Different types of wiring refer to the specific insulation materials, jacket configurations, and conductor groupings used to safely route electrical current from a panel to a load based on the physical environment. When you select a cable type, you are not just picking a color or a gauge; you are locking in the maximum operating temperature, the allowable physical protection, and the exact ampacity derating rules dictated by the National Electrical Code (NEC). Choosing the wrong type changes the thermal limits of your circuit, alters voltage drop calculations, and can instantly fail an inspection or create a fire hazard.
The Core Cable Families: NM-B, THHN, UF-B, and MC
Before pulling any wire, you need to understand the four primary families of residential and light-commercial wiring. Each is governed by a specific NEC Article that dictates exactly where it can and cannot be installed.
| Cable Type | Full Name / Description | Max Voltage | Typical Environment | NEC Article |
|---|---|---|---|---|
| NM-B | Non-Metallic Sheathed Cable (Romex) | 600V | Interior dry, concealed wall cavities | Article 334 |
| THHN/THWN-2 | Thermoplastic High Heat-resistant Nylon-coated | 600V | Inside raceways, conduit, and panelboards | Article 310 |
| UF-B | Underground Feeder and Branch-Circuit Cable | 600V | Direct burial, damp/wet outdoor locations | Article 339 |
| MC | Metal-Clad Cable (Armor) | 600V+ | Commercial exposed runs, fire-rated assemblies | Article 330 |
Where You Meet These Different Types of Wiring in Practice
Knowing the definitions is only half the battle; recognizing where each type belongs on a jobsite is what separates a competent DIYer from a code-violating hack.
NM-B (Romex) in Interior Framing
You will use NM-B for 90% of standard indoor residential branch circuits. It is stapled to the sides of wooden studs, run through bored holes in joists, and terminated at outlets and switches. The outer PVC jacket is rated for 90°C, but the internal conductors are legally limited to the 60°C ampacity column in NEC Table 310.16 for standard residential sizing. It is strictly forbidden in wet locations, embedded in concrete, or run outdoors.
THHN/THWN-2 in Conduit Systems
When you transition from a drywall cavity to an exposed basement ceiling, a garage wall, or an outdoor run, you switch to THHN wires pulled through EMT (Electrical Metallic Tubing) or PVC conduit. The nylon coating on THHN makes it incredibly slick, allowing you to pull long runs through conduit bends without tearing the insulation. Because it is rated for 90°C, it offers higher baseline ampacity, though termination limits at breakers and devices usually cap you at the 75°C column.
UF-B for Direct Burial and Outbuildings
If you are feeding a detached shed, running power to a landscape lighting transformer, or wiring an outdoor receptacle post, UF-B is your standard. Unlike NM-B, the UF-B jacket is solid gray, UV-resistant, and completely encapsulates the internal conductors in a solid matrix of moisture-resistant plastic. According to EC&M's breakdown of NEC Article 334 and 339, UF-B can be direct-buried without conduit (typically at a minimum depth of 24 inches for residential branch circuits), though the segment emerging from the ground must be protected by conduit up to 8 feet.
MC Cable in Commercial and Exposed Runs
Metal-Clad cable features aluminum or steel interlocking armor. You will see this in commercial drop ceilings, hospital corridors, and exposed industrial walls where physical damage is a risk. It provides the grounding path (when equipped with an internal grounding wire and approved fittings) and eliminates the need to pull individual wires through conduit, saving massive amounts of labor on commercial fit-outs.
What Changes When You Swap Cable Types: A Numeric Example
The most critical thing that changes when you select different types of wiring is the ampacity derating requirement. Because THHN wires are bundled tightly inside a conduit, they trap heat. The NEC mandates that as you add more current-carrying conductors to a single raceway, you must reduce (derate) their allowable ampacity to prevent the insulation from melting.
Imagine you are wiring five separate 20A, 120V circuits to a detached garage. You decide to pull all the wires through a single 1-inch PVC conduit to save time. That means you have 10 current-carrying conductors (5 black hots, 5 white neutrals) in one pipe.
