Electrical wiring refers to the specific insulated or armored conductor assemblies used to route power from a source to a load, defined by their insulation material, jacket type, and approved installation environment. The kind of wiring you pull fundamentally changes your allowable ampacity, dictates your physical protection requirements, and determines whether the run can legally survive in damp basements, direct burial trenches, or exposed stud bays. Before we get into the weeds, we need to clear up the most common confusion: makers and DIYers constantly mix up a wire (a single insulated conductor, like THHN) with a cable (a factory-assembled bundle of wires with an outer jacket, like NM-B). Understanding the difference between these kinds of electrical wiring is the difference between a safe, code-compliant installation and a melted terminal lug.
The Core Kinds of Electrical Wiring at a Glance
Here is the spec-sheet breakdown of the five most common wiring types you will encounter in residential and light commercial work. Keep in mind that ampacity figures below assume standard conditions (not more than three current-carrying conductors in a raceway, ambient temperature of 30°C/86°F).
| Cable/Wire Type | Full Name & Description | Max Temp Rating | Standard Jacket / Armor | Approved Environments | Common AWG Sizes |
|---|---|---|---|---|---|
| NM-B | Non-Metallic Sheathed (Romex). Flat PVC jacket over THHN wires + bare ground. | 90°C (but ampacity limited to 60°C column) | PVC (Gray, Yellow, Orange, White) | Dry, concealed indoor locations only. | 14, 12, 10, 8, 6, 4, 2 |
| THHN/THWN-2 | Thermoplastic High Heat / Water Resistant Nylon. Individual conductors. | 90°C (Dry) / 75°C (Wet) | PVC insulation with slick Nylon outer coating | Must be installed inside conduit or approved raceways. | 14 through 4/0 and larger |
| UF-B | Underground Feeder. Solid PVC jacket encasing individually insulated wires. | 90°C (but ampacity limited to 60°C column) | Thick, solid gray PVC | Direct burial, wet/damp locations, outdoor feeds. | 14, 12, 10, 8, 6 |
| MC | Metal-Clad Cable. Interlocking aluminum or steel armor over THHN wires. | 90°C | Corrugated or interlocking metal armor | Exposed or concealed, commercial, where physical damage is a risk. | 14, 12, 10, 8, 6, 4 |
| SE | Service Entrance Cable. Heavy-duty jacket, often with bare concentric ground. | 90°C (but often limited to 75°C for terminations) | Thick gray PVC or rubberized cloth | Main feeder runs from meter to panel, or panel to subpanel. | 4, 2, 1/0, 2/0, 4/0 |
Worked Numeric Example: Feeding a 60A Garage Subpanel
Let's look at how the kind of wiring you choose changes the math, the materials, and the budget. Suppose you are running a 240V feeder from your main panel to a new 60A subpanel in a detached garage, exactly 100 feet away. You need two hot wires, one neutral, and one ground.
Option A: Pulling NM-B (Indoor routing through a shared drywall partition)
Because NM-B ampacity is locked to the 60°C column, a 6 AWG copper wire is only rated for 55A. That is not enough for a 60A breaker. You must step up to 4 AWG copper NM-B, which is rated for 70A at 60°C.
- Voltage Drop Check: 4 AWG copper has a resistance of 0.308 ohms/kFT. At a continuous load of 48A (80% of 60A), the voltage drop is: (2 × 100ft × 48A × 0.308) / 1000 = 2.95V. That is a 1.2% drop on a 240V circuit, well under the 3% NEC recommendation.
- 2026 Material Cost: 100 feet of 4-4-4-6 NM-B aluminum is cheaper, but if you insist on copper, 100 feet of 4 AWG copper NM-B will run you roughly $380 to $420.
Option B: Pulling THHN in PVC Conduit (Underground or surface mount)
When you pull individual THHN/THWN-2 wires in conduit, you get to use the 75°C column for terminations. A 6 AWG copper THHN is rated for 65A at 75°C, which is perfectly legal for a 60A breaker.
- Voltage Drop Check: 6 AWG copper resistance is 0.491 ohms/kFT. At 48A continuous: (2 × 100ft × 48A × 0.491) / 1000 = 4.71V. That is a 1.96% drop. Still perfectly acceptable.
- 2026 Material Cost: 100 feet of 6 AWG THHN (three rolls) plus a bare 10 AWG ground is about $130. Add $60 for 100 feet of 1-inch Schedule 40 PVC conduit and fittings. Total: ~$190.
The Verdict: By choosing THHN in conduit over NM-B, you save over $200, gain wet-location rating (if using THWN-2), and get superior physical protection. The only trade-off is the labor time required to glue PVC and pull the wires.
Where You Meet This in Practice
Theory is fine, but the jobsite is where wiring types reveal their true nature. Here is what you actually deal with when your hands are on the material.
NM-B (Romex) Realities
NM-B is the undisputed king of interior residential rough-in because it is fast. You unroll it, staple it, and strip it. But it has two major failure modes in the hands of amateurs. First, UV degradation. If you leave NM-B exposed to sunlight in an attic or on the exterior of a house, the PVC jacket will become brittle and crack within a single season. Second, improper stapling. NEC 334.30 requires NM-B to be secured within 12 inches of every box and every 4.5 feet thereafter. I have seen DIYers use standard office staples that slice right through the 14 AWG jacket, creating a dead short inside the wall.
Stripping THHN/THWN-2
THHN features a PVC inner insulation and a slick nylon outer skin. That nylon skin is what allows it to slide easily through tight conduit bends. When stripping THHN, never use standard wire strippers that clamp down hard; you will score the PVC underneath. Once the PVC is nicked, the conductor is compromised. Use a dedicated wire stripping tool calibrated for THHN, or carefully ring it with a utility knife and pull, keeping the blade angled away from the copper.
UF-B Trenching Rules
UF-B looks like NM-B, but the PVC jacket is solid and encases the wires completely, making it waterproof. However, waterproof does not mean 'invincible.' If you are direct-burying UF-B for a residential branch circuit, Southwire's installation guidelines and NEC Table 300.5 dictate a minimum burial depth of 24 inches. If you protect the circuit with a GFCI breaker and limit it to 120V/20A max, you can reduce that depth to 12 inches. Always lay warning tape 12 inches above the cable before you backfill the trench.
Common Confusions and Code Caveats
Q: What is the difference between MC and AC cable?
A: This is a classic trap. AC (Armor-Clad, often called BX) contains a bare aluminum bonding strip running under the metal armor, which allows the armor itself to act as the equipment grounding conductor. MC (Metal-Clad) does not have this strip. Standard MC relies on a separate green insulated or bare ground wire inside the armor. If you use standard MC and try to ground through the armor fittings without a dedicated ground wire, your circuit will lack a low-impedance fault path, and the breaker won't trip during a short.
Q: Can I use NM-B inside conduit?
A: Technically, the NEC allows NM-B to be run through conduit for physical protection (like running down a block wall to a disconnect). However, you cannot use it for long conduit runs. The jacket traps heat, and if you stuff multiple NM-B cables into a single conduit, you must apply severe ampacity derating factors. If you are pulling through conduit, strip the NM-B jacket and pull individual THHN wires instead.
Q: Why does my 10 AWG wire keep tripping the breaker?
A: 10 AWG copper is rated for 30A. If you are running a continuous load (like a space heater or EV charger that runs for 3+ hours), NEC 210.20 requires the breaker to be sized at 125% of the continuous load. A 24A continuous load requires a 30A breaker, but the wire will run hot. If you are experiencing nuisance trips, check if your load is continuous; you may need to step up to 8 AWG wire and a 40A breaker.
