House wiring types refer to the specific classifications of electrical cables and individual conductors used in residential construction, defined by their insulation material, jacketing, and approved installation environments (dry, damp, or wet). What this classification changes in a real installation is everything from your physical routing path to the thermal derating limits of the conductors and the physical protection required by code. The most common mistake DIYers and junior apprentices make is confusing the cable assembly type (like NM-B) with the individual wire insulation rating (like THHN) or the conductor gauge (like 12 AWG). Knowing the difference dictates whether your circuit passes inspection or becomes a fire hazard.
The Master Spec Sheet: Residential Cable Types Compared
Before you pull any wire, you need to know the environmental and thermal limits of your cable. The National Electrical Code (NEC) strictly governs where each type can be installed. Below is the reference table for the five most common residential wiring types.
| Cable / Wire Type | Outer Jacket / Armor | Approved Environment | Max Insulation Temp | Ampacity Column Limit | Typical Residential Use |
|---|---|---|---|---|---|
| NM-B (Romex) | PVC Non-Metallic Sheath | Dry, Indoor Only | 90°C | 60°C (NEC 334.80) | Interior branch circuits (outlets, lights) inside finished walls |
| UF-B | Solid PVC Underground Feeder | Wet, Direct Burial | 90°C | 60°C (NEC 333.5) | Underground feeds to sheds, landscape lighting, exterior post lamps |
| THHN / THWN-2 | None (Individual Conductors) | Dry (THHN) / Wet (THWN-2) | 90°C | 75°C or 90°C* | Pulled through EMT/PVC conduit for exposed walls, long runs, subpanels |
| MC (Metal Clad) | Interlocking Aluminum/Steel Armor | Dry/Damp (Wet if rated) | 90°C | 75°C or 90°C* | Exposed runs in basements/garages where physical damage is a risk |
| SE (Service Entrance) | Reinforced PVC / Bare Neutral | Dry (SE) / Wet (USE-2) | 75°C / 90°C | 75°C | Main panel feeders, electric range/dryer branch circuits |
Where You Meet These Wiring Types in Practice
Understanding the table above is only half the battle. You need to know which cable to grab when facing specific jobsite scenarios.
Interior Finished Walls (NM-B)
If you are wiring a new bedroom, adding a receptacle to a living room, or running a switch leg inside standard drywall, NM-B (commonly referred to by the brand name Romex) is your default. It is cheap, easy to strip, and staples neatly to studs. However, it cannot be left exposed in unfinished basements or garages where it is subject to physical damage, and it absolutely cannot be run outdoors or inside conduit that is exposed to the elements.
Exposed Garage and Basement Walls (MC or THHN in Conduit)
When wiring a workshop or finishing a garage where the studs are exposed, NM-B is a code violation if run below 8 feet or where subject to damage. Here, you transition to either MC (Metal Clad) cable or individual THHN/THWN-2 wires pulled through EMT (Electrical Metallic Tubing) or PVC conduit. MC cable offers built-in physical protection via its aluminum armor, while THHN in EMT allows you to easily pull additional wires later if you upgrade your tools or add a 240V welder outlet.
Underground and Outdoor Feeds (UF-B or THHN in PVC)
Running power to a detached shed or a backyard pergola requires wet-rated wiring. UF-B can be direct-buried in a trench (typically 24 inches deep for residential branch circuits, per NEC Table 300.5). However, for longer runs or larger subpanel feeds, electricians prefer pulling individual THHN/THWN-2 wires through buried Schedule 80 PVC conduit. It is vastly easier to pull three individual 4 AWG wires through a 1-inch pipe than to drag a stiff, heavy 4/3 UF-B cable through the dirt.
Worked Example: Sizing a 48A EV Charger Circuit
To see how house wiring types directly impact your material costs and installation labor, let us look at a real-world scenario: installing a hardwired Level 2 Electric Vehicle (EV) charger.
The Scenario: You are installing a 48A continuous-load EV charger in a garage. The run is 65 feet from the main panel. The first 40 feet will go through the finished drywall of the basement ceiling, and the remaining 25 feet will be surface-mounted in EMT conduit up the garage wall to the charger.
The Math: Because an EV charger is a continuous load (running for 3 hours or more), NEC 210.20(A) requires the circuit to be sized at 125% of the load.
48A × 1.25 = 60A.
You need a 60A breaker, and your wire must have an allowable ampacity of at least 60A.
Option A: Running NM-B for the entire run
If you attempt to use NM-B cable for the whole run (transitioning to a junction box in the garage), you are bound by NEC 334.80, which limits NM-B ampacity to the 60°C column, regardless of the 90°C insulation printed on the jacket.
• 6 AWG NM-B at 60°C = 55A. (Fails: 55A < 60A)
• 4 AWG NM-B at 60°C = 70A. (Passes)
Result: You must buy expensive, incredibly stiff 4 AWG NM-B cable, which is difficult to bend into standard junction boxes and hard to terminate on a 60A breaker.
Option B: Running THHN/THWN-2 in conduit for the entire run
If you run EMT conduit from the panel all the way to the charger and pull individual THHN wires, you can use the 75°C column (assuming your breaker and charger terminals are rated for 75°C).
• 6 AWG THHN at 75°C = 65A. (Passes: 65A > 60A)
Result: You can use 6 AWG wire. It is significantly cheaper per foot than 4 AWG, and pulling three individual 6 AWG wires through 3/4-inch EMT is vastly easier than wrestling with a 4/2 NM-B cable.
Common Confusions and Code Traps
Even experienced DIYers stumble over a few specific nuances regarding residential wiring types. Keep these rules in mind to avoid failing an inspection.
"Conduit is waterproof, so I can run Romex inside it outdoors."
This is one of the most frequent code violations. The NEC considers the interior of any conduit exposed to the outdoors to be a wet location because condensation will inevitably form inside the pipe. NM-B is strictly rated for dry locations. If you are running wire through outdoor conduit, you must strip the NM-B jacket at the transition box and pull individual THWN-2 (note the 'W' for wet) conductors through the pipe.
Confusing "Romex" with a Universal Standard
"Romex" is a trademarked brand name owned by Southwire, much like "Kleenex" is for tissues. When you ask for Romex at the hardware store, you are asking for NM-B (Non-Metallic Sheathed Cable, Type B). Older homes might contain NM or NM-A, which had lower temperature ratings (60°C max insulation) and different jacket materials. Always verify the printing on the jacket says "NM-B" to ensure you are working with modern, 90°C-rated insulation.
Using the 90°C Column for NM-B Breaker Terminations
Because the jacket of NM-B is printed with "90°C", many builders assume they can use the 90°C ampacity column to downsize their wire. You cannot. NEC 334.80 explicitly states that while you can use the 90°C rating for derating (e.g., if you bundle four NM-B cables together through a single framing hole), your final, derated ampacity cannot exceed the value listed in the 60°C column. A 12 AWG NM-B cable will never be legally permitted on a breaker larger than 20A, no matter how many mathematical derating loopholes you try to apply.