1. Baseline Ampacity: 12 AWG THHN is rated for 30A in the 90°C column of NEC Table 310.16.
2. Derating Factor: According to NEC Table 310.15(C)(1), 10 to 20 conductors in a raceway require a 50% derating factor.
3. Adjusted Ampacity: 30A × 0.50 = 15 Amps.
The Result: Your wire can now only safely carry 15A. You cannot protect this circuit with a standard 20A breaker. To maintain a 20A circuit with 10 conductors in a pipe, you must upsize to 10 AWG THHN (40A at 90°C; 40A × 0.50 = 20A).
If you had used NM-B cable instead, you would not apply this specific conduit derating chart because NM-B is not pulled through conduit in this manner (and stuffing multiple NM-B cables into a single bored hole requires a different set of thermal calculations under NEC 310.15(C)(2)). This single numeric reality dictates why commercial electricians often pull 10 AWG wire for standard 20A receptacle circuits when running long conduit home-runs.
Common Confusions and Code Traps
When evaluating different types of wiring, DIYers and junior apprentices frequently fall into two specific traps that violate the National Electrical Code published by the NFPA.
Confusion 1: Treating THHN as a standalone cable. Because THHN insulation is tough and rated for high heat, many assume it is simply a 'better' version of Romex and try to fish it through wall cavities or staple it directly to joists without conduit. This is a severe violation. THHN is a single conductor wire; NEC 300.15 and 310.10 strictly require it to be installed within a recognized raceway (like EMT, PVC, or flexible metal conduit) or a cable assembly. Exposed THHN is easily nicked by drywall screws or chewed by rodents, leading to dead shorts inside walls.
Confusion 2: Burying NM-B in the dirt. A common mistake when wiring a backyard feature is running standard yellow NM-B out of the basement, wrapping it in a plastic bag or slit PVC pipe, and burying it. NM-B is not rated for wet or damp locations. Ground moisture will eventually wick through the paper filler inside the NM-B jacket, corroding the copper and causing ground faults. You must transition to UF-B or individual THWN-2 wires inside a sealed, glued PVC conduit schedule 80 before the wire ever touches the earth.
Frequently Asked Questions About Different Types of Wiring
What are the different types of wiring allowed inside residential walls?
For standard concealed, dry residential wall cavities, NM-B (Non-Metallic Sheathed Cable) is the universally accepted standard. In areas where the walls are subject to physical damage (like an unfinished basement where studs are exposed), you must either use MC (Metal-Clad) cable, run THHN wires inside EMT conduit, or protect the NM-B with running boards or physical guards. You cannot use UF-B inside walls as a substitute for NM-B unless it is specifically required for a damp location, such as behind a tiled shower surround, though standard practice favors sealed conduit in those zones.
Which different types of wiring should I use for an underground shed run?
For an underground run to a detached shed, you have two primary code-compliant options. The first is direct-burial UF-B cable, which must be trenched to a minimum depth of 24 inches (for 120V residential branch circuits) and protected by rigid conduit where it emerges from the ground. The second, and generally preferred method for future-proofing, is pulling individual THWN-2 wires through a continuous, glued Schedule 40 or 80 PVC conduit buried at 18 inches. The conduit method allows you to easily pull new wires or upgrade the circuit amperage later without having to dig up your yard again.
Can I mix different types of wiring in the same junction box?
Yes, you can transition between different types of wiring inside a properly sized, accessible junction box. A classic example is running NM-B through a framed wall, terminating it inside a metal junction box, and then transitioning to THHN wires pulled through EMT conduit for an exposed garage run. The critical requirement is that the junction box must remain permanently accessible (you cannot drywall over it) and must have enough cubic inch volume to accommodate the physical bulk of the cable jackets and wire nuts, as calculated by NEC Article 314 box fill rules.
