Types of electrical wiring for homes refers to the specific cable assemblies and insulation materials—primarily NM-B, UF-B, and individual THHN/THWN conductors—used to distribute power from the main panel to branch circuits based on environmental and load requirements. For standard 15A and 20A interior branch circuits, 14/2 and 12/2 NM-B (commonly called Romex) is the default choice, while underground or wet locations demand UF-B or THWN-2 pulled through conduit. What you choose dictates your physical routing options, maximum safe current (ampacity), and whether the installation survives moisture or physical damage. Homeowners and junior DIYers frequently confuse the cable jacket type (like NM-B) with the wire gauge (like 12 AWG) or the conductor metal (copper vs. aluminum), but these are distinct properties that interact to define the circuit's limits.
The Core Cable Types: NM-B, UF-B, and THHN/THWN
To wire a house safely, you need to understand the big three cable and wire categories recognized by the National Electrical Code (NEC). Each has a specific temperature rating and environmental boundary.
- NM-B (Non-Metallic Sheathed, Type B): The standard yellow, white, or black 'Romex' cable. It contains two or more insulated conductors and a bare copper ground wrapped in a PVC jacket with a paper filler. It is strictly rated for dry, indoor locations. While the internal THHN wires are rated for 90°C, NEC 334.80 restricts the overall ampacity to the 60°C column for sizing purposes.
- UF-B (Underground Feeder, Type B): Looks like NM-B but features a solid, moisture-resistant grey PVC jacket that encapsulates the conductors directly (no paper filler). It is rated for wet locations and direct burial. Like NM-B, its ampacity is limited to the 60°C column.
- THHN/THWN-2 (Thermoplastic High Heat-resistant Nylon-coated): Individual, single-conductor wires. THHN is rated for 90°C in dry locations, while the dual-rated THWN-2 is rated for 75°C in wet locations. These are pulled through raceways (like EMT or PVC conduit) and offer the highest ampacity and best physical protection.
Where You Meet This in Practice
The type of wiring you select changes how and where you can physically route the circuit through a building. Here is how these materials map to real jobsite scenarios:
- Drywall Cavities and Stud Bays: NM-B is the undisputed king here. It is flat, flexible, and easy to staple to the center of a 2x4 stud.
- Trench to a Detached Garage: You will use either UF-B direct burial (minimum 24 inches deep) or individual THWN-2 wires pulled through Schedule 40 PVC conduit (minimum 18 inches deep).
- Exposed Basement Joists: If NM-B is run perpendicular to joists, it must be protected by running board or conduit sleeves. For exposed vertical runs down a wall where physical damage is likely, electricians switch to MC (Metal-Clad) cable or EMT conduit with THHN.
Material costs also shift dramatically based on type. 12/2 NM-B costs roughly $0.65 per foot, while outfitting the same circuit with 3/4-inch EMT conduit and three strands of 12 AWG THHN pushes material costs to about $2.10 per foot, though the conduit offers vastly superior physical protection and future upgradeability.
Worked Numeric Example: Ampacity and Derating
Choosing the wrong wire type can severely limit your circuit's capacity when multiple wires are grouped together. Let us look at a derating scenario, which is where wire type drastically changes the outcome of a real installation.
The Setup: You are pulling wires through a single 3/4-inch PVC conduit to feed two separate 3-way switch loops and a shared neutral. This results in 5 current-carrying conductors in one raceway. According to NEC Table 310.15(C)(1), 4 to 6 conductors require an 80% derating factor.
Scenario A: Using 12 AWG THHN
- Base ampacity (90°C column): 30A
- Derated ampacity: 30A × 0.80 = 24A
- Result: You can safely protect this circuit with a standard 20A breaker.
Scenario B: Using 12 AWG NM-B (Hypothetical/Code Violation)
Suppose you tried to stuff a 12/3 NM-B cable into that same conduit (which violates code, but illustrates the thermal physics). NM-B ampacity is locked to the 60°C column.
