Cable wires are assemblies of multiple insulated conductors bound together within a protective outer jacket, designed to route electrical power or signals from a source to a load. The specific jacket material and inner insulation dictate exactly where the cable can be legally installed (wet vs. dry, buried vs. exposed) and its maximum safe current capacity. Beginners frequently confuse a single "wire" (one conductor) with a "cable" (multiple conductors), or mistakenly assume the 90°C temperature rating printed on the inner conductors of standard NM-B cable applies to the cable's overall ampacity.
The Core Rule: Termination Temperatures and the 60°C Limit
To understand the different types of cable wires, you must first understand the "weakest link" rule of electrical terminations. Think of a circuit like a chain: the ampacity is limited by the lowest temperature rating of any connected component, conductor, or device.
Under NEC 110.14(C), standard residential breakers, switches, and receptacles rated 100A or less are generally tested and rated for 60°C or 75°C terminations. Even if the individual THHN wires inside a standard yellow NM-B (Romex) cable are printed with a 90°C rating, NEC Article 334.80 strictly mandates that the ampacity of NM-B cable must be determined using the 60°C column of the ampacity tables. This single rule changes how you size wire in a real installation, often forcing you to buy a thicker, more expensive gauge of NM-B than you would need if using individual wires in a conduit.
Spec Sheet: Common Residential Cable Types Compared
Below is a data-dense comparison of the four primary cable assemblies you will encounter in residential and light-commercial rough-ins. Prices reflect average 2026 retail costs for 250-foot spools of 12 AWG, 2-conductor with ground.
| Cable Type | Outer Jacket Material | Conductor Insulation | Ampacity Column Used | Wet Location Rated? | Avg Cost (12/2 w/ Gnd) |
|---|---|---|---|---|---|
| NM-B (Non-Metallic) | PVC (Color-coded by AWG) | THHN/THWN-2 (90°C) | 60°C Column | No (Dry only) | $0.45 - $0.55 / ft |
| UF-B (Underground Feeder) | Solid Gray PVC (Embedded) | THHN/THWN-2 (90°C) | 60°C Column | Yes (Direct Burial) | $0.65 - $0.80 / ft |
| MC (Metal Clad) | Interlocked Aluminum Armor | THHN/THWN-2 (90°C) | 75°C or 90°C* | Depends on fittings | $0.85 - $1.10 / ft |
| AC (Armored Cable / BX) | Interlocked Steel/Alu Armor | THHN/THWN-2 (90°C) | 75°C or 90°C* | No (Dry only) | $0.90 - $1.15 / ft |
*Note: MC and AC cables can utilize the 75°C or 90°C column for derating purposes, provided the terminations at both ends (breaker and device) are rated for that temperature. However, the final overcurrent protection is still capped by the termination rating.
Worked Example: Sizing a 40A EV Charger Circuit
Let's apply these cable types to a real-world scenario. You are installing a hardwired Level 2 EV charger rated for 40 amps of continuous draw. According to NEC 210.20(A), continuous loads require the branch circuit to be sized at 125% of the load.
The Math: 40A × 1.25 = 50A. You need a 50A breaker and wire with an allowable ampacity of at least 50A.
Many DIYers look at an ampacity chart and see that 8 AWG copper at 75°C is rated for 50A. They buy 8/2 NM-B. This will fail inspection. Because NM-B is restricted to the 60°C column, 8 AWG NM-B is only rated for 40A. You must upgrade to 6 AWG NM-B, which is rated 55A at 60°C. Cost for 50 feet: ~$95.
MC cable contains individual THHN conductors and is not bound by the NM-B 60°C restriction. Assuming your EV charger's internal terminal block and your panel's breakers are rated for 75°C, you can use the 75°C column. 8 AWG copper at 75°C is rated for exactly 50A. 8 AWG MC cable passes. Cost for 50 feet: ~$50. You save money and pull an easier, more flexible cable.
Where You Meet This in Practice
Understanding the physical and legal boundaries of these cables prevents the most common jobsite mistakes and failed inspections.
The "NM-B in Conduit" Trap
A frequent error is stripping the outer jacket off NM-B cable to pull the individual THHN wires through PVC conduit outdoors, or running intact NM-B through outdoor conduit to a detached garage. NM-B contains a paper filler and a jacket not rated for wet locations. Even if the conduit is sealed, temperature swings cause condensation inside the pipe. The paper wicks moisture, degrading the insulation and creating a ground-fault hazard. If you transition from indoor NM-B to outdoor conduit, you must terminate the NM-B in a junction box and splice to individual THWN-2 wires or use UF-B/MC cable for the outdoor run.
MC vs. AC: The Grounding Difference
While Metal Clad (MC) and Armored Cable (AC, historically called BX) look identical from the outside, their grounding mechanisms differ fundamentally. MC cable contains a dedicated green insulated (or bare) equipment grounding conductor (EGC). AC cable, however, relies on a thin, internal aluminum bonding strip pressed against the inside of the metal armor; the armor itself acts as the ground path. You cannot use standard MC fittings on AC cable without risking a poor ground connection, and vice versa. Always check the print legend on the jacket before buying fittings.
Direct Burial Depth Requirements
When routing power to an outdoor structure or EV pedestal, UF-B cable is the standard choice for direct burial. However, the cable type dictates the trench depth. Under NEC Table 300.5, UF-B cable buried directly in the earth requires a minimum cover of 24 inches. If you instead run individual THWN-2 wires inside Schedule 80 PVC conduit, the required burial depth drops to 18 inches (or 12 inches if using Rigid Metal Conduit). Choosing the right cable assembly directly changes your trenching labor.
Frequently Asked Questions
Can I use NM-B cable inside a metal stud wall?
Yes, but you must use listed plastic bushings or grommets in every knockout hole where the cable passes through the metal stud. The sharp, stamped edges of metal studs can easily slice through the soft PVC jacket of NM-B during or after installation, exposing the bare ground wire or hot conductors to the grounded metal framing.
Is it legal to paint over exposed NM-B cable in a basement?
The NEC does not explicitly forbid painting NM-B, but paint solvents can chemically degrade the PVC jacket over time, making it brittle. Furthermore, painting over the jacket obscures the manufacturer's print legend (AWG, voltage rating, and type), which inspectors rely on to verify compliance. It is best practice to leave the jacket its original color.
Why does my 10 AWG UF-B cable keep tripping the 30A breaker?
If you are running a 30A continuous load (like a heavy-duty space heater or workshop welder) on 10 AWG UF-B, you are violating the 125% continuous load rule. 10 AWG at 60°C is rated for exactly 30A. A 30A continuous load requires a 40A circuit and 8 AWG wire. The breaker is tripping because it is thermally overloaded, doing exactly what it was designed to do.
