The Direct Answer: Electrical wiring types refer to the specific combinations of conductor material, insulation temperature rating, and outer jacketing designed to safely carry current in distinct physical environments. The wiring type you choose dictates exactly where the cable can be routed (wet vs. dry, exposed vs. concealed), how many conductors you can pull through a single conduit before requiring ampacity derating, and the physical protection the installation requires. The most common mistake DIYers and junior apprentices make is confusing a cable assembly (like a sheathed NM-B "Romex" cable) with the individual wire inside it (like a THHN conductor), or assuming all gray-jacketed cables are rated for direct burial.
The Core Types of Electrical Wiring in Modern Homes
When you walk into an electrical supply house, you are not just buying "wire." You are buying a specific insulation and jacketing system engineered for a specific environment. According to the NFPA 70 (National Electrical Code), the application determines the allowable wiring method. Here are the primary types of electrical wiring you will encounter in residential and light commercial work.
| Wire/Cable Type | Jacket / Insulation | Max Temp Rating | Primary Application | NEC Article |
|---|---|---|---|---|
| NM-B (Non-Metallic) | PVC outer jacket, THHN inner wires | 90°C (but ampacity limited to 60°C) | Concealed, dry indoor walls and ceilings | Article 334 |
| THHN / THWN-2 | Nylon outer coat, PVC insulation | 90°C (dry) / 75°C (wet) | Inside raceways, conduit, and panels | Article 310 |
| UF-B (Underground Feeder) | Solid gray PVC encapsulating conductors | 90°C (but ampacity limited to 60°C) | Direct burial, outdoor damp/wet locations | Article 339 |
| MC (Metal-Clad) | Interlocking aluminum or steel armor | 90°C | Exposed runs, commercial, physical protection | Article 330 |
| SE / USE (Service Entrance) | Heavy black PVC or cross-linked polyethylene | 75°C to 90°C depending on variant | Main service feeds, subpanel feeders | Article 338 |
Think of the outer jacket like a weatherproof coat: the copper inside is the body generating heat, and the jacket determines whether that body can stand in the rain (THWN-2), get buried in the mud (UF-B), or needs to stay inside the house (NM-B).
Where You Meet This in Practice: Environment Dictates the Jacket
Theory is fine, but on the jobsite, the physical environment forces your hand on which types of electrical wiring you can legally and safely install. Here is where the distinctions matter most:
- Inside Finished Walls (Dry & Concealed): This is the domain of NM-B. It is cheap, fast to pull, and requires no conduit. However, NEC 334.12 strictly forbids NM-B in wet locations, embedded in poured concrete, or exposed to physical damage.
- Attic Runs and Crawlspaces: You can use NM-B here, but if the cable is run across the face of joists where it could be stepped on or used for storage, it requires physical protection (like a running board or guard strips) per NEC 334.23.
- Underground Trenches: If you are feeding a detached garage or a landscape lighting transformer, you must use UF-B for direct burial (minimum 24 inches deep for residential 120V/240V circuits) or pull THWN-2 through buried Schedule 80 PVC conduit (minimum 18 inches deep).
- Exposed Basement Ceilings and Garages: Local AHJs (Authorities Having Jurisdiction) frequently reject NM-B stapled to the underside of floor joists in unfinished basements because it is considered "subject to physical damage." In these zones, you must transition to MC cable or pull THHN through EMT (Electrical Metallic Tubing) conduit.
Worked Numeric Example: Sizing a 60-Amp Subpanel Feeder
To understand why wire type changes your material list, let us look at a real-world sizing scenario. You need to run a 60-amp subpanel feeder from your main breaker panel to a detached workshop 60 feet away. You are deciding between pulling individual THHN wires through a PVC conduit versus trenching a single UF-B cable.
Scenario A: Using 6 AWG THHN in PVC Conduit
- Base ampacity for 6 AWG copper in the 90°C column is 75 amps.
- Because standard breakers and panel lugs are rated for 75°C, we use the 75°C column for our termination limits. 6 AWG at 75°C is rated for 65 amps.
- 65 amps is greater than our 60-amp load. Result: 6 AWG THHN passes.
Scenario B: Using 6 AWG UF-B Direct Burial Cable
- UF-B cable ampacity is strictly governed by the 60°C column per NEC 339.5, regardless of the fact that the individual conductors inside might have 90°C insulation.
- Looking at the 60°C column (Table 310.16), 6 AWG copper is only rated for 55 amps.
- 55 amps is less than our 60-amp load. Result: 6 AWG UF-B fails. You must upgrade to 4 AWG UF-B (rated 70A at 60°C) to legally carry 60 amps.
This single distinction in wire type forces you to buy significantly more expensive 4 AWG copper if you choose the direct burial cable route, whereas the conduit route allows you to use cheaper 6 AWG. For a comprehensive breakdown of voltage drop and sizing, the Cerrowire Sizing Calculator is an excellent bench reference.
Frequently Asked Questions About Types of Electrical Wiring
What is the exact difference between NM-B cable and THHN wire?
NM-B (Non-Metallic Sheathed Cable, commonly called Romex) is a cable assembly. It is a bundle containing two or more insulated THHN wires and a bare copper ground, all wrapped together in a flexible, non-metallic PVC jacket. THHN (Thermoplastic High Heat-resistant Nylon-coated) is a single individual wire. You cannot staple bare THHN wire directly to a stud; it must be pulled inside a raceway like conduit. Conversely, you cannot pull a whole NM-B cable through a long conduit run because the outer jacket creates too much friction and violates conduit fill capacity rules.
Can I run standard indoor Romex (NM-B) outside if I put it inside a PVC conduit?
No. This is one of the most frequent code violations seen in DIY electrical work. NM-B is strictly rated for dry locations only. Even if you slide it inside a waterproof PVC conduit, the interior of that conduit is considered a "wet location" by the NEC because condensation will inevitably build up inside the pipe due to temperature swings. The paper wrap inside NM-B will wick moisture, and the PVC jacket will degrade. If you are running wire through outdoor conduit, you must strip the NM-B jacket and pull individual THWN-2 wires, or use a wet-rated cable assembly like UF-B or XHHW.
Which types of electrical wiring are required for a 200-amp main service entrance?
For the service entrance conductors (the wires running from the utility meter to your main 200-amp breaker panel), you must use wire rated for wet locations and physical stress. The standard choices are SE (Service Entrance) cable or individual THHN/THWN-2 wires pulled through rigid metal or Schedule 80 PVC conduit. For a 200-amp residential service, you will typically need 4/0 AWG Aluminum or 2/0 AWG Copper conductors. NM-B and UF-B are never permitted for exterior service entrance mast runs.
Why does my 10 AWG THHN wire have a 90°C rating printed on it, but I have to use the 60°C ampacity table?
This is due to the "weakest link" rule in NEC 110.14(C). While the THHN wire insulation can physically handle 90°C without melting, the circuit breakers, receptacles, and panel lugs you are terminating the wire to are generally only tested and rated for 60°C or 75°C. If you push 90°C heat into a 60°C rated breaker lug, the breaker will trip prematurely or the lug will oxidize and fail. Therefore, you use the 90°C column only for derating purposes (like adjusting for high ambient temperatures or bundling multiple wires in a conduit), but your final adjusted ampacity cannot exceed the value listed in the 60°C or 75°C column for the termination equipment.
