Wire cable types refer to the specific combinations of conductor material, stranding, insulation chemistry, and outer jacketing designed to safely carry electrical current in distinct physical environments. What this changes in a real installation is everything from your maximum allowable ampacity to whether the cable can survive being buried in wet soil or baked in a 120°F attic. Beginners commonly confuse "wire" (a single solid or stranded conductor) with "cable" (two or more insulated conductors bundled inside a single outer jacket), but in residential wiring, selecting the correct assembled cable type dictates your entire rough-in strategy, your material costs, and your compliance with the National Electrical Code (NEC).
The Big Three: NM-B, THHN, and UF-B
Walk into any electrical supply house or big-box store, and you will immediately encounter three dominant cable categories. Each is engineered for a specific thermal and moisture profile. The insulation temperature rating—typically 60°C, 75°C, or 90°C—determines how much heat the jacket can withstand before degrading, which directly impacts ampacity derating when you bundle conductors together.
| Cable Type | Common Name | Insulation Rating | Ampacity Column | Approved Environments | Avg. Cost (250ft, 12 AWG) |
|---|---|---|---|---|---|
| NM-B | Romex / Indoor Cable | 90°C | 60°C | Dry, indoor, concealed | $115 - $135 |
| THHN/THWN-2 | Conduit Wire | 90°C (Dry) / 75°C (Wet) | 90°C (Derating) / 75°C (Terminations) | Inside raceways, conduit, wet/dry | $65 - $85 (per conductor) |
| UF-B | Underground Feeder | 90°C | 60°C | Direct burial, wet, outdoor | $220 - $260 |
Where You Meet This In Practice
You will make wire cable type decisions at three critical phases of any residential or light-commercial project:
- Branch Circuit Rough-In: When wiring standard 15A and 20A indoor receptacles and lighting, 14/2 and 12/2 NM-B is the default. It is fast to pull through bored stud holes and requires no separate conduit.
- Feeder Runs to Subpanels: When pulling power from a 200A main panel to a 100A detached garage subpanel, NM-B is rarely used due to its stiffness in larger gauges and lack of wet-location rating. Instead, you will pull individual 1/0 AWG or 2 AWG THHN/THWN-2 conductors through 1.5-inch or 2-inch Schedule 80 PVC conduit.
- Outdoor and Wet Locations: If you are wiring a landscape lighting transformer, a detached shed, or an outdoor hot tub, any cable exposed to soil moisture or rain must be UF-B (if direct buried) or THHN inside liquid-tight or PVC conduit. NM-B will wick moisture through its paper filler and fail catastrophically.
Worked Numeric Example: Voltage Drop on a 100-Foot Shed Run
Selecting the right wire cable type isn't just about the insulation jacket; it is also about managing voltage drop over distance. The NEC recommends a maximum 3% voltage drop on branch circuits for efficiency. Let's calculate a 100-foot one-way run to a detached shed pulling a continuous 20A load at 120V nominal.
The Formula: Voltage Drop = (2 × Length × Current × Resistance per 1000ft) / 1000
- Attempt 1: 12 AWG Copper (Standard 20A circuit)
Resistance of 12 AWG uncoated copper = 1.93 Ω/kft.
VD = (2 × 100 × 20 × 1.93) / 1000 = 7.72V
Percentage: (7.72 / 120) × 100 = 6.43%. Result: Fails. Voltage at the shed is only 112.2V, which will cause motors and compressors to overheat. - Attempt 2: 10 AWG Copper (Upsized)
Resistance of 10 AWG = 1.21 Ω/kft.
VD = (2 × 100 × 20 × 1.21) / 1000 = 4.84V
Percentage: (4.84 / 120) × 100 = 4.03%. Result: Marginal. Better, but still exceeds the 3% ideal branch circuit recommendation. - Attempt 3: 8 AWG Copper (Correctly Sized for Distance)
Resistance of 8 AWG = 0.764 Ω/kft.
VD = (2 × 100 × 20 × 0.764) / 1000 = 3.05V
Percentage: (3.05 / 120) × 100 = 2.54%. Result: Passes.
The Installation Reality: You cannot direct-bury 8 AWG NM-B. For this trench, you must either buy 8/2 UF-B (which costs roughly $4.50 per foot) or pull three individual 8 AWG THHN conductors plus a 10 AWG green ground wire through buried PVC conduit (costing roughly $2.10 per foot in wire, plus conduit labor). This numeric reality is why THHN in conduit dominates long outdoor runs.
Common Installation Mistakes and Code Violations
Even experienced DIYers make critical errors when transitioning between wire cable types. Avoid these frequent NEC violations:
- Stripping NM-B for Conduit: Some builders strip the yellow or white outer jacket off NM-B cable to "save space" when pulling it through conduit. This violates NEC 334.12. The individual THHN wires inside NM-B are not rated for the physical stress of conduit pulling without their protective jacket, and the bare ground wire lacks insulation.
- Using NM-B in Wet Conduit: If you run NM-B through PVC conduit that exits the house and enters a wet location (like a patio cover), condensation will form inside the conduit and travel via capillary action into the NM-B paper wrap. Always transition to THWN-2 or use UF-B when the raceway enters a wet zone.
- Ignoring the 1.25x Continuous Load Rule: If your shed draws a continuous 16A load (running for 3+ hours), you cannot use a 20A breaker and 12 AWG wire. You must size the wire and breaker at 125% of the continuous load (16A × 1.25 = 20A minimum wire ampacity, requiring a 25A or 30A breaker and 10 AWG wire).
What are the best wire cable types for direct outdoor burial?
For direct burial without conduit, UF-B (Underground Feeder) is the standard for branch circuits up to 600V. It features a solid PVC jacket that encases the conductors, eliminating the paper filler found in NM-B. For larger feeder runs (like a 100A subpanel), direct burial MH feeder (Mobile Home feeder) or USE-2 (Underground Service Entrance) cables are preferred due to their robust, moisture-resistant XLPE insulation and higher ampacity ratings. Always check local codes for minimum burial depths, typically 24 inches for direct-buried cables.
Which wire cable types are required for a 200-amp main service panel?
For a 200-amp residential service entrance, you typically need 4/0 AWG aluminum or 2/0 AWG copper conductors. The standard cable type for overhead service drops is triplex or quadruplex aerial cable. For underground service laterals, URD (Underground Residential Distribution) or USE-2 cable is used. Inside the home, from the meter base to the main panel, you will pull individual THHN/THWN-2 conductors in rigid metal conduit or use SE-U (Service Entrance) cable, which features a bare concentric ground wire wrapped around the insulated phase conductors.
Can I mix different wire cable types in the same conduit run?
Yes, you can mix different insulation types (like THHN and THWN-2) in the same raceway, provided they are all rated for the environment (e.g., all must be wet-rated if the conduit is outdoors). However, when calculating ampacity derating for bundling, NEC 310.15 requires you to use the lowest temperature rating among the mixed conductors. You cannot mix NM-B cable and individual THHN wires in the same conduit run, as NM-B is not approved for use inside raceways.
