Residential wiring encompasses the specific categories of insulated conductors and cable assemblies—primarily NM-B, UF-B, and individual THHN/THWN wires—used to distribute electrical power from the main service panel to branch circuits and appliances within a dwelling. The specific wire type you choose dictates your installation method (stapling to wooden studs versus pulling through metallic or PVC conduit), environmental limits (dry indoor air versus wet underground soil), and the physical protection the conductors require to meet National Electrical Code (NEC) safety standards. The most common point of confusion for DIYers is mixing up a cable assembly (like NM-B, which contains multiple current-carrying wires and a bare ground wrapped in a single outer jacket) with individual conductors (like THHN), leading to dangerous mistakes like attempting to run indoor-rated NM-B inside buried outdoor conduit.
The Core Types of Residential Wiring (And What They Actually Change)
When you walk into an electrical supply house, you aren't just buying 'wire.' You are buying a specific insulation system engineered for a specific environment. Here is how the primary residential wiring types break down in practice.
| Wire/Cable Type | NEC Article | Jacket / Insulation | Approved Locations | Common Use Case |
|---|---|---|---|---|
| NM-B (Romex) | Art. 334 | PVC outer jacket, paper wrap, THHN inner wires | Strictly dry, indoor locations | Standard 15A/20A wall outlets and lighting circuits |
| UF-B | Art. 339 | Solid gray PVC jacket embedding all conductors | Dry, wet, and direct underground burial | Feeding detached garages, landscape lighting, well pumps |
| THHN / THWN-2 | Art. 310 | PVC insulation with a nylon outer skin (individual wires) | Dry (THHN) and Wet (THWN-2) inside conduit | Subpanel feeders, long runs, commercial-style residential pulls |
| MC / AC Armor | Art. 330 / 320 | Interlocking metal armor with THHN inner wires | Dry locations, exposed runs | Basement ceiling runs, retrofits where physical damage is a risk |
Where You Meet This in Practice: The Dry, Wet, and Buried Divide
The most critical decision point when selecting residential wiring is determining the moisture level of your installation path. The NEC draws a hard line between dry, damp, and wet locations, and using the wrong cable type is an immediate fail on any electrical inspection.
Dry Locations: This is the inside of your finished walls, ceilings, and floors where moisture is controlled. NM-B is the undisputed king here because it is cheap, easy to strip, and staples directly to wooden framing.
Wet Locations: Anywhere subject to saturation with water or other liquids. This includes direct burial in soil, inside masonry block walls in contact with the earth, and—crucially—inside underground conduit. Water inevitably condenses inside buried PVC pipes. Therefore, underground conduit is legally classified as a wet location. You must use UF-B (direct burial) or individual THWN-2 conductors pulled through the conduit.
A Real-World Scenario: The 120-Foot Garage Circuit Mistake
To understand how wire type and circuit physics interact, let's look at a common real-world failure involving a detached garage build.
The Setup: A homeowner needed to run a 120V, 20A circuit from the main house panel to a detached garage 120 feet away to power workbench tools and lighting. They planned to bury a 1.5-inch Schedule 40 PVC conduit from the house to the garage and pull wire through it.
The Numbers: They initially selected 12 AWG NM-B cable. Let's calculate the voltage drop. According to standard wire resistance tables, 12 AWG copper has a resistance of roughly 1.93 ohms per 1,000 feet. For a 120-foot run, the total loop length (hot and neutral) is 240 feet.
Resistance = (1.93 ohms / 1000 ft) * 240 ft = 0.463 ohms.
Voltage Drop = Current * Resistance = 20A * 0.463 ohms = 9.26 Volts.
Percentage Drop = (9.26V / 120V) * 100 = 7.7%.
The Outcome: A 7.7% voltage drop far exceeds the NEC's recommended maximum of 3% for branch circuits. At this drop, table saw motors would overheat, and LED drivers would flicker. The homeowner correctly realized they needed to step up to 10 AWG wire to bring the drop down to an acceptable 2.9%.
What Went Wrong: The homeowner bought 10 AWG NM-B and pushed it through the buried PVC conduit. During the rough-in inspection, the local Authority Having Jurisdiction (AHJ) red-tagged the job. NEC Article 334.12(B)(4) explicitly prohibits the use of NM-B in wet or damp locations. Because underground conduit is classified as a wet location due to condensation, the NM-B jacket could eventually degrade and short out. The homeowner had to pull 240 feet of NM-B out of the mud, throw it away, and purchase individual 10 AWG THWN-2 conductors to pull through the conduit instead.
Worked Numeric Example: Sizing a 60A Subpanel Feeder
Let's apply the temperature column rules to a 60A subpanel feeder to see why wire type changes your required gauge.
You are installing a 60A subpanel in a finished basement. You need to run a 240V feeder from the main panel. You have two choices: run 6 AWG NM-B cable through the studs, or run 6 AWG THHN/THWN-2 individual conductors inside a 1-inch EMT metal conduit.
Here is how the math breaks down based on manufacturer ampacity charts:
- Option A: 6 AWG NM-B Cable. Per NEC 334.80, the ampacity of NM-B cable is strictly limited to the 60°C column of Table 310.16, regardless of the inner wire insulation. Looking at the 60°C column for 6 AWG copper, the maximum ampacity is 55A. Because 55A is less than your 60A breaker, 6 AWG NM-B is illegal for this installation. You would be forced to buy much more expensive and harder-to-staple 4 AWG NM-B (rated 70A at 60°C).
- Option B: 6 AWG THHN in Conduit. Individual conductors in conduit are not bound by the NM-B 60°C restriction. While the wire insulation is 90°C, your breaker terminals are rated for 75°C. Looking at the 75°C column for 6 AWG copper, the maximum ampacity is 65A. Because 65A is greater than your 60A breaker, 6 AWG THHN is perfectly legal and safe.
Frequently Asked Questions About Home Wire Types
Can I splice NM-B and THHN inside a junction box?
Yes. It is very common to transition from NM-B cable to THHN conductors inside an accessible junction box. For example, you might run NM-B through your attic, terminate it in a metal junction box, and then transition to THHN to pull down through a metal conduit to a garage receptacle. Just ensure the box is large enough to meet NEC fill capacity rules and that you use proper wire nuts or push-in connectors rated for the wire gauges.
Why does UF-B cable feel so much harder to strip than NM-B?
NM-B features a loose PVC outer jacket with a paper slip-sheet and individual insulated wires inside, making it easy to slit with a cable ripper. UF-B (Underground Feeder) has a solid PVC jacket that is extruded tightly around and between the conductors to prevent moisture ingress. You cannot use a standard cable ripper on UF-B; you must carefully score the jacket with a utility knife and peel it back, taking extreme care not to nick the inner conductor insulation.
Is it code-compliant to paint or plaster over exposed NM-B cable in a basement?
No. NM-B is not designed to be embedded in plaster, concrete, or cementitious materials, as the alkaline nature of these materials can degrade the PVC jacket over time. Furthermore, if the cable is run exposed along the surface of basement walls, it must be protected from physical damage. If it is run along the face of studs in an unfinished basement, it is highly vulnerable and should be run through bored holes in the studs or protected by conduit.
