Modern home wiring primarily consists of nonmetallic sheathed cable (NM-B) for interior branch circuits and individual THHN/THWN-2 conductors pulled through conduit for exposed, wet, or high-capacity feeder runs. If you are asking what type of wiring is used in homes today, the direct answer is that copper NM-B (commonly known by the brand name Romex) handles 90% of interior 15A and 20A receptacle and lighting circuits, while aluminum or copper SER (Service Entrance Rated) cable handles the heavy 100A to 200A main panel feeders.
Understanding the exact cable type dictates not just how you route the wire, but the maximum breaker size you can legally install. The physical jacket and the chemical makeup of the inner insulation completely change the thermal limits and environmental tolerances of the circuit.
The Core Cable Types: NM-B, THHN, and Feeders
To select the right wire, you must match the insulation type to the environment. Below is the definitive spec sheet for the four cable families you will encounter in any modern residential rough-in or remodel.
| Cable Type | Insulation / Jacket | Max Temp Rating | Standard Ampacity (12 AWG / 2 AWG) | Primary Residential Application |
|---|---|---|---|---|
| NM-B (Nonmetallic Sheathed) | PVC jacket, THHN inner conductors, bare copper ground | 90°C (conductors) / 60°C (ampacity limit) | 20A (12 AWG) / N/A | Interior dry-location branch circuits (outlets, lights, switches) |
| THHN/THWN-2 (Individual Conductors) | PVC with nylon jacket, dual-rated for heat and water | 90°C (dry) / 75°C (wet) | 30A (12 AWG) / 115A (2 AWG Cu) | Conduit runs, exposed basement ceilings, panel pigtails |
| UF-B (Underground Feeder) | Solid PVC jacket encasing conductors, moisture resistant | 90°C (conductors) / 60°C (ampacity limit) | 20A (12 AWG) / N/A | Direct burial for exterior lighting, detached garage subpanels |
| SER (Service Entrance Round) | Gray PVC jacket, bare aluminum or copper stranded conductors | 75°C (standard terminations) | N/A / 100A-125A (2 AWG Al) | Main service entrance, heavy appliance feeders (ranges, EV chargers) |
According to the National Electrical Code (NEC), the temperature rating of the wire is only half the equation; the temperature rating of the terminations (the breaker lugs and receptacle screws) is what ultimately caps your circuit capacity.
What Insulation Ratings Change in a Real Circuit
The most critical concept in residential wiring is that a wire's printed temperature rating does not automatically grant you its maximum ampacity. What the insulation type changes in a real installation is your derating headroom and your routing permissions.
Worked Numeric Example: The 12 AWG THHN Illusion
Imagine you are wiring a 20A kitchen small-appliance branch circuit. You decide to pull individual 12 AWG THHN wires through a metal EMT conduit because you prefer the ease of pulling individual conductors over wrestling with stiff NM-B cable.
- The Wire: 12 AWG THHN has a 90°C insulation rating. Looking at the 90°C column in the ampacity table, 12 AWG is rated for 30 amps.
- The Mistake: A novice might assume they can install a 30A breaker because the wire can handle 30A of heat.
- The Reality: The breaker's termination lug and the kitchen receptacle screws are rated for 60°C (or 75°C at best). Looking at the 60°C column, 12 AWG is capped at 20 amps.
- The Result: You must install a 20A breaker. The 90°C rating of the THHN is essentially 'wasted' on a simple branch circuit.
Where does the 90°C rating actually matter? It matters for derating. If you pull five current-carrying 12 AWG THHN conductors through a single conduit to feed a multi-wire branch circuit or a smart home hub, NEC Chapter 9 requires you to apply an 80% derating factor.
Calculation: 30A (90°C base) × 0.80 (derating) = 24A. Because 24A is still higher than the 20A termination limit, your 12 AWG wire remains perfectly legal on a 20A breaker. If you had used a wire with a lower thermal rating, the derated value might have dropped below 20A, forcing you to upsize to 10 AWG.
Where You Meet This in Practice
On the jobsite or in your own attic, the transition between these wiring types is dictated by physical exposure and moisture.
