A wire is a single solid or stranded electrical conductor, while a cable is a group of two or more wires bundled together inside a single protective sheath. In a real circuit or installation, the distinction between wire and cable—and the specific insulation types used—dictates your maximum ampacity, physical routing limits, and whether the run will pass local electrical inspection. Most DIYers commonly confuse the physical stranding of a conductor (solid vs. stranded) with the wire-vs-cable distinction, assuming any bundled conductors are 'wires' and any single conductor is a 'cable,' which is exactly backward.
The Core Distinction: Wire vs. Cable in Home Wiring
To understand the different types of wires and cables, we have to look at how the National Electrical Code (NEC) defines them. According to EC&M's breakdown of NEC Article 100, a wire is a single conductor, bare or insulated. A cable is an assembly of two or more conductors. When you buy a spool of single blue THHN, you are buying wire. When you buy a 250-foot roll of 14/2 Romex, you are buying cable.
This distinction changes everything about your installation. Single wires must be routed inside a raceway (like EMT or PVC conduit) to protect them from physical damage. Cables, because they have their own integrated protective jacket, can often be stapled directly to framing lumber or run through bored holes in studs without additional raceway protection.
| Feature | Single Wire (e.g., THHN) | Cable (e.g., NM-B, UF-B) |
|---|---|---|
| Conductors | One (Hot, Neutral, or Ground) | Two or more bundled together |
| Protection | Requires conduit or raceway | Integrated outer jacket |
| Terminations | Pigtails or lugs in junction boxes | Direct to device or panel lugs |
| Typical Use | Commercial, long runs, wet locations | Residential branch circuits, dry locations |
Where You Meet Different Types of Wires and Cables in Practice
On a jobsite or in your garage, you will primarily encounter three main categories of residential wiring materials. Knowing which one to pull from the truck prevents failed inspections and fire hazards.
1. NM-B (Non-Metallic Sheathed Cable)
Commonly known by the brand name Romex, NM-B is the standard for indoor, dry-location residential branch circuits. It contains bare copper ground wires, insulated hot(s), and a neutral, all wrapped in a PVC jacket. It is strictly limited to dry locations and cannot be embedded in concrete, buried underground, or run through wet conduit.
2. THHN / THWN-2 (Single Conductor Wire)
This is the workhorse of commercial wiring and residential conduit runs. Cerro Wire's THHN/THWN-2 specifications note that modern dual-rated wire features a nylon outer skin for easy pulling through conduit and is rated for both dry (90°C) and wet (75°C) locations. You buy these as individual wires and pull three or four of them through a pipe to create a circuit.
3. UF-B (Underground Feeder Cable)
UF-B looks like NM-B, but the PVC jacket is solid gray and extruded tightly around the conductors, leaving no paper fillers. It is rated for direct burial and wet locations, making it the go-to cable for running power to a detached shed, a landscape lighting transformer, or an outdoor receptacle post.
Think of conduit fill like a multi-lane highway: if you pack too many cars (wires) into the lanes, traffic (current) slows down and generates excess heat. When you pull more than three current-carrying THHN wires in a single conduit, NEC Chapter 9 and Article 310 require you to derate the ampacity of the wires to prevent the insulation from melting inside the pipe.
Numeric Breakdown: Ampacity and Temperature Columns
The insulation type on your wire or cable dictates which temperature column in NEC Table 310.16 you must use to determine ampacity. Let us look at a worked numeric example for a 40A Level 2 EV charger installed 60 feet from the main panel.
The Setup: You need to deliver 40 amps continuous (which requires sizing the wire for 125% of the load, meaning 50A minimum wire ampacity). You are deciding between running 6/2 NM-B cable through the attic, or pulling three 8 AWG THHN wires through 3/4-inch EMT conduit along the garage wall.
- Option A: 6/2 NM-B Cable. NEC 334.80 mandates that NM-B ampacity must be based on the 60°C column, regardless of the fact that the internal wires might technically have 90°C insulation. Looking at the 60°C column for 6 AWG copper, the ampacity is 55A. This safely handles the 50A minimum requirement.
- Option B: 8 AWG THHN in Conduit. THHN is rated in the 90°C column (55A for 8 AWG). However, because the breaker and the EV charger terminations are likely only rated for 75°C, NEC 110.14(C) forces us to use the 75°C column for the final ampacity. In the 75°C column, 8 AWG copper is rated for 50A. This also perfectly meets the 50A minimum requirement.
The Verdict: You can use smaller, cheaper 8 AWG wire if you pull it through conduit, but you must use bulkier 6 AWG if you are stapling NM-B cable to the studs. The physical installation method changes the allowable current capacity.
Real-World Scenario: The 50A Hot Tub Feed Mistake
Theory is clean; the jobsite is messy. Here is a scenario that illustrates what happens when you confuse cable ratings with wire ratings in a wet environment.
Setup: A homeowner is wiring a new 50A, 240V hot tub located 60 feet from the main panel. To save money and avoid pulling individual wires, they buy 6/2 NM-B Romex with a ground. They dig a trench, lay down 1-inch PVC conduit from the house to the tub pad, and pull the NM-B cable through the conduit underground, terminating it in the tub's disconnect box.
Numbers: 6 AWG NM-B is rated 55A at 60°C. The breaker is 50A. Mathematically, the ampacity is fine for the load.
Outcome: The local electrical inspector red-tags the installation and fails it. Alternatively, if un-inspected, the GFCI breaker begins nuisance-tripping after a few months of rain, or the cable shorts out entirely.
What Went Wrong: The homeowner treated a wet location like a dry one. According to NEC 310.10, underground conduit is classified as a wet location because condensation and groundwater inevitably seep in. NM-B cable is strictly prohibited in wet locations. The paper wrap inside the NM-B jacket wicks moisture like a sponge, degrading the insulation and causing ground faults. Furthermore, pulling flat NM-B cable through 60 feet of PVC conduit is a physical nightmare that often damages the outer jacket, violating the cable's physical protection rating. The correct approach was to pull individual 6 AWG THWN-2 wires (rated for wet locations) through the PVC conduit.
Frequently Asked Questions
Can I strip the outer jacket off NM-B cable and use the individual wires inside conduit?
No. The individual insulated conductors inside NM-B cable are not marked with the THHN/THWN print required for conduit pulls. An inspector will fail this immediately because you cannot verify the insulation rating or wet-location suitability once the outer jacket and paper fillers are removed.
Why is stranded wire used in some cables and solid in others?
Solid wire (like standard 14 and 12 AWG NM-B) is cheaper to manufacture, holds its shape when stapled to studs, and pushes easily into back-wired receptacle holes. Stranded wire is used in larger gauges (like 6 AWG and larger NM-B) or in appliance cords because solid copper that thick is too stiff to bend around corners or flex without work-hardening and snapping.
Is THHN and THWN-2 the exact same wire?
Historically, they were different. THHN was for dry locations only (90°C), while THWN was for wet locations (75°C). Today, almost all manufacturers produce a dual-rated THHN/THWN-2 wire that is approved for both dry (90°C) and wet (75°C) locations. Always check the printing on the wire jacket to confirm the dual rating before pulling it through outdoor conduit.
