Types of electrical wiring refer to the specific combinations of conductor material, insulation type, and outer jacket configuration used to safely route current from a breaker to a load. What you choose dictates the maximum safe current (ampacity), the environments where the cable can be installed (wet, dry, or buried), and the physical routing methods permitted by the National Electrical Code (NEC). Most DIYers confuse the outer jacket (like a yellow NM-B sheath) with the actual conductor insulation (like the PVC inside), or use the terms "wire" (a single conductor) and "cable" (multiple conductors bundled together) interchangeably. Getting this wrong means failing inspection, tripping breakers constantly, or starting a hidden electrical fire inside your walls.
The Core Differences: Cable Jackets vs. Conductor Insulation
To make the right choice, you have to separate the outer protective layer from the inner conductive layer. The four primary wiring configurations you will encounter in residential and light commercial work are NM-B, THHN/THWN-2, UF-B, and MC cable.
- NM-B (Non-Metallic Sheathed Cable): Commonly known by the brand name Romex. It consists of bare copper conductors wrapped in paper, enclosed in a flexible PVC jacket. The jacket color indicates the gauge: white for 14 AWG (15A), yellow for 12 AWG (20A), and orange for 10 AWG (30A).
- THHN/THWN-2 (Thermoplastic High Heat Nylon): This is individual conductor insulation, not a cable. It is a single copper wire coated in PVC and a thin nylon skin. It must be pulled through a raceway (conduit) and cannot be run loose inside walls.
- UF-B (Underground Feeder): Looks like NM-B but uses a solid, moisture-resistant PVC jacket that encases the wires completely without the paper wrap. It is rated for direct burial and wet locations.
- MC (Metal-Clad Cable): Conductors wrapped in an interlocking aluminum or steel armor jacket. It provides physical crush resistance and is often used in exposed commercial settings or residential garages where cables might be impacted.
Where You Meet This in Practice
You will rarely use all four types on a single project. Your physical environment and local code dictate which type belongs where.
Interior Drywall (NM-B): If you are adding a receptacle to a bedroom, running a switch leg to a bathroom vanity, or wiring a ceiling fan in a dry, climate-controlled space, NM-B is the standard. It is cheap, flexible, and staples directly to wooden framing.
Conduit Runs and Subpanels (THHN): When you transition from a main panel to a detached garage via underground PVC, or run exposed EMT conduit across a masonry basement wall, you pull individual THHN wires. The conduit provides the physical protection; the THHN provides the 90°C heat resistance.
Outdoor and Buried (UF-B): Wiring a landscape lighting transformer, a detached shed via shallow trench, or an outdoor post light requires UF-B. Standard NM-B will wick moisture through its paper wrap and rot if exposed to damp soil or weather.
Exposed and High-Impact (MC): If you are wiring surface-mounted outlets in an unfinished basement, a workshop, or a commercial retail space where the cable is exposed below 8 feet, MC cable prevents accidental snags from ladders, toolboxes, or shelving units.
Worked Example: Ampacity and Derating in Conduit
Choosing the wrong wiring type changes your allowable ampacity, especially when bundling wires. Let us look at a real-world scenario: running a 30A, 240V circuit to a workshop welder, requiring two hot wires, one neutral, and one ground (four total wires, three current-carrying) inside a single 3/4-inch EMT conduit.
If you use 10 AWG THHN inside the conduit, you start with a base ampacity of 40A. Because you have three current-carrying conductors in one pipe, NEC Table 310.15(C)(1) requires no derating (the 80% derating factor kicks in at 4-6 conductors). However, per NEC 110.14(C), you must size your breaker based on the termination temperature of your equipment. Most standard breakers and receptacles are rated for 75°C. Looking at the 75°C column for 10 AWG, the limit is 35A. The next standard breaker size down is 30A. Your 10 AWG THHN setup is perfectly legal and safe on a 30A breaker.
