Electrical wiring types refer to the specific combinations of conductive metal, insulation material, and outer jacketing that dictate where and how much current a cable can safely carry. The specific wiring type you choose changes the allowable ampacity, the permitted installation environment (wet versus dry, concealed versus exposed), and the physical protection required by code. The most common mistake DIYers and junior apprentices make is confusing a cable assembly (a factory-bundled group of wires with an outer jacket, like 12/2 NM-B) with an individual wire (a single insulated conductor, like green THHN pulled through EMT conduit).
The Core Categories of Residential Wiring
When you walk into an electrical supply house, you will encounter dozens of variations, but 95% of residential and light-commercial work relies on four primary categories. Below is a breakdown of the most common types of electrical wiring, their construction, and their NEC article references.
| Wiring Type | Common Name | Construction & Insulation | Temperature Rating | NEC Article |
|---|---|---|---|---|
| NM-B | Romex (Brand) | 2+ THHN/THWN wires, bare ground, PVC jacket | 90°C (Dry), restricted to 60°C ampacity | Art. 334 |
| THHN / THWN-2 | Stranded/Solid Wire | Single copper conductor, PVC insulation, nylon coat | 90°C (Dry) / 75°C (Wet) | Art. 310 |
| UF-B | Underground Feeder | Similar to NM-B, but solid PVC jacket encapsulates wires | 90°C (Dry), restricted to 60°C ampacity | Art. 339 |
| MC | Metal-Clad Cable | THHN/THWN wires wrapped in interlocking aluminum/steel armor | 90°C (Dry), typically uses 75°C ampacity | Art. 330 |
Notice the temperature ratings. While THHN wire boasts a 90°C insulation rating, the NEC requires you to use the 60°C or 75°C ampacity columns for sizing breakers because standard residential terminals (breakers, receptacles, switches) are rarely rated for 90°C heat.
How Insulation and Bundling Change Ampacity (Numeric Example)
To understand why the different types of electrical wiring behave differently in real circuits, we need to look at a worked numeric example involving NEC derating rules. Let's compare running a 240V circuit using 10 AWG THHN in a conduit versus using 10/3 NM-B cable in a standard wall cavity.
Scenario A: 10 AWG THHN in EMT Conduit
You are pulling 9 current-carrying 10 AWG THHN conductors through a single 3/4-inch EMT conduit to feed multiple circuits.
- Base Ampacity: 10 AWG at 90°C is rated for 40A.
- Derating Factor: Per NEC Table 310.15(C)(1), 7 to 9 current-carrying conductors in a raceway requires a 70% adjustment factor.
- Calculation: 40A × 0.70 = 28A.
- Final Breaker Size: You must round down to the next standard breaker size, which is 25A.
Scenario B: 10/3 NM-B in a Wall Cavity
You are running a single 10/3 NM-B cable through standard 2x4 stud bays (no bundling with other cables for more than 24 inches).
- Base Ampacity: Per NEC 334.80, NM-B ampacity must be determined using the 60°C column, regardless of the 90°C wire inside. 10 AWG at 60°C is rated for 30A.
- Derating Factor: None. Standard residential wall cavities do not trigger conduit fill derating.
- Final Breaker Size: 30A.
Where You Meet This in Practice
Choosing the right wiring type is entirely dependent on the physical environment of the installation. Here is where you will deploy each type on a standard jobsite:
- Indoor Dry & Concealed (NM-B): This is your standard interior wall and ceiling wiring. It is cheap, fast to staple, and easy to strip. It is strictly forbidden in wet locations, embedded directly in concrete, or exposed in unfinished commercial spaces where it is subject to physical damage.
- Outdoor & Underground (UF-B or THWN-2 in PVC): If you are burying a feed to a detached shed or a landscape lighting transformer, NM-B will rot and fail. You must use UF-B for direct burial (typically at 24 inches deep) or pull individual THWN-2 wires through buried Schedule 40 PVC conduit (at 18 inches deep).
- Exposed Basements & Garages (MC or EMT+THHN): When wiring surface-mounted outlets on a concrete block wall or running across exposed floor joists in a garage where vehicles might impact the run, NM-B is vulnerable. Metal-Clad (MC) cable or THHN pulled through rigid/EMT conduit provides the necessary crush resistance.
- Multi-Family & Commercial (MC/AC): Fire-rated assemblies in apartment complexes often prohibit NM-B due to the flammability of its PVC jacket. MC cable with a fire-rated jacket is the standard here.
For comprehensive installation rules and safety standards, always refer to the latest edition of the National Electrical Code (NEC) published by the NFPA, keeping in mind that your local Authority Having Jurisdiction (AHJ) may have specific amendments.
Frequently Asked Questions
Can I use standard NM-B wiring inside an outdoor conduit?
No. This is one of the most common and dangerous code violations in DIY electrical work. The interior of a conduit installed outdoors is classified by the NEC as a "wet location" due to condensation and temperature swings. NM-B is strictly rated for dry locations. If you are pulling wire through outdoor conduit, you must use individual conductors rated for wet locations, specifically marked as THWN or THWN-2.
What is the exact difference between THHN and THWN-2 wire?
THHN stands for Thermoplastic High Heat-resistant Nylon-coated, rated for 90°C in dry locations. THWN stands for Thermoplastic Heat and Water-resistant Nylon-coated, rated for 75°C in wet locations. Today, almost all single-conductor wire manufactured by major brands like Southwire and Cerrowire is dual-rated as THHN/THWN-2, meaning you can safely use it in both dry and wet environments up to its respective temperature limits. Always check the printing on the wire jacket to confirm the dual rating.
When should I use MC cable instead of NM-B?
You should upgrade to Metal-Clad (MC) cable when the wiring will be exposed to potential physical damage (like low on a garage wall), when local code prohibits NM-B in multi-family dwellings for fire safety, or when you need an armored ground path (though modern MC includes a separate green grounding wire). MC is significantly more expensive and requires specialized metal-cutting tools to strip, so it is only used where NM-B is legally or physically insufficient.
Is "Romex" a type of wire or a brand name?
Romex is a registered trademark owned by Southwire, much like Kleenex is for facial tissues. The actual technical designation for the cable is NM-B (Non-Metallic Sheathed Cable, Type B). While electricians use "Romex" as a catch-all term on the jobsite, your local building inspector and the NEC will only recognize the NM-B classification.
