Electrical wiring types refer to the specific insulation materials, jacket configurations, and conductor groupings used to safely route current from a panel to a load. Choosing the correct type dictates your installation method, maximum ampacity, and whether the circuit can survive damp environments or physical damage. Beginners commonly confuse cable (a factory-assembled bundle of insulated wires plus a bare ground, like NM-B) with individual wire (single conductors like THHN pulled through conduit), or they mistake a cable's voltage rating (600V) for its temperature ampacity derating column.
The Big Three: NM-B, THHN, and MC Cable
While there are dozens of specialized wiring methods in the National Electrical Code (NEC), residential and light-commercial DIYers will encounter three main workhorses. Here is how they stack up in 2026, factoring in current material costs and code applications.
| Wiring Type | Jacket / Armor | Wire Temp Rating | NEC Ampacity Column Used | Avg. Cost (12 AWG / 20A) | Best Application |
|---|---|---|---|---|---|
| NM-B (Non-Metallic) | PVC Jacket, Paper filler | 90°C (Wire) | 60°C (NEC 334.80) | $0.45 - $0.60 / ft | Indoor, dry residential branch circuits (walls, ceilings). |
| THHN/THWN-2 | Nylon-coated Thermoplastic | 90°C (Dry) / 75°C (Wet) | 75°C (Terminations) | $0.18 - $0.25 / ft (plus conduit) | Conduit runs, subpanel feeders, commercial lighting. |
| MC (Metal-Clad) | Interlocked Aluminum/Steel | 90°C (Wire) | 75°C (Terminations) | $1.10 - $1.40 / ft | Exposed runs, garages, commercial, physical damage risk. |
The most critical takeaway from this table is the Ampacity Column Used. Even though the individual wires inside a standard Southwire Romex SIMpull NM-B cable are rated for 90°C, the NEC forces you to use the 60°C column for sizing the breaker. This is a frequent trap for new builders who assume they can push higher currents through NM-B because of the 90°C printing on the jacket.
Where You Meet This in Practice: A Numeric Example
Understanding wiring types isn't just about picking what fits in the wall; it changes how you calculate voltage drop and size your conductors for long runs. Let's look at a real-world scenario: running a dedicated 20A, 120V circuit for a basement dehumidifier located 80 feet from the main panel.
The Scenario: You plan to use standard 12/2 NM-B cable. The dehumidifier draws a continuous 16A load (80% of the 20A breaker capacity).
The Math (Voltage Drop):
The formula for single-phase voltage drop is: Vd = (2 × K × I × L) / CM
- K (Copper constant) = 12.9
- I (Current) = 16A
- L (One-way length) = 80 ft
- CM (Circular Mils for 12 AWG) = 6,530
Vd = (2 × 12.9 × 16 × 80) / 6530 = 5.05V
A 5.05V drop on a 120V circuit is a 4.2% voltage drop. The NEC recommends a maximum of 3% for branch circuits to ensure motor longevity and efficient operation. If you blindly install the 12/2 NM-B because "it's rated for 20A," your dehumidifier compressor will run hot and trip on internal thermal overloads.
The Fix: You must upsize to 10/2 NM-B. The Circular Mils (CM) for 10 AWG is 10,380. Running the math again yields a 3.17V drop (2.6%), which safely clears the 3% threshold. This is where wiring type and physical gauge intersect: because NM-B is restricted to the 60°C ampacity column, 10 AWG is rated 30A, meaning you can still terminate it on a 20A breaker (using the "next size up" or standard termination rules) while gaining the physical copper mass needed to defeat voltage drop.
Common Confusions and Code Traps
When transitioning between wiring types on the jobsite, a few specific code violations trip up even experienced hobbyists.
Confusion 1: "THHN is 90°C, so I can use the 90°C ampacity table."
False. While THHN wire insulation won't melt until 90°C, NEC 110.14(C) termination rules dictate that the lugs on your breakers and receptacles are generally only rated for 75°C. Therefore, your circuit's maximum ampacity is bottlenecked by the weakest link (the termination), forcing you to use the 75°C column for THHN in conduit.
Confusion 2: Running NM-B inside conduit for long distances.
NM-B is a complete wiring method on its own. You are permitted to use a short sleeve of EMT conduit to protect NM-B from physical damage (e.g., dropping down a basement wall to a receptacle). However, you cannot use NM-B as a substitute for THHN to pull through 50 feet of underground PVC conduit. The jacket traps heat, and the paper filler acts like a sponge if condensation forms inside the conduit. For long conduit runs, strip the jacket and pull individual THWN-2 wires, or use individual wires from the start.
Confusion 3: UF-B vs. NM-B ampacity.
Underground Feeder (UF-B) looks like NM-B but has a solid PVC jacket that encases the wires individually, making it safe for damp locations and direct burial. However, because UF-B dissipates heat poorly compared to individual wires in conduit, NEC Article 339.5 mandates that UF-B must also use the restrictive 60°C ampacity column, exactly like indoor NM-B.
Frequently Asked Questions
What types of wiring electrical panels accept for main feeders?
For main service entrance feeders and large subpanel feeds (100A to 200A), electricians typically use either SER (Service Entrance Round) cable or individual XHHW-2 wires pulled through rigid or PVC conduit. In 2026, AA-8000 series aluminum is the dominant choice for feeders over 60A due to copper pricing. A 200A residential feeder usually requires 4/0 AWG aluminum XHHW-2 or SER. Never use standard NM-B for a 200A main feed; it is not rated for service entrance use and lacks the physical mass and thermal dissipation required.
Can I mix NM-B and THHN in the same junction box?
Yes, this is standard practice when transitioning from an interior wall (NM-B) to an exposed garage ceiling (THHN in EMT conduit). You must use a properly sized, accessible junction box. Inside the box, the bare ground from the NM-B must be bonded to the metal box (if metal) and pigtailed to the green grounding THHN wire. Ensure the box fill calculations (NEC Article 314) are respected; the bulky NM-B jacket and paper filler take up significant physical volume, so use a deep 4x4 or 4x11 box to avoid crowding the THHN pigtails.
Which types of electrical wiring are safe for direct outdoor burial?
For direct burial without conduit, you must use UF-B (Underground Feeder) for branch circuits (up to 30A typically) or USE-2 / XHHW-2 for larger feeders. Standard NM-B will rot and fail within months if buried. THHN/THWN-2 is rated for wet locations only if it is inside a raceway (conduit); the thin nylon coating provides no mechanical protection against rocks, shovels, or soil settling, so it cannot be direct-buried on its own. Always bury UF-B at least 24 inches deep, or 12 inches if using GFCI protection and restricted to 120V/20A (NEC Table 300.5).
Why is MC cable preferred in commercial buildings over NM-B?
Metal-Clad (MC) cable features an interlocked aluminum or steel armor jacket that provides inherent protection against physical damage, rodents, and accidental impact. Furthermore, commercial buildings often require plenum-rated cables (MC with special low-smoke zero-halogen jackets) to run through HVAC return air spaces without emitting toxic smoke in a fire. NM-B is strictly prohibited in commercial plenums and is generally not allowed in commercial construction over three stories due to fire-stop and physical damage requirements outlined in NEC Article 334.
