House wiring types refer to the specific combinations of conductor insulation, outer jacketing, and armor designed to protect electrical circuits from physical damage, moisture, and heat in distinct residential environments. Choosing the correct cable type dictates three critical variables in your installation: the allowable ampacity (how much current the wire can safely carry), the physical routing methods (whether it must be inside a conduit or can be stapled directly to studs), and the environmental rating (whether it can survive wet or buried conditions). The most common mistake DIYers make is confusing the wire type (the jacket and insulation material) with the wire gauge (the physical thickness of the copper, like 14 AWG vs. 12 AWG), or falsely assuming that all yellow "Romex" cable is waterproof and can be buried underground.
The Core Types of House Wiring Defined
To understand why we have different cables, you have to look at the anatomy of the wire. A standard electrical conductor consists of the copper metal itself, an inner layer of plastic insulation (like PVC or XLPE) that prevents the current from escaping, and an outer jacket or armor that protects the insulation from physical abrasion, UV light, and moisture.
The National Electrical Code (NEC) categorizes these cables based on their thermal limits and physical protection. For instance, Non-Metallic Sheathed Cable (NM-B) features individual conductors insulated at 90°C, but the overall cable assembly is strictly limited to the 60°C ampacity column per NEC Article 334.80. This means even though the plastic inside won't melt at higher temperatures, the heat dissipation properties of the bundled cable require us to treat it as a lower-temperature wire for safety.
Where You Meet This in Practice: The Big Five Cables
When you walk into the electrical aisle of a big-box store or open a supply house catalog, you will predominantly encounter these five types of house wiring. Each solves a specific physical problem on the jobsite.
| Cable Type | Outer Jacket / Armor | Max Temp Rating | Primary Use Case | Limitations |
|---|---|---|---|---|
| NM-B (Romex) | PVC with paper filler | 90°C (Derated to 60°C) | Interior dry walls, stapled to studs | Cannot be used in wet locations or buried |
| THHN / THWN-2 | None (Individual wires) | 90°C Dry / 75°C Wet | Pulled through EMT, PVC, or flex conduit | Must be enclosed in a raceway; no exposed runs |
| UF-B | Solid gray PVC (no paper) | 90°C (Derated to 60°C) | Direct burial, outdoor sheds, damp areas | Stiff and difficult to strip; higher voltage drop over distance |
| MC (Metal Clad) | Interlocking aluminum armor | 90°C | Commercial walls, exposed basement ceilings | Requires specialized MC cutters to avoid nicking wires |
| AC (Armored Cable) | Steel or aluminum spiral | 90°C (Derated to 60°C) | Retrofit work in older masonry or plaster walls | Relies on the armor for grounding; harder to pull |
Worked Numeric Example: Sizing and Derating a Multi-Wire Feeder
Let’s look at a real-world calculation where wire type and installation method intersect. Suppose you are running a feeder from your main panel to a detached garage subpanel. You need to supply 40 Amps of continuous load capacity. You decide to use individual 8 AWG THHN copper wires pulled through a 1.5-inch PVC conduit.
Your conduit contains four current-carrying conductors (two hots, one neutral, and one equipment ground—wait, the ground doesn't count for derating, so let's say you are pulling two separate 120V circuits: two hots, two neutrals = 4 current-carrying conductors).
- Base Ampacity: Looking at NEC Table 310.16, 8 AWG copper in the 90°C column (which THHN is rated for) has a base ampacity of 50A.
- Derating Factor: Because you have 4 current-carrying conductors in the same raceway, they generate mutual heat. Think of current-carrying conductors in a conduit like cars in a tunnel; the more cars you pack in, the more heat builds up, forcing everyone to slow down. NEC Table 310.15(C)(1) dictates an 80% adjustment factor for 4-6 conductors.
- The Math: 50A × 0.80 = 40 Amps.
- Outcome: Your 8 AWG THHN is perfectly sized for a 40A breaker. If you had tried to use 8 AWG NM-B cable for this, you would be forced to use the 60°C column, which limits 8 AWG to just 40A *before* any derating, and NM-B cannot legally be pulled through long conduit runs anyway.
Real-World Scenario: The Buried Conduit Mistake
Theory is great until you are standing in a trench with muddy boots. Here is a classic failure mode I see frequently in DIY forum post-mortems.
The Setup
A homeowner wants to power a shed 60 feet away. They dig a trench and lay down 1.5-inch Schedule 40 PVC conduit. To save money and avoid stripping individual wires, they buy a 250-foot roll of 10 AWG 3-conductor NM-B (Romex) and push the entire cable assembly through the buried PVC conduit, terminating it at a 30A double-pole breaker.
The Numbers
The 10 AWG copper is rated for 30A at the 60°C column. The breaker is 30A. The shed loads (lights, a small heater, a battery charger) pull about 22A. On paper, the circuit shouldn't trip.
The Outcome
Six months later, the breaker starts nuisance-tripping, and the homeowner notices a faint burning plastic smell near the shed junction box. When they pull the wire back out, the yellow PVC jacket is brittle, and the bare copper ground wire is heavily corroded.
What Went Wrong
The homeowner confused physical protection (the PVC conduit) with environmental rating. Conduit buried underground is classified by the NEC as a "wet location" because water will get inside it eventually. NM-B is strictly rated for "dry locations" only. The correct installation would have been to either use individual THWN-2 wires (the "W" stands for water-resistant) pulled through the conduit, or to use UF-B cable, which uses a solid, moisture-impervious gray PVC jacket with no paper filler, rated for direct burial.
Frequently Asked Questions
Can I mix NM-B and THHN in the same junction box?
Yes, this is incredibly common. A standard practice is to run NM-B cable through the dry wall cavities to a surface-mounted junction box, and then transition to individual THHN wires to run through EMT conduit to an exposed appliance or outdoor disconnect. Just ensure the junction box is large enough to accommodate the fill volume of both cable types, and use proper wire nuts or Wago connectors rated for the mixed strand/solid copper.
Why is my 12 AWG NM-B wire limited to 20A if the insulation says 90°C?
This is governed by NEC 334.80. While the cross-linked polyethylene (XLPE) or PVC insulation inside the jacket can physically withstand 90°C without melting, the outer PVC jacket and the bundled nature of the cable trap heat. The code mandates that the final overcurrent protection (the breaker) must be sized according to the 60°C column of the ampacity tables. For 12 AWG, that is strictly 20A, regardless of the 90°C print on the inner wires.
Is MC cable a suitable ground on its own?
It depends on the specific type. Standard interlocking aluminum MC cable requires a separate internal green equipment grounding conductor (EGC). However, "MC-AP" (Armor Protected) or continuous corrugated armor MC cable is listed to use the metal armor itself as the ground path, provided you use the correct listed MC fittings that bite into the armor to maintain equipotential bonding. Always check the manufacturer's spec sheet—usually from brands like Southwire or AFC Cable Systems—before relying on the armor for grounding.
For deeper reading on conductor ampacity and the National Electrical Code requirements for residential wiring, consult the NFPA 70 NEC guidelines or reference local authority having jurisdiction (AHJ) amendments, as local inspectors always have the final say on approved wiring methods in your specific municipality.
