoor Subpanel: SER cable routed through bored joist holes.
Worked Example: The 60A Subpanel Feeder Trap
Let's look at a real-world scenario where misunderstanding wiring types costs you money and labor. You are running a 60A subpanel to a detached workshop 100 feet away. You need to supply 240V. Which wire do you buy?
Option A: Copper NM-B (The DIY Default)
Because NM-B is locked to the 60°C ampacity column, 6 AWG copper is only rated for 55A. It will fail a 60A inspection. You must upsize to 4 AWG copper NM-B (rated 70A at 60°C).
Material Cost: 100 ft of 4/3 NM-B at ~$4.80/ft = $480.
Labor Reality: 4 AWG NM-B is incredibly stiff. Pulling it through bored studs and making tight bends in a panel is a knuckle-busting nightmare.
Option B: Aluminum XHHW-2 in PVC Conduit (The Pro Move)
Because individual wires in conduit use the 75°C column, 4 AWG Aluminum is rated for 65A—perfectly fine for a 60A breaker. You pull four wires (two hots, one neutral, one ground) through 1-inch Schedule 40 PVC buried in the trench.
Material Cost: 400 ft of 4 AWG Aluminum XHHW-2 at ~$0.90/ft ($360) + 100 ft of 1" PVC and fittings ($110) = $470.
Labor Reality: Pulling individual wires through glued PVC is vastly easier than wrestling a rigid 4 AWG cable through framing. You also gain the 90°C rating for derating if you have multiple circuits in the same trench.
The Verdict: For heavy gauge feeders over 50 feet, abandon NM-B. Switch to individual aluminum conductors in conduit. You save money, save your wrists, and adhere to industry best practices for aluminum terminations (just remember to use an anti-oxidant compound like Noalox on the lugs).
The Decision Tree: Which Cable to Pull?
Use this matrix to terminate your planning phase and pick a specific part number for your next hardware store run.
| Installation Scenario (IF...) | Required Cable Type (THEN...) | Concrete Pick / Specification |
|---|---|---|
| Standard 15A/20A receptacle circuit inside finished interior walls. | NM-B (Romex) | Southwire SIMpull 12/2 or 14/2 NM-B (Always prefer 12 AWG to minimize voltage drop and allow future 20A upgrades). |
| Wiring a 3-way switch loop in an existing finished wall (retrofit). | THHN in flexible metal conduit (FMC) or ENT (Smurf tube). | 14 AWG THHN (Black, Red, White, Green) pulled through 1/2" ENT. |
| Powering an outdoor post light or gate motor 40 feet from the house. | UF-B Direct Burial | 12/2 UF-B, buried 24 inches deep, transitioning to 1/2" PVC conduit 8 feet above grade. |
| Running a 240V, 50A EV charger circuit across exposed garage ceiling joists. | MC Cable or EMT + THHN | 6/2 MC Cable with interlocked aluminum armor, strapped every 6 feet. |
| Feeding a 100A subpanel in a finished basement (indoor, dry). | SER (Service Entrance Round) | 2-2-2-4 Aluminum SER cable, routed through bored joists, secured every 30 inches. |
FAQ: Clearing Up Common Wiring Confusions
Can I mix NM-B and THHN inside the same junction box?
Yes. It is extremely common to run NM-B through a wall cavity into a metal junction box, then transition to individual THHN wires to pull through conduit for an exposed run to a motor or appliance. The junction box serves as the legal transition point between the cable assembly and the raceway.
Why do I have to bury UF-B at 24 inches, but THWN-2 in PVC only needs 18 inches?
The NEC grants a shallower burial depth to wires inside rigid PVC conduit because the conduit provides a secondary layer of physical protection against shovels and digging equipment. If you want to dig a shallower trench (12 inches), you must use Rigid Metal Conduit (RMC) or IMC. If you only have 12 inches of cover and want to use PVC, you must encase the PVC in 2 inches of concrete.
Is 'Romex' the same thing as NM-B?
'Romex' is a trademarked brand name owned by Southwire, much like 'Kleenex' is for tissues. NM-B is the actual National Electrical Code classification for Non-Metallic Sheathed Cable. When you buy Southwire brand cable, you are buying Romex; if you buy Cerro or Encore, you are buying NM-B. They are functionally identical.
Can I use NM-B in a damp location like a crawlspace?
Yes, but with a major caveat. NM-B can be run through the joists of a damp crawlspace, but it cannot be buried in insulation that traps moisture against the jacket, nor can it be used where it is subject to standing water. If the crawlspace is prone to flooding or constant condensation, upgrade to UF-B or pull THWN-2 through PVC.
Stop guessing at the hardware store counter. Match the physical environment to the jacket rating, respect the 60°C vs 75°C ampacity columns, and pull the right wire the first time.