Now, what if you tried to pull a 10/3 NM-B cable through that same conduit instead of individual THHN wires? This is where DIYers fail inspections. NEC 334.80 strictly limits NM-B cable ampacity to the 60°C column, regardless of the insulation rating inside the jacket. In the 60°C column, 10 AWG is capped at exactly 30A. Furthermore, NM-B inside a conduit acts as a heat trap. While technically permissible if conduit fill limits are met, pulling a flat, sheathed cable through metal conduit generates massive friction, often tearing the jacket and ruining the wire.
Decision Tree: Picking the Right Wiring for Your Project
Use this matrix to eliminate guesswork. Follow your physical environment down the table to find your exact material requirement.
| Installation Environment | Physical Protection Required? | Moisture / Wet Location? | Required Wiring Type | Concrete Pick / Part Example |
|---|---|---|---|---|
| Interior stud bays (drywall) | No (Drywall covers it) | No | NM-B | Southwire 12/2 NM-B (Yellow) |
| Underground trench (18"+ deep) | No (PVC conduit optional) | Yes | UF-B or THWN-2 in PVC | Southwire 10/2 UF-B (Gray) |
| Surface-mounted in garage/shop | Yes (Impact risk) | No | MC Cable or THHN in EMT | AFC Industries 12/2 MC (Armor) |
| Inside PVC/EMT conduit | Yes (Conduit provides it) | Depends on conduit seal | THHN / THWN-2 | Southwire 12 AWG THHN (Spool) |
| Default Residential Branch | Standard Interior | Dry | NM-B | Buy: 12/2 NM-B (Part #55042122) |
The Default Pick: If you are wiring standard 20A receptacles or lighting circuits inside the conditioned, dry envelope of a residential home, buy 12/2 NM-B with a ground. The 12 AWG size gives you a 20A capacity, which is the modern standard for general-purpose receptacles, and the non-metallic jacket is the fastest, most cost-effective material to staple and pull through wooden framing.
Common Confusions and Code Caveats
Can I use THHN wire without conduit inside a wall?
No. THHN is only an insulation type, not a complete cable assembly. It lacks the outer jacket required to protect it from physical damage, nails, and insulation friction inside a wall cavity. Running bare THHN inside a wall is a direct violation of NEC 300.3 and will immediately fail a rough-in inspection. It must be housed in a raceway (conduit) or be part of a manufactured cable assembly.
Is it legal to run NM-B (Romex) through conduit?
Technically yes, but practically it is a terrible idea for long runs. The NEC allows NM-B to be sleeved in conduit for physical protection (like dropping down a masonry wall to a receptacle). However, you must calculate conduit fill based on the entire outer diameter of the NM-B cable, not just the individual wires. A 12/2 NM-B cable takes up vastly more cross-sectional area than three individual 12 AWG THHN wires, meaning you will need much larger, more expensive conduit. Furthermore, the friction of pulling a flat cable through bends often destroys the jacket.
Why is 14 AWG wire banned in some local codes?
While the baseline NEC guidelines permit 14 AWG NM-B on 15A breakers, many local Authorities Having Jurisdiction (AHJs) and modern municipal codes have amended the rules to require a minimum of 12 AWG (20A) for all general-purpose receptacle circuits. This reduces voltage drop on long runs and prevents homeowners from accidentally overloading a 15A lighting circuit with space heaters or vacuum cleaners. Always default to 12 AWG NM-B for receptacles to ensure you pass inspection anywhere in the country.
Does the ground wire count for conduit fill calculations?
No. When calculating conduit fill capacity (NEC Chapter 9, Table 1), equipment grounding conductors do not count toward the percentage fill limit. However, they do count when calculating ampacity derating for heat dissipation if they carry current in specific fault scenarios, though standard practice generally excludes the bare ground from the "current-carrying conductor" count for standard derating tables. Always count the hots and the neutral (if it carries unbalanced load) for derating.
