Types of electrical wire refer to the specific combinations of conductor material, insulation chemistry, and outer jacketing designed for distinct environmental and thermal conditions. While the copper inside carries the current, it is the outer layers that dictate where the wire can legally and safely be installed, how much heat it can withstand, and how it must be protected.
The Core Types of Electrical Wire (Spec Sheet)
Before pulling any wire through a stud bay or trench, you need to know exactly what you are working with. The table below breaks down the four most common wire types you will encounter in residential and light commercial projects, based on current 2026 market availability and NFPA 70 (National Electrical Code) classifications.
| Wire Type | Jacket / Insulation | Max Temp Rating | Primary Use Case | 2026 Avg Cost (250ft) |
|---|---|---|---|---|
| NM-B (Non-Metallic) | PVC outer jacket, THHN inner conductors | 60°C (Assembly limit) | Indoor dry residential branch circuits (stapled to studs) | $115 (12/2 w/ ground) |
| THHN/THWN-2 | PVC insulation with nylon outer skin | 90°C (Dry) / 75°C (Wet) | Conduit runs, commercial wiring, panel pigtails | $48 (12 AWG solid) |
| UF-B (Underground Feeder) | Solid gray PVC encapsulating conductors | 60°C | Direct burial, outdoor lighting, detached garages | $165 (12/2 w/ ground) |
| MC (Metal-Clad) | Aluminum interlocking armor, THHN inner | 90°C | Commercial exposed runs, residential retrofit where NM-B is banned | $185 (12/2 w/ ground) |
What Wire Type Actually Changes in a Real Circuit
It is a common misconception that the copper conductor is the only thing that matters. In reality, the wire type dictates the temperature column you must use for ampacity calculations. This changes everything about how you route and protect the circuit.
Think of the insulation like a winter coat: a light windbreaker (60°C rating) is fine for a climate-controlled house, but you need a heavy parka (90°C rating) if you are packing multiple wires tightly together in a conduit where heat builds up. If you exceed the thermal limit of the insulation, it becomes brittle, cracks, and eventually causes a short circuit or arc fault inside the wall.
What people commonly confuse it with: The most frequent mistake DIYers make is confusing wire gauge (AWG size) with wire type (insulation rating). A 12 AWG wire is not universally rated for 20 Amps. A 12 AWG NM-B cable is limited to the 60°C column (20A), while a 12 AWG THHN wire is rated in the 90°C column (30A). However, NEC 240.4(D) caps small conductors at specific overcurrent limits regardless of the 90°C rating, which is why 12 AWG is still protected by a 20A breaker. The type matters most when derating for heat or bundling.
Worked Numeric Example: Conduit Derating Math
To see exactly what wire type changes in a real installation, let us look at a scenario where you must run a 20-Amp, 120V branch circuit through a 1.5-inch PVC conduit that already contains four other current-carrying conductors (six total).
According to NEC Chapter 9, Table 310.15(C)(1), bundling 4 to 6 current-carrying conductors requires an 80% derating factor. You must size the wire so that its derated ampacity still supports the 20A load.
Scenario A: Using 12 AWG NM-B (Inner conductors pulled into conduit)
- Per NEC 334.80, NM-B cable ampacity is strictly limited to the 60°C column, even though the inner wires are technically THHN.
- Base ampacity of 12 AWG at 60°C = 20A (Source: Cerrowire Ampacity Tables).
- Derating math: 20A × 0.80 = 16A.
- Result: 16A is less than the required 20A load. This installation fails code and creates a fire hazard.
Scenario B: Using 12 AWG THHN/THWN-2
- THHN in conduit utilizes the 90°C column for derating purposes.
- Base ampacity of 12 AWG at 90°C = 30A.
- Derating math: 30A × 0.80 = 24A.
- Result: 24A is greater than the 20A load. The wire safely handles the heat buildup, and the circuit is properly protected by the 20A breaker.
This single numeric example proves why you cannot simply strip the jacket off NM-B and pull it through a long, crowded conduit run. The wire type fundamentally changes the thermal headroom of the circuit.
Where You Meet This in Practice (and Common Confusions)
You will encounter these different types of electrical wire at specific phases of a build or retrofit:
- Rough-in Phase: You will mostly handle NM-B (often referred to by the brand name Romex). It is fast to staple to wooden studs, easy to strip, and cost-effective for standard indoor dry locations.
- Conduit and Bends: When routing through EMT (metal conduit) or PVC underground, you will pull individual THHN/THWN-2 wires. The nylon coating on THHN acts as a lubricant, allowing it to slide through tight conduit bends without tearing the insulation.
- Outdoor Trenching: When feeding a detached shed or landscape lighting, you will use UF-B. Unlike NM-B, which has a hollow void between the wires, UF-B has solid PVC molded tightly around every conductor, preventing moisture from wicking down the cable into your outdoor junction boxes.
Frequently Asked Questions
Q: Can I use THHN wire without conduit in my garage?
A: No. THHN is a single-conductor wire type that lacks the physical protection required for exposed runs. NEC 300.3 requires conductors to be installed in a recognized wiring method, such as a cable assembly (NM-B, MC) or a raceway (conduit). Leaving bare THHN exposed across garage rafters is a severe code violation and a physical damage hazard.
Q: Is stranded wire a different 'type' than solid wire?
A: Stranded vs. solid refers to the stranding of the copper conductor, not the wire type. You can buy 12 AWG THHN in both solid and stranded varieties. Stranded is vastly preferred for pulling through conduit because it is flexible, while solid is preferred for NM-B because it is stiff, making it easier to push into the back-wire holes of standard 15A/20A receptacles.
Q: Why is UF-B so much more expensive than NM-B?
A: The manufacturing process for UF-B requires extruding a solid, moisture-resistant PVC matrix completely around the conductors under high pressure, rather than simply wrapping a thin jacket around loosely bundled wires. The material cost and extrusion energy drive the price up by roughly 40-50% compared to standard NM-B.
Choosing the correct type of electrical wire is not just about passing an inspection; it is about matching the physical and thermal environment of your installation to the chemical limits of the insulation. Always verify your local AHJ (Authority Having Jurisdiction) amendments, as some municipalities ban NM-B entirely in favor of MC or conduit systems for fire resistance.