1. Interior Walls and Ceilings (NM-B)
For 95% of your interior rough-in, you will use 14-2 or 12-2 NM-B. The yellow (12 AWG) and white (14 AWG) PVC jackets provide enough physical protection for wires hidden behind drywall. The Southwire NM-B datasheets confirm that the jacket includes anti-fungal and moisture-resistant properties, but it is strictly rated for 'dry, indoor locations'.
2. Unfinished Basements and Exposed Joists (THHN in Conduit)
If a wire runs along the face of a floor joist in an unfinished basement where it could be physically damaged, NM-B is a code violation. Here, you transition to individual THHN/THWN-2 conductors pulled inside rigid metal or EMT conduit. The conduit provides the physical armor that the soft NM-B jacket lacks.
3. Underground Trenches (UF-B vs. THWN-2)
Running power to a detached shed or a backyard pergola requires underground wiring. You have two choices:
- UF-B Direct Burial: Trenched at 24 inches deep. It is stiff, difficult to strip, and expensive, but requires no conduit.
- THWN-2 in PVC Schedule 80: Trenched at 18 inches deep. Individual conductors are much easier to pull through swept PVC bends, making this the preferred method for runs over 50 feet.
4. Heavy Feeders and Subpanels (SER Aluminum)
While branch circuits are almost exclusively copper, modern homes use aluminum SER cable for the 200A service entrance from the utility meter to the main panel, and for large subpanel feeders. According to the Copper Development Association, while copper has higher conductivity per volume, aluminum's massive cost-to-weight advantage makes it the undisputed standard for heavy feeders (typically 4/0 AWG or 2/0 AWG aluminum for 200A services).
Common Confusions and Code Violations
When DIYers and junior apprentices ask what type of wiring is used in homes, they often carry a few persistent misconceptions that lead to failed inspections or fire hazards.
- 'Romex' vs. NM-B: Romex is a trademarked brand name owned by Southwire. NM-B is the actual NEC cable designation. You will also see brands like Cerro or Southwire's own generic labels. They are identical in specification.
- UF-B in the Attic: UF-B is rated for wet locations, but its gray jacket is softer and more susceptible to physical damage than NM-B. If you use UF-B to feed an exterior camera from an attic, it must be protected by conduit sleeves if it runs within 6 feet of the attic access scuttle hole where someone could step on it.
- Mixing Copper and Aluminum: You cannot directly splice copper branch circuit wires to aluminum feeder wires using standard wire nuts. This creates galvanic corrosion, leading to high-resistance arcing faults. You must use listed mechanical lug connectors (like Polaris connectors) or apply anti-oxidant paste (Noalox) in rated compression lugs.
- The Missing Neutral: Modern smart switches and WiFi-enabled relays require a neutral wire to power their internal radios. Older homes wired with 14-2 or 12-2 cable to a switch loop only have a hot and a switched-hot. When upgrading these homes, electricians often have to pull new 14-3 or 12-3 NM-B cable to provide the required neutral.
Frequently Asked Questions
Can I use aluminum wire for standard 15A and 20A wall outlets?
No. While aluminum is perfectly safe and code-compliant for heavy feeders (like a 100A subpanel run using 2 AWG SER), it is virtually never used for 12 AWG or 14 AWG branch circuits in modern construction. The smaller gauge aluminum wires are prone to creep and oxidation at the small screw terminals of standard receptacles, which historically caused fires. Stick to copper NM-B for all branch circuits.
What is the difference between NM-B and NMD90?
NM-B is the standard designation used in the United States under the NEC, rated for 90°C conductors but limited to 60°C ampacity. NMD90 is the Canadian equivalent (CEC standard), which features a slightly different jacket formulation designed to withstand colder temperatures during winter installation without the PVC cracking. They are not strictly interchangeable across borders without inspector approval.
Do I need to replace my old cloth-sheathed wiring?
If your home was built before 1950, you may have knob-and-tube or early rubber/cloth-sheathed wiring. While not inherently dangerous if perfectly preserved and unmodified, the insulation becomes brittle, and it lacks a modern equipment grounding conductor (EGC). Insurance companies frequently require a full rewire with modern NM-B or THHN in conduit to issue a policy on these older properties.
