Electrical wiring in homes refers to the specific insulated conductor assemblies and cables used to distribute power from the main panel to branch circuits, chosen based on environmental exposure and physical protection needs. The specific type you select dictates your installation method, material cost, and where the cable can legally be routed under the National Electrical Code (NEC). The most common mistake DIYers make is confusing a wire (a single insulated conductor like THHN) with a cable (a factory-assembled bundle like NM-B), or assuming indoor NM-B can be buried directly in the dirt like UF-B.
The Core Types of Electrical Wiring in Homes
To choose the right material, you need to understand the physical construction and temperature ratings of the standard options available at your local electrical supply house. Think of NM-B cable as a pre-packaged multi-lane highway, while THHN in conduit is like laying individual asphalt strips inside a concrete tunnel.
| Wiring Type | Construction & Jacket | Approved Locations | Max Temp Rating | Avg Cost (12 AWG) |
|---|---|---|---|---|
| NM-B (Romex) | 2+ insulated wires + bare ground in PVC jacket | Dry, indoor, concealed walls/ceilings | 90°C (derated to 60°C) | $0.65 / ft |
| UF-B | Solid PVC jacket encapsulating all conductors | Damp, wet, direct burial underground | 90°C (derated to 60°C) | $0.85 / ft |
| THHN/THWN-2 | Single conductor, PVC insulation + nylon jacket | Inside raceways (conduit) only | 90°C dry / 75°C wet | $0.20 / ft (per wire) |
| MC (Metal Clad) | Insulated wires + ground inside interlocking metal armor | Indoor, exposed, commercial, some residential | 90°C | $1.40 / ft |
The most critical takeaway from this table is the 60°C ampacity derating rule for NM-B and UF-B. Even though the wire insulation inside the jacket can handle 90°C, NEC Article 334.80 requires you to size the breaker based on the 60°C column of NEC Table 310.16. This means a 12 AWG NM-B cable is strictly limited to a 20A breaker, regardless of the 90°C rating printed on the jacket.
Worked Example: Sizing a 20A Garage Workbench Circuit
Let’s apply this to a real-world scenario. You need to run a 120V, 20A branch circuit from your main panel to a garage workbench located 80 feet away. The garage walls are unfinished, meaning the wiring will be exposed to potential physical damage from tools and lumber.
Step 1: Check Voltage Drop
Using the standard voltage drop formula: VD = (2 × Length × Current × K) / Circular Mils.
For 12 AWG copper (K=12.9, CM=6530):
VD = (2 × 80 × 20 × 12.9) / 6530 = 6.32V.
A 6.32V drop on a 120V circuit is 5.2%. The NEC recommends a maximum 3% drop for branch circuits. We must upsize to 10 AWG (CM=10380).
Recalculating with 10 AWG: VD = 41,280 / 10,380 = 3.97V (3.3%). This is acceptable for a garage workbench running heavy saws.
Step 2: Select the Wiring Method
Because the garage walls are unfinished, NEC 300.4 requires protection against physical damage. You have two choices:
- Option A: 10/2 NM-B inside EMT conduit. Material cost: 10/2 NM-B at $0.95/ft + 1/2" EMT at $0.65/ft = $1.60/ft. Problem: Pulling flat NM-B through long conduit runs is physically difficult, and conduit fill limits restrict how many cables you can jam inside.
- Option B: 10 AWG THHN inside EMT conduit. Material cost: Three 10 AWG THHN wires (Hot, Neutral, Ground) at $0.35/ft each = $1.05/ft + 1/2" EMT at $0.65/ft = $1.70/ft.
The Verdict: Option B (THHN in conduit) costs only $0.10 more per foot but saves hours of labor, pulls smoothly through the conduit, and offers superior heat dissipation. You will run NM-B from the panel to a junction box near the garage entry, then transition to THHN in EMT for the exposed wall runs.
Where You Meet This In Practice
Understanding the theory is only half the battle; installing these materials correctly requires adherence to specific mechanical rules that inspectors check immediately.
Securing and Supporting
NM-B cable must be secured within 8 inches of every single-gang electrical box without a cable clamp, and within 12 inches of boxes with clamps. After that, it must be stapled every 4.5 feet. Use insulated cable staples; bare metal staples can crush the PVC jacket and short the conductors over time.
Bending Radius
Never kink NM-B or MC cable. The NEC mandates that the bending radius of the inner edge of a cable must not be less than five times the diameter of the cable. For standard 12/2 NM-B, this means a minimum bend radius of about 1.5 inches. Kinking damages the internal paper separator and weakens the copper.
Transitioning Between Types
You cannot simply tape THHN wires to the end of an NM-B cable. Transitions must occur inside an approved, accessible junction box. Strip the NM-B jacket back at least 1/4 inch inside the box, secure it with a proper Romex connector, and use wire nuts or Wago lever nuts to bond the NM-B conductors to the THHN pigtails.
Frequently Asked Questions
Can I use indoor NM-B wiring for outdoor underground runs?
No. NM-B (Non-Metallic Sheathed Cable) is strictly rated for dry, indoor locations. The paper wrapping inside the jacket acts like a wick; if the outer PVC jacket is nicked and buried in damp soil, moisture will travel up the cable and cause a short or ground fault. For direct burial, you must use UF-B (Underground Feeder) cable buried at least 24 inches deep, or pull THWN-2 rated individual wires inside buried PVC conduit at an 18-inch depth.
What is the actual difference between AC (BX) and MC cable?
While both feature flexible metal armor, their grounding methods differ entirely. AC (Armor Clad, historically called BX) relies on the metal armor itself and a thin internal aluminum bonding strip to carry fault current back to the panel; it does not contain a dedicated green ground wire. MC (Metal Clad) cable contains a dedicated, full-sized green insulated or bare equipment grounding wire inside the armor. Modern residential construction heavily favors MC because the dedicated ground wire provides a more reliable, lower-impedance fault path.
Why do electricians pull THHN through conduit instead of just using NM-B?
Electricians use THHN in conduit for three primary reasons: physical protection, heat dissipation, and future-proofing. In exposed areas like garages, basements, or exterior walls, NM-B is vulnerable to impact damage. Conduit protects the wires. Furthermore, if a circuit needs to be upgraded from 15A to 20A in the future, an electrician can simply pull the old 14 AWG THHN out of the conduit and pull new 12 AWG THHN in, without having to rip open drywall to replace a solid NM-B cable.
Is aluminum wiring still used in modern residential construction?
Yes, but strictly for high-amperage feeder circuits, not standard branch circuits. While solid aluminum branch wiring from the 1970s caused fires due to oxidation and thermal expansion at termination points, modern AA-8000 series aluminum alloy is highly stable. It is routinely used today for service entrance cables (e.g., 4/0 AL for a 200A main service) and subpanel feeders (e.g., 2/0 AL for a 100A detached garage subpanel) because it is significantly lighter and up to 40% cheaper than equivalent copper. It must, however, be terminated using connectors rated specifically for aluminum (marked AL/CU) and treated with anti-oxidant paste.
