The types of wiring in electrical systems refer to the specific physical cable constructions and insulation materials—such as NM-B, THHN, UF-B, and MC—used to safely route current from the panel to loads based on environmental and mechanical requirements. Choosing the correct wiring type dictates your circuit's maximum ampacity, the physical protection required, and whether the installation passes local code inspections. People commonly confuse the physical cable type (like NM-B) with the circuit topology (like a multi-wire branch circuit or a ring main), but in residential and light commercial work, 'wiring types' almost always means the physical jacket and insulation you are pulling through your framing.
The Core Wiring Methods You Need to Know
Before you buy a single spool or coil, you need to understand the four primary wiring methods approved for most residential and light commercial projects under the National Electrical Code (NEC). Each has a specific temperature rating, jacket type, and approved installation environment.
| Wiring Type | Full Name / Description | Insulation Temp Rating | Primary Use Case |
|---|---|---|---|
| NM-B | Non-Metallic Sheathed Cable (Romex) | 90°C (but ampacity limited to 60°C) | Interior dry locations, concealed behind drywall. |
| THHN/THWN-2 | Thermoplastic High Heat-resistant Nylon-coated | 90°C dry / 75°C wet | Pulled through conduit (EMT, PVC) for exposed or long runs. |
| UF-B | Underground Feeder and Branch-Circuit Cable | 90°C (but ampacity limited to 60°C) | Direct burial, outdoor sheds, damp/wet locations. |
| MC | Metal-Clad Cable | 90°C | Commercial exposed runs, areas subject to physical damage. |
What This Changes in a Real Circuit (A Numeric Example)
The wiring type you choose fundamentally changes how many conductors you can run together and what size breaker you can use. Let's look at a real-world scenario where confusing these types leads to a code violation.
The Scenario: You need to run two separate 20A, 120V receptacle circuits from your panel to a workshop wall. You want to run both circuits through a single 3/4-inch EMT conduit to save space.
Approach A: Using 12 AWG THHN in Conduit
- You pull six individual 12 AWG THHN wires (two black, two white, two green/bare grounds). Only the four black and white wires are 'current-carrying conductors'.
- Base ampacity for 12 AWG THHN in the 90°C column is 30A.
- NEC Table 310.15(C)(1) requires an 80% adjustment factor for 4 to 6 current-carrying conductors in a single raceway.
- Adjusted ampacity: 30A × 0.80 = 24A.
- Since 24A is greater than your 20A breaker, this installation is perfectly legal.
Approach B: Using Two 12/2 NM-B Cables Bundled Together
- Instead of conduit, you try to staple two 12/2 NM-B cables tightly together through the studs for a distance of 30 inches.
- Base ampacity for 12 AWG NM-B is strictly limited to the 60°C column, which is 20A.
- NEC 334.80 states that if NM cables are bundled for more than 24 inches, you must apply the same derating factors. You have 4 current-carrying conductors, so the 80% factor applies.
- Adjusted ampacity: 20A × 0.80 = 16A.
- Because 16A is less than your 20A breaker, this is a severe code violation and a fire hazard. You would have to upsize to 10/2 NM-B (30A × 0.80 = 24A) or separate the cables.
Where You Meet This in Practice
Understanding the theoretical differences is only half the battle. Here is where you will physically encounter these wiring types on the jobsite or in your own home, along with the common mistakes DIYers make.
- Interior Walls and Ceilings: You will almost exclusively use NM-B (commonly referred to by the brand name Romex). It is cheap, easy to strip, and flexible. Common mistake: Running NM-B through a wet basement wall or burying it directly in concrete. The paper wrapping inside NM-B wicks moisture and rots, leading to short circuits.
- Outdoor Trenches and Detached Sheds: You will use UF-B. The conductors are fully encased in a solid, moisture-resistant gray plastic jacket. Common mistake: Burying UF-B too shallow. NEC requires a minimum of 12 inches of cover for a 120V/20A GFCI-protected circuit, or 18 inches for standard circuits. Always lay warning tape 12 inches above the wire.
- Exposed Basement Ceilings and Commercial Spaces: You will use MC (Metal-Clad) cable or individual THHN wires in EMT conduit. MC offers physical protection against impacts. Common confusion: People frequently confuse MC cable with older AC (Armored Cable / BX). AC cable relies on the metal armor itself and a thin internal bonding strip for grounding, which is prone to high impedance. Modern MC cable contains a dedicated, fully-sized green grounding wire inside the armor. Never use old AC cable for new circuits.
For a deeper visual breakdown of identifying these cables in older homes, Family Handyman's guide to electrical wire types offers excellent cross-sectional photography of legacy vs. modern jackets.
Frequently Asked Questions
What are the different types of wiring in electrical homes?
In modern residential construction, the vast majority of wiring is NM-B (Non-Metallic Sheathed Cable) for interior dry locations, and UF-B (Underground Feeder) for outdoor or damp locations. For service entrance feeders or heavy subpanel runs, you will also see SE-U (Service Entrance Cable) or individual THHN/THWN-2 wires pulled through large PVC or metal conduit. Older homes (pre-1960s) may still contain Knob-and-Tube or cloth-sheathed wiring, which lacks a grounding conductor and usually requires replacement or specialized insurance.
Which type of electrical wiring is best for outdoor use?
For direct burial underground without conduit, UF-B is the standard choice. If you are running power up an exterior wall or across a roof where the wire is exposed to UV light and physical damage, you must use individual THWN-2 wires inside a rigid or intermediate metal conduit (IMC), as standard THHN is not rated for wet locations and UV degrades standard NM-B jackets rapidly. Never use standard indoor NM-B for any outdoor application, even if it is under a soffit, as ambient humidity and condensation will compromise the paper insulation.
Can I mix NM-B and THHN wiring in the same circuit?
Yes, you can transition between NM-B and THHN in the same circuit, provided the transition happens inside an approved, accessible junction box or panel. A very common practice is to run 12/2 NM-B from the breaker panel to a ceiling junction box, and then transition to individual 12 AWG THHN wires in EMT conduit to drop down a masonry or finished wall to a receptacle. When doing this, your overcurrent protection (breaker size) must be sized to the lowest ampacity of the wire types used in the circuit, and all splices must be made with proper wire nuts or Wago connectors inside the box.