- Base ampacity (60°C column): 20A
- Derated ampacity: 20A × 0.80 = 16A
- Result: You can no longer use a 20A breaker. You would have to drop down to a 15A breaker, severely limiting the circuit's utility.
This numeric example proves that the insulation type (THHN vs NM-B) directly dictates your thermal headroom and breaker sizing when wires are bundled.
Real-World Scenario: The Buried Conduit Mistake
Theory is fine, but insulation failure in the field is expensive. Here is a walkthrough of a common, costly error involving wire types.
Setup: A homeowner runs a 120V, 20A circuit to a backyard shed 60 feet away. They dig a trench and lay grey Schedule 40 PVC conduit. To save time and avoid fishing individual wires, they pull standard yellow 12/2 NM-B (Romex) through the underground conduit.
Numbers: The trench sits in a low area with a high seasonal water table. The conduit is not perfectly sealed at the joints. Over the winter, the conduit fills entirely with groundwater. The NM-B jacket is rated for 90°C dry, but the internal paper filler and standard PVC jacket are not rated for continuous submersion.
Outcome: Fourteen months later, the 20A breaker starts tripping randomly, especially after heavy rain. A megohmmeter (megger) test reveals < 1 megohm of insulation resistance between the black conductor and the bare ground wire.
What Went Wrong: Underground conduit is classified as a wet location by the NEC (Article 314.15 and 334.12). Moisture wicked through the NM-B sheath, degrading the paper wrap and causing the insulation to break down, resulting in a ground fault. The homeowner had to dig up the landscaping, pull out the ruined NM-B, and fish three individual strands of 12 AWG THWN-2. The fix cost roughly $850 in labor and landscape repair, entirely wiping out the $20 they 'saved' by not buying THWN-2 initially.
Common Confusions: Cable Type vs. Wire Gauge vs. Material
When shopping at the electrical supply house, mixing up these three concepts will result in buying the wrong product. Here is how to separate them, as detailed in standard wiring classification guides.
| Property | What It Defines | Examples | Why It Matters |
|---|---|---|---|
| Cable Type | The jacket, assembly, and environmental rating. | NM-B, UF-B, THHN, MC | Determines if you can bury it, expose it, or run it in wet locations. |
| Wire Gauge | The physical cross-sectional area of the conductor. | 14 AWG, 12 AWG, 10 AWG, 4 AWG | Determines the baseline ampacity (current capacity) and voltage drop over distance. |
| Material | The metal used to carry the current. | Copper, Aluminum (AA-8000 series) | Aluminum requires larger gauges for the same ampacity and specific anti-oxidant paste at terminations. |
A common mistake is asking for '12-gauge Romex' when you actually need '12-gauge UF-B' for an outdoor run. The gauge (12 AWG) dictates the current, but the type (UF-B) dictates whether it will survive the rain.
FAQ: Home Wiring Selection
Can I use UF-B cable inside my house instead of NM-B?
Technically, yes. UF-B is listed for both wet and dry locations. However, it is a terrible idea for interior rough-in. UF-B is incredibly stiff, the solid jacket makes stripping it tedious and risky (you are more likely to nick the conductor), and it costs roughly 40% more than NM-B. Stick to NM-B for interior stud bays.
Is THHN the exact same thing as THWN?
Historically, they were different (THHN was dry-only 90°C; THWN was wet-location 75°C). Today, almost all modern single-conductor wire sold at big-box and supply stores is dual-rated as THHN/THWN-2. Always check the printing on the wire jacket; if it says THHN/THWN-2, you are cleared for both dry conduits and wet, underground conduits.
Do I need to strip the NM-B jacket when it enters the breaker panel?
Yes, but only up to the point where the cable enters the panel's knockout. NEC 334.40 requires the outer jacket to be removed inside the panel to prevent crowding and heat buildup, but you must leave at least 1/4 inch of the jacket extending past the cable clamp to protect the individual wire insulation from the sharp metal edges of the clamp.
