Electrical wires are single conductors—either bare or insulated—that carry current, while electrical cables are two or more wires bundled together inside a unified protective outer sheath. Understanding the different types of electrical cables and wires dictates your circuit's ampacity, voltage drop, and physical protection, separating a safe, code-compliant installation from a hazardous one. When you are standing in the electrical aisle, knowing exactly which spool or box to grab prevents return trips and ensures your breaker panel operates safely under load.
Wire vs. Cable: What Changes in a Real Installation
People commonly confuse the terms "wire" and "cable" in casual conversation, referring to a spool of "Romex wire" when they actually mean Romex cable. In a real installation, the distinction changes how you route, protect, and terminate the conductors. Think of a single wire as a lone vehicle on a road, while a cable is a convoy traveling together inside a single protective tunnel.
If you are pulling individual wires (like THHN), the National Electrical Code (NEC) requires you to install them inside a raceway, such as PVC conduit, EMT, or flexible metal conduit. The conduit provides the physical protection. If you are using a cable (like NM-B or UF-B), the outer jacket acts as the protective raceway, allowing you to staple it directly to studs or joists without additional conduit. Choosing between them changes your material cost, labor time, and the fill-capacity calculations you must perform before pulling the run.
The Core Types of Electrical Cables and Wires for Residential Use
While industrial and commercial sites use a massive variety of armored and shielded products, residential and light-commercial DIYers primarily deal with four main categories. Here is the breakdown of what you will actually use on a jobsite.
| Type | Conductors | Jacket / Armor | Primary Use Case | Max Temp / Ampacity Rule |
|---|---|---|---|---|
| NM-B (Romex) | 2, 3, or 4 + Ground | PVC Outer Jacket | Dry, indoor, concealed framing | 60°C column limit |
| THHN / THWN-2 | 1 (Individual) | PVC with Nylon coating | Inside conduit (EMT, PVC, ENT) | 90°C insulation (termination limits apply) |
| UF-B | 2, 3, or 4 + Ground | Solid, moisture-resistant PVC | Underground, wet locations, outbuildings | 60°C column limit |
| MC (Metal Clad) | 2, 3, or 4 + Ground | Aluminum interlocking armor | Exposed walls, garages, drop ceilings | 90°C insulation (termination limits apply) |
Where You Meet This in Practice: Sizing and Voltage Drop
The type of cable you choose directly impacts voltage drop over distance, which changes how your equipment performs. Let’s look at a worked numeric example to see how this plays out on the bench and in the walls.
The Scenario: You are wiring a 120V dedicated circuit for a workshop table saw that draws 16 Amps under continuous load. The run from the panel to the outlet is 85 feet. You initially plan to use standard 12 AWG NM-B cable, which is rated for a 20A breaker.
The Calculation: To find the voltage drop (VD), we use the standard single-phase formula: VD = (2 × K × I × D) / CM.
- K (resistivity of copper) = 12.9
- I (current) = 16A
- D (one-way distance) = 85 ft
- CM (circular mils for 12 AWG) = 6,530
VD = (2 × 12.9 × 16 × 85) / 6530 = 35,088 / 6530 = 5.37V.
The Result: A 5.37V drop on a 120V circuit is a 4.47% drop. According to NEC-style guidance and industry standards, branch circuit voltage drop should ideally not exceed 3%. At 4.47%, your table saw motor will run hot, lose torque, and potentially trip its internal thermal overload.
The Fix: You must upgrade the wire size. If you switch to 10 AWG NM-B (or pull 10 AWG THHN in conduit), the CM increases to 10,380.
VD = 35,088 / 10,380 = 3.38V.
This yields a 2.81% drop, safely under the 3% threshold. This is exactly where understanding your cable types and physical dimensions prevents equipment failure.
Ampacity Derating and Temperature Column Traps
The most common mistake DIYers make with cable types is misunderstanding the temperature rating printed on the jacket versus the ampacity table in NEC 310.16. If you strip back a 12/2 NM-B cable, you will see the internal individual wires are printed with "THHN 90°C".
However, because the overall PVC jacket of NM-B cannot dissipate heat as efficiently as individual wires in open air or large conduit, NEC Article 334.80 mandates that NM-B ampacity must be determined using the 60°C column. Therefore, 12 AWG NM-B is strictly limited to 20 Amps, and 14 AWG is limited to 15 Amps. You cannot use the 90°C column to uprate NM-B, even if your math suggests the copper can handle it. For a deeper dive into conductor ampacities and thermal limits, the Southwire Voltage Drop Calculator and wire data sheets provide excellent baseline reference values for copper and aluminum.
FAQ: Types of Electrical Cables and Wires
What type of electrical cable is used for underground outdoor wiring?
For direct burial without conduit, you must use UF-B (Underground Feeder) cable, buried at the depth specified by your local AHJ (typically 24 inches for residential branch circuits). If you prefer to dig a shallower trench (18 inches for PVC, or just 6 inches for rigid metal conduit), you should pull individual THWN-2 rated wires inside a continuous, glued Schedule 40 or 80 PVC conduit. Never use standard indoor NM-B cable underground or inside outdoor conduit, as the paper filler and standard jacket will absorb moisture and fail.
Can I mix NM-B Romex and THHN wire in the same circuit?
Yes, you can transition from NM-B cable to THHN wires inside an approved junction box or panel. For example, you might run 12/2 NM-B through your wall framing, terminate it in a metal junction box, and then pull 12 AWG THHN through EMT conduit down a surface-mounted garage wall to a receptacle. The circuit's maximum allowable ampacity will always be limited by the weakest link—in this case, the 20A limit of the 12 AWG conductors and the 60°C/75°C termination ratings of your devices.
What is the difference between 12/2 and 12/3 cable?
The first number (12) refers to the AWG wire gauge, while the second number (2 or 3) refers to the number of current-carrying insulated conductors inside the sheath, not counting the bare ground wire. A 12/2 cable contains one black (hot), one white (neutral), and one bare ground. A 12/3 cable adds a red (second hot) wire. You use 12/2 for standard 120V, 20A receptacle circuits, while 12/3 is used for 3-way/4-way switch legs, split-wire receptacles, or 240V baseboard heaters that require a neutral.
Why is MC cable preferred over NM-B in some garage or basement builds?
Under NEC 334.15, NM-B cable cannot be installed where it is subject to physical damage. In unfinished basements, garages, or utility rooms where the cable might be exposed on the surface of block walls or lower framing, inspectors will often flag NM-B as vulnerable to impact from tools, vehicles, or storage. MC (Metal-Clad) cable features an interlocking aluminum armor jacket that provides robust physical protection, allowing it to be surface-mounted in these exposed areas without needing to be hidden behind drywall or run inside rigid conduit.
