Types of home wiring refer to the specific insulated conductors and cable assemblies—such as NM-B, THHN, and UF-B—approved by the National Electrical Code (NEC) for routing electrical power through residential structures. What this choice changes in a real installation is the physical routing method, the allowable ampacity based on insulation temperature ratings, and whether the wire can be buried, exposed, or pulled through conduit.
The Core Categories of Residential Cable and Wire
When you walk into an electrical supply house, you will not just ask for 'wire.' You must specify the assembly type, insulation rating, and gauge. The NEC strictly categorizes these based on their jacket construction and thermal limits. According to the National Fire Protection Association (NFPA), using the wrong cable type in a specific environment is a primary cause of residential electrical fires due to insulation degradation or physical damage.
| Wire Type | Full Name / Description | Max Insulation Temp | Primary Residential Use Case | Avg Cost (per 250ft roll) |
|---|---|---|---|---|
| NM-B | Non-Metallic Sheathed Cable (Romex) | 90°C (rated at 60°C) | Interior dry-wall branch circuits | $90 - $140 |
| THHN/THWN-2 | Thermoplastic High Heat-resistant Nylon-coated | 90°C (dry) / 75°C (wet) | Pulled through EMT conduit, exposed runs | $60 - $90 (per 250ft spool) |
| UF-B | Underground Feeder and Branch Circuit | 90°C (rated at 60°C) | Direct burial, outdoor sheds, landscape | $160 - $220 |
Where You Meet These Wiring Types in Practice
The physical environment dictates your wire choice. You cannot simply use the cheapest option everywhere; the NEC mandates specific protections based on physical exposure and moisture.
Inside Finished Walls (NM-B)
NM-B (often called by the brand name Romex) is the standard for 90% of indoor residential wiring. Its flat, flexible PVC jacket wraps around paper-insulated copper conductors. It is designed to be stapled to wooden studs and covered by drywall. Because the drywall provides physical protection, the relatively soft NM-B jacket does not need to withstand impact or UV light. You will meet this in every bedroom, living room, and interior kitchen wall.
Exposed Basement and Garage Walls (THHN in Conduit)
NEC Article 334 prohibits NM-B from being installed where it is subject to physical damage. If you are wiring outlets on an unfinished basement wall or a garage workshop where the studs are exposed, you must use individual THHN/THWN-2 conductors pulled inside rigid metal or EMT (Electrical Metallic Tubing) conduit. The steel conduit takes the impact; the THHN provides the 90°C insulation.
Outdoor and Underground Feeds (UF-B)
If you are running power to a detached shed, a well pump, or landscape lighting, NM-B will fail. Moisture will rot the paper wrapping and corrode the copper. UF-B cable embeds the conductors directly in a solid, gray, moisture-resistant plastic jacket. It is rated for direct burial without conduit (typically at a minimum depth of 24 inches, per NEC Table 300.5).
Worked Example: Sizing and Voltage Drop for a 50A EV Charger
Let us look at how wire type and sizing intersect in a real-world scenario. You are installing a hardwired Level 2 EV charger in your garage. The charger draws a continuous 40A load, requiring a 50A breaker (per the NEC 125% continuous load rule). The run from your main panel to the charger is 80 feet. You are deciding between 8 AWG and 6 AWG copper NM-B.
Step 1: Ampacity Check (The 60°C Rule)
Even though modern NM-B wire has 90°C insulation, NEC Article 334.80 and 110.14(C) dictate that for residential branch circuits under 100A, the final allowable ampacity is locked to the 60°C column of the Cerrowire ampacity tables.
- 8 AWG NM-B: Rated at 40A in the 60°C column. Because your continuous load is exactly 40A, this wire is maxed out and cannot be protected by a 50A breaker. Result: Fails code.
- 6 AWG NM-B: Rated at 55A in the 60°C column. This safely exceeds the 50A breaker requirement. Result: Passes code.
Step 2: Voltage Drop Calculation
Now we verify that 6 AWG will not suffer excessive voltage drop over the 80-foot distance. We use the standard single-phase voltage drop formula: VD = (2 x K x I x D) / CM.
- K (Copper constant) = 12.9
- I (Current) = 40A (the actual continuous draw)
- D (Distance) = 80 feet
- CM (Circular Mils for 6 AWG) = 26,240
VD = (2 x 12.9 x 40 x 80) / 26,240 = 82,560 / 26,240 = 3.14 Volts.
On a 240V circuit, a 3.14V drop is just 1.3%. The NEC recommends keeping branch circuit voltage drop under 3%. Therefore, 6 AWG NM-B is the correct, code-compliant, and electrically efficient choice for this installation.
Common Confusions and Costly Mistakes
What people commonly confuse when selecting types of home wiring usually boils down to insulation ratings and jacket materials. Here are the most frequent mistakes that fail inspections or create fire hazards.
Confusion 1: THHN vs. THWN-2
Older stock might be labeled strictly THHN (Thermoplastic High Heat-resistant Nylon-coated), which is only rated for dry locations. If you pull pure THHN into an outdoor conduit where condensation forms, the insulation will degrade. Today, almost all modern wire is dual-rated as THHN/THWN-2, making it safe for both dry and wet locations. Always check the jacket printing for the 'W' (Water-resistant) designation if there is any chance of moisture.
Confusion 2: Using NM-B Inside Conduit
DIYers often try to pull entire NM-B cables through PVC conduit to protect them in a garage. This is a massive mistake. The friction of the paper wrapping will tear the jacket, and the conduit fill capacity (NEC Chapter 9) is calculated based on individual wires, not flat cables. If you need conduit, strip the NM-B and use individual THHN wires, or just buy THHN spools from the start.
Confusion 3: Aluminum vs. Copper Sizing
For large feeder wires (like a 100A subpanel), aluminum SER cable is significantly cheaper than copper. However, people confuse the sizing. Aluminum has higher resistance and lower ampacity. To carry 100A, you need 4 AWG Copper, but you must step up to 2 AWG Aluminum. Furthermore, aluminum requires anti-oxidant paste (like Noalox) at the terminations to prevent thermal expansion and arcing over time.
Frequently Asked Questions
What types of home wiring are allowed in exposed garage walls?
NM-B (Romex) is not permitted where subject to physical damage, which includes exposed studs in a garage. You must use individual THHN/THWN-2 wires pulled through EMT or PVC conduit, or use rigid metal conduit (RMC). Alternatively, you can frame the wall with drywall to protect the NM-B, or use MC (Metal Clad) cable, which features an interlocking aluminum armor jacket that provides physical protection without needing separate conduit.
Can I mix NM-B Romex and THHN wire in the same conduit?
Technically, the NEC does not explicitly forbid mixing cable types in a single conduit, but it is highly impractical and strongly discouraged by inspectors. NM-B is a flat cable assembly; calculating conduit fill (NEC Chapter 9, Table 1) for a flat cable is complex and usually results in requiring massively oversized conduit. The standard, code-friendly practice is to transition from NM-B to individual THHN wires inside a junction box, and then pull only the THHN wires through the conduit.
Which types of home wiring are safe for direct burial underground?
UF-B (Underground Feeder) cable is the standard for direct burial without conduit, provided it is buried at the depth required by NEC Table 300.5 (typically 24 inches for residential branch circuits). Alternatively, you can bury individual THWN-2 wires inside a continuous run of Schedule 80 PVC conduit, which allows for a shallower trench depth (usually 18 inches) and makes future wire upgrades much easier without having to dig up the yard again.
